spectroscopy

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This entry is made by Compilation and application of scientific encyclopedia entries of "Science Popularization China" to examine.

Spectroscopy is an important interdisciplinary subject mainly involving physics and chemistry spectrum To study electromagnetic wave With matter Interaction 。 Light is a kind of wavelength (or frequency) of electromagnetic waves electromagnetic radiation 。 Spectrum is a kind of grating 、 Prism 、 Fourier transform Isometric means is used to analyze a certain property of a beam of electromagnetic radiation into a chart showing the contribution of each constituent wavelength of the radiation to this property. For example, an absorption spectrum can list the absorption degree of the substance to the corresponding wavelength in the order of wavelength from low to high in a certain wave band. With the development of science and technology, the electromagnetic wave band involved in spectroscopy is becoming wider and wider. From gamma rays with a wavelength of picometers to X-rays, ultraviolet rays, visible light regions, infrared rays, microwaves, and radio waves with a wavelength of several kilometers, there are characteristic forms of interaction with matter. According to the interaction form of light and matter, spectrum can be generally divided into absorption spectrum, emission spectrum, scattering spectrum, etc. Through spectroscopic research, people can analyze the energy levels and geometric structures of atoms and molecules, the reaction rate of specific chemical processes, the concentration distribution of a substance in a specific area of space, and other microscopic and macroscopic properties.

Chinese name: spectroscopy
Foreign name: spectroscopy
Purpose: Study the interaction between light and matter

History: Newton's prism splitting experiment in the 17th century
Status: Important interdisciplinary subjects of physics and chemistry
Involved: electromagnetic wave

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brief introduction

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With the development of science and technology, the electromagnetic wave band involved in spectroscopy is becoming wider and wider Pimi Grade Gamma ray , to X-ray ， ultraviolet rays ， visible light Area, infrared , microwave, and then to the wavelength of several kilometers radio waves They all have the characteristic form of their interaction with matter. According to the interaction form of light and matter, the spectrum can generally be divided into absorption spectrum 、 emission spectrum , scattering spectrum, etc. Through spectroscopic research, people can analyze atom And molecule Of energy level Related to geometry and specific chemical process reaction rate , a substance in Space Various microscopic and macroscopic properties such as the concentration distribution in a specific area of the. People can also use the specific composition of the material to produce a spectrum with special optical properties, such as a specific frequency of laser. Spectroscopy is not only a basic science, but also an important qualitative and quantitative measurement method in daily applications, such as the analysis of the content of various substances in water, disease prevention and monitoring by analyzing the content of protein in blood, and optical fiber communication using optimal wavebands. Since the discovery of laser in the middle of last century, human control of light has reached a new stage, which can generate electromagnetic radiation with unprecedented brightness, frequency distribution and time resolution, opening the way to nonlinear optics With the door of nonlinear spectroscopy, spectroscopy is in a new era of rapid development.

brief history

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spectroscopy

Spectroscopy has been studied for more than 300 years. In 1666, 1. Newton Pass through the glass prism sunlight Displaying a spectrum of colors ranging from red light to purple light, he found that white light was composed of light of various colors. This is the earliest study of spectra. Then until 1802, W. H. Wollaston and J. von in 1814 Fraunhofer They observed each other independently Spectral line 。 Each spectral line represents only one "color" of light. Here the word color is generalized. Newton could not observe the spectral line because he made the sunlight pass through the circular hole rather than through the slit. Between 1814 and 1815, Fraunhofer Announced Solar spectrum Many hidden lines are named after letters, some of which are still in use today. These lines are called Fraunhofer hidden line 。

Practical spectroscopy is composed of Kirchhoff, G·R And Bunsen, R·W.E. Developed in the 1860s; They proved that spectroscopy can be used as qualitative chemical analysis This method was also used to discover several unknown elements at that time, and proved that there are many known elements in the sun.

