Femtosecond photodissociation dynamics of nitroethane and 1-nitropropane in the gas and solution phases from resonance Raman intensity analysis

We have obtained resonance Raman spectra of nitroethane and 1-nitropropane in the gas phase and in cyclohexane solution with excitation within the broad B-band absorption around 200 nm. The resonance Raman spectra are dominated by a long overtone progression in the nominal NO2symmetric stretch vibrational mode. The initial short-time (femtosecond) photodissociation dynamics of the excitedelectronic state associated with the B-band absorption changes mainly in the two N–O bond lengths in the Franck–Condon region. Time-dependent wavepacket calculations have been carried out to simulate both the absorption spectra and the resonance Raman intensities. These calculations show that very fast predissociation competes with wavepacket motion out of the Franck–Condon region, and or there is a significant change in the transition dipole moment along the nominal NO2symmetric stretch coordinate, that must be taken into account to correctly model the higher overtone resonance Raman intensities. Since the resonance Raman intensity patterns and calculation parameters are very similar for both gas and solution phase nitroethane and 1-nitropropane, electronic dephasing due tosolvent collisions does notappear tobe a significantfactor in determining the resonance Raman overtone intensities up to v = 9.