Probing the vibrational dynamics of high-lying electronic states using pump-degenerate four-wave mixing.

To investigate the vibrational dynamics in high-lying electronic states of molecules, a degenerate four-wave mixing (DFWM) process in combination with an initial pump pulse was employed. Applying this technique, the vibrational dynamics occurring in the high-lying E(0(2)g) ion-pair state of molecular bromine were investigated. The initial pump pulse is used to excite the B(3)Pi(0(+)u) state, from which the E(0(2)g) ion-pair state can be accessed in a subsequent DFWM process. By introducing an internal time delay in the DFWM process, the vibrational dynamics of the E(0(2)g) and B(3)Pi(0(+)u) states can be probed and analyzed. In most cases, the signals of the E(0(2)g) and B(3)(0(+)u) state dynamics are overlapping, which makes it difficult to identify the contributions of the respective states. To this end we show that it is possible to extract the contributions of the E(0(2)g) state by a spectral decomposition of the DFWM signal. These findings are circumstantiated by quantum dynamical calculations.