Since the middle of the 19th century, it has been one of the important subjects of spectroscopy research. Trying to explain hydrogen atom In the process of spectrum, the achievements quantum mechanics The rules have been established greatly Facilitation 。 These laws can be applied not only to hydrogen atom Can also be applied to other atoms, molecules and Condensed Matter 。 In fact, they have finally become Modern chemistry , solid state physics and even applied disciplines such as electronics The foundation of.

Atomic hydrogen spectroscopy The strongest spectral line in 1853 Sweden Physicist A J. Esteran Detected（ light wave The unit of wavelength is named after his surname, 1 angstrom is 10-8 cm). In the next 20 years, more hydrogen atomic lines were observed in the spectra of stars. In 1885, engaged in Astrometry Of Switzerland Scientist J J. balmer Find an empirical formula to explain the position of the known hydrogen atomic spectral line. Since then, this group of lines has been called Balmer system 。 Following Balmer's achievements, in 1889, Swedish spectroscopist J R. Rydberg Linear spectral systems of many elements have been found, the most obvious of which is alkali The spectral system of metal atoms can all satisfy a simple formula—— Rydberg formula 。 This formula was later written as

(1)1/λ=R[(1/m²)-(1/n²)]+bn+c

among λ Is the wavelength in angstroms. m and n Take positive integers 1, 2, 3. b And с are constants, and their values depend on different elements and different linear systems. R The value of is almost the same for all elements, called Rydberg constant 。 In equation (1) b ， When с is equal to zero, formula (1) is simplified as Balmer formula 。

1/ λ = R [(1/m²)-(1/ n² )](2)

With appropriate m and n The wavelength of all spectral lines in the hydrogen atom spectrum is obtained by substituting the value of.

Although the expression (2) of the wavelength of the atomic hydrogen spectral line is very simple, its cause was unknown at that time. Until 1913, N. Bohr Only then has it been clearly explained. Bohr not only derived equation (2), but also calculated the value of Rydberg constant. although Bohr theory Although much progress has been made in concept, Bohr theory cannot explain the various features of the observed atomic spectrum, and even the further interpretation of the hydrogen atomic spectrum is difficult. For example, as early as 1892, A A. Michelson It is found that the strongest line in the Balmer system is actually composed of two lines close together, and the interval between them is about 0.14 angstrom.

Can satisfactorily explain the splitting of spectral lines and other Complex atomic spectrum It is quantum mechanics developed in the 20th century. The electron has not only orbital angular momentum, but also spin Angular momentum. The combination of these two kinds of angular momentum successfully explains the splitting phenomenon of spectral lines. electron spin In 1925, G E. Ulenbeck And S A. Guzmit Introduced as a hypothesis to explain Alkali metal atomic spectrum Of measurement result 。 At P A.M. dirac In relativistic quantum mechanics, electron spin (including proton Spin vs neutron The concept of spin has a solid theoretical basis, which is the natural result of the basic equation rather than a special assumption.

In 1896, P. Zeeman Put the light source on magnetic field To observe the influence of magnetic field on spectral lines. It is found that the spectral lines studied are split into dense triple lines, and these spectral lines are polarized. Modern times call this phenomenon Zeeman effect 。 In 1897, H.A. Lorentz This effect has been satisfactorily explained, and its basic concept is that light Isotropy Of Harmonic oscillator The movement of these harmonic oscillators is affected by the magnetic field Magnetic line of force The role of the Zeeman split. But in 1898, T. Preston observed that the zinc wire (4722 A) and cadmium The line (4800 A) splits into quadruple rather than triple lines in the magnetic field. Similar phenomena have been observed by others. Later, people put Spectral line The triple line splitting of Normal Zeeman effect And all exceptions are called Anomalous Zeeman effect 。

Zeeman effect is not only important in theory, but also in practice. In the classification of complex spectra, Zeeman effect is a very useful method, which effectively helps people understand complex spectra. On the other hand, it is called Stark effect The spectral line of electric field The splitting in (1913) has only theoretical significance, but has no practical use for the analysis of spectral lines.

