A femtosecond velocity map imaging study on B-band predissociation in CH3I. II. The 2_0121 and 3-0131 vibronic levels

Abstract

Femtosecond time-resolved velocity map imaging experiments are reported on several vibronic levels of the second absorption band (B-band) of CH3I, including vibrational excitation in the ν2 and ν3 modes of the bound 3R1(E) Rydberg state. Specific predissociation lifetimes have been determined for the 2_01201 and 3_01$301 vibronic levels from measurements of time-resolved I*(2P1/2) and CH3 fragment images, parent decay, and photoelectron images obtained through both resonant and non-resonant multiphoton ionization. The results are compared with our previously reported predissociation lifetime measurements for the band origin 0_0 0 000 [Gitzinger et al., J. Chem. Phys. 132, 234313 (2010)10.1063/1.3455207]. The result, previously reported in the literature, where vibrational excitation to the C-I stretching mode (ν3) of the CH3I 3R1(E) Rydberg state yields a predissociation lifetime about four times slower than that corresponding to the vibrationless state, whereas predissociation is twice faster if the vibrational excitation is to the umbrella mode (ν2), is confirmed in the present experiments. In addition to the specific vibrational state lifetimes, which were found to be 0.85 ± 0.04 ps and 4.34 ± 0.13 ps for the $2_0^1$201 and $3_0^1$301 vibronic levels, respectively, the time evolution of the fragment anisotropy and the vibrational activity of the CH3 fragment are presented. Additional striking results found in the present work are the evidence of ground state I(2P3/2) fragment production when excitation is produced specifically to the 3_0^1 301 vibronic level, which is attributed to predissociation via the A-band 1Q1 potential energy surface, and the indication of a fast adiabatic photodissociation process through the repulsive A-band 3A1(4E) state, after direct absorption to this state, competing with absorption to the 3_0^1 301 vibronic level of the 3R1(E) Rydberg state of the B-band.