Direct Observation of the Transition-State Region in the Photodissociation of CH3I by Femtosecond Extreme Ultraviolet Transient Absorption Spectroscopy

Femtosecond extreme ultraviolet (XUV) pulses produced by high harmonic generation are used to probe the transition-state region in the 266 rim photodissociation of CH3I by the real-time evolution of core-to-valence transitions near the iodine N-edge at 45-60 eV. During C-I bond breaking, new core-to-valence electronic states appear in the spectra, which decay concomitantly with the rise of the atomic iodine resonances of I(P-2(3/2)) and I*(P-2(1/2)). The short-lived features are assigned to repulsive valence-excited transition-state regions of (3)Q(0) and (1)Q(1), which can connect to transient core-excited states via promotion of 4d(I) core electrons. A simplified one-electron transition picture is described that accurately predicts the relative energies of the transient states observed. The transition-state resonances reach a maximum at similar to 40 fs and decay to complete C-I dissociation in similar to 90 fs, representing the shortest-lived chemical transition state observed by core-level, XUV, or X-ray spectroscopy.