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According to different research methods of spectroscopy, it is customary to divide spectroscopy into Emission spectroscopy 、 Absorption spectroscopy And Scattering spectroscopy 。 These different kinds of spectroscopy provide material microstructure knowledge and different chemical analysis methods from different aspects.

Emission spectroscopy

The emission spectrum can be divided into three different categories: Linear spectrum , band spectrum and Continuous spectrum 。 Linear spectrum is mainly generated from atoms, band spectrum is mainly generated from molecules, and continuous spectrum is mainly generated from Incandescence Discharge of solid or gas.

Millions of spectral lines of atomic emission have been observed in modern times. Each atom has its own unique spectrum, just like human fingerprint The same is different. According to the theory of spectroscopy, each atom has its own series of discrete Energy state Each energy state has a certain energy. Taking hydrogen atom as an example, the energy can be expressed as (3)

spectroscopy

Where n Take from 1 to ∞ The positive integer of is called the main quantum number. From the classical point of view, n It describes the size of the orbit of the electron around the atomic nucleus. every last n The value determines an energy level. R H Is the Rydberg constant of hydrogen atom. h by Planck constant 。 The minus sign in the formula is used to correspond to n = ∞ The energy of is set as the highest value and is equal to zero, which corresponds to n =The energy of 1 is defined as the lowest energy, and this energy state is called ground state , corresponding energy level It is called the basic energy level. When an atom is raised from the ground state to a higher energy state in some way, the internal energy of the atom increases, and the atom will emit this excess energy in the form of light, thus generating atomic emission spectrum 。 The frequency of light emitted by atoms v Yes, (4)

spectroscopy

Where W h Is the energy of higher energy level, W i Is the energy of lower energy level. Frequency is often counted backwards in centimeters (cm -1 ）To measure; The reciprocal of 1 cm is the number of wavelengths included in the length of 1 cm. When describing the electronic motion in atoms in detail, except for the principal quantum number n In addition, there are three other quantum numbers, namely l 、 m and s express. l It describes the magnitude of the angular momentum of the electronic orbital movement, called Angular quantum number , its value is（ n -1) A positive integer between and 0. l =The electron of 0 is called s electron, l =The electron of 1 is p electron, l =The electron of 2 is d electron, etc. Quantum number m It describes the orientation of the motion orbit of the electron in space, called Magnetic quantum number , its value cannot be greater than l But it can be negative. For a given l For example, m = l ， l -1,…，- l 。 s Is the electron spin quantum number, which describes the angular momentum of the electron spin, etc. It produces multiple lines in the spectrum. If there is more than one electron producing spectrum in the atom, the vector sum of the spin of each electron S Is the total spin of the electron configuration. Quantum number S It is always a positive number. The same is true for orbital angular momentum. Their sum gives the total orbital angular momentum of the atom (quantum number is L ）。 L =0, the relevant energy state is called S state; L =1 is called P state; L =2 is called D state, etc. Multielectron Atomic energy level Depends on S , dependent on L , also depends on S And L Interaction between. S And L Combined to give the total angular momentum of atoms J 。 Quantum number J Take a positive value, whose range is L And S Minimum sum of differences L + S Between the maximum values of. In spectroscopy, the symbol MLJ is often used to represent the atomic energy level or spectral term. among M =2 S +1 is the multiplicity of the spectrum, which represents the number of energy level splits. For example, then M =2, which means that each energy level (except the S level) is split into two components, which is the case for the energy level of alkali metal atoms. For two electron atoms, S =0 or 1, depending on whether the spin directions of the two electrons are parallel or antiparallel, because the multiplicity of energy levels is either 1 (no splitting of energy levels) or 3 (each energy level is split into three components). Alkaline soil This is the case for the energy levels of metal atoms. And so on. The transition of atoms between energy levels produces spectral lines, and the transition from high energy level to low energy level produces emission spectrum; Conversely, an absorption spectrum is generated. According to the law of quantum mechanics, the transition of atoms between energy levels follows the selection rules, which are Δ L =0，±1； Δ S =0； Δ J =0 (except J =0），±1。 Symbol Δ L Representing the initial and final states in the transition L Value. Δ S And Δ J The meaning of is the same.

