A femtosecond velocity map imaging study on B -band predissociation in CH 3 I. II. The $2_0^1$ 2 0 1 and $3_0^1$ 3 0 1 vibronic levels

Femtosecond time-resolved velocity map imaging experiments are reported on several vibronic levels of the second absorption band ( B -band) of CH 3 I, including vibrational excitation in the ν 2 and ν 3 modes of the bound 3 R 1 ( E ) Rydberg state. Specific predissociation lifetimes have been determined for the $2_0^1$ 2 0 1 and $3_0^1$ 3 0 1 vibronic levels from measurements of time-resolved I*( 2 P 1/2 ) and CH 3 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 $0_0^0$ 0 0 0 [ Gitzinger , 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 (ν 3 ) of the CH 3 I 3 R 1 ( 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 (ν 2 ), 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 $2_0^1$ 2 0 1 and $3_0^1$ 3 0 1 vibronic levels, respectively, the time evolution of the fragment anisotropy and the vibrational activity of the CH 3 fragment are presented. Additional striking results found in the present work are the evidence of ground state I( 2 P 3/2 ) fragment production when excitation is produced specifically to the $3_0^1$ 3 0 1 vibronic level, which is attributed to predissociation via the A -band 1 Q 1 potential energy surface, and the indication of a fast adiabatic photodissociation process through the repulsive A -band 3 A 1 (4 E ) state, after direct absorption to this state, competing with absorption to the $3_0^1$ 3 0 1 vibronic level of the 3 R 1 ( E ) Rydberg state of the B -band.