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Raman spectroscopy has been widely used in recent years due to the intensive development of the following technologies. These technologies are: CCD detection system in the near infrared region of high sensitivity, small size and high power diode laser, with excitation laser and signal filtering integrated optical fiber probe. These products, together with high calibre and short focal length spectrophotometers, provide high quality Raman spectra with a low fluorescence background and a small, easy-to-use Raman spectrometer.

Meaning

Elastic and inelastic scattering will occur when light hits the material. Elastic scattering of scattering light is the same as the excitation wavelength of light composition. Inelastic scattering of scattered light than the excitation waves are long and the short composition, collectively known as the Raman effect when using the wavelength of sample particle size smaller than monochromatic light irradiation gases, liquids, or transparent specimen, most of the light will be according to the original direction of transmission, while a small portion of the scattered in different point of view, producing light scattering. In the observation of the vertical direction, in addition to rayleigh scattering with the same frequency as the original incident light, there are also a series of very weak Raman spectral lines symmetrically distributed with the displacement of the incident light frequency, which is called the Raman effect. Because of the number of Raman lines, the displacement and the length of the line are directly related to the molecular vibration or rotational energy levels of the sample. Therefore, similar to infrared absorption spectrum, the study of Raman spectrum can also obtain information about molecular vibration or rotation. Raman spectroscopy has been widely used in the identification of substances and the study of spectral line characteristics of molecular structures.

Raman scattering spectrum has the following obvious characteristics:

A. Although the wave number of Raman scattering spectrum line varies with the wave number of incident light, for the same sample, the displacement of the same Raman spectrum line has nothing to do with the wavelength of incident light, but only with the vibration rotational energy level of the sample;

B. In Raman spectra with wave numbers as variables, stokes lines and anti-stokes lines are symmetrically distributed on both sides of rayleigh scattering lines, due to the corresponding gain or loss of energy of a vibrational quantum in both cases.

C. In general, stokes lines are stronger than anti-stokes lines. This is because of the boltzmann distribution, the number of particles in the vibrational ground state is much larger than that in the vibrational excited state.

Advantages of Raman spectroscopy

It provides fast, simple, repeatable, and more importantly, damageless qualitative and quantitative analysis that requires no sample preparation and can be measured directly through a fiber optic probe or through glass, quartz, and fiber optics.

1. Because of the weak Raman scattering of water, 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. Conversely, the grating, beam splitter, filter, and detector must be changed to cover the same range.

3. Raman spectrum peak is clear and sharp, which is more suitable for quantitative research, database search, and qualitative research using difference analysis. In chemical structure analysis, the strength of the independent Raman interval can be correlated with the number of functional groups.

4. Because the diameter of the laser beam is usually only 0.2-2 mm at its focal point, only a small number of samples are needed for conventional Raman spectra. This is a great advantage of Raman spectrum over conventional infrared spectrum. Moreover, the Raman microscope objective can further focus the laser beam to 20 microns or less, allowing analysis of even smaller samples.

5. The resonance Raman effect can be used to selectively enhance the vibration of large biomolecular specific color groups, whose Raman light intensity can be selectively enhanced by 1,000 to 10,000 times.

Characteristics of Raman spectrum of laser source

The application of laser light source Raman spectroscopy application of laser has the characteristics of good monochromatic, strong direction, high brightness, good coherence, and so on, combined with the surface enhancement Raman effect, produced the surface enhancement Raman spectrum. The sensitivity is 104~107 times higher than that of conventional Raman spectra, and the signal to noise ratio of the analysis is greatly improved because of the inhibition of fluorescence emission by selected adsorbent molecules on the surface of the active carrier. It has been applied to the detection of trace substances in biological, pharmaceutical and environmental analysis. Resonance Raman spectroscopy is another laser Raman spectroscopy method based on resonance Raman effect. Resonance Raman effect occurs when the excitation light frequency is close to or coincide with the electron absorption peak of the molecule to be measured, and the strength of one or several characteristic Raman bands of this molecule can be 104~106 times that of the normal Raman bands, which is conducive to the detection of low concentration and trace samples. It has been used for the determination and research of inorganic, organic, biological macromolecules, ions and even in vivo composition. Laser Raman spectroscopy and fourier transform infrared spectroscopy have become the main means of molecular structure research

1. Characteristics of resonance Raman spectrum:

(1) the intensity of fundamental frequency can reach the intensity of rayleigh line.

