Femtosecond Extreme Ultraviolet Ion Imaging of Ultrafast Dynamics in Electronically Excited Helium Nanodroplets

A novel femtosecond extreme ultraviolet (EUV) ion-imaging technique is applied to study ultrafast dynamics in electronically excited helium nanodroplets. Ion mass spectra recorded by single-photon EUV ionization and by transient EUV-pump/IR-probe two-photon ionization differ significantly for EUV photon energies below and above similar to 24 eV, in agreement with recently performed synchrotron measurements. Pump-probe time-delay-dependent ion kinetic energy (KE) spectra exhibit two major contributions: a decaying high KE component and a rising low KE component, which are attributed to the different excitation regimes. A model is presented that describes the excitation energy dependence of the relaxation and ionization dynamics within the framework of bulk and surface states. The model is supported by recent ab initio calculations on electronically excited states of 25-atom clusters. An intraband relaxation mechanism is proposed that proceeds on a similar to 10-20-ps time scale and that corresponds to the transfer of electronic excitation in the Rydberg n = 2 manifold from bulk to surface states.