Raman spectra

Scattering spectrum

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Raman spectrum is a kind of scattering spectrum. Raman spectra analytical method Is based on India Scientist C 5. Raman Raman scattering Effect incident light frequency Different scattering spectra are analyzed to obtain information about molecular vibration and rotation, and are applied to molecular structure An analytical method of research.

Chinese name: Raman spectra
Foreign name: Raman spectra

Category: Scattering spectrum
Purpose: molecular structure An analytical method of research

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Raman spectra

1928 C 5. Raman experiments show that when light passes through transparency medium cover Molecular scattering The frequency of light changes, which is called Raman scattering , later in the same year Soviet Union and France It was also observed. In the scattering spectrum of transparent medium incident light Frequency ν zero The same ingredient is called Rayleigh scattering ； The frequency is symmetrically distributed in ν zero Spectral lines or bands on both sides zero ±υ one Namely Raman spectrum, in which the component with smaller frequency is ν zero －υ one Also known as Stokes line, the component with larger frequency is ν zero +υ one It is also called anti Stokes line. near Rayleigh scattering The spectral lines on both sides of the line are called small Raman spectra; The spectral lines on both sides far from the Rayleigh line are called large Raman spectra. Rayleigh scattering The strength of the line is only incident light 10% of strength -3 Raman spectrum intensity is only about 10% of Rayleigh line -3 。 Small Raman Spectra and Molecular Rotation energy level The large Raman spectrum is related to the vibrational rotational energy levels of molecules. The theoretical interpretation of Raman spectrum is that the incident photon and molecule are inelastic scattering, and the molecular absorption frequency is zero Of photons, emitting ν zero －υ one Photons of（ That is, the absorbed energy is greater than the released energy ）At the same time, the molecule transitions from low energy state to high energy state (Stokes line); The molecular release frequency is ν zero Of photons, emitting ν zero +υ one Photons of（ That is, the energy released is greater than the energy absorbed ）At the same time, the molecule transitions from the high energy state to the low energy state (anti Stokes line) Molecular energy level The transition of only involves Rotational energy level , emitting small Raman spectra; When it comes to vibration rotation energy levels, large Raman spectra are emitted. And molecules infrared spectrum Different, polar molecules and Nonpolar molecule Can produce Raman spectrum. The advent of laser provides high quality and high intensity Monochromatic light , which strongly promotes the research and application of Raman scattering. Raman spectroscopy can be used in chemistry physics 、 biology And medicine, for pure qualitative analysis, highly quantitative analysis and determination molecular structure They are of great value.

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Electrochemical in-situ Raman spectroscopy is to use the phenomenon that the frequency of incident light is greatly changed due to the scattering of material molecules, to excite monochromatic incident light (including circularly polarized light and linearly polarized light) to the electrode surface modulated by electrode potential, and to measure the scattered Raman spectrum signal (changes in frequency, intensity and polarization performance) Change relation with electrode potential or current intensity. The Raman spectrum of general substance molecules is very weak. In order to obtain enhanced signals, the electrode surface can be used Coarsening 10% higher strength can be obtained four -10 seven Surface Enhanced Raman Scattering (SERS) spectrum, when molecules with resonance Raman effect are adsorbed on the roughened electrode surface, the surface enhanced resonance Raman scattering (SERRS) spectrum is obtained, and its intensity can be enhanced by 10 two -10 three 。

The measurement device of electrochemical in-situ Raman spectroscopy mainly includes two parts: Raman spectrometer and in-situ electrochemical Raman cell. Raman spectrometer consists of laser source, collection system Spectroscopic The system and detection system are composed, and the light source is generally energy Centralized power density High laser, the collection system is composed of lens groups, and the light splitting system uses grating or notch filter combined with grating to filter Rayleigh scattering and Stray light And the photomultiplier tube is used in the spectroscopic detection system detector 、 semiconductor Array detector or multi-channel charge coupled device. In situ electrochemical Raman cells generally have Working electrode 、 Auxiliary electrode and Reference electrode And breather. To avoid corrosive solutions and Gas erosion Instrument, Raman cell must be equipped with Optical window Sealing system. If experimental conditions permit, thin layer solution shall be used to avoid interference of solution signal as far as possible（ electrode The distance from the window is 0.1~1mm), which is very important for the micro Raman system. If the optical window or solution layer is too thick, the optical path of the micro system will be changed, and the collection efficiency of the surface Raman signal will be reduced. The most common method for electrode surface roughening is electrochemical oxidation -The reduction cycle (ORC) method can generally be used for in-situ or non in-situ ORC treatment.

