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Photoelectron spectroscopy

Photoelectron spectroscopy reflecting solid surface information

photoelectron Photoelectricity spectroscopy, using photoelectric effect Principle measurement of Monochromatic radiation The kinetic energy of photoelectrons hit from the sample (and measured from it binding energy ）Photoelectron intensity and the angular distribution And use this information to study the electronic structure of atoms, molecules, condensed phases, especially solid surfaces. For solids, photoelectron spectroscopy is a surface sensitive technique. Although the incident photon can penetrate into the deep part of the solid, only the photoelectrons in a thin layer of 20-30 angstrom below the solid surface can escape (photon Inelastic scattering Mean free path 10~10 times larger than electrons), so photoelectrons reflect the information of solid surface.

Chinese name: Photoelectron spectroscopy
Foreign name: photoelectron spectroscopy

Fundamentals: utilize Einstein Photoelectric effect law of
Purpose: surface analysis

catalog

seven experimental technique
eight application

brief introduction

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Photoelectron spectroscopy is mainly used for surface analysis , with certain energy emitted by the excitation source X-ray ， electron beam ， ultraviolet ， ion beam or Neutron beam When acting on the sample surface, electrons of different energy levels in the atoms on the sample surface can be excited to produce photoelectrons or Auger electron Etc. These free electron With sample surface information and characteristic kinetic energy. Collect and study their energy distribution through an energy analyzer, and record the electron signal intensity Relation curve with electron energy. This is the photoelectron spectrum

Fundamentals

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Photoelectron spectroscopy

The basic principle of photoelectron spectroscopy is Einstein's photoelectric effect Laws. The material is exposed to a sufficiently short wavelength (high photon energy) electromagnetic wave The emission of electrons can be observed. This is because the electrons in the material are bound to different quantized energy levels Light quanta When the sample is irradiated valence electron or Core electronics After absorbing a photon, make a dipole transition from the initial state to high excited state Leaving the atom. At first, this phenomenon was observed Photocurrent And called photoelectric effect ； Now, the more common term is photoionization or photoemission. If the sample is irradiated by monochromatic photons with a fixed frequency, the energy of this process can be specified by Einstein relation:

hν=Ek+Eb

Where h ν is the energy of the incident photon, Ek is the energy of the electron struck by the incident photon, and Eb is the energy of the electron ionization energy , or called binding energy 。 Photoionization The action requires a certain minimum photon energy, which is called threshold photon energy h ν 0. For solid samples work function This term is called φ.

For the photon whose energy h ν significantly exceeds the threshold photon energy h ν 0, it has the ability to ionize various electrons with different ionization energies (as long as Eb<h ν). One photon ionizes one electron separately. A photon may knock out a loosely bound electron and transfer high kinetic energy to it; Another photon with the same energy may ionize a tightly bound electron and generate a photoelectron with lower kinetic energy. Therefore, photoionization, even if a fixed frequency excitation source is used, will also produce polychromatic, that is, multi energy photoemission. Because the energy level occupied by the electron is quantized, the photoelectron has a kinetic energy distribution n (E), which is separated by a series of energy band form. This fact essentially reflects that the electronic structure of the sample is a "shell" structure. Using the method of analyzing photoelectron kinetic energy, the experimental determination of n (E) is called photoelectron spectroscopy (PES). N (E) is plotted against E to form a photoelectron spectrum. So simple Photoelectron spectrum Fig. provides the most direct and convincing evidence for the orbital model of electronic structure. Strictly speaking, the photoelectron spectrum should be explained by the multi electron state method of the ionized system M+, which is better than the occupied single electron state (orbit) of the neutral system M.

classification

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According to different light sources, photoelectron spectrum can be divided into: 1 Ultraviolet photoelectron spectroscopy UPS（Ultroviolet Photoelectron Spectrometer）； 2、 X-ray photoelectron spectroscopy XPS（ X-Ray Photoelectron Spectrometer ）3、 Auger electron spectroscopy AES(Auger Electron Spectrometer)。

X-ray Photoelectron Spectrum : used for (qualitative) analysis of atomic Valence state , and the combined form. The instrument is simple and the spectral analysis is simple.

Ultraviolet photoelectron spectroscopy Analyze the energy and function of the electrons in the valence shell orbit. We can get a lot about the stability of molecules, Reactivity And other information. But due to the electronic transition and Vibrational energy level Yes, and Molecular symmetry Very closely related. The spectrum analysis is complex. Instrument requirements are high.

Auger Electron spectroscopy : It belongs to secondary electron spectroscopy. Mainly used for solid, or Condensed matter Analysis of elements and valence states of substances. The atlas is simple and the instrument requirements are high. Commonly used to and X-ray photoelectron spectroscopy , fluorescence spectrum, complementary and combined use.

Instrument composition

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Photoelectron spectrometer It is mainly composed of six parts: excitation source, sample ionization chamber Electronic energy analyzer , electronic detector, vacuum system and data processing system Etc. Excitation source Common ultraviolet radiation source and X-ray source 。 Using ultraviolet radiation source as excitation source is called Ultraviolet photoelectron spectroscopy , using X-ray The is called X-ray photoelectron spectrum, and collectively called photoelectron spectrum.

