Dimer photofragmentation and cation ejection dynamics in helium nanodroplets

We present femtosecond pump-probe photoionization experiments with indium dimers (In₂) solvated in helium nanodroplets (He_N). At short pump-probe time delays, where the excited In₂* is still located inside the droplet, we surprisingly observe detachment of InHe⁺ ions with n = 1 to ∼30 from the droplet. These ions indicate that fragmentation of In₂ 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⁺ signal reveals vibrational wave packet motion in neutral In₂*. By correlating the InHe⁺ signal with the corresponding photoelectrons through covariance detection, we unequivocally identify the ionization pathway leading to InHe⁺: pump-excitation from the ground-state In₂ creates a vibrational wave packet in In₂*, followed by probe-ionization to the cationic ground state In₂⁺. Subsequently, a further probe photon promotes the molecule to an excited ionic state In₂+* 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.