Dimer photofragmentation and cation ejection dynamics in helium nanodroplets

Abstract

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_n⁺ 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_n⁺ signal reveals vibrational wave packet motion in neutral In₂*. 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 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.