VIBRONIC DEPENDENCE OF THE B-OVER-TILDE-STATE LIFETIMES OF CH3I AND CD3I USING FEMTOSECOND PHOTOIONIZATION SPECTROSCOPY

Tunable deep UV femtosecond photoionization spectroscopy with single p hoton excitation and wavelengths longer than 192 nm has been used to d etermine predissociation-mediated excited state lifetimes for many vib ronic levels of the (B) over tilde (6s[2] Rydberg) state of CH3I and C D3I. These include states with vibrational excitation in the nu(1), nu (2), nu(3), and nu(6) modes. We have previously reported lifetime meas urements 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 re sults corroborate the counterintuitive result from the resonance Raman work by Wang and Ziegler [J. Chem. Phys. 95, 288 (1991)] that the lev el singly excited in the C-I stretching mode (3(1)) dissociates more s lowly (we measure similar to 4.0 ps for both CH3I and CD3I) than the v ibrationless levels (1.38 and 1.90 ps, respectively). In contrast to t he resonance Raman results and similar to those from resonance enhance d multiphoton ionization linewidth studies by Syage [Chem. Phys. Lett. 212, 124 (1993)], we find a faster predissociation rate upon excitati on in the nu(6) 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 m easured for the 31 compared to the vibrationless level. This work prov ides further evidence of the multidimensional nature of the (B) over t ilde state predissociation mechanism and an opportunity to critically test high level calculations of the Rydberg state photodissociation dy namics. (C) 1998 American Institute of Physics. [S0021-9606(98)01009-5 ].