At present, the research progress of electrochemical in-situ Raman spectroscopy is mainly as follows: First, the detection system is extended to transition metal and semiconductor electrodes through surface enhancement treatment. Although electrochemical in-situ Raman spectroscopy is a more sensitive method for on-site detection, only silver, copper and gold can be used electrode It can give a strong SERS in the visible light region. Many scholars try to realize surface enhanced Raman scattering on transition metal electrodes and semiconductor electrodes with important application background. The second is to study the electrode through analysis Surface adsorption Structure and orientation of species and SERS spectra of objects electrochemistry The relationship between the parameters, the electrochemical adsorption phenomenon for the molecular level description. Third, by changing the frequency of the modulation potential, we can get the "time-resolved spectrum" that changes under two potentials to analyze the relationship between the SERS peak of the system and the potential, and solve the problem caused by the change of the SERS active site on the electrode surface with the potential.

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The elastic scattering and inelastic scattering occur when the light shines on the material The elastic scattering light has the same wavelength as the excitation light, and the inelastic scattering light has longer and shorter wavelength than the excitation light, which is collectively called Raman effect. Raman effect is the result of the interaction between photons and optical phonons.

Raman spectroscopy - Principle Raman effect originates from Molecular vibration (and lattice vibration) and rotation, so molecules can be obtained from Raman spectra Vibrational energy level (lattice vibration energy level) and rotational energy level structure. The Raman effect can be explained by the concept of virtual upper level:

Suppose that the scatterer molecule is in the original basic electronic state, and the vibration energy level is shown in the figure. When irradiated by the incident light, the polarization caused by the interaction between the excited light and this molecule can be regarded as virtual absorption, which is expressed as Electron transition To the virtual state, the electrons on the virtual level immediately jump to the lower level and emit light, that is Scattered light 。 If we still return to the initial electronic state, there are three cases as shown in the figure. thus Scattered light There are both spectral lines with the same frequency as the incident light and spectral lines with different frequencies. The former is called Rayleigh line, and the latter is called Raman line. In Raman lines, the spectral lines whose frequency is less than the frequency of the incident light are called Stokes lines, and the spectral lines whose frequency is greater than the frequency of the incident light are called anti Stokes lines.

The additional frequency value related to the vibration energy level is called a large Raman shift, and the value related to the rotation energy level in the same vibration energy level is called a small Raman shift:

Large Raman shift: (is the frequency of vibration energy band)

Small Raman shift: (where B is the rotation constant)

Simple derivation of small Raman shifts: using rotation constants

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Raman scattering spectrum has the following obvious characteristics

a. Raman scattering wave number Although it varies with the wave number of the incident light, for the same sample, the shift of the same Raman spectrum line is different from that of the incident light wavelength It is only related to the vibration and rotation energy level of the sample;

b. On the Raman spectrogram with wave number as a variable, Stokes line and anti Stokes line are symmetrically distributed on both sides of Rayleigh scattered ray, because in the above two cases, the energy of a vibrating quantum is obtained or lost respectively.

c. In general, the strength of the Stokes line is greater than that of the anti Stokes line. This is because the Boltzmann distribution is in vibration ground state On Number of particles Far greater than in vibration excited state Number of particles on.

spectral analysis

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Spectrogram made in the experiment (see the attached figure, in wavelength)

Standard spectrogram (as follows, in wave number)

Interpretation of spectra by structural analysis:

