Dimer photofragmentation and cation ejection dynamics in helium nanodroplets.

We present femtosecond pump-probe photoionization experiments with indium dimers (In 2 ) solvated in helium nanodroplets (He N ). At short pump-probe time delays, where the excited In 2 * is still located inside the droplet, we surprisingly observe detachment of InHe n + ions with n = 1 to ∼30 from the droplet. These ions indicate that fragmentation of In 2 occurs and that the kinetic energy release enables In + to overcome the attractive He N potential, which typically prevents ion ejection from the droplet. We find that the transient InHe n + signal reveals vibrational wave packet motion in neutral In 2 *. By correlating the InHe n + signal with the corresponding photoelectrons through covariance detection, we unequivocally identify the ionization pathway leading to InHe n + : pump-excitation from the ground-state In 2 creates a vibrational wave packet in In 2 *, followed by probe-ionization to the cationic ground state In 2 + . Subsequently, a further probe photon promotes the molecule to an excited ionic state In 2 +* of nonbonding character, leading to fragmentation and kinetic energy release. This interpretation is additionally supported by probe power- and droplet-size dependencies, as well as energetic considerations. Unambiguous assignment of the ionization path to absorption-ionization-dissociation (fragmentation of the ion) in contrast to absorption-dissociation-ionization (fragmentation of the neutral) is enabled by ion ejection and electron-ion correlation. This complementary observable for ultrafast photochemical processes inside He N will be particularly valuable for more complex systems.