The photoelectric effect isphysicsIt is an important and magical phenomenon.At frequencies higher than a certain frequencyelectromagnetic wave(This frequency is called the threshold frequency)ElectronicsIt is formed after absorbing energy and escapingelectric currentThat is, light generates electricity.
Photoelectric phenomenonGerman physicisthertzIt was discovered in 1887, and the correct interpretation isEinsteinProposed.In the process of scientists studying photoelectric effect, physicistsphotonOfquantumA deeper understanding of the nature ofWave particle dualityThe concept has a significant impact.[1]
Chinese name
photoelectric effect
Foreign name
Photoelectric effect
Discovery date
1887
Correct interpretation
Einstein
Discipline
physics
Phenomenon
Some substances generate photocurrent under the irradiation of light
Light irradiation on metal causes changes in the electrical properties of the material.This kind of phenomenon of light variable calling is collectively called photoelectric effect.Photoelectric effect is divided into photoelectron emission, photoconductive effect and barrier photoelectric effect, also known as photovoltaic effect.The former phenomenon occurs on the surface of an object, also known as the photoelectric emission.The latter two phenomena occur inside the object, which is called internal photoelectric effect.
According to the particle theory, light is composed of discontinuous photons one by one. When a photon irradiates a light sensitive substance (such asselenium）When it goes up, its energy can be completely absorbed by an electron in the material.After the electron absorbs the energy of the photon, the kinetic energy increases immediately;If the kinetic energy is increased enough to overcome the gravity of the atomic nucleus, it can fly out of the metal surface in one billionth of a second, become photoelectrons, and formPhotocurrent。In unit time, the larger the number of incident photons, the more photoelectrons will fly out, and the stronger the photocurrent will be. This phenomenon of automatic discharge from light energy to electrical energy is called photoelectric effect.[1]
hertzThe photoelectric effect was discovered in 1887,EinsteinThe first one successfully explained the photoelectric effect (the effect of metal surface emitting electrons under light irradiation, and the emitted electrons are called photoelectrons).Only when the light frequency is greater than a certain critical value can electrons be emitted, that is, the cut-off frequency. The corresponding light frequency is called the limiting frequency.The critical value depends on the metal material, while the energy of emitted electrons depends on the wavelength of light and has nothing to do with the light intensity, which cannot be explained by the fluctuation of light.Another point contradicts with the fluctuation of light, that is, the instantaneity of photoelectric effect. According to the fluctuation theory, if the incident light is weak, the irradiation time should be longer, so that the electrons in the metal can accumulate enough energy and fly out of the metal surface.But the fact is that as long as the frequency of light is higher than the limit frequency of metal, the brightness of light is almost instantaneous, no matter how strong or weak, and the generation of electrons is not more than ten minus nine seconds.The correct explanation is that light must be composed of strictly specified energy units (i.e. photons or photons) related to wavelength.
photoelectric effect
In the photoelectric effect, the emission direction of electrons is not completely directional, but most of them are emitted perpendicular to the metal surface, independent of the light direction.Light is an electromagnetic wave, but it is an orthogonal electromagnetic field with high-frequency oscillation, and its amplitude is very small, so it will not affect the direction of electron emission.
Photoelectric effect shows that light is particle.Correspondingly, the most typical example of the wave nature of light is the interference and diffraction of light.
As long as the frequency of light exceeds a certain limit frequency, photoelectrons will immediately escape from the metal surface exposed to light, resulting in photoelectric effect.When a closed circuit is added outside the metal and a positive power supply is added, all these escaped photoelectrons reach the anode and form the so-called photocurrent.When the incident light is constant, increase the forward voltage of the two poles of the photocell, increase the kinetic energy of the photoelectron, and the photocurrent will increase accordingly.However, the photocurrent will not increase infinitely. It is restricted by the number of photoelectrons. There is a maximum value, which isSaturation current。Therefore, when the intensity of the incident light increases, according to the photon hypothesis, the intensity of the incident light (that is, the light energy passing through the unit vertical area in unit time) is determined by the number of photons passing through the unit vertical area in unit time, and the number of photons passing through the metal surface in unit time also increases, so the number of collisions between photons and electrons in the metal also increases,As a result, the number of photoelectrons escaping from the metal surface per unit time increases, and the current also increases.
Research History
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The photoelectric effect was first developed by German physicistsHeinrich Hertz It was discovered in 1887, which played a fundamental role in the development of quantum theory and the assumption of wave particle duality.Philipp Lenard The important rule of photoelectric effect was found by experiment.Albert EinsteinAnd put forward the correct theoretical mechanism.
nineteenth century
In 1839, at the age of 19, Alexandre Becquerel discovered the photovoltaic effect while assisting his father in studying the effects of light waves on the electrolytic cell.Although this is not an optical effect, it plays a great role in revealing the close relationship between the electrical properties of matter and light waves.In 1873, Willoughby Smith carried out a task related to underwater cables to test the high resistance property of selenium cylinder, and found that it has photoconductivity, that is, illuminating a beam of light on the selenium cylinder will increase its conductivity.
Heinrich Hertz
In 1887, German physicist Heinrich Hertz made experiments to observe the photoelectric effect and the emission and reception of electromagnetic waves.There is a spark gap in the Hertz transmitter, which can generate and emit electromagnetic waves by making sparks.There is a coil and a spark gap in the receiver. When the coil detects electromagnetic waves, sparks will appear in the spark gap.Because the spark was not very bright, he put the whole receiver into an opaque box in order to observe the spark more easily.He noticed that the maximum spark length was therefore reduced.In order to clarify the reason, he removed part of the box and found that the opaque plate between the receiver spark and the transmitter spark caused this shielding phenomenon.If glass is used for separation, this shielding phenomenon will also be caused, while quartz will not.After using a quartz prism to decompose light waves according to their wavelengths, and carefully analyzing the shielding behavior of light waves at each wavelength, he found that ultraviolet light caused the photoelectric effect.Hertz published these experimental results in《Physical yearbook》He did not do further research on this effect.