The molecule is a tetrahedral structure, one carbon atom At the center, four chlorine atoms are at the four vertices of the tetrahedron. When the tetrahedron rotates around its own axis for a certain angle, or memory inversion (r-r), or rotation plus inversion, the operation of keeping the geometric configuration of the molecule unchanged is called Symmetrical operation , which Rotation axis become Axis of symmetry 。 CCI four There are 13 symmetry axes and 4 on file Symmetrical operation 。 As we know, the molecule composed of N atoms has (3N-6) internal Vibration degree of freedom 。 So there can be 9 molecules (3 × 5-6) freedom , or 9 independent normal vibrations. According to the symmetry of molecules, these nine normal vibrations can be classified into the following four categories:

In the first category, there is only one mode of vibration. Four chlorine atoms stretch and vibrate along the line connecting with C atoms, which is recorded as v1, representing nondegenerate vibration.

In the second category, there are two modes of vibration. Two adjacent pairs of CI atoms simultaneously move in the opposite direction of the C atom or in the vertical direction of the C atom, which is recorded as v2, representing double degenerate vibration.

In the third category, there are three modes of vibration. Four CI and C atoms move in the opposite direction, recorded as v3, representing triple degenerate vibration.

In the fourth category, there are three modes of vibration. The adjacent pair of CI atoms make an extension motion, and the other pair makes a compression motion, which is recorded as v4, indicating another triple degenerate vibration.

The "degeneracy" mentioned above refers to that in the same type of vibration, although there are different vibration modes, they have the same energy, and they correspond to the same spectral line in the Raman spectrum. Therefore, there should be four basic spectral lines in the molecular vibration Raman spectrum. According to the relative intensity of each spectral line measured in the experiment, the order is v1>v2>v3>v4. The spectral line of benzene is also shown in the attached figure. The analysis is similar and will not be repeated here.

Advantages

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Advantages of Raman spectroscopy

Raman spectra

It provides fast, simple, repeatable, and more importantly, nondestructive qualitative and quantitative analysis. It does not require sample preparation. Samples can be directly passed through optical fiber probes or glass quartz , and optical fiber measurement. in addition

Because the Raman scattering of water is very weak, Raman spectroscopy is an ideal tool for studying biological samples and chemical compounds in aqueous solutions.

2 Raman can cover the range of 50-4000 wave numbers at the same time, and can analyze organic and inorganic substances. On the contrary, if the infrared spectrum covers the same range, it must be changed grating , beam splitter wave filter And detectors.

3 The Raman spectrum peak is clear and sharp, which is more suitable for quantitative research, database search, and qualitative research using difference analysis. stay Chemical structure In the analysis, the intensity of independent Raman intervals can be related to the number of functional groups.

Because the diameter of the laser beam is usually only 0.2-2 mm at its focusing position, conventional Raman spectroscopy can be obtained with only a few samples. This is a great advantage of Raman spectroscopy over conventional infrared spectroscopy. Moreover, Raman microscope objective lense The laser beam can be further focused to 20 microns or even smaller, and the sample with smaller area can be analyzed.

Resonance Raman effect can be used to selectively enhance the specific coloration of macromolecules Group The Raman intensity of these chromophores can be selectively enhanced by 1000 to 10000 times.

a spectrometer

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brief introduction

Raman spectrometer generally consists of the following five parts.

Raman spectra

light source

Its function is to provide incident light with good monochromaticity, high power and preferably multi wavelength operation. At present, all the light sources in Raman spectroscopy experiments have used lasers instead of mercury lamps used in history. For conventional Raman spectroscopy experiments, common Gas laser It can basically meet the needs of the experiment. In some Raman spectroscopy experiments, the intensity of incident light is required to be stable, which requires Laser Of output power stable.

External optical path

The external light path part includes condensing, light collecting, sample holder, filter, polarization and other parts.

(1) Spotlight: use one or two focal length With a suitable convergent lens, the sample is placed at the waist of the convergent laser beam to increase the irradiation power of the sample light, which can increase the irradiation power of the sample in unit area by 105 times than that before the lens is used for convergence.

