Ultrafast dynamics in helium nanodroplets probed by femtosecond time-resolved EUV photoelectron imaging Page: 4 of 34
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electronically excited states is monitored by subsequent ionization using a 1.58 eV
(785 nm) probe pulse that is delayed by up to 100 ps relative to the pump pulse.
With this setup, both the droplet EUV photoionization mechanism as well as the
desorption of helium Rydberg atoms and molecules can be probed with femtosecond
time-resolution. For example, most He*(lsnl) Rydberg states with n>2 can be ionized by
the probe pulse. These states have very distinct photoelectron images,35 which permits
direct probing of their rates of production. In addition, the probe pulse should deplete the
population of the electronically excited droplets created by the EUV pump pulse,
resulting in depletion of the low energy electron signal at early times and recovery at later
times. The presented data indeed show evidence for these and other effects, yielding new
insights into the decay mechanisms of electronically excited droplets. Specifically, a
depletion of the low energy photoelectron signal at pump-probe time delays below
-200 fs is observed, followed by an enhancement at longer delays. The findings are
interpreted in terms of a relaxation mechanism in which the droplet undergoes a
transition from an initially excited electronic band to an energetically lower-lying band
from which it can be re-excited into the upper band by the probe pulse. This picture is
supported by the observation of a broad photoelectron band that decays on a time scale
that is comparable to the rise time of the low energy photoelectron enhancement.
II. Experiment
The experiments are carried out using a new apparatus designed for femtosecond
time-resolved photoelectron imaging studies in the EUV (Fig. 2). A femtosecond EUV
light source based on high-order harmonic generation is interfaced to a previously4
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Kornilov, Oleg; Wang, Chia C.; Buenermann, Oliver; Healy, Andrew T.; Leonard, Mathew; Peng, Chunte et al. Ultrafast dynamics in helium nanodroplets probed by femtosecond time-resolved EUV photoelectron imaging, article, July 9, 2010; Berkeley, California. (https://digital.library.unt.edu/ark:/67531/metadc1015855/m1/4/: accessed April 19, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.