In the emission spectrum of molecules, the main research contents are Diatomic molecule In molecules, electrons surround two or more Nucleus Motion, like atoms, each has its own specific energy level 。 In addition to the movement of electrons, atomic nuclei act on each other around their center Periodic vibration ; In addition, these nuclei as a whole will rotate in space around certain axes. All these movements will be shown in the molecular spectrum, so the molecular spectrum becomes very complex.

An electron in a molecule, like an electron in an atom, has four quantum numbers. But in diatomic molecules, the electron is shared by two atomic nuclei, so the quantum number m

spectroscopy

From a new quantum number λ Instead. λ Represents the orientation of the electronic orbit to the molecular axis, which can be positive or negative, but cannot be greater than l 。 In diatomic molecules, λ with spin s The combination of l And s The combination of. The algebra they are combined together and represents the projection of the total angular momentum of the electron on the molecular axis, whose value is determined by Λ express, Λ =0，1，2，…。 Corresponds to Λ The electronic states of different values of Δ, Ф. The symbol of molecular energy state is similar to that of atom M Λ， M Still equal to 2 S +1。 E.g. H two 、 N two 、 The lowest electronic state of HCl, etc. is 1 ∑, O two Is 3 ∑, For NO, 2 Π, etc. In the molecule, Electronic state The energy of vibration is 50~100 times larger than that of vibration, and the energy ratio of vibration Dynamic state 50~100 times more energy. Therefore, the transition between the electronic states of molecules is always accompanied by vibration transition and rotation transition, so many spectral lines are concentrated together to form a band spectrum.

from emission spectrum We can get the atomic and molecular energy level Knowledge of structures, including measurements of important constants. also Atomic emission spectrum It is widely used in chemical analysis.

Absorption spectroscopy

When a beam of light with continuous wavelength passes through a substance, some components in the beam will be weakened. When the beam absorbed by passing through the substance is spread into a spectrum by the spectrometer, the absorption spectrum of the substance will be obtained. Almost all substances have their own unique absorption spectra. The knowledge about the energy level structure given by the atomic absorption spectrum is complementary to that given by the emission spectrum.

In general, absorption spectroscopy studies the absorption of light at those wavelengths, the degree of absorption, and why. The object of study is basically molecule.

Spectroscopic Spectral range It is very broad, from about 10nm to 1000 μ m. At 200 nm The absorption of solid, liquid and solution can be observed within the spectral range of 800nm. Some of these absorption are continuous, which is called general absorption spectrum; Some show one or more Absorption band , called selective absorption spectrum. All these spectra are due to changes in the electronic states of molecules.

spectroscopy

Selective absorption spectroscopy has a wide range of applications in organic chemistry, including identification of compounds, control of chemical processes, determination, characterization and Quantitative chemical analysis Etc. The infrared absorption spectrum of molecules is generally used to study molecules Vibration spectrum And Rotational spectrum Among them, molecular vibration spectroscopy has always been a major research topic.

The study of molecular vibration spectrum shows that many vibration frequencies are basically the vibration frequencies of some small atomic groups inside the molecule, and these frequencies are Atomic cluster Regardless of the rest of the molecule. This is very similar to the absorption spectrum of the color group in the visible light region, and this fact is very important in the application of molecular infrared absorption spectroscopy. It has been used for many years to study the structure of polyatomic molecules, quantitative and qualitative analysis of molecules, etc.

