Development history, principle and application of laser Raman spectroscopy
Abstract: Raman spectra is a kind of scattering spectrum. Raman spectroscopy is based on the Raman scattering effect discovered by Indian scientist CV Raman (Raman). It analyzes the scattering spectrum at a frequency different from the incident light to obtain molecular vibration and rotation information, and is used in molecular structure research. An analytical method.
1 Development history of Raman spectroscopy
Indian physicist Raman irradiated benzene liquid with a mercury lamp in 1928 and discovered a new radiation spectrum: there are symmetrically distributed sharp bands with frequencies ω0-ω and ω0 ω on both sides of incident light frequency ω0 It belongs to a new type of molecular radiation called Raman scattering, where ω is the elementary excitation frequency of the medium. Raman won the Nobel Prize in Physics in 1930 for discovering this new molecular radiation and many achievements in light scattering research. At the same time, the former Soviet Union Landsberger and Mandelstad reported that a similar phenomenon was found in quartz crystals, namely Raman scattering caused by optical phonons, which is called combined scattering.
French Rocate, Carbens and American Wood confirmed the results of Raman's observational research. However, by 1940, the status of Raman spectroscopy had plummeted. Mainly because the Raman effect is too weak (about 10-6 of the incident light intensity), it is difficult for people to observe and study the weak Raman scattering signal, let alone measure to study the higher order Raman scattering effect above the second level. And requires that the volume of the sample to be tested must be large enough, colorless, dust-free, non-fluorescent, etc. So by the mid-1940s, the advancement and commercialization of infrared technology made the application of Raman spectroscopy decline for a time. After 1960, the emergence of ruby ​​laser has made the study of Raman scattering into a whole new period. Because the laser has good monochromaticity, strong directivity and high power density, using it as an excitation light source greatly improves the excitation efficiency. Become the ideal light source of Raman spectrum. With the improvement of the detection technology and the reduction of the requirements for the samples to be tested, Raman spectroscopy has been widely used in various fields such as physics, chemistry, medicine, industry, etc., and it has been paid more and more attention by researchers.
In the mid-1970s, the emergence of laser Raman probes gave vitality to micro-area analysis. Since the 1980s, Spex in the United States and Rrinshow in the United Kingdom have successively launched the bitman probe confocal laser Raman spectrometer. Because the notch filter is used to filter the excitation light, the stray light is suppressed, so it is no longer necessary. Double monochromator and even triple monochromator, and only need to use a single monochromator, so that the efficiency of the light source is greatly improved, so that the power of the incident light can be very low, and the sensitivity is greatly improved. Dilo has launched a multi-point online Raman system for industrial use. The optical fiber used can reach 200m, which makes the application range of Raman spectroscopy wider.
2 Principles of Raman spectroscopy
2.1 Rayleigh scattering and Raman scattering
When a photon of excitation light interacts with the molecule as the scattering center, most of the photons only change direction and scatter, and the frequency of the light is still consistent with the excitation light source. This scattering is called Rayleigh scattering. But there are also very few photons that not only change the direction of light propagation, but also change the frequency of light waves. This scattering is called Raman scattering. The intensity of the scattered light accounts for about 10-6 to 10-10 of the total scattered light intensity. The reason for Raman scattering is that the energy exchange between photons and molecules changes the energy of photons.
