Photoionization Dynamics in Pure Helium Droplets Page: 3 of 46
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This paper presents a study of the photoionization of pure helium droplets above the
ionization energy of atomic helium. It is motivated by a desire to understand the energetics
and dynamics of ionization and electron escape of in pure helium droplets, and by extension,
the differences between ionization in pure and doped helium droplets. Photoelectron imaging
and photoionization time-of-flight mass spectrometry are combined to examine ionization
processes in helium droplets. The two measurements, taken over a wide range of droplet
sizes, provide new insight into the complex ionization phenomena that occur within helium
There have been numerous investigations on the electron impacti-8 and photoionization9-12
of pure and doped helium droplets in which ion mass spectrometry was used as the detection
method. An essential feature common to rare gas clusters, especially helium, is that the
neutral species are only weakly bound, at long range, by van der Waals interactions, while
the cationic systems are strongly bound by covalent interactions, and have greatly reduced
equilibrium bond distances. As a result, the cation, when formed, is highly vibrationally
excited, typically with > 1 eV of internal energy. The relaxation of vibrational energy within
the cluster leads to extensive fragmentation. The presence or absence of any particular ion in
the mass spectra is then related of many factors, but more often reflects the fragmentation
dynamics of the cluster and the relative stability of daughter ions, rather than the initial state
of the cluster. The appearance energy of the fragment ions also does not necessarily correlate
with the either the adiabatic ionization energy or the vertical ionization energy of the nascent
cluster. In helium droplets, both electron-impact ionization and photoionization lead to
extensive fragmentation of the cluster, with the resulting cluster ion distributions only weakly
dependent on the initial droplet size.4'9
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Peterka, Darcy S.; Kim, Jeong Hyun; Wang, Chia C.; Poisson,Lionel & Neumark, Daniel M. Photoionization Dynamics in Pure Helium Droplets, article, February 4, 2007; Berkeley, California. (https://digital.library.unt.edu/ark:/67531/metadc900443/m1/3/: accessed April 19, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.