(2) Light collection: lens group or reflective concave mirror is usually used as the collecting mirror of scattered light. Usually by Relative aperture Values around 1 lens form. To collect more Scattered light For some experimental samples, a reflector can be added on the opposite side of the collector and in the direction of illumination light propagation.

(3) Sample rack: The design of the sample rack shall ensure the most effective illumination and the least stray light, especially avoid the incident laser entering the incident slit of the spectrometer. Therefore, for transparent samples, the best sample layout scheme is to make the illuminated part of the sample appear as a long cylinder in the shape of an incident slit of the spectrometer, and make the direction of the collected light perpendicular to the propagation direction of the incident light. Several typical sample racks Space configuration See the right figure.

(4) Light filtering: the main purpose of placing light filtering components is to suppress stray light and improve Raman scattering Signal-to-noise ratio 。 stay sample In front, the typical filter components are pre monochromator or interference filter, which can filter out the light source Central African Most of the light energy at the laser frequency. The small aperture optical barrier has a good effect on filtering the plasma line generated by the laser. stay sample Later, a large part of the unwanted Rayleigh line can be filtered out with a suitable interference filter or absorption box energy And increase the relative intensity of Raman scattering.

(5) Polarization: When measuring polarization spectrum, polarization elements must be inserted in the external optical path. Adding polarization rotator can change the incidence Polarization of light Direction; Incident at spectrometer slit A polarizer is added before the spectrometer to change the Scattered light Polarization; The depolarization interference of the spectrometer can be eliminated by setting a polarization scrambler behind the polarizer.

Dispersion system

dispersion The system separates the Raman scattering light according to the wavelength in space, usually using a monochromator. Because the Raman scattering intensity is very weak, the Raman spectrometer is required to have a good Stray light Horizontal. Of various optical components defect , especially the defect of grating instrument Stray light The main source of. When the instrument's Stray light When the skill is less than 10-4, only Gas 、 Clear liquid And transparent crystals.

Raman spectra

receiving system

There are two types of Raman scattering signal reception: single channel reception and multi-channel reception. Photomultiplier tube reception is a single channel reception.

information processing

In order to extract Raman scattering information, the commonly used electronic processing methods are DC amplification, frequency selection and photon counting, and then use the recorder or computer interface software to draw the spectrum.

Raman effect

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The elastic scattering and inelastic scattering occur when the light shines on the material The elastic scattering light is the same component as the excitation light. The inelastic scattering light has longer and shorter components than the excitation light, which are collectively called Raman effect 。

When a gas, liquid or transparent sample is irradiated with a monochromatic light whose wavelength is much smaller than the particle size of the sample, most of the light will be transmitted in the original direction, while a small part will be scattered at different angles to generate scattered light. When viewed in the vertical direction, except for the original incident light In addition to Rayleigh scattering with the same frequency, there is a series of symmetrically distributed Raman lines that are very weak and shift with the frequency of the incident light. This phenomenon is called Raman effect. Due to the number of Raman spectral lines, the size of the displacement, and the length of the spectral lines Rotational energy level of Therefore, compared with infrared absorption spectrum Similarly, the study of Raman spectroscopy can also obtain information about molecular vibration or rotation. at present Raman spectrum analysis The technology has been widely used in the identification of substances, molecular structure The study of spectral line characteristics.

Analytical techniques

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type

Several important Raman spectroscopy analysis techniques

1. Single channel detection Raman spectroscopy

Raman spectra

2. Raman Spectrum Analysis Technology of Multi channel Detector Represented by CCD

3. Adopt Fourier transformation FT-Raman spectral analysis technology

4. Resonance Raman Spectroscopy

5. Surface enhanced Raman effect analysis technology

Advantages and disadvantages of Raman spectroscopy for analysis

advantage

1. Advantages of Raman spectroscopy for analysis

The Raman spectrum analysis method does not require pretreatment of the sample, nor does it have the sample preparation process, which avoids some errors, and is easy to operate in the analysis process, with short determination time, sensitivity Advanced advantages.

