Probing the contributions of hot vibrational states using pump-degenerate four-wave mixing

Femtosecond pump-degenerate four-wave mixing (DFWM) spectroscopy was used to investigate the dynamics in the excited B state of bromine via transitions to ion-pair states. By varying the DFWM wavelength different vibrational energy spacings of the excited B state can be observed. This arises from a substantial contribution of a hot vibrational state of the electronic ground state. Specific Franck–Condon overlaps between the B and an ion-pair state can be used to probe different parts of the excited state dynamics individually and therefore indirectly the contributions from different vibrational states in the electronic ground state. To understand the details of this complex scenario, quantum dynamical calculations were performed.