Femtosecond Time-Resolved Photofragment Rotational Angular Momentum Alignment in Electronic Predissociation Dynamics

This Letter presents an experimental and theoretical study of femtosecond time-resolved vector correlations in methyl iodide (CH₃I) electronic predissociation via the second absorption B-band at 201.2 nm. The time evolution of the phenomenological anisotropy parameters β_l was determined from time-resolved photofragment angular distributions obtained by means of the femtosecond laser pump-probe technique coupled with velocity map imaging detection of vibrational ground-state CH₃(ν = 0) fragments and spin-orbit excited I*(²P_1/2) atoms. Theoretical interpretation of the experimental results was performed on the basis of a fitting procedure using quasiclassical theory, which elucidates vector correlations in photodissociation of symmetric top molecules. The results of the fitting are in very good agreement with the experimental data and demonstrate the important role of molecular excited-state lifetimes, parent molecule and methyl fragment rotations, and methyl fragment angular momentum alignment on the time-dependent electronic predissociation dynamics.