Insufficient

2. Shortcomings of Raman spectroscopy for analysis

(1) Raman scattering area

(2) Different vibration peak overlapping and Raman scattering intensity are easily affected by optical system Influence of parameters and other factors

(3) Interference of fluorescence on Fourier transform Raman spectroscopy

(4) It is often found that curve The nonlinear problem of

(5) The introduction of any substance will bring some degree of pollution to the measured body system, which is equal to the possibility of introducing some errors, and will have a certain impact on the analysis results.

Signal selection

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Selection of Raman signal

The power of the incident laser, the thickness of the sample cell, and the parameters of the optical system also have a great impact on the intensity of the Raman signal. Therefore, the matrix or the molecule of an external substance that can generate a strong Raman signal and whose Raman peak does not overlap with the Raman peak to be measured is often used as the internal standard for correction. The selection principle and quantitative analysis method It is basically the same as other spectral analysis methods.

Stokes Linear energy Decrease, wavelength becomes longer

The anti Stokes line energy increases and the wavelength becomes shorter

Application direction

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Applications of Raman spectroscopy

Through the analysis of Raman spectrum, we can know the vibration and rotation energy levels of substances, so that we can identify substances and analyze their properties. Here are some examples:

L The Raman spectrum of natural chicken blood stone is essentially different from that of imitation chicken blood stone. The former is mainly the Raman spectrum of dickite and cinnabar, and the latter is mainly the Raman spectrum of organic matter. The two can be distinguished by Raman spectrum.

Raman Spectra of Natural Chicken Blood Stone

Raman spectrum of natural chicken blood stone: Raman spectrum of imitation chicken blood stone:

In the above two figures, a is the ground (black), and b is the blood (red)

Raman Spectra of Imitated Chicken Blood Stone

After consulting the data and comparing the Raman spectra of different substances, it can be seen that the main component of natural chicken blood stone "ground" is dickite, and the natural chicken blood stone sample "blood" has both cinnabar and dickite, which is actually the aggregate of cinnabar and dickite. The main component of the imitated chicken blood stone "ground" is polystyrene acrylonitrile, and the Raman spectrum of "blood" is basically consistent with that of a red organic dye called PermanentBorde.

Identification of drugs: drugs and some white powders were analyzed by Raman spectroscopy, and the spectrogram is as follows:

common narcotics There are quite abundant Raman characteristic shift peaks, and the signal to noise ratio of each peak is high, which indicates that Raman spectroscopy is feasible for drug composition analysis, and the quality of the spectrum obtained is high. because Laser Raman spectroscopy It has micro analysis function. Even if drugs and other white powder substances are mixed together, they can also be identified by microscopic analysis technology, and Raman spectra of drugs and other white powders can be obtained respectively.

Analysis of Drugs and Some White Powders by Raman Spectroscopy

Raman spectroscopy can be used to monitor the preparation of materials: supported molybdenum sulfide and tungsten sulfide catalysts are supported by corresponding metallic oxide It is prepared by temperature programmed method in H2 and H2S atmosphere and is mainly used as hydrofining catalyst in industry. Under such industrial conditions, two-dimensional surface metal oxides are transformed into two-dimensional or three-dimensional metal sulfide 。 Compared with the supported metal oxides, the Raman spectra of the supported metal sulfides are relatively less, which is due to the strong absorption of black sulfides relative to visible light, resulting in weak signals. However, Raman spectroscopy can easily detect small metal sulfide microcrystals. The figure below shows the unloaded Crystalline phase MoS two Raman spectrum of

(Figure) Non loaded crystalline MoS two Raman spectrum of

At 380 and 450 cm -1 There are two spectral peaks belonging to the sum of the crystalline phases at, and the spectral peak of the supported crystalline molybdenum sulfide is much wider than that of the crystalline molybdenum sulfide. Cobalt promoter The addition of molybdenum sulfide led to the shift of the spectral peak and the weakening of the intensity, which was caused by the formation of phase and black phase.

