Torsional selection rules, Raman tensors, and cross sections for degenerate modes of C2H6

16 pages, 12 figures.-- PACS nrs.: 33.20.Tp, 33.15.Mt, 33.70.Fd.We analyze the peculiarities induced by the torsional motion on the Raman spectra of the degenerate vibrational bands of ethane. Selection rules for the Raman transitions between the torsionally split energy levels are derived first in terms of symmetry arguments. Then, their associated intensities are calculated with a model for the torsional dependence of the molecular polarizability. New experimental spectra of the Raman degenerate bands of C2H6, some recorded in jet expansions, are also included and analyzed to show the current state of the problem.This work was supported by the Spanish DGICYT and DGES, research projects PB94-1526 and PB97-1203.Peer reviewe