  The predissociation dynamics of lithium iodide (LiI) in the first excited
A-state is investigated for molecules in the gas phase and embedded in helium
nanodroplets, using femtosecond pump-probe photoionization spectroscopy. In the
gas phase, the transient Li+ and LiI+ ion signals feature damped oscillations
due to the excitation and decay of a vibrational wave packet. Based on
high-level ab initio calculations of the electronic structure of LiI and
simulations of the wave packet dynamics, the exponential signal decay is found
to result from predissociation predominantly at the lowest avoided X-A
potential curve crossing, for which we infer a coupling constant V=650(20)
reciprocal cm. The lack of a pump-probe delay dependence for the case of LiI
embedded in helium nanodroplets indicates fast droplet-induced relaxation of
the vibrational excitation.