The discovery that ultraviolet rays incident on the spark gap can help generate sparks immediately aroused the curiosity of physicists, including Wilhelm Hallwachs, Augusto Righi, Aleksandr Stoletov, etc.They carried out a series of investigations on the effects of light waves on charged objects, especially ultraviolet rays.These research investigations have confirmed that if the newly cleaned zinc metal surface is negatively charged, no matter how much it is, it will quickly lose this negative charge when exposed to ultraviolet radiation;If the neutral zinc metal is irradiated by ultraviolet light, it will quickly become positively charged, and the electrons will escape to the gas around the metal. If the strong wind blows on the metal, the number of positive charges can be greatly increased.
Johann elster and Hans Geitel first developed the first practical photoelectric vacuum tube, which can be used to measure irradiance.Esther and Gatel used it to study the effects of light waves on charged objects, and achieved great results.They arrange various metals according to the discharge capacity of photoelectric effect from large to small:rubidium, potassium, sodium potassium alloy, sodium, lithium, magnesiumthallium, Zinc.For copper, platinum, lead, iron, cadmium, carbon and mercury, the photoelectric effect caused by ordinary light waves is very small, and no effect can be measured.The above metals are arranged in the same order asAlexandro FudaThe more positive the metal is, the greater the photoelectric effect will be.
Thomson Photoelectric effect experimental device for measuring the charge mass ratio of particles.
At that time, various experiments to study the "Hertz effect" were accompanied by the phenomenon of "photoelectric fatigue", which made the research more complicated.Photoelectric fatigue refers to the phenomenon of gradual decline of photoelectric effect observed from clean metal surface.According to the research results of Holfax, ozone plays an important role in this phenomenon.However, other factors, such as oxidation, humidity, polishing mode, etc., must be taken into consideration.
From 1888 to 1891, Stoletov completed many experiments and analyses on photoelectric effect.He designed a set of experimental device, especially suitable for quantitative analysis of photoelectric effect.With this experimental device, he found the direct ratio of irradiance and induced photocurrent.In addition, Stolertov and Rigi also jointly studied the relationship between photocurrent and air pressure. They found that the lower the air pressure, the greater the photorheology, until the optimal air pressure;Below this optimal pressure, the lower the pressure, the smaller the photoelectric rheology.
Joseph John Thomson On April 30, 1897Royal Institute of Great Britain(Royal Institution of Great Britain) said in his speech that by observing the fluorescence irradiance caused by the cathode ray in the Krux tube, he found that the transmission capacity of the cathode ray in the air is far greater than that of particles of ordinary atomic size.Therefore, he advocated that cathode rays were composed of negatively charged particles, later called electrons.Shortly thereafter, he measured the charge mass ratio of cathode ray particles by observing the deflection of cathode ray caused by the action of electric and magnetic fields.In 1899, he irradiated the zinc metal with ultraviolet light, and measured the charge mass ratio of the emitted particles to be 7.3 × 10emu/g, which was roughly consistent with the value of 7.8 × 10emu/g of the cathode ray particles measured in the previous experiment.He therefore correctly concluded that the two particles were the same kind of particle, that is, the electron.He also measured the negative charge contained in the particle.From these two data, he successfully calculated the mass of the electron: about one thousandth of the mass of hydrogen ion.The electron was the smallest particle known at that time.
Philip Leonard discovered in 1900 that ultraviolet light can induce ionization of gases.Because this effect widely occurs in the air over a wide area of several centimeters, and many large positive ions and small negative ions are produced, this phenomenon is naturally interpreted as the photoelectric effect occurs in solid particles or liquid particles in the gas, which is how Thomson interprets this phenomenon.In 1902, Leonard published several important experimental results on photoelectric effect.First, by changing the distance between the ultraviolet light source and the cathode, he found that the number of photoelectrons emitted from the cathode per unit time was proportional to the incident irradiance.Second, using different materials as cathode materials, it can be shown that each kind of material emits photoelectrons with its specific maximum kinetic energy (maximum speed). In other words, the maximum kinetic energy of photoelectrons isspectrumComposition related.Third, by adjusting the voltage difference between cathode and anode, he observed that the maximum kinetic energy of photoelectrons is proportional to the cut-off voltage, independent of irradiance.
Since the maximum speed of photoelectrons is independent of irradiance, Leonard believes that light waves do not give these electrons any energy. These electrons already have this energy. The role of light waves is like a trigger, which selects and releases electrons bound to atoms instantly. This is Leonard's famous "triggering hypothesis".During that period, the academic community widely accepted the trigger hypothesis as the mechanism of photoelectric effect.However, this hypothesis encountered some serious problems. For example, if the electron had already possessed the kinetic energy after escaping the binding and emission in the atom, then heating the cathode should give more kinetic energy, but physicists did not measure any different results in their experiments.
The brilliant Einstein published six epoch-making papers in 1905 (Einstein's miracle year).