(2) the intensity of overfrequency and frequency is sometimes greater than or equal to the intensity of the fundamental frequency.

(3) selectively study a substance by changing the excitation frequency so that it only resonates with a substance in the sample.

(4) compared with ordinary Raman, its scattering time is short, generally 10-12~ 10-5s.

2. Disadvantages of resonance Raman spectrum: Continuously adjustable lasers are needed to meet the absorption of different samples in different regions.

The application direction of Raman spectrum

Raman spectroscopy is a molecular structure characterization technique based on Raman effect. Its signal source is the vibration and rotation of molecules. The analysis direction of Raman spectrum includes: Qualitative analysis: Different substances have different characteristic spectra, so qualitative analysis can be carried out through the spectrum.

Structural analysis: The analysis of spectral bands is the basis of material structure analysis.

Quantitative analysis: According to the characteristics of the absorbance of the material to the spectrum, can be a very good analysis of the quantity of the material.

Advantages and disadvantages of Raman spectrum for analysis

1. Advantages of Raman spectrum for analysis

The analysis method of Raman spectrum does not require sample pretreatment, nor does it have sample preparation process, which avoids some errors, and has the advantages of simple operation, short determination time and high sensitivity in the analysis process.

2. Shortcomings of Raman spectrum for analysis

Different vibration peaks overlap and Raman scattering intensity are easily affected by optical system parameters. In fourier transform spectral analysis, the nonlinear problem of curves often appears. The introduction of any substance will bring some degree of pollution to the system of the measured body, which is equivalent to introducing some possibility of error, which will have a certain impact on the results of the analysis.

New progress and development prospects

Over the past decade, although there have been some reports on the study of surface Raman spectra of single crystal metal systems in high vacuum system, atmosphere, and solid/liquid system (electrochemical system), it was not until recent years that the sers research of smooth single crystal electrode system made important progress. Current research can be used for single crystal surface electrode system used in the sers is limited to the Raman scattering cross section very few molecules, still needs further improvement and look for ways to experiment, to broaden the researchable molecular system. If you can successfully used of the single crystal surface electrode sers signals and treated with different ways of rough surface of the electrode signal systematically comparison and research, not only used for quantitative study sers mechanism and to distinguish the contribution of different strengthening mechanism of great benefits, and will be conducive to accurate and reliable Raman spectra of surface is put forward to choose law.

With the rapid development of nano science and technology, all kinds of differences in the preparation of nanoparticles and two-dimensional orderly pattern of nanometer technology and method will be increasingly mature, people can through the grope for the appropriate surface treatment and adopt a new generation of high sensitivity of Raman spectrometer, Raman spectroscopy can be expanded to a series of important system of transition metal and semiconductor, and the technology to develop into a wide applicability, and strong research capacity of the surface (interface) spectroscopy tool, at the same time promote the the development of the theory of surface (interfacial) spectroscopy. Related testing and research methods are also likely to get more rapidly development and improve. On the basis of improving the detection sensitivity, people are not content with merely detection electrode surface species, but pay attention to by raising its detecting resolution (including band resolution, time and spatial resolutions) to study the electrochemical interfacial structure and details of the surface molecules and dynamic process. Raman spectroscopy study of another development direction is to use laser Raman micro area to carry out the space resolution microscopy research and then to carry out the structure and behavior of the micro electrode surface area. Fujishima and others using the confocal micro Raman system used as sers technique developed surface enhanced Raman imaging technique, and used to study the sers active silver surface adsorption and self-assembled film begawan images. This technique and confocal Raman spectroscopy with three-dimensional spatial resolution will play an important role in the study of conductive polymers, l-b films and self-assembled film electrodes, as well as electrode passivation films and micro-zone corrosion. In order to obtain detailed information in multiple directions and learn from each other's strengths and weaknesses, it is imperative to conduct research on the combination of Raman spectroscopy and other advanced technologies. Optical fiber technologies can play a key role in coupling, such as surface Raman spectroscopy combined with scanning probe microscopy in real time. The combined technique is expected to comprehensively study complex systems and accurately explain difficult experimental phenomena, provide experimental data for various theoretical models and surface selection laws, and promote the development of related theories of spectrochemistry and electrochemistry and surface quantum chemistry.

It can be predicted that, with the rapid development of surface detection technology, sers and its application in electrochemical research will enter a new stage in the near future.