Observation of enhanced excitation of I22+ by strong laser fields

Using pump-probe spectroscopy with ultrashort laser pulses, we see an enhancement of the charge-asymmetric dissociation (CAD) channel, (I-2(2+))(*)-> I2++I, over a narrow range of internuclear separation. The enhancement of the CAD channel appears to come from the excitation of the symmetric ground state dissociation channel (I++I+) for two reasons. First, there is a depletion in the symmetric channel at approximately the same pump-probe delay as the asymmetric enhancement. Second, for a fixed delay, the asymmetric channel increases as a function of probe intensity while the symmetric channel decreases. In addition, we find that the kinetic energy of the extra I2++I ions decreases for increasing delay. To explain this dependence of the kinetic energy release on delay, we introduce model potential energy curves. Based on these curves, we conclude that the excitation is produced by a resonant three-photon transition within I-2(2+) rather than by ionization of I-2(+).