Rotational Raman excitation profiles of symmetric tops: Subpicosecond rotation dependent lifetimes in the à state of ammonia

Abstract

The analysis of Raman excitation profiles of rotational scattering transitions is shown to be a sensitive measure of rotationally specific intramolecular dynamics in the subpicosecond time regime. O, P, Q, R, and S Raman REPs of ammonia are reported and theoretically treated using previously derived resonance rotational scattering intensity expressions. Resonance rovibrational Raman spectra are obtained with UV Raman excitation continuously resonant with the v^’₂ =2 vibronic band (207–210 nm) of the NH₃ X̃ → à absorption spectrum. The resulting REP analysis reveals that the rates of photodissociation on the à state surface have a significant rotational quantum dependence in the 2² band. Rovibronic lifetimes decrease from 140 to 70 fs as J increases from 2 to 8 in this resonant à state vibrational level. These lifetime effects are largely ascribed to centrifugal effects.