In scattering spectroscopy, Raman spectroscopy It is the most common spectroscopy technology. When light passes through a substance, in addition to the transmission and absorption of light, it is also observed that Light scattering 。 In addition to the original frequency of the incident light (Rayleigh scattering and Tindell scattering), the scattered light also includes some new frequencies. This kind of scattering that produces a new frequency is called Raman scattering, and its spectrum is called Raman spectrum.

The intensity of Raman scattering is very small, about Rayleigh scattering One thousandth of. Raman frequency and intensity polarization Wait for a sign scattering The nature of matter. From these materials, we can derive knowledge of material structure and composition. This is pull Mann spectrum It is widely used.

spectroscopy

Because Raman scattering is very weak, it was discovered by Indian physicist Raman et al. in 1928. They're using mercury lamps Monochromatic light A new spectral line with a frequency lower than that of the incident light is observed in the scattered light of the liquid when it is used to irradiate some liquids. A few months after Raman and others announced their discovery, Soviet physicists Lenz see grid The existence of this effect in crystals was also independently reported by et al.

Raman effect originates from molecular vibration (and lattice Vibration) and rotation, so the tension between Mann spectrum The molecule can be obtained from Vibrational energy level (lattice vibration energy level) and Rotational energy level Knowledge of structure.

The intensity of Raman scattering is very weak. Before the advent of lasers, it often takes time to get a perfect spectrum. Since the development of lasers, Raman spectroscopy technology has undergone great changes using lasers as excitation sources. The laser output from the laser has good monochromaticity, directionality and high intensity, so they become an almost ideal light source for obtaining Raman spectrum, especially continuous wave argon Ions Laser And ammonia ion laser. So the research of Raman spectroscopy has become very active, and its research scope has also been greatly expanded. In addition to expanding the variety of substances studied, we have also made efforts in the research of combustion process, detection of environmental pollution, analysis of various materials, etc Mann spectrum Technology has also become a useful tool.^[1]

Beam foil spectroscopy

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spectroscopy

Beam foil spectroscopy It is a new discipline developed internationally in the 21st century. The main content is to study basic atomic physics and measurement by means of accelerating ions hitting thin foils of different elements Electronic energy level Average life of. Many people in the modern world will accelerator Modified to study the beam foil problem. Energy at 10 two ～10 three The beam foil experiment in the eV range can reveal that the accelerated elements are low ionization The range of 8~10MeV/nucleon can produce high Z Single electron and double electron of Electronic system This transition can be tested sensitively Quantum electrodynamics 。 Application of beam foil technology to Astrophysics As for the problem, we can Corona The nature of and Galaxy The abundances of elements in are well understood.

Engaged in atomic physics, spectroscopy Astrophysics Experimenters and theorists who are engaged in research work like this will get a lot of rich information about atomic systems when reading this book. Teachers, students and graduate students of the Department of Physics will also benefit from reading this book.

Photoacoustic spectroscopy

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spectroscopy

A new spectral analysis and detection technology based on photoacoustic effect. Illuminate the sample sealed in the photoacoustic cell with a beam of monochromatic light with adjustable intensity, and the sample absorbs Light energy And de excitation by releasing heat energy, which makes the sample and surrounding medium Modulation frequency Periodical heating will occur, which will lead to periodic heating of the medium pressure Fluctuation, such pressure fluctuation can be realized by sensitive microphone or Piezoelectric ceramics The microphone detects and amplifies the photoacoustic signal, which is Photoacoustic effect 。 If the wavelength of the incident monochromatic light is variable, the photoacoustic signal spectrum that varies with the wavelength can be measured, which is photoacoustic spectrum. If the incident light is a focused light beam, press sample Of x - y When the axis scanning mode is moved, the photoacoustic signal can be recorded to change with the sample position, which is photoacoustic imaging technology. The photoacoustic spectrum equipment and its principle are shown in the figure. incident light For monochromatic light whose intensity has been modulated, light intensity modulation can use a chopper. Photoacoustic cell It is a closed container, containing samples and Microphone 。 The figure shows a solid sample, which is surrounded by non absorbable ray radiation A gaseous medium, such as air. If it is a liquid or gas sample, fill the photoacoustic cell with the sample. The microphone should be very sensitive. For gas samples, capacitive type Electret microphone It is suitable. It can measure 10% with an electronic detection system -6 ℃ temperature rise Or 10 -9 J/（cm three ·s) Heat input. For liquid and solid samples, it is better to use Piezoelectric ceramic detector 。

