In 1928, C.V.Raman found that there were some different frequencies with the incident light's frequency when he did the experiments about the interaction of light with liquid, gas and solid 1. This is called Raman spectrum, and is one kind of scattering spectrum. The difference of frequency with the incident light's frequency is called roman shift. And the roman shift of one medium is only determined by the molecular structure of the medium, and is independent of the incident light. Degree of depolarization is one important parameter of the Raman spectrum, it is used to characterize the polarization of the Raman spectrum and is an important criterion of molecular normal symmetry vibration modes. Raman spectrum is often used in the research of the medium molecular structure, such as medicine, identification of ancient writing, identification of ancient painting, identification of gems, and so on 2-5. Raman spectrum is often used as one important experiment for the students in the majors of physics, chemistry and optics 6-7. The liquid medium often used in the experiment is CCl4. There are two questions about the Raman spectrum, one is the position of peak which can be resolved by shifting. Another is the degree of depolarization, there is large differences with theoretical value. In this paper, the polarization components are focused on to research the reason of difference of degree of depolarization. The performance of the polarization components has great effect on it.
The structure of CCl4 is tetrahedron, one carbon atom is in the center and four chlorine atoms are in the tetrahedral four vertices. There are nine dependent harmonic oscillations. These nine oscillations are classified into four categories. There are eight Raman peaks in CCl4 Raman spectrum. To describe degree of depolarization, one plane is determined first, it's called scattering plane which includes the direction of the incidence light propagation and the direction of observation, such as the XOY plane shown in figure 1. Degree of depolarization is the ratio of the spectrum intensity Iy of parallel scattering plane to the intensity Iz of vertical scattering plane, shown in equation 1. The intensity Iy and Iz can be obtained using one line polarizer by rotating the polarizer to change the direction of polarizing.
If the incident light is Linear polarized and the polarization direction is Ez, the theoretical value about the degree of polarization is shown in talbe1.
CHARACTERIZATION OF POLARIZATION COMPONENTS
There are three square polarizers and three circle analyzers, which is named as Q1, Q2, Q3 and J1, J2, J3. Before doing experiments, the polarization components are measured using the spectrometer of cary5000. For the polarization of the polarizer, the transmittance is measured with the s polarization light and p polarization light at 0 ° incident angle, respectively. The results are shown in figure1. Figure 1a is the result for square polarizers and figure 1b is for circle analyzers. Comparing the transmittance, the quality of Q1 is best, Q2 is worst. And J3 is best, J1 is worst. The polarization is defined using equation2:
Using equation1, the polarization is obtained with the data in figure1, shown in figure2, figure 2a is for square polarizers and figure 2b is for circle analyzers. The polarizers work in the wavelength range of 532 nm± 5 nm in the experiment for CCl4 Raman spectrum, so the quality of Q3 is higher than Q1, and Q2 is worst. The analyzers work in the wavelength range of 505-560 nm for CCl4 Raman spectrum. So it is difficult to obtained high quality analyzer in so broad wavelength. From figure 2b, it can be seen that the polarization of J3 is worst, its change in the broad wavelength is smallest, the polarization of J1 is best, its change is bigger, and the polarization of J2 is middle but its change in biggest. Considering two parameters of polarization and transmittance for polarizers and analyzers, Q1 is best, Q2 is worst, and J3 is best, J1 and J2 is near.
EXPERIMENT AND RESULTS
The experiment is done using the Raman spectrometer of LS-II. To reduce the environment effect on data, the spectrometer is always covered by black-out cloth when the experiment is doing. At the same time, the other parameters such as step, integral time, are not changed when doing different experiments. Using the best quality of Q1 and worst quality Q2 with three analyzers, six groups experiments are done to comparing the effect of the characterization of polarizers and analyzers. According to the definition of degree of depolarization for Raman spectrum in equation 1 and the structure of the used Raman spectrometer in the experiments, 0 is expressed the data for the spectrum intensity Iy of parallel scattering plane and 90 is expressed the data for the intensity Iz of vertical scattering plane. Figure3 shows the data of CCl4 Raman spectrum for the group of Q1 and J1. Degree of depolarization for CCl4 Raman spectrum is calculated using the data shown in figure3 and lists in talbe1. The data for spectrum and degree of depolarization for other groups also are obtained using the same method, and the degree of depolarization for other groups are listed in table1.
Degree of depolarization of CCl4 Raman spectrum for six groups and theoretical value
|Calculated value ρ||QI||J1||0.07||0.60||0.53||0.65||0.66||0.08||0.55|
|Theoretical value ρ||0||0.75||0.75||0.75||0.75||0||0.75|
The results show that the degree of depolarization is closer to the theoretical value when the polarization of analyzer is flatter than the other two in the broadband wavelength using the same polarizer, just like Q1 and J2, Q2 and J2, these two data are worst in their individual group. At the same time, higher of the transmittance and polarization of the polarizers, closer to the theoretical value using the same analyzer.
Degree of depolarization of CCl4 Raman spectrum is investigated by changing the different group of polarizer and analyzer. Polarizer works in a narrow wavelength band, so it's easy to obtain high depolarization (P>0.995) with high transmittance (T>70%) for Raman spectrum experiment. Analyzer works in a very broadband wavelength, depending on the structure of the measured medium. It's very difficult to obtain the broadband analyzer with high polarization and high transmittance in such broadband wavelength. But its feature effects on degree of depolarization for Raman spectrum. It is one important work to look for one method to fabricate the analyzer with high transmittance and the flat polarization curve in the required wavelength region as far as possible.
This work is supported by Special fund for experimental teaching reform of Tongji University.
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