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 nm 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(2P3/2) and I*­(2P1/2). The short-lived features are assigned to repulsive valence-excited transition-state regions of 3Q0 and 1Q1, 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 ∼40 fs and decay to complete C–I dissociation in ∼90 fs, representing the shortest-lived chemical transition state observed by core-level, XUV, or X-ray spectroscopy.