Raman spectroscopy can monitor pesticide residues on fruit surface

Raman Spectra of Unbonded Crystal MoS2

Tear a small piece of peel from the treated fruit surface, and drop a drop of different pesticides on the surface of the fruit, and the pesticides will soak into the peel. Wipe the pesticide liquid on the peel with absorbent paper, then press the peel with pesticide residue into the small groove of the aluminum sheet to ensure that the surface of the peel with pesticide residue appears outside the small groove of the aluminum sheet, and then wipe the juice out with absorbent paper. The spectrum is as follows:

Raman spectra obtained by dropping postdoctoral doctors on the surface of different kinds of fruits (see the left figure). Obviously, in addition to the original Raman peak of fruit, the characteristic peak of Doctor of Plant Protection is 993cm -1 、1348cm -1 、1591cm -1 However, pesticide residues are still clearly displayed, which indicates that this method is sensitive and applicable. Quantitative analysis pesticide residue Can be obtained from pesticide Characteristic spectral line And the relative intensity ratio of the characteristic spectral lines of fruit.

Application of Laser Raman Spectroscopy

Laser Raman spectroscopy There are several applications of: Organic chemistry Applications in polymer, biology, surface and film.

Organic chemistry: Raman spectroscopy is mainly used as a means of structural identification in organic chemistry. The size and intensity of Raman shift and the shape of Raman peak are important Chemical bond Important basis for functional groups. utilize polarization Raman spectrum can also be used as the basis for judging the cis trans structure.

Raman spectra of different kinds of fruits after dropping plant protection

high polymer Raman spectroscopy can provide structural information about carbon chains or rings. In determining Isomer (Single break heterogeneous, location heterogeneous Geometric isomerism Raman spectroscopy can play a unique role in the research of stereoisomerism. Electroactive polymers such as polypyrrole pth Raman spectroscopy is often used as a tool in the research of polymer industrial production, such as extrusion linear Raman spectroscopy has been used to study the morphology of polyethylene, the observation of tight beam molecules in high-strength fibers, and the measurement of the crystallinity of polyethylene wear debris.

Biology Raman spectroscopy is a powerful means to study biological macromolecules. Because the Raman spectrum of water is very weak and the spectrum is very simple, Raman spectroscopy can study the structure and changes of biological macromolecules in a state close to the natural state and active state. Raman spectrum in Protein secondary structure The research on the relationship between DNA and carcinogen molecules, the structural changes of rhodopsin in the light cycle, calcification deposition and Erythrocyte membrane The application of this method in many studies has been reported.

There are many successful examples of using FT Raman to eliminate fluorescence interference of biological macromolecules.

Surface and film

Raman spectroscopy can be used to study materials, phase composition interface, grain boundary and other topics.

Recently, for Raman spectroscopy diamond and Diamond-like carbon film The interest of scholars at home and abroad is increasing.

Raman spectrum has become CVD（ Chemical vapor deposition Methods) Detection and identification methods of the prepared films.

In addition, Raman spectroscopic studies of LB films and nitridation of silicon dioxide films have been reported.

Although Raman scattering is weak and Raman spectrum is usually not sensitive enough, resonance or Surface enhanced Raman Technology can greatly enhance the sensitivity of Raman spectroscopy. Surface enhanced Raman spectroscopy (SERS) has become an active field in Raman spectroscopy.

development

The traditional grating spectroscopic Raman spectrometer, which is point by point scanning and single channel recording, is a waste of time. Moreover, the laser used in the laser Raman spectrometer can easily excite fluorescence, affecting the determination. In order to avoid the disadvantages of traditional laser spectrometers, two new spectrometers have been developed recently:

Fourier transform near-infrared laser Raman spectrometer and confocal laser spectrometer.

Fourier Raman spectrometer consists of laser source, sample chamber Michelson Interferometer, special filter and detector.

Fourier Raman spectrometer and optical path Fourier infrared spectrometer The light paths of the two are similar. The detected signal is transferred from the computer Collect and dispose.

Development prospect

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Laser technology

Raman spectroscopy has developed rapidly in recent years, which should benefit from two aspects.

