Photoionization Dynamics in Pure Helium Droplets Page: 4 of 46
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Though the mass spectra resulting from electron impact ionization and photoionization of
droplets are similar, photoionization provides two significant advantages over electron-
impact: it deposits a well defined amount of energy into the droplets, and it