Ultrafast dissociative ionization and large-amplitude vibrational wave packet dynamics of strong-field-ionized...

We employ few-cycle pulses to strong-field-ionize di-iodomethane (CH₂I₂) and femtosecond extreme ultraviolet (XUV) transient absorption spectroscopy to investigate the subsequent ultrafast dissociative ionization and vibrational wave packet dynamics. Probing in the spectral region of the I 4d core-level transitions, the time-resolved XUV differential absorption spectra reveal the population of several electronic states of CH₂I₂ ⁺ by strong-field ionization. Global analysis reveals three distinct time scales for the observed dynamics: 20 ± 2 fs, 49 ± 6 fs, and 157 ± 9 fs, ascribed to relaxation of the CH₂I₂ ⁺ parent ion from the Franck-Condon region, dissociation of high-lying excited states of CH₂I₂ ⁺ to I⁺ (³P₂), CH₂I, and I₂ ⁺ (²Π_3/2,g), and dissociation of CH₂I₂ ⁺ to I (²P_3/2) and CH₂I⁺, respectively. Oscillatory features in the time-resolved XUV differential absorption spectra point to the generation of vibrational wave packets in both the residual CH₂I₂ and the CH₂I₂ ⁺ parent ion. Analysis of the oscillation frequencies and phases reveals, in the case of neutral CH₂I₂, C-I symmetric stretching induced by bond softening and I-C-I bending driven by a combination of bond softening and R-selective depletion. In the case of CH₂I₂ ⁺, both the fundamental and first overtone frequencies of the I-C-I bending mode are observed, indicating large-amplitude I-C-I bending motion, in good agreement with results obtained from ab initio simulations of the XUV transition energy along the I-C-I bend coordinate. These results show that femtosecond XUV absorption spectroscopy is well-suited for studying ultrafast photodissociation and vibrational wave packet dynamics.