Raman spectra

On the one hand, it is the development of laser technology. I recently participated in the London, UK At the 21st International Raman Spectroscopy Conference, it was felt that the nonlinear Raman spectroscopy technology based on ultrafast laser has become more and more mature. In the past, only a few units could engage in such sophisticated and expensive technology. In particular, the laser part is built by ourselves, and has to be adjusted every day, which is very unstable. Now this situation does not exist, and the price of the instrument is relatively low. The ultrafast (femtosecond or picosecond) lasers launched internationally to engage in nonlinear spectral research have reached a relatively mature stage in technology, and can be purchased in complete sets and are also relatively stable. Nonlinear Raman spectroscopy technology has been applied life sciences It plays a unique and important role in field research. For example, Harvard University's Xie Xiaoliang The professor has made a series of important achievements in developing and applying CARS Microscopy to study the three-dimensional structure of living cells. I think that high-quality ultrafast lasers also promote the development of another promising surface spectroscopy technology, namely, frequency combining (SFG) technology. As a nonlinear spectroscopy method with unique interface selectivity, it has played an increasingly important role in the research of interface and surface science, materials and even life fields.

Nanotechnology

The second important aspect is the rapid development of nanotechnology, which makes the Surface enhanced Raman spectroscopy (SERS) and tip enhanced Raman spectroscopy (TERS) have made great progress in ultra-high sensitivity detection, which has promoted Raman spectroscopy to become one of the few technologies that can reach the level of single molecule detection. At present, SERS is one of the most concerned topics in both Raman spectroscopy publications and Raman spectroscopy conferences. In recent international Raman spectroscopy conferences, SERS branch is the largest branch. In recent years, the number of papers on SERS has also shown a significant upward trend. SERS and TERS will have great development potential not only in the field of surface science research, but also in the field of life science, which can be used to study various important Life science system And solving basic problems. One of the advantages of Raman spectroscopy over infrared spectroscopy is that it is convenient to use Raman spectroscopy to study aqueous solutions, which are often needed for many studies in life sciences. Resonance Raman, surface enhanced Raman and nonlinear Raman spectroscopy, as well as their combination, will become valuable research methods in the forefront of life science, because the 21st century is the century of life science, and I think it is also the century of nanotechnology and laser technology.

Related technologies

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Surface enhancement method

Raman spectra

Since 1974, Fleischmann et al found that pyridine molecules adsorbed on the roughened Ag electrode have a huge Raman scattering phenomenon. In addition, the selective adsorption of molecules on the active carrier surface inhibits fluorescence emission, which greatly improves the signal noise ratio of laser Raman spectroscopy analysis. This surface enhancement effect is called surface enhanced Raman scattering (SERS). SERS technology is a new surface measurement technology, which can study the structural information of material molecules at the molecular level.

Pyrometry

High temperature laser Raman technology is used in metallurgy, glass geology It is used in chemistry, crystal growth and other fields to study the high-temperature phase transformation process of solids and the bonding structure of melts. However, these tests need to be carried out at high temperatures, and the conventional Raman analyzer must be technically modified.

Resonance method

Laser resonance Raman spectroscopy (RRS) generates laser frequency and an electron of the molecule to be measured absorption peak When approaching or overlapping, the intensity of one or several characteristic Raman bands of this molecule can reach 104~106 times of the normal Raman band, and overtones and combined vibration spectra that are difficult to appear in the normal Raman effect and whose intensity can be comparable to the fundamental frequency are observed. Compared with the normal Raman spectrum, the resonance Raman spectrum is highly sensitive. Combined with the surface enhancement technology, the sensitivity has reached the level of single molecule detection.

