UC Berkeley

Ultrafast soft X-ray transient absorption spectroscopy has exhibited remarkable sensitivity to electronic and molecular structures with element specificity and short temporal resolution. At the carbon K-edge, X-ray probe pulses enabled the capability to investigate molecular dynamics of photoinduced organic chemicals. In the study of the photodynamics of pentamethylcyclopentadiene (C5Me5H), two timescales of C5Me5• radical formation following UV excitation are directly observed, which are attributed to a fast channel likely coming from C5Me5H with an active C-H vibration stretching mode and a slow channel taking time for the vibrational energy to redistribute into this vibrational mode for the dissociation. The transition state 1B2 and the photoproduct C5Me5• are spectroscopically characterized by carbon 1s core-to-valence transitions.

Additionally, the ultrafast state crossing in the photorelaxation of benzaldehyde upon 267 nm excitation was investigated. Two well-separated core level absorption features at 281.8 and 282.8 eV are tentatively assigned to the population transfer to S1 (n*) followed by the intersystem crossing to the triplet state in about 12 ps. The evolution of the core-to-valence resonances at the carbon K-edge identifies the excited singlet and triplet states involved in the photorelaxation of benzaldehyde.

Finally, the electronic structure of C2F4I• radical, the photodissociation fragment of C2F4I2, is characterized by a carbon 1s X-ray feature at lower energy, 286.4 eV, resulting from a transition to the singly occupied molecular orbital (SOMO). After the loss of an iodine atom, the shift in the carbon 1s orbital is greater than the shift in the antibonding orbital upon going from the closed-shell molecule to the radical.