Electronic relaxation after resonant laser excitation of Cr in superfluid helium nanodroplets

Abstract

Chromium (Cr) atoms embedded into helium nanodroplets (HeN) are ejected from the droplets upon photoexcitation. During ejection they undergo electronic relaxation resulting in bare Cr atoms in various excited states. In a study of the relaxation process we present absorption spectra observed via laser induced fluorescence and beam depletion as well as dispersed fluorescence spectra and time-resolved fluorescence measurements. Broad and shifted absorption structures were found for the strong z7P° ← a7S3 and y7P° ← a7S 3 excitations from the ground state. Emission lines are, in contrast, very narrow, which indicates that fluorescence is obtained from bare excited Cr atoms after ejection. Upon excitation into the y7P° 2,3,4 states we observed fluorescence from y7P° 2 , z5P°1,2,3, and z7P° 2,3,4, indicating that these states are populated by electronic relaxation during the ejection processes. Relative population ratios are obtained from the intensities of individual spectral lines. Excitation into the z7P°2,3,4, states resulted in fluorescence only from z7P°2. Estimates of the time duration of the ejection process are obtained from time-resolved measurements. © 2013 American Chemical Society.