In 1905, Einstein published a paper "A tentative view on the generation and transformation of light", giving another explanation for the photoelectric effect.He described the light beam as a group of discrete quanta, now called photons, rather than continuous waves.aboutMax Planck Previously under studyBlackbody radiationEinstein gave another interpretation of the Planck relation found in: the energy of a photon with frequency f is E=hf;Where the h factor isPlanck constant。Einstein believed that the energy of each quantum of light beam is equal to the frequency times Planck's constant.If the frequency of a photon is greater than a certain limit frequency, the photon has enough energy to make an electron escape, causing the photoelectric effect.Einstein's argument explains why the energy of photoelectrons is only related to frequency, but not irradiance.Although the irradiance of the beam is very weak, as long as the frequency is high enough, some high-energy photons will be generated to facilitate the escape of bound electrons.Although the irradiance of the beam is very strong, if the frequency is lower than the limit frequency, it is still unable to give any high-energy photons to facilitate the escape of bound electrons.
Einstein's argument is very imaginative and persuasive, but it met strong resistance from the academic community, because it is closely related toJames Clerk Maxwell The wave theory of light, which has been strictly theoretical tested and proved by precise experiments, is contradictory. It cannot explain the refraction and coherence of light waves. More generally, it is contradictory to the "infinite separability hypothesis" of energy in physical systems.Even after the experiment proved that Einstein's photoelectric effect equation was correct, the strong resistance continued for many years.Einstein's discovery opened the door to quantum physics. Einstein won the honor of 1921 for his achievements in theoretical physics, especially the discovery of the law of photoelectric effectThe nobel prize in physics。
The figure shows the relationship between the maximum energy and the frequency obtained from Millikan's photoelectric effect experiment.The vertical axis is the cut-off voltage that can prevent the maximum energy photoelectron from reaching the anode, P is the work escaped, and PD is the potential difference.
Einstein's paper soon attracted the attention of American physicist Robert Millikan, but he also disagreed with Einstein's theory.In the next ten years, he spent a lot of time doing experiments to study the photoelectric effect.He found that the maximum energy of photoelectrons would not increase with the increase of cathode temperature.He also confirmed that the photoelectric fatigue phenomenon was caused by impurities generated by oxidation. If the clean cathode could be stored in a high vacuum, this phenomenon would not occur.In 1916, he confirmed the correctness of Einstein's theory, and directly calculated Planck's constant using the photoelectric effect.Millikan was awarded the 1923 Nobel Prize in Physics for his work on fundamental charges and photoelectric effects.
According to the wave particle duality, the photoelectric effect can also be analyzed with the wave concept, without the photon concept.Willis Lamb and Marlan Scully proved this theory in 1969.
Mathematical derivation
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The energy of photons in a beam is proportional to the frequency of light.If the free electron in the metal absorbs the energy of a photon, and the energy is greater than or equal to a certain energy threshold related to the metal (called the metal's escape work), the electron will escape from the metal because it has enough energy, becomingphotoelectron；If the energy is insufficient, the electron will release energy, and the energy will become a photon again to leave. The electron energy can not escape from the metal until it recovers to absorption.Increasing the irradiance of the beam will increase the "density" of photons in the beam, which will excite more electrons at the same time, but will not make each excited electron gain more energy by absorbing more photons.In other words, the energy of photoelectrons is not related to irradiance, but only to the energy and frequency of photons.
The electron irradiated by the beam will absorb the energy of the photon, but the mechanism follows the criterion of "all or nothing". All the energy of the photon must be absorbed to overcome the escaping work, otherwise the energy will be released.If the energy absorbed by the electron can overcome the escaping work, and there is residual energy, the residual energy will become the kinetic energy of the electron after being emitted.
The escape work W is the minimum energy required to emit a photoelectron from the metal surface.From the perspective of frequency, the frequency of photons must be greater than the limit frequency of metal characteristics to give electrons enough energy to overcome the work escaping.The relationship between the work escaping and the limiting frequency v0 is
W=h*v0
Where h is the Planck constant and W is the energy of the photon whose light frequency is v0.
After overcoming the work escaping, the maximum kinetic energy Kmax of photoelectron is
Kmax=hv-W=h（v-v0）
Where hv is the energy carried by the photon whose light frequency is v and absorbed by the electron.
The actual physics requires that the kinetic energy must be positive. Therefore, the light frequency must be greater than or equal to the limit frequency before the photoelectric effect can occur.
Photoelectric effect original
An enlightening view on the generation and transformation of light
Einstein
March 1905
There are profound formal differences between physicists' theoretical concepts of gases or other heavy objects and Maxwell's theory of so-called electromagnetic processes in empty space.That is, we believe that the state of an object is completely determined by the coordinates and velocities of a large but limited number of atoms and electrons;On the contrary, in order to determine the electromagnetic state of a space, we need to use continuous spatial functions. Therefore, in order to completely determine the electromagnetic state of a space, we cannot consider that a finite number of physical quantities are enough.According to Maxwell's theory, for all pure electromagnetic phenomena and therefore for light, energy should be regarded as a continuous spatial function. According to physicists, the energy of a heavy object should be expressed by the sum of the energy carried by atoms and electrons.The energy of a heavy object cannot be divided into any number of small parts. According to Maxwell's theory of light (or more generally, according to any wave theory), the energy of the light beam emitted from a point light source is continuously distributed in an increasing volume.
The wave theory of light, which is calculated by continuous space function, has been proved to be very excellent in describing pure optical phenomena, and it seems difficult to replace it with any other theory.However, it should not be forgotten that optical observation is related to the time average rather than the instantaneous value, and although the theories of diffraction, reflection, refraction, dispersion and so on are completely confirmed by experiments, it can still be envisaged that when people apply the theory of light calculated by continuous space functions to the phenomenon of light generation and transformation,This theory will lead to conflict with experience.
Indeed, in my opinion, aboutBlackbody radiation，PhotoluminescenceUV light generates cathode rays, as well as other observations on the generation and transformation of light. If we use the hypothesis that the energy of light is not continuously distributed in space to explain it, it seems to be better understood.According to the assumption envisaged here, the energy of the light beam emitted from the point light source is not continuously distributed in the growing space during the propagation, but consists of a limited number of energy particles confined to each point in space. These energy quanta can move, but can not be divided, and can only be absorbed or generated as a whole.
