An approach is presented and demonstrated for determining predissociation lifetimes of small molecules in the vapor phase at room temperature through measurement of resonance Raman depolarization ratios. Depolarization ratios for lowhyphen;pressure CS2vapor have been measured using ten excitation wavelengths between 1995 and 2001 Aring;, spanning one of the strongest vibrational features of theS3larr;S0electronic transition. The excitation wavelengths are obtained by antihyphen;Stokes Raman shifting in H2the fourth harmonic of a Nd:YAG laser and the second harmonic of a YAGhyphen;pumped dye laser. The Raman depolarization ratio rgr;(Iperp;/Ipar;) varies from less than 0.4 with excitation on the highhyphen;frequency side of the vibronic band to nearly 0.6 on the lowhyphen;frequency side. Numerical simulation of the depolarization dispersion curve using literature values for the rotational constants shows that this behavior can be attributed straightforwardly to partially selective excitation of initialJstates for which theS3predissociation lifetime is either long or short relative to rotational periods. The best fit between the experimental and calculated depolarization dispersion curves is obtained with a homogeneous linewidth of 6 cmminus;1(excited state lifetime 0.9 ps). The relevance of this result to measurements of dissociation times through photofragment anisotropy analysis is discussed.
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