The Effect of Resonance Effect and Electron-Phonon Coupling on Resonance Raman Spectra of Linear Polymers
LI Shuo1, 2, WANG Jun-xing1, HE Yue1, LI Zheng-qiang2, SUN Cheng-lin1*
1. College of Physics, Jilin University, Changchun 130012, China
2. Key Laboratory for Molecular Enzymology and Engineering, Ministry of Education, Jilin University, Changchun 130012, China
Abstract：Linear polymers are characterized by their high intensity and information-rich resonance Raman spectroscopy，which has applications in biology, photoelectric materials and medicine. However, β-carotene molecules with conjugated double bonds are the most representative of polyene molecules. It is worth researching the effects of π-electron and CC vibration interaction on absorption spectrum and Raman spectroscopy, and the effects of resonance enhancement effect and electron-phonon coupling on the relative intensities, frequency and line shape Raman spectra. The resonance Raman spectra and UV-visible spectrum of β-carotene in 1,2-dichloroethane solution are obtained from the 283 to 223 K temperature range. We research the effects of the resonance effect and electron-phonon coupling on absorption and Raman spectra. With decreasing temperature, the Huang-Rhys decreases, indicates the CC bond vibration weakened and the molecular system energy decreases, which cause the absorption spectra redshift; in addition, with decreasing temperature, the degree of structural order of molecule increases, indicates the electron-phonon coupling interaction increases, which enhance the effect of electronic energy gap on the CC vibration, so, the Raman frequency shifts to the lower wavenumber, namely, the Raman spectra redshift. Calculations show that as the temperature decreases, the Raman scattering cross-section of the CC bonds increases, the Raman bandwidths of the CC bonds narrow, the ratio of overtone to fundamental mode increases. We compare and analyze the effects of resonance effect and electron-phonon coupling on the Raman cross-section, linewidth and the intensity ratio of overtone to the fundamental mode of Raman spectra. Although both the resonance effect and electron-phonon coupling can influence the Raman spectra at different temperatures, the resonance effect is more significant than the electron-phonon coupling for the Raman spectra, and electron-phonon coupling is more negligible effective on harmonics. With the temperature decreasing, the redshift of the absorption spectrum makes the 514.5nm excitation light in the Raman spectra closer to the 00-absorption peak, which significantly enhances the resonance effect of the molecule and makes the Raman scattering cross-section, linewidth, and the intensity ratio of the frequency of the overtone to fundamental modes significantly change with the temperature. The researches of electron-phonon coupling and resonance effect provide some experimental and theoretical basis for studying the effects of temperature on the properties of linear polyenes such as carotene.
李 硕，王俊星，何 越，李正强，孙成林. 共振拉曼效应和电子-声子耦合对线性多烯分子共振拉曼光谱的影响[J]. 光谱学与光谱分析, 2022, 42(02): 454-458.
LI Shuo, WANG Jun-xing, HE Yue, LI Zheng-qiang, SUN Cheng-lin. The Effect of Resonance Effect and Electron-Phonon Coupling on Resonance Raman Spectra of Linear Polymers. SPECTROSCOPY AND SPECTRAL ANALYSIS, 2022, 42(02): 454-458.
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