Photoelectron spectroscopy

(1) Vacuum system: the purpose is to prevent electrons from being trapped by residual gas Molecular scattering And avoid residual gas Molecular adsorption Samples caused Surface contamination 。 Generally, in experiments, gas pressure 1.33 * 10-7Pa is acceptable. The existing air extraction system includes: with liquid nitrogen Cold trap Water cooling of Oil diffusion pump ， Turbomolecular pump , sputter ion pump, Titanium sublimation pump ， Cryopump 。 The first three types of pumps are often used in the main air extraction system, and the rest are often used as auxiliary pumps. Evacuate in this way and pass 100~160C º; The base pressure of 1.33 * 10-8 or even lower can be obtained by the system that bakes overnight under the condition. In addition, this system needs to be baked regularly.

（2） Sample handling Part: including three vacuum chambers, the first Vacuum chamber It is used to enter and exit samples, and the second one is vacuumized Buffering effect And prepare and treat the sample inside, and the sample is irradiated by X-ray in the third vacuum chamber to obtain photoelectrons.

（3） X-ray source : Thermal filament Emit electrons, accelerate by electric field, bombard anode (usually Al or Mg), emitting X-ray (Al Characteristic spectral line Is 1486.6 ev, and the characteristic spectral line of Mg is 1253.6 ev). Such X-ray is composed of multiple frequencies of X-ray. Often used in experiments quartz crystal Monochromator , monochromate the obtained X-ray.

(4) Electronic energy analyzer: used to measure the energy distribution emitted from the sample surface Photoelectron spectrum Is one

Amplitude Electron flow Graph of intensity versus kinetic energy.

(5) Detector

brief history

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In 1905, Einstein explained in his paper photoelectric effect , and P Auger discovered the Auger effect in 1923, and these two effects constitute the current chemical analysis Electron spectroscopy The foundation of learning. Instruments for analyzing the kinetic energy of electrons have also appeared very early, even in the First World War The experiment of using magnetic field to analyze beta rays has been carried out before. However, the energy of the electrons needed to be analyzed in chemical research is generally low, so high resolution measurement Low-energy electron It is possible to make full use of the technology of Electron spectroscopy The technological achievements in the 1960s met the requirements of high resolution. In 1981, Kai M. Siegbahn (1918 -) developed high resolution Electron spectrometer And used to study photoelectron spectroscopy and chemical element Quantitative analysis, and Blombergan (Nicolaas Bloombergen, 1920 -) and Arthur L. Schawlow (1921-1999) shared this year The nobel prize in physics 。 In chemical analysis electron spectroscopy, the most important Photoelectron spectroscopy 。

Historically, photoelectron spectroscopy was originally developed by Sweden K. of Uppsala University Siegbahn and its collaborators were established after about 20 years of efforts. It is often called Electron spectroscopy for chemical analysis (ESCA), however, because the original light source uses the characteristics of aluminum, magnesium, etc X-ray , this method is gradually known as X-ray photoelectron spectroscopy （XPS）。 In addition, London D. of Imperial College W. Turner et al. created the He I resonance line as a vacuum in 1962 Ultraviolet light source Of Photoelectron spectrometer , within the analytical molecule valence electron Has achieved great success in the state of Price band In the study of, this party's application area It is gradually expanding. Compared with X-ray photoelectron spectroscopy, this method is called Ultraviolet photoelectron spectroscopy (UPS) to show the difference.

Experimental mode

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Experimental methods and techniques

Due to the continuous adjustability of photon energy, synchrotron radiation Photoelectron spectroscopy (photoemission spectroscopy) experiments can be carried out in several modes to obtain Material surface Different electronic structure information. (1) EDC mode: Photon energy Fixed energy distribution curve (Energy Distribution Curves) experiment, that is, a certain energy photon is used as the excitation source to measure the sample surface conduction band and Price band Electronics of Energy state Distribution.

(2) CFS mode: fixed Final state Constant Final state Spectra experiment, that is to use photon energy scanning to constantly detect the kinetic energy of a final state Photoelectron spectrum , which can be used to measure the surface in the process of interface formation Band structure and Energy band bending 。

(3) CIS mode: Constant Initial state Spectra experiment, that is, select and fix the initial state energy that makes the core energy level transition to the empty surface state strongest, and combine the photon energy with the detection photoelectron The kinetic energy of Empty surface state 。

experimental technique

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(1) XPS, AES: that is to use conventional X-ray (Mg ka, Al ka) or electron as excitation source to determine the elements, composition and chemical state of the sample surface.

(2) LEED: used to determine the order of material surface.

(3) SRPES: synchrotron radiation Light is the excitation source of photoelectron spectroscopy to determine the electronic structure of material surface.

application

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The application of photoelectron spectroscopy mainly includes the following two aspects:

(1) Measured on each occupied track Electron ionization The required energy is Molecular orbital theory Provide experimental basis.

(2) Study the composition and structure of solid surface a The chemical state of the surface, including the type and content of elements, Chemical valence Sum of states Chemical bond The formation of;

b. surface structure , including macro and surface morphology, Phase Distribution, Element distribution And microscopic atomic surface arrangement;

c. Surface electronic state Electronic cloud Distribution and energy level structure.

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