because Photoacoustic spectrum It measures the amount of light energy absorbed by the sample, so reflected light 、 Scattered light The photoacoustic spectrum is suitable for measuring high scattering samples, opaque samples, absorbed light intensity and incident light Weak absorption samples and low concentration samples with very small specific strength values powder 、 colloid Can be measured, which is impossible for ordinary spectra. The photoacoustic effect is related to the modulation frequency. Changing the modulation frequency can obtain the information of different depths of the sample surface, so it is a non-destructive detection method to provide the structural information of different depths of the surface.

Photoacoustic spectroscopy Spectrum technology and Calorimetry The combination product is a new method developed in the early 1970s to detect substances and study their properties. Photoacoustic technology is developing continuously, and there have been CO2 laser Light source infrared photoacoustic spectrometer, xenon lamp ultraviolet visible photoacoustic spectrometer for solid and liquid analysis, and Fourier transform photoacoustic spectrometer 。 Photothermal deflection Spectroscopy , photoacoustic Raman spectroscopy Photoacoustic microscope , Laser Thermal lens Methods and thermal wave imaging technology are developing rapidly. Photoacoustic spectrum Technology is widely used in physics, chemistry, biology, medicine, geology and material science.

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According to the action mode of matter and light, it can be divided into the following three categories ：

① Emission spectroscopy

spectroscopy

Atomic emission spectrum Conduct research. Each atom and molecule has a specific energy level structure and spectrum series. Through the study of emission spectrum, we can get the information about atoms and Molecular energy level A lot of knowledge of structure, measuring various important constant And qualitative and quantitative analysis Etc.

② Absorption spectroscopy

Molecule or atomic group band All have characteristic absorption, which is mainly represented by the band absorption spectrum unique to the molecular spectrum (see spectrum). The widely used infrared absorption spectrum is composed of different vibrations and rotations within the same electronic state of molecules Transitions between energy levels Generate. Infrared absorption spectrum It is mainly used to study the energy level structure and molecular structure of molecules, or conduct qualitative and quantitative analysis of molecules. For absorption spectrum and emission spectrum The research of is often complementary to each other.

③ Raman spectroscopy

stay Raman scattering The Raman lines originate from scattering The vibration and rotation of material molecules reflect the internal structure and movement of molecules. Through Raman spectroscopy, compounds can be qualitatively and quantitatively analyzed, the vibration and rotation frequencies and related constants of molecules can be measured, the internal or inter molecular forces can be understood, and the symmetry and geometric shape of molecular structure can be inferred. Raman spectroscopy is widely used in many fields of physics, chemistry and biology. The application of new light source laser has strongly promoted the development of Raman spectroscopy.

According to different light sources, it can be divided into the following two categories ：

① Laser spectroscopy

The branch of spectroscopy that uses lasers as light sources. Laser Line width Narrow, high strength and directional Good and other unique advantages have brought a new look to spectroscopy, which not only has extremely high spectral resolution And detection sensitivity nonlinear effect And coherent Raman spectroscopy.