2.2 Generation of Raman scattering
The interaction between photons and sample molecules can be analyzed from the transition between energy levels. The sample molecule is in the ground state of the electronic energy level and the vibrational energy level. The energy of the incident photon is much larger than the energy required for the vibrational energy level transition, but it is not enough to excite the molecule to the excited state of the electronic energy level. In this way, the sample molecules absorb a photon and reach a quasi-excited state, also known as the virtual energy state. The sample molecule is unstable in the quasi-excited state, and it will return to the ground state of the electronic energy level. If the molecule returns to the ground state of the vibration level in the ground state of the electron level, the energy of the photon does not change, and Rayleigh scattering occurs. If the sample molecule returns to a higher vibrational energy level in the ground state of the electronic energy level, that is, some vibrational excited state, the energy of the scattered photon is less than the energy of the incident photon, and its wavelength is greater than the incident light. At this time, the lower frequency side of the Rayleigh scattering line of the scattering spectrum will appear a line of Raman scattered light, called Stokes line. If the sample molecule is not at the lowest vibrational level in the ground state of the electronic level before the interaction with the incident photon, but at a certain vibrational level excited state in the ground state of the electronic level, the incident photon action causes it to transition to a quasi After the excited state, the molecule is de-excited back to the ground state of the vibrational level of the electronic level, so that the scattered light energy is greater than the incident photon energy. Stokes line and anti-Stokes line are located on both sides of Rayleigh spectrum line with equal spacing. The Stokes line and the anti-Stokes line are collectively referred to as the Raman line. Because the vibration level spacing is still relatively large, according to Boltzmann's law, at room temperature, most molecules are in the ground state of the vibration level, so the strength of the Stokes line is much stronger than the anti-Stokes line. The Raman spectrometer generally records only the Stokes line.
2.3 RamanShift
The difference Δν between the frequency of Stokes and anti-Stokes scattered light and the frequency of the excitation light source is collectively called RamanShift. The intensity of Stokes scattering is usually much stronger than that of anti-Stokes scattering. In Raman spectroscopy, Stokes scattered light is usually measured. The Raman shift depends on the change of the vibrational energy level of the molecule. Different chemical bonds or ground states have different vibration modes, which determine the energy change between their energy levels. Therefore, the corresponding Raman shift is characteristic. This is the theoretical basis for the qualitative analysis of molecular structure by Raman spectroscopy.
2.4 Raman spectrum parameters
The parameters of Raman spectrum are mainly the position and intensity of the peak. The peak position is a reflection of the vibrational dynamic nature of the ground state of the electronic level of the sample molecule. It is expressed by the difference between the wave number of the incident light and the scattered light. The movement of the peak position is independent of the frequency of the excitation light. The intensity of Raman scattering is related to the concentration of the specific substance that produces the spectral line, and is in direct proportion. In the infrared spectrum, the intensity of the spectrum has an exponential relationship with the sample concentration. ) The molecular weight of the sample is also related to Raman scattering. As the molecular weight of the sample increases, the intensity of Raman scattering generally increases. For a certain sample, the intensity I has the following relationship with the incident light intensity I0, scattered light frequency ns, molecular polarizability a: I = CI0ns4a2 (where C is a constant).
2.5 Selection rule of Raman scattering
The applied alternating electromagnetic field acts on the nuclei and extranuclear electrons in the molecule, which can distort the shape of the molecular charge distribution and generate an induced dipole moment. Polarizability is a measure of the magnitude of the induced dipole moment produced by molecules under the action of an applied alternating electromagnetic field. The high polarizability indicates that the molecular charge distribution is prone to change. If the molecular polarizability also changes during the vibration of the molecule, the molecule can generate Raman scattering to the electromagnetic wave, which is said to have Raman activity. During the vibration of infrared-active molecules, the dipole moment changes, while the vibration of Raman-active molecules is accompanied by changes in the molecular polarizability. Therefore, polarized groups with inherent dipole moment generally have obvious infrared activity, while non-polarized groups have no obvious infrared activity. Raman spectroscopy is complementary to infrared spectroscopy. Any molecule or group with a symmetric center has no Raman activity if it has infrared activity; on the contrary, if there is no infrared activity, the Raman activity is more obvious. Generally, most molecules or groups do not have symmetry centers, so many groups often have both infrared and Raman activities. Of course, specific to a certain vibration of a group, infrared activity and Raman activity may be different. Some groups, such as the torsional vibration of ethylene molecules, have neither infrared activity nor Raman activity.
