Time-domain measurement of pure rotational Raman collisional linewidths of ethane C₂H₆

The pure rotational Raman linewidths of the symmetric-top molecule C₂H₆ are determined from a time-resolved femtosecond experiment at room temperature and moderate pressure in a pump-probe scheme. After excitation by a strong 100 fs-duration nonresonant near infrared pump pulse, the molecular sample exhibits periodic post-pulse alignment revivals that produce an anisotropic angular distribution of the molecular axis. Such molecular motion induces in turn a time-dependent optical birefringence, which can be probed using a weak time-delayed probe pulse. The Fourier transform of the temporal signal, recorded over a wide time delay range (several hundred picoseconds) for high frequency resolution, is directly related to the rotational Raman spectrum of ethane molecules. The Raman lines are analyzed with a frequency lineshape taking into account the collisional relaxation and the apparatus function. The linewidths are measured for rotational quantum numbers J=3 to J=30 in the pressure range 0.69–1.06 bar. A decrease of the collisional broadening coefficient is observed as J increases. These first measurements of pure rotational Raman linewidths of ethane are compared with previously reported infrared and far-infrared values.