② Non laser spectroscopy

overview

The branch of spectroscopy that deals with eliminating Doppler broadening of spectral lines. Only the nonlinear field is involved here. Improving the resolution of spectroscopy has always been one of the subjects that spectroscopists try to solve. early stage Atomic spectrum The research work of was limited by the resolution of the spectrometer, and used Michelson interferometer and Fabry Perot interferometer The precision of measuring the wavelength of light wave is effectively improved after such sensitive instruments are used. However, the spectral lines close to each other are still indistinguishable, such as Atomic hydrogen spectroscopy The composition of the Balmer line in. This is not because the performance of the interferometer is not perfect, but because Spectral line Not sharp enough. The width of the spectral line conceals its fine structure. Spectral line broadening Part of the reason is the increase of natural width. Even under the best observation conditions, spectral lines are not absolutely monochromatic. The reason is that the stable state of the atom is not really stable. After the atom is excited, it will Radiant energy In other words, atoms in excited state always decay. The finite lifetime of the excited state increases the natural width of the spectral line.

The natural width of the spectral line sets a limit for spectral resolution. But until Laser It is almost impossible to reach this limit before it is applied in spectral research, because in gas samples, spectral lines are Doppler effect As a result, ordinary spectroscopic techniques cannot effectively eliminate the Doppler broadening Therefore, it is difficult to improve the resolution of spectroscopy.

Since 1970, Laser spectroscopy The technology has developed rapidly. One of the effects of this technology is that the resolution of spectroscopy has been significantly improved by several orders of magnitude. Laser spectroscopy can effectively eliminate Doppler broadening of spectral lines. These methods are mainly Saturation spectroscopy 、 Polarization spectroscopy And double photon Spectroscopy.

Saturation spectroscopy

According to the two main properties of laser: narrow spectral line width and high intensity. Most of the lasers used are CW frequency modulated lasers, especially dye laser 。 Under intense laser light beam Medium, gas atomic absorption Photonic rate It exceeds the rate at which the atom returns to the original energy level, thus reducing the number of atoms that can absorb photons of a given frequency. That is to say, the laser beam "sweeps" the atoms absorbing this frequency on the road. When another beam with the same frequency passes through the gas sample along the same path, it will be found that Absorption of light Less. In fact, using this effect High resolution spectroscopy The research of the frequency modulated laser is to split the beam into a strong saturated beam and a weak test beam. The so-called saturated beam, roughly speaking, is able to excite a large number of atoms, so that the excitation of atoms presents a saturated state. These two beams pass through the gas atomic sample along the same path, but their propagation directions are opposite. When the output frequency of the laser is scanned and just adjusted to the frequency of atomic energy level transition, the strong saturated beam is absorbed by a group of specific atoms, and the velocity component of these atoms in the beam direction is zero. The atoms with velocity component in the direction of the beam will not absorb photons in the saturated beam due to the Doppler effect. The saturated beam reduces the number of atoms in the selected state, so when the test beam passes through the atomic sample, it is correspondingly subject to small absorption. This absorbed frequency range Narrow due to no Doppler effect. If the frequency of the laser beam slightly deviates from Atomic transition If the frequency of the beam is exactly the atomic transition frequency, the two beams will interact with the same atom. Therefore, the saturated beam has no effect on the absorption of the test beam. It can be seen that the width of the test beam signal given by the saturation spectrum technology is very narrow, almost close to Spectral line The natural width of the.

Saturation spectroscopy technology eliminates spectral lines Doppler broadening It is one of the effective methods, and its use is very wide. One example is the Balmer alpha line used to study the spectrum of hydrogen atom fine structure The research results are much more accurate than before. In addition, on the absorption spectrum 2S full and 2P full were observed for the first time in energy level Of Lamb shift 。 The precise data of the fine structure of the hydrogen atom spectrum improves the precision of the Rydberg constant. The Rydberg constant determined from such research R =(109737.311 ± 0.012) cm, which is nearly 10 times higher than the previous accuracy.

