Femtosecond spectroscopy of fragment cage dynamics: I-2 in Kr

We report femtosecond pump-probe spectra for the B state of I-2 in solid Kr with systematic variation of both pump and probe wavelength. The observed oscillations strongly depend on the probe wavelength and we show that it must be tuned together with the pump pulses to obtain the correct vibrational frequencies. We construct an RKR potential for the B state that includes the solvent response. The ionic E state surface is directly constructed from the measurements. Both are compared to DIM potentials. We report rates for vibrational energy relaxation in the B state that increase by three orders of magnitude on going from low excitation to excitation beyond the gas phase dissociation limit. By systematic variation of the probe window we record snapshots of a mean trajectory at the dissociation limit that clearly displays the strong interaction with the cage.