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

Abstract

Ultrafast dynamics in helium nanodroplets probed by femtosecond time-resolved EUV photoelectron imaging Oleg Kornilov 1,2 , Chia C. Wang 1,2 + , Oliver Bunermann 1,2 , Andrew T. Healy 1 x , Mathew Leonard 1,2 , Chunte Peng 1,2 † , Stephen R. Leone 1,2 ♦ , Daniel M. Neumark 1,2 , and Oliver Gessner 1 ∗ Ultrafast X-ray Science Laboratory, Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720 Department of Chemistry, University of California, Berkeley, CA 94720 Abstract The dynamics of electronically excited helium nanodroplets are studied by femtosecond time-resolved photoelectron imaging. EUV excitation into a broad absorption band centered around 23.8 eV leads to an indirect photoemission process that generates ultraslow photoelectrons. A 1.58 eV probe pulse transiently depletes the indirect photoemission signal for pump-probe time delays <200 fs and enhances the signal beyond this delay. The depletion is due to suppression of the indirect ionization process by the probe photon, which generates a broad, isotropically emitted photoelectron band. Similar time scales in the decay of the high energy photoelectron signal and the enhancement of the indirect photoemission signal suggest an internal relaxation process that populates states in the range of a lower energy droplet absorption band located just below the droplet ionization potential (IP~23.0 eV). A nearly 70% enhancement of the Current addresses: + Department of Chemistry, University of British Columbia, Vancouver, British Columbia, Canada V6T 1Z1, x Department of Chemistry, University of Minnesota, Minneapolis, MN 55455, † Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA 02139 On appointment as a Miller Research Professor in the Miller Institute for Basic Research in Science. Corresponding author. Email: ogessner@lbl.gov