Next, I will describe my thinking process and quote some facts that led me to this path. I hope that the points to be explained here may be useful for some researchers in their research.
§ 1 A difficulty in the theory of "blackbody radiation"
Let's still usemaxwellTheory and electronic theory.In a space enclosed by a completely reflective wall, there are a certain number of gas molecules and electrons that can move freely, and when they are very close to each other, they exert a conservative force on each other, that is, they can collide with each other like gas molecules in the theory of gas molecule motion.In addition, it is assumed that a certain number of electrons are bound to some distant points in this space by some force, and the direction of the force points to these points, whose size is proportional to the distance between the electrons and each point.When free gas molecules and electrons are very close to these bound electrons, these electrons should also interact conservatively with free molecules and electrons.We call these electrons bound on the space point "vibrators";They emit electromagnetic waves of a certain period and also absorb electromagnetic waves of the same period.
According to the modern point of view on the generation of light, in the space we are investigating, the radiation under the condition of dynamic equilibrium according to Maxwell's theory should be identical with the "blackbody radiation" - at least when we regard all oscillators with frequencies that should be considered as existence.
We will not consider the radiation emitted and absorbed by the oscillator for the time being, but will deeply discuss the conditions of dynamic equilibrium corresponding to the interaction (or collision) of molecules and electrons.The kinetic theory of gas [molecule] puts forward the condition for dynamic equilibrium: the average kinetic energy of an electronic vibrator must be equal to the average kinetic energy of a gas molecule in translational motion.If we decompose the motion of the electronic oscillator into three mutually perpendicular partial vibrations, then we can find the average value of the energy of such a linear partial vibration
by
Here R is the absolute gas constant, N is the "actual molecule" number of gram equivalent, and T isAbsolute temperature。Since the average value of the kinetic energy and potential energy of the vibrator with respect to time is equal, the energy
Equal to the kinetic energy of free monatomic gas molecules
。If in the 21st century, for any reason - in our case, due to the radiation process - the energy of a vibrator is greater or less than
Then, its collision with free electrons and molecules will cause the gas to gain or lose energy whose average is not equal to zero.Therefore, in the case we investigated, only when each oscillator has an average energy
Dynamic balance is possible.
We further consider the interaction between the oscillator and the radiation existing in space in a similar way.PlonkMr. (Planck) once assumed that radiation can be regarded as the most disordered process imaginable. Under this assumption, he deduced the conditions for dynamic equilibrium in this case.He found:
here
Is the average energy of a vibrator (each vibration component) with eigenfrequency ν, c is the speed of light, ν is the frequency, and
Is the frequency between ν and
The amount of energy radiated per unit volume between.
If the energy of radiation with frequency ν is neither continuously increasing nor continuously decreasing, then
It must be true.
The relationship found as a condition of dynamic balance is not only not in line with experience, but also shows that it is impossible to talk about any definite energy distribution between the ether and matter in our picture.Because the wider the range of vibration number of the vibrator is selected, the greater the radiant energy in space will become. In the limit case, we can get:
§ 2. Planck's determination of basic constants
Now we would like to point out that Mr. Planck's determination of the basic constant is, to a certain extent, irrelevant to the blackbody radiation theory he founded.
So far, all experience can satisfy
The Planck formula of is:
Among them,
For large
Value, that is, for large wavelength and radiation density, this formula becomes the following form in the limit case:
It is seen that this formula is consistent with the formula obtained by Maxwell's theory and electron theory in § l.By making the coefficients of these two formulas equal, we get:
perhaps
That is, one hydrogen atom weighs
gram
Gram.This is exactly the value obtained by Mr. Planck, and it is in satisfactory agreement with the value of this quantity obtained by other methods.
We therefore conclude that the greater the energy density and wavelength of radiation, the more applicable the theoretical basis we use;However, for small wavelength and small radiation density, our theoretical basis is completely inapplicable.
equation
The following equation is used in the quantitative analysis of photoelectric effect in Einstein's way: photon energy=the energy required to remove an electron+the algebraic form of kinetic energy of the emitted electron: wherehIs Planck's constant,νIs the frequency of the incident photon, is the work function, is the minimum energy required to remove an electron from the atomic bonding, is the maximum kinetic energy of the emitted electron,ν0 is the threshold frequency of photoelectric effect,mIs the static mass of the emitted electron,vmIs the speed of the emitted electron,notes: If the energy of the photon（hν) If it is not greater than the work function (ϕ), no electrons will be emitted.The work function sometimes takesWTag.When this equation is inconsistent with the observation (that is, no electrons are emitted or the kinetic energy of electrons is less than expected), it may be because some energy is lost in the form of heat energy or radiation.
effect
（1） Abnormal photovoltaic effect:
Photovoltaic effect
Generally, the photogenerated voltage will not exceed Vg=Eg/e, but some thin film semiconductors will have much higher photogenerated voltage than Vg when exposed to strong white light, which is called abnormal photogenerated voltage effect.(5000V photogenerated voltage has been observed)
In the 1970s, light was discoveredFerroelectricsThe anomalous photovoltaic effect (APV) ofpolarizationIn the direction,
Photo generated voltage: V=(Jc/(σ D+△ σ l)) l
（2） Becquerel effect:
Immerse two identical electrodes in electrolyte, and one of them will be irradiated by light, which will produce potential difference between the two electrodes, called Becquerel effect.
