  The nonadiabatic photodissociation dynamics of alkali halide molecules
excited by a femtosecond laser pulse in the gas phase are investigated
theoretically, and it is shown that the population of the photoexcited
molecules exhibits power-law decay with exponent -1/2, in contrast to
exponential decay, which is often assumed in femtosecond spectroscopy and
unimolecular reaction theory. To elucidate the mechanism of the power-law
decay, a diagrammatic method that visualizes the structure of the nonadiabatic
reaction dynamics as a pattern of occurrence of dynamical events, such as
wavepacket bifurcation, turning, and dissociation, is developed. Using this
diagrammatic method, an analytical formula for the power-law decay is derived,
and the theoretical decay curve is compared with the corresponding numerical
decay curve computed by a wavepacket dynamics simulation in the case of lithium
fluoride. This study reveals that the cause of the power-law decay is the
quantum interference arising from the wavepacket bifurcation and merging due to
nonadiabatic transitions.