Confocal microscopy

Micro Raman spectroscopy is an application technology that combines Raman spectroscopy analysis technology with micro analysis technology. Compared with other traditional technologies, it is easier to directly obtain a large amount of valuable information. Confocal micro Raman spectroscopy not only has the characteristics of conventional Raman spectroscopy, but also has its own unique advantages. With high power optical microscope, it has micro, in-situ, multiphase state, good stability spatial resolution Advanced features, can realize point by point scanning, and obtain high-resolution three-dimensional images. In recent years, confocal micro Raman spectroscopy has been used in tumor detection, cultural relics archaeology Public Security Law and other fields have a wide range of applications.

fourier transformation

Fourier transform Raman spectroscopy is a new technology developed in the 1990s. In 1987, Perkin Elmer Company launched the first near-infrared excited Fourier transform Raman spectrometer (NIR FT-R), which uses Fourier transform technology to collect signals and accumulate them for many times to improve the signal to noise ratio, and uses 1064mm near-infrared laser to illuminate samples, greatly reducing the fluorescence background. Since then, Fr Raman has been engaged in biomedical science The non-destructive structural analysis of samples shows great vitality.

Optical fiber method

The introduction of optical fiber makes it possible for Raman spectrometer to be used for industrial online analysis and on-site telemetry analysis. Huy et al. used two optical fibers with a length of 10m and a diameter of 100 μ m, and the laser wavelength was 514 5nm, the benzene/heptane mixture was analyzed, and very good results were obtained. Benoit et al Optical waveguide sensor When used in Raman spectrometer, the Raman signal of liquid sample is enhanced 50 times. Cooney et al. compared the results of single fiber and multiple fibers used in Raman spectrometer, and found that the application of multiple fibers will improve the effectiveness of collecting Raman light. Cooper et al. analyzed xylene isomers in petroleum dyes by fiber remote Raman technology. In recent years, 1550nm fiber laser and EDFA fiber amplifier technology have been applied to Raman scattering distribution abroad Optical fiber temperature sensor The system has achieved good results. Distributed fiber Raman photon temperature sensor has become Optical fiber sensing technology And the development trend of detection technology. Because of its unique performance, it has become a new detection device in industrial process control and developed into an industrial automatic measurement network.

Solid photoacoustic method

Raman spectra

Photoacoustic Raman technology is a nonlinear optical storage technology that directly detects the energy stored in samples due to coherent Raman processes through photoacoustic methods. The photoacoustic Raman signal is proportional to the imaginary part of the third-order Raman polarizability of the solid medium, and has nothing to do with the nonresonant Raman polarizability, so it completely avoids the influence of nonresonant Raman scattering, and overcomes Rayleigh scattering in traditional optical methods, Brillouin scattering The disadvantages of interference are high sensitivity (Raman coefficient of 10 - 6cm - 1 can be detected), high resolution and basically no optical background. The ideal results have been obtained in the detection and analysis of gas and liquid samples. Unlike coherent Stokes Raman process, which has strict phase matching angle requirements, it is also suitable for studying the characteristics of solid media. Barrett et al. theoretically analyzed the process of photoacoustic Raman spectroscopy in gas samples, but unlike it, the photoacoustic Raman effect in solid media is caused by coherence raman gain The local thermal energy generated by the process is coupled to the thermoelastic process of the vibration mode of the sample itself. For the anisotropic structure of the medium, the third-order nonlinear Raman susceptibility tensor form shows symmetry, so the situation is much more complex. Using the parallel model and the thermoelastic theory, the analytical formula of photoacoustic Raman signal in the solid medium sample is derived, Some characteristics of photoacoustic Raman effect in solids are analyzed.

Associativity

In the past two years, Raman has been combined with a variety of other micro area analysis and testing instruments, including Raman and scanning electron microscopy (Raman SEM); Raman and atomic force microscope /Near field optical microscope (Raman AFM/NSOM); Raman IR; Raman and Laser scanning confocal microscope Raman CLSM (Raman CLSM), which focuses on the in-situ detection of micro areas. Through joint use, more information can be obtained and reliability can be improved.^[1]

technical analysis

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1. Single channel detection Raman spectroscopy

2. Raman Spectrum Analysis Technology of Multi channel Detector Represented by CCD

3. FT Raman spectral analysis technology using Fourier transform technology

4. Resonance Raman Spectroscopy

5. Surface enhanced Raman effect analysis technology

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