(It is possible to imitate photosynthesis to make high-efficiency solar cells)
（3） Photon traction effect:
When the energy of a beam of photons is not enough to cause the laser generated by the electron hole to shine on the sample, the potential difference VL can be established at both ends of the sample in the direction of the beam, and its size is proportional to the optical power, which is called the photon pulling effect.
（4） Auger effect(1925 French Auger)
Use high-energy photons or electrons to knock out electrons from the inner layer of atoms, and generate electrons with certain energy at the same time（Auger electron ）The phenomenon that makes atoms and molecules called high-order ions is called Auger effect.
Application: Auger electron spectrometer is used for surface analysis to identify the "fingerprint" of different molecules.
photoelectric effect
（5） Photocurrent effect(Panning, 1927)
Photocurrent effect refers to photoinduced voltage (current) change between two stages of discharge tube.
(1) : Low pressure gas can be discharged (about 100Painert gas）
(2) : Space charge effect and glow discharge:
There are 7 different areas from cathode to anode in the discharge tube:
1: Aston dark area: a thin dark area near the cathode.Reason: The kinetic energy of the secondary electrons bombarded from the cathode by positive ions is very small, which is not enough to excite atomic luminescence.
2: Cathode glow zone: a thin light-emitting layer following Aston dark zone.
3: Cathode dark area: when electrons reach this area from the cathode, they get more and more energy, which exceeds the atomic ionization energy, causing a large number of collision ionization, and avalanche ionization process is concentrated here.After ionization, the electrons leave quickly. A strong positive space charge is formed here, causing distortion of the electric field distribution. Most of the tube pressure drops between here and the cathode
The above three areas are cathode potential drop areas.
4: Negative glow area: the area with the strongest luminescence.The electrons generate many excitation collisions in the negative glow area and emit bright glow.
5: Faraday dark area: electrons lose energy in the negative glow area, and there is not enough energy to generate excitation when entering this area.
6: Positive column area: in this area, the electron density is equal to the positive ion density, and the net space charge is zero, so it is also called the plasma area.
7: Anode area: Anode dark area and anode bright area can be seen.Application: Gas discharge devices, such as gas discharge lamps (fluorescent lamps, neon lamps, atomic spectrum lamps, neon bubbles), voltage stabilizing tubes, cold cathode thyratrons, etc.The positive column area is used in the laser to realize the inversion of particle beam, the cold cathode ion source in the particle beam device, plasma etching in the semiconductor process, thin film sputtering deposition, plasma chemical deposition, etc.
Photocurrent effect mechanism: metastable (with a life of about 10 ^ (- 4) s to 10 ^ (- 2) s) atoms are easier to ionize than neutral atoms, and more excited atoms are generated, especially metastable atoms, which may change the carrier concentration in the discharge tube.
Photocurrent spectroscopy technology application: Photocurrent spectroscopy does not require the optical system of conventional spectrometers, and can produce photocurrent effects from ultraviolet, visible, infrared to microwave.Photocurrent spectroscopy has a dynamic range of 8 orders of magnitude, high sensitivity, low noise, and is an ultra sensitive spectroscopy technology.(Green et al. used laser to verify photocurrent spectrum in 1976)
Josh effect: when the gas capacitor with air or insulating gas as the dielectric is continuously irradiated by visible light, the low-frequency current flowing through the capacitor will change, which is called Josh effect.
Marte effect: when there is a metal oxide film on the cathode surface of the discharge tube, when positive ions bombard the surface, the secondary electron emission effect will be enhanced, which is called Marte effect.
experimental verification
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In 1887, Hertz accidentally discovered the photoelectric effect when doing the spark discharge experiment to confirm Maxwell's electromagnetic theory.Hertz made experiments with two sets of discharge electrodes, one of which generated oscillation and electromagnetic waves;The other set serves as the receiver.He accidentally found that if the electrode receiving electromagnetic waves was exposed to ultraviolet radiation, spark discharge would become easy to produce.After the publication of Hertz's paper "The Influence of Ultraviolet on Discharge", it has attracted extensive attention in the field of physics, and many physicists have carried out further experimental research.
In 1888, Wilhelm Hallwachs, a German physicist, confirmed that this was due to the presence of a charge in the discharge gap.
photoelectric effect
In 1899, J.J. Thomson measured the photocurrent generated byCharge mass ratioThe value obtained is close to the charge mass ratio of cathode ray particles, which means that the photocurrent generated is the same as that of cathode ray.In this way, physicists realize that the essence of this phenomenon is due to the fact that light (especially ultraviolet light) shines on the metal surface and makes the metal insidefree electronA phenomenon in which greater kinetic energy is obtained and thus escapes from the metal surface.From 1899 to 1902,Leonard(P. Lenard, 1862-1947) systematically studied the photoelectric effect, and first called this phenomenon "photoelectric effect".In order to study the energy of photoelectrons escaping from the metal surface, Leonard added an adjustable reverse voltage between the electrodes until the photocurrent was cut off. From the cut-off value of the reverse voltage, the maximum speed of electrons escaping from the metal surface can be calculated.He selected different metal materials and used different light sources to illuminate, studied the cut-off value of reverse voltage, and summarized some experimental laws of photoelectric effect.According to the kinetic energy theorem: qU=mv ^ 2/2, the energy of the emitted electron can be calculated.It can be concluded that hf=(1/2) mv ^ 2+I+W
There are many contradictions between the laws discovered by in-depth experiments and the classical theory, but many physicists still want to explain the experimental laws of photoelectric effects within the framework of the classical electromagnetic theory.Some physicists try to explain photoelectric effect as a resonance phenomenon.Leonard put forward the trigger hypothesis in 1902. It is assumed that in the process of electron emission, light only plays a trigger role. The electron originally moves in the atom at a certain speed, and the light shines on the atom. As long as the frequency of light is consistent with the vibration frequency of the electron itself, resonance will occur, and the electron will escape from the atom at its own speed.Leonard believed that the vibration frequency of the electron in the atom is specific, and only light with appropriate frequency can trigger.Leonard's hypothesis was very influential at that time and was accepted by some physicists.However, Lernerd's trigger hypothesis was soon denied by his own experiment.
