Ultrafast Dynamics in Helium Nanodroplets Probed by Femtosecond Time-Resolved EUV Photoelectron and Ion Imaging

Femtosecond time-resolved EUV photoelectron and ion imaging are employed to study the relaxation dynamics of electronically excited helium nanodroplets. Excitation into a broad droplet absorption band ($\sim $23.8 eV) is followed by ionization with a delayed IR pulse. The transient photoelectron spectra and angular distributions indicate that electronically excited helium atoms are predominantly emitted in either an aligned 1s4p Rydberg state within less than $\sim $100~fs or in a non-aligned 1s3d state within $\sim $200~fs. The transient ion imaging results suggest that different Rydberg atoms are emitted with significantly different kinetic energy distributions that closely resemble Maxwell-Boltzmann distributions with temperatures of 2700~K (1s3d) and 490~K (1s4p). The results are interpreted in terms of a dynamic model that is based on the local density dependent blue shift of atomic Rydberg states in the droplet environment.