Selective probing of vibrational hot states in bromine using time-resolved coherent anti-Stokes Raman scattering

In previous work (Scaria, A.; et al. Chem. Phys. Lett. 2009, 470, 39-43) it was shown that the excitation of the electronic B state in bromine can be characterized by transitions starting from vibrational hot states of the electronic ground X state. This contribution is strongly depending on the specific Franck-Condon factors for the chosen wavelength (in that work 540 nm) used for excitation. For the investigation of the resulting excited state dynamics, a pump-degenerate four-wave mixing (pump-DFWM) experiment was applied. To increase the vibrational selectivity, in the present work we have performed temperature-dependent time-resolved coherent anti-Stokes Raman scattering (CARS) spectroscopy to probe the B state dynamics of bromine. Also here, the wavelength of the excitation (in this case, the pump laser of the CARS process) was set to 540 nm for all measurements. The hot state contribution is small, even at high temperatures. It can be probed by tuning the Stokes wavelength to resonance. The time delay between the probe pulse and the time-coincident pump/Stokes pulse pair of the CARS process is scanned, giving access to the wave packet dynamics in the excited B state. The experimental observations are supported by quantum dynamical calculations.