After Einstein put forward a theoretical explanation of photoelectric effect with the light quantum theory, the initial reaction of the scientific community was cold, and even some physicists who believed in quantum concepts did not accept the light quantum hypothesis.Although the theory does not contradict the existing experimental facts, there were not enough experiments to support the quantitative relationship given by Einstein's photoelectric effect equation at that time.It was not until 1916 that the quantitative experimental study of the photoelectric effect was completed by American physicist Millikan.
Millikan has conducted long-term research on photoelectric effect. After ten years of experiment, improvement and learning, he effectively eliminated the influence of surface contact potential difference and other factors, and obtained better monochromatic light.His experiment was excellent. In 1914, he first verified the accuracy of Einstein's equation by experiment, and made a direct photoelectric measurement of Planck's constant h for the first time, with an accuracy of about 0.5% (within the experimental error range).In 1916, Millikan published his accurate experimental results. He measured the relationship between the cut-off value of the reverse voltage and the frequency curve with six monochromatic lights of different frequencies. This is a good straight line. Planck constant can be calculated from the slope of the straight line.The results are in good agreement with the values obtained by Planck from blackbody radiation in 1900.
discover the law of sth
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Through a large number of experiments, the photoelectric effect has the following experimental laws:
1. Each metal has a limit frequency (or cut-off frequency) when producing photoelectric effect, that is, the frequency of light irradiation cannot be lower than a certain critical value.The corresponding wavelength is called the limiting wavelength (or red limit wavelength).When the frequency of the incident light is lower than the limit frequency, no matter how strong the light is, the electrons cannot escape.
2. The speed of photoelectrons generated in the photoelectric effect is related to the frequency of light, but not to the light intensity.
3. Instantaneous of photoelectric effect.It is found that the photocurrent is generated almost immediately when the metal is irradiated.The response time shall not exceed ten negative ninth power seconds (1ns).
photoelectric effect
4. The intensity of the incident light only affects the intensity of the photocurrent, that is, it only affects the number of photoelectrons escaping in a unit area per unit time.When the light color remains unchanged, the stronger the incident light, the greater the saturation current, that is, the stronger the incident light, the more electrons will be emitted in a certain time.
application area
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Manufacturing photomultiplier tubes
When the formula is inconsistent with the observation (that is, no electrons are emitted or the kinetic energy of the electrons is less than expected), it may be because the system is not completely efficient, and some energy is lost by turning into heat energy or radiation.
Light control apparatus
PV controller
The light control electrical appliance made of photocell can be used for automatic control, such as automatic counting, automatic alarm, automatic tracking, etc. The diagram on the right is the schematic diagram of the light control relay. Its working principle is that when the light shines on the photocell, the photocell circuit generates electro-optic flow, which is amplified by the amplifier to magnetize the electromagnet M, while the armature N is attracted,When there is no light on the photocell, and there is no current in the photocell circuit, electromagnet M will automatically control. The photoelectric effect can also be used to measure the speed of some rotating objects.
Photomultiplier tube
Using photoelectric effect, we can also manufacture a variety of photoelectric devices, such as photomultiplier tubes, television camera tubes, photoelectric tubes, electro-optic meters, etc. Here we introduce photomultiplier tubes.This tube can measure very weak light.The following figure on the right shows the general structure of the photomultiplier tube. In addition to a cathode K and an anode A, there are several multiplier electrodes K1.K2.K3.K4.K5, etc.When using, not only the voltage should be applied between the cathode and anode, but also the voltage should be applied to each multiplier electrode to minimize the cathode potential, increase the potential of each multiplier electrode in turn, and maximize the anode potential. In this way, there is an accelerating electric field between two adjacent electrodes. When the cathode is exposed to light, it will emit photoelectrons, and under the effect of the accelerating electric field,Hit the first multiplier electrode with greater kinetic energy, and photoelectrons can excite more electrons from this multiplier electrode. Under the action of electric field, these electrons hit the second multiplier electrode, which excites more electrons. In this way, the number of excited electrons increases continuously,Finally, the number of electrons collected by the rear anode will be many times more than the number of electrons emitted from the cathode at first (generally 105~108 times).Therefore, as long as the tube is exposed to very weak light, it can generate a large current, which plays an important role in engineering, astronomy, military and other aspects.
Agricultural pest control
The management of agricultural pests needs to put forward a technical system and key technologies that are suitable for the environment and compatible with the ecology according to the characteristics of the pests.Harmful insects show phototaxis behavior characteristics with individual differences and group consistency to sensitive light sources, and show the essence of biological photoelectric effect through the way of visual nerve signal response and physiological photon energy demand.Based on the induced gain characteristics of insects, some photoelectric induced insecticidal lamp techniques and insect induced trapping techniques have been widely used in the control of agricultural pests, with good application prospects.
Law influence
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The photoelectric effect is confirmed by HertzmaxwellThis phenomenon has become an important evidence for breaking through Maxwell's electromagnetic theory.
Einstein
Einstein's explanation of light quanta in the study of photoelectric effect not only extendsPlonkIt proves that wave particle duality is not only possessed by energy, but also quantized by light radiation itself. At the same time, it provides natural scientific evidence for the law of the unity of opposites in materialistic dialectics, which has immeasurable philosophical significance.This theory also laid the foundation for Bohr's atomic theory and de Broglie's theory of matter waves.Millikan's quantitative experimental research not only proved the light quantum theory from an experimental perspective, but also provided evidence for Bohr atomic theory.