Polarization spectroscopy

Another way to eliminate Doppler effect is polarization spectroscopy. This technology is characterized by measuring Polarization of light It is much easier to measure the small change of the intensity than to measure the change of the intensity, so the measurement sensitivity can be significantly improved. As in saturation spectroscopy light beam It is also divided into two bundles, one of which is much stronger than the other and passes through the studied sample in the opposite direction. However, in polarization spectroscopy, the weak test beam is linearly polarized and passes through the gas sample placed between the cross polarizers. If the test beam does not change its polarization when passing through the sample, it will not reach the detector. But saturated beams can cause this change. Because when it first passes Quarter wave plate It becomes Circularly polarized light Has. Circularly polarized Electric field direction It rotates, either clockwise or counterclockwise. The probability of atoms absorbing circularly polarized light depends on the orientation of the atomic angular momentum. The initial atomic orientation is random, but when some atomic orientation can absorb a circularly polarized light, it is saturated light beam The energy levels of these atoms become empty, and there are more atoms with opposite angular momentum orientation. When the linearly polarized test beam passes through the same area of the gas, the oriented atoms will change the propagation of the test beam. This reason is easy to understand. Linearly polarized light It can be seen as the superposition of two circularly polarized light with equal intensity, and the electric field of one circularly polarized light is along Clockwise When rotating, the electric field of another circularly polarized light rotates counterclockwise. When the test beam passes through the gas, the atoms it encounters will absorb a kind of circularly polarized light too much, because the number of these atoms is relatively large. As a result, the intensity of one kind of circularly polarized light is weakened, while the other kind is relatively stronger. Therefore, the test beam from the gas sample is no longer linearly polarized, but becomes Elliptically polarized light Has. In this way, the test beam has a component that can pass through the crossed polarizer. However, all these conditions must occur when the saturated beam and the test beam act on the same atom, that is, none Doppler Displaced atoms. In this respect, polarization spectroscopy is the same as saturation spectroscopy. In fact, polarization spectroscopy is derived from saturation spectroscopy. The biggest feature of this spectroscopy technology is that there is basically no noise. Using this technology, more precise knowledge of energy level structure can be obtained. For example, the measurement results have tripled the accuracy of the Rydberg constant, making it the most accurate known basic constant.

Two-photon spectroscopy

It is also a good method to eliminate spectral Doppler broadening. This technology was first reported in 1974. In this technology, a beam of light is reflected back by the mirror along the original path, so that they travel in the opposite direction along the same optical axis, and after superposition standing wave 。 The gas sample is placed in the standing wave field. If the frequency of the laser beam is adjusted to half of the selected atomic transition frequency, under certain conditions, each atom interacting with the beam will simultaneously absorb a photon from each of the two beams propagating in opposite directions.

Suppose that when an atom moving along the optical axis in a standing wave field absorbs two photons from the opposite direction Doppler The shift is in the direction of purple light, that is, it has a higher frequency, while the Doppler shift of another photon is in the direction of red light, and the shift size is the same as that of the previous photon. Therefore, the total energy of the two absorbed photons is constant, regardless of the atomic Movement speed how. Therefore, Two-photon absorption The Doppler effect of atomic motion is canceled, and the sum of light frequencies absorbed by atoms is just the atomic transition frequency. If the output frequency of the laser slightly deviates from half of the atomic transition frequency, the atom will not absorb photons in two opposite directions. Therefore, spectral lines are eliminated Doppler broadening The effect of is not good. That is, Doppler broadening can be effectively eliminated only when the laser frequency matches the atomic transition frequency.

In two-photon absorption spectroscopy, all atoms interacting with the laser beam can contribute to the signal without Doppler effect, not limited to atoms moving in the direction perpendicular to the optical axis, so the signal without Doppler effect is very strong. This is different from saturation spectroscopy and polarization spectroscopy. In these two kinds of spectra, the atoms without Doppler effect are selective, and their motion should be perpendicular to the optical axis; And in Two-photon absorption In spectroscopy, all atoms that interact with the beam can eliminate its Doppler effect.^[2]

application area

High resolution spectroscopy is widely used in the study of the energy level structure of atoms and molecules. Until now, all the data obtained belong to the prediction of verification theory. However, many of the major changes in physics in the 20th century were caused by the discovery of small differences between theory and observation. High resolution spectroscopy may make contributions in this regard.

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