Infrared spectroscopic study of rovibrational states of perdeuterated methane (CD_4) trapped in parahydrogen crystal

摘要

The triply degenerate stretching (v_3) and bending (v_4) modes of CD_4 in solid parahydrogen at cryogenic temperatures are studied by Fourier transform infrared spectroscopy to reveal crystal field split rovibrational structures. The observed spectra are analyzed by taking into account the crystal field potential which is constructed by the summation of the pairwise (dispersive) potential between the methane and the surrounding hydrogen molecules. By the least-squares fitting of that observed to a theoretical model the molecular constants of CD_4 as well as the potential coefficient are determined as in the previous work on the CH_4/p-H_2 system. The potential is approximated to be proportional to the product of the polarizability of hydrogen molecule, the dipole-quadrupole polarizability of methane, and the inverse of the seventh power of the intermolecular distance between methane and hydrogen molecules. From the ratio of the potential coefficients of CH_4 and CD_4 the dipole-quadrupole polarizability of CD_4 is found to be 0.875 times that of CH_4 under the assumption that the quantum renormalization effect is negligible. A novel temperature dependence of the line shape of the v_3 and v_4 modes is discovered over the range of 4.5 to 8.0 K.