In 1921, Einstein won the Nobel Prize in Physics for establishing the quantum theory of light and successfully explaining the photoelectric effect.
In 1922, Bohr's atomic theory was also supported by experiments because Millikan confirmed the quantum theory of light, thus winning the Nobel Prize in physics.
In 1923, Millikan won the Nobel Prize in Physics for "measuring basic charges and studying photoelectric effects".
contradiction
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photoelectric effect
In the photoelectric effect, it obviously needs enough energy to release photoelectrons.According to the classical electromagnetic theory, light is an electromagnetic wave, and the energy of the electromagnetic wave depends on its strength, that is, it is only related to the amplitude of the electromagnetic wave, but not the frequency of the electromagnetic wave.However, the first and second points of the experimental law can't be explained by classical theory.The third point cannot be explained, because according to the classical theory, there must be an energy accumulation process for very weak light to get enough energy to escape, and it is impossible to produce photoelectrons instantaneously.
In the photoelectric effect, the emission direction of electrons is not completely directional, but most of them are emitted perpendicular to the metal surface, independent of the light direction. Light is an electromagnetic wave, but light is an orthogonal electromagnetic field with high-frequency oscillation, and the amplitude is very small, which will not affect the emission direction of electrons.
In fact, all these have exposed the defects of the classical theory. If you want to explain the photoelectric effect, you must break through the classical theory.
classification
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The photoelectric effect is divided into external photoelectric effect and internal photoelectric effect.
Internal photoelectric effect refers to the phenomenon that the carriers (free electrons or holes) generated by light excitation still move inside the substance, which changes the conductivity of the substance or generates photovoltaic.
External photoelectric effect refers to the phenomenon that electrons generated by light excitation escape from the surface of matter and form electrons in vacuum.
External photoelectric effect
Under the action of light, electrons in the object escape from the surface of the object and emit outward, which is called the external photoelectric effect.
Some experimental rules of external photoelectric effect
a. Only when the light frequency of the irradiated object is not less than a certain value, the object can emit photoelectrons. This frequency is called the limit frequency (or the cut-off frequency), and the corresponding wavelength λ 0 is called the limit wavelength.The limiting frequency of different substances and the corresponding limiting wavelength λ 0 are different.
Limiting wavelength of some metals (in angstroms):
b. Initial velocity of the photoelectron when it comes out of the object is related to frequency of the light but not to luminous intensity.That is to say, the initial kinetic energy of photoelectron is only related to the frequency of light irradiation and has nothing to do with the luminous intensity.
c. When the frequency of light is constant, the stronger the incident light is, the more photoelectrons will be emitted by the cathode (the metal material that emits photoelectrons) in the same time
photoelectric effect
d. From the experiment, we know that the process of photocurrent generation is very fast, generally not more than 10 - 9 power seconds;When you stop using light, the photocurrent will stop immediately.This shows that the photoelectric effect is instantaneous.
e. Einstein equation: h ν=(1/2) mv ^ 2+I+W
Where (1/2) mv ^ 2 is the initial kinetic energy of the photoelectron detached from the object.There are a large number of free electrons in the metal, which is the characteristic of the metal. Therefore, for the metal, the term I can be omitted. The Einstein equation is h ν=(1/2) mv ^ 2+W If h ν<W, the electrons cannot be detached from the metal surface.For a certain metal, the minimum light frequency that produces photoelectric effect（Limiting frequency) u0。Determined by h ν 0=W.The corresponding limiting wavelength is λ 0=C/ν 0=hc/W.The increase of luminous intensity increases the number of photons irradiated on the object, so the number of photoelectrons emitted is proportional to the intensity of the irradiated light.Equation When quantitatively analyzing photoelectric effects in Einstein's way, the following equation is used: photon energy=the energy required to remove an electron+the algebraic form of the kinetic energy of the emitted electron: hf=φ+Em φ=hf0 Em=(1/2) mv ^ 2, where h is the Planck constant, h=6.63 × 10 ^ - 34 J · s, and f is the frequency of the incident photon,φ is the work function, the minimum energy required to remove an electron from the atomic bond, f0 is the threshold frequency of photoelectric effect, Em is the maximum kinetic energy of the emitted electron, m is the static mass of the emitted electron, and v is the speed of the emitted electron
Note: If the photon energy (hf) is not greater than the work function (φ), no electrons will be emitted.The work function is sometimes marked with W.When this formula is inconsistent with observation (that is, no electrons are emitted or the kinetic energy of electrons is less than expected).Einstein won the 1921 Nobel Prize in Physics for his successful explanation of the photoelectric effect.
Electronic components based on external photoelectric effect include photocell and photomultiplier.The photomultiplier tube can convert each flash into an amplified electric pulse, and then send it to the electronic circuit for recording.
Internal photoelectric effect
When the light shines on the object, the conductivity of the object will change, or the phenomenon of photogenerated electromotive force will occur.It is divided into photoconductive effect and photovoltaic effect (photovoltaic effect).
1 Photoconductivity effect
Under the action of light, electrons absorb photon energy fromBondingThe transition from the state to the free state causes the change of the conductivity of the material.
When the light shines on the photoconductor, if the photoconductor is an intrinsic semiconductor material and the light radiation energy is strong enough, the electrons on the valence band of the photoconductor will be excited to the conduction band, making the conductivity of the photoconductor larger.
Photoelectric devices based on this effect have photoresistors.