3 Application of laser Raman spectroscopy in catalysis research
Raman spectroscopy using a laser light source, because the laser has the characteristics of good monochromaticity, strong directivity, high brightness, coherence, etc., therefore, the combination of laser Raman spectroscopy and Fourier transform infrared spectroscopy has become a molecular structure The main means. Laser Raman spectroscopy has been applied in the field of catalysis for decades, and has achieved rich results in the research of supported metal oxides, molecular sieves, in-situ reaction and adsorption.
The application of laser Raman spectroscopy in molecular sieve research: skeleton vibration of molecular sieve, characterization of heteroatom molecular sieve, synthesis of molecular sieve. Research on catalyst surface adsorption: At present, one of the main uses of Raman spectroscopy in the study of catalyst surface adsorption behavior is to use pyridine as an adsorption probe to study the surface acidity of the catalyst. Research on catalyst surface species: Raman spectroscopy plays a very important role in the study of supported metal oxides. Not only can it obtain the structural information of surface species, but also can well correlate the structure with the reactivity and selectivity. This is very important in the study of catalysis. However, due to the generally strong fluorescence interference of the carrier, the conventional Raman spectroscopy of some oxides, especially low-load oxides, has encountered great difficulties. Research on the phase change of the catalyst surface: Research on the coordination structure and dispersion state of metal oxides, etc.
Reflect the TO film structure, D means ~ layer film, at ℃, only the anatase Raman peak; 1 layer, 2 layers of thin film crystallization is not good, because of Fe diffusion, short burning time, thin film, etc .; 3, 4 layers The difference is not big, all have Raman peaks of well-crystallized anatase; the position of Raman peaks will change with the particle size and pore size. The smaller the particle size will shift the peak position, the peak asymmetry widens, the peak strength becomes weaker, and the pore size of the TiO2 film becomes smaller. The size of the displayed particle size is 10 nm. Figure 3 shows the UV Raman spectrum of 2% molY at different calcination temperatures, and the laser source is 244nm. After roasting the sample at 500 ℃, five main peaks of 340, 374, 476, 613 and 640 cm-1 were given. From the relative intensities of the peaks at 340 and 374cm-1 and 476 and 630cm, it can be seen that after calcination at 500 ℃, they mainly exist in the form of monoclinic phase. With the increase of calcination temperature, the spectrum peaks have almost no change in intensity, width and frequency. Only the peak of the monoclinic phase of zirconia was observed during the calcination at 500 to 800 ° C.
The spectrum peak broadens, and two new spectrum peaks appear at 930 and 1070 cm. The spectral peak at 930cm is the V = O symmetric stretching vibration peak of the polymerized vanadium oxide outside the framework, and the spectral peak is the V = O symmetric stretching vibration peak of the framework four-coordinated vanadium oxide. The author believes that the two peaks of 930 and 1070 cm-1 are not found in the visible Raman spectrum, because the excitation line of the 244vim wavelength excites the charged transition of the framework vanadium and non-frame vanadium species, so the resonance effect makes the intensity of these two peaks. Enhancement, so that the peaks of ultraviolet Raman spectrum of skeleton alum and non-framework alum species are obtained at the same time. Figure 4 is the ultraviolet Raman spectrum of Silielite 1 and Fe-ZSM-5. Compared with Silicalite-1's UV Raman spectrum, Fe-ZSM-55's UV Raman spectrum shows five new peaks at 516, 580, 1026, 1126 and 1185 cm-1. Since the ultraviolet excitation line is located in the charged transition region (250 rim) between the framework iron and oxygen, these spectral peaks can be attributed to the resonance Raman peak of the framework iron species. In addition, the authors have also detected the presence of trace iron in sili-calite1 and ZSM-5 molecular sieves using ultraviolet Raman spectroscopy, which shows that ultraviolet resonance Raman spectroscopy is a sensitive and reliable means to characterize the framework heteroatoms in molecular sieves.
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