Comparison of Calculated Chain-Length Dependence of the Anisotropy and Depolarization Ratio in n-Alkane Gases with Measurements of the Laser Light Scattering by Photon Counting

Theoretical optical anisotropy and depolarization ratio calculations of n-alkanes are critically reviewed. New depolarization ratio measurements on gaseous n-alkanes are reported from a new method employing a continuously rotating analyzer, an argon ion laser, and photon counting electronics. The experimental and theoretical depolarization ratios are compared. The experiments show an increase of depolarization ratio with increasing number of C–C bonds in contrast to the theoretical results which show a general decrease of depolarization ratio. A new formula for depolarization ratio calculation of ethane which is a nonlinear symmetric top molecule is suggested. This formula reduces ethane's theoretical depolarization ratio by 25%. Our measurements suggest nonzero depolarization ratios for the spherical molecules methane and neopentane in contrast to the theoretical predictions. Possible sources of errors are discussed.