Photovoltaic effect
"Photovoltaic effect", referred to as "photovoltaic effect".It refers to the phenomenon that the light causes a potential difference between heterogeneous semiconductors or between different parts of semiconductors and metals.First of all, it is a process of transforming photons (light waves) into electrons and light energy into electrical energy;Secondly, it is the process of voltage formation.With voltage, it is like building a high dam. If the two are connected, a current loop will be formed.
The basic principle of photovoltaic power generation is "photovoltaic effect".The task of solar energy experts is to complete the work of making voltage.Because it is necessary to produce voltage, solar cells that complete photoelectric conversion are the key to solar power generation.
In short, it is a phenomenon that an object can generate an electromotive force in a certain direction under the action of light.Devices based on this effect include photocells and photodiodestriode。
When the light irradiates the PN junction, if hf ≥ Eg, the electrons in the valence band will jump to the conduction band, and an electron hole pair will be generated. Under the effect of the electric field in the barrier layer, the electrons will be biased to the outside of the N area, and the holes will be biased to the outside of the P area, making the P area positively charged and the N area negatively charged, forming a photogenerated electromotive force.
② Lateral photoelectric effect (Danpei effect)
WhenSemiconductor optoelectronic deviceWhen the illumination is uneven, the illumination part will produce electron hole pairs, and the carrier concentration is larger than that of the non illumination part. A carrier concentration gradient will appear, causing carrier diffusion. If the electrons diffuse faster than the holes, the illumination part will be positively charged, and the non illumination part will be negatively charged, thus generating an electromotive force, that is, the lateral photoelectric effect.
③ Photomagnetic effect
When a semiconductor is exposed to strong light and a magnetic field is applied in the vertical direction of the light, the phenomenon that an electric potential is generated between the two ends of the semiconductor perpendicular to the light and magnetic field is called the photoelectromagnetic effect, which can be regarded as the photodiffusion currentHall effect。
④ Becquerel effect
It refers to the photovoltaic effect in liquid.When light irradiates one of the two same electrodes immersed in electrolyte, the phenomenon of generating potential between the two electrodes is called Becquerel effect.The operating principle of the photocell is based on this effect.
photoelectric effect
⑤ Ultraviolet photoelectric effect
When the ultraviolet ray shines on the surface of some metals, the free electrons inside the metal escape from the metal surface, and this ultraviolet photoemission constitutes one of the contents of the ultraviolet photoelectric effect.As early as 1887, when German physicists (1857-1894) were studying ultraviolet radiation, they first discovered the phenomenon of photoelectric emission.In 1888, photoelectric emission was proved by the Russian physicist Stolitov (1839-1896) with experiments.
3 photon pull effect
When photons interact with free carriers in semiconductors, photons transfer momentum to free carriers, and free carriers will move relative to the lattice along the propagation direction of light.As a result, in the case of an open circuit, the semiconductor sample will generate an electric field, which will prevent the movement of carriers.This phenomenon is called the photon pull effect.
Quantum interpretation
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In 1905, Einstein further popularized Planck's concept of quantization.He pointed out that not only the energy exchange between the blackbody and the radiation field is quantized, but the radiation field itself is composed of discontinuous light quanta. The energy of each light quanta and the frequency of the radiation field satisfy ε=h ν, that is, its energy is only related to the frequency of the light quanta, but not to the intensity (amplitude).
According to Einstein's quantum theory of light, the light emitted on the metal surface is essentially a photon flow with energy ε=h ν.If the frequency of light irradiation is too low, that is, the energy of each photon in the photon flow is small. When it shines on the metal surface, the electron absorbs this photon, and its increased energy ε=h ν is still less than the work that the electron needs to escape from the metal surface, the electron cannot escape from the metal surface, so it cannot produce photoelectric effect.If the frequency of light irradiation is high enough to make electrons absorb enough energy to overcome the work escaping and leave the metal surface, photoelectric effect will be generated.At this time, the relationship between the kinetic energy of the escaping electron, the photon energy and the escaping work can be expressed as: photon energy - the energy required to remove an electron（Escape work）=The maximum initial kinetic energy of the emitted electron.
That is:Εk(max)=hv-Wzero
This is Einstein's equation of photoelectric effect.
Among them,hIs Planck's constant;vIs the frequency of the incident photon
photoelectric effect
work function
Φ is the work function, which refers to the minimum energy required to remove an electron from the atomic bonding. The expression is shown in the right figure, where f0 is the threshold frequency of photoelectric effect, namely the limit frequency;The work function is sometimes marked with W or A.
Kinetic energy expression
E (kmax) is the maximum kinetic energy of the escaped electron, as shown in the right figure;M is the static mass of the emitted electron;Vm is the initial velocity at which the emitted electron escapes.
Note: When this formula is inconsistent with the observation (that is, no electrons are emitted or the kinetic energy of electrons is less than expected).
Experimental circuit
According to Einstein's quantum theory of light, the energy of photoelectron in photoelectric effect depends on the frequency of light irradiation, but has nothing to do with the intensity of light irradiation, so it can explain the first and second two experimental laws.The limiting frequency means that the energy of the optical quantum just meets the frequency of the optical quantum to overcome the work escaping from the metal, and different metal electrons need different energy to escape, so the limiting frequency of different metals is different.For the third point, when the energy of the light quantum is sufficient, no matter how strong the light is (only depending on the number of light quanta), the electrons can escape immediately after absorbing the light quanta, so the photoelectric effect can be generated immediately without the accumulation process.When the light shines on the metal surface, the greater the intensity is, theLight quantaThe more the number is, the more likely it is to be absorbed by the electrons in the metal, which can explain why the number of electrons ejected is only related to the intensity of light, but not the frequency of light.
