Vibronic dependence of the B̃ state lifetimes of CH3I and CD3I using femtosecond photoionization spectroscopy

Abstract

Tunable deep UV femtosecond photoionization spectroscopy with single photon excitation and wavelengths longer than 192 nm has been used to determine predissociation-mediated excited state lifetimes for many vibronic levels of the B̃ (6s[2] Rydberg) state of CH3I and CD3I. These include states with vibrational excitation in the ν1, ν2, ν3, and ν6 modes. We have previously reported lifetime measurements for the origin bands [Chem. Phys. Lett. 222, 335 (1994)]. The vibronic and isotopic dependences presented here qualitatively agree with various aspects of results from two indirect measurements. Our results corroborate the counterintuitive result from the resonance Raman work by Wang and Ziegler [J. Chem. Phys. 95, 288 (1991)] that the level singly excited in the C–I stretching mode (31) dissociates more slowly (we measure ∼4.0 ps for both CH3I and CD3I) than the vibrationless levels (1.38 and 1.90 ps, respectively). In contrast to the resonance Raman results and similar to those from resonance enhanced multiphoton ionization linewidth studies by Syage [Chem. Phys. Lett. 212, 124 (1993)], we find a faster predissociation rate upon excitation in the ν6 mode. The lifetimes are considerably longer than those measured for the higher lying 6p and 7s Rydberg states by femtosecond mass-resolved photoionization studies by Janssen et al. [Chem. Phys. Lett. 214, 281 (1994)]. In that case, a faster dissociation rate was measured for the 31 compared to the vibrationless level. This work provides further evidence of the multidimensional nature of the B̃ state predissociation mechanism and an opportunity to critically test high level calculations of the Rydberg state photodissociation dynamics.