Elastic and photo-elastic constants of CH4 and CD4 obtained by Brillouin scattering

The elastic constants and Pockels coefficients of CH4 and CD4 single crystals near their triple points have been determined using high-resolution Brillouin spectroscopy. For CH4 at 90.4 K, the elastic constants are C11 = 19.57 ± 0.30, C12 = 14.46 ± 0.20, C44 = 9.20 ± 0.15 kbar, and the ratios of the Pockels coefficients are p12/p11 = 1.031 ± 0.035 and p44/p11 = 0.069 ± 0.010. For CD4 at 89.2 K, C12 = 20.04 ± 0.30, C12 = 15.00 ± 0.25, C44 = 9.15 ± 0.15 kbar, and p12/p11 = 1.027 ± 0.020, P44/p11 = 0.138 ± 0.010. The velocities of the low-frequency transverse modes in CH4 and CD4 are anomalously slow in the [Formula: see text] direction, indicative of rotational–translational coupling of molecules. Isotopic differences are also evident: the Pockels coefficient p44 in CD4 is double that in CH4; and the velocities of both transverse modes in the principle directions [Formula: see text], [Formula: see text], and [Formula: see text] are in the ratio V(CH4)/V(CD4) = 1.130 instead of 1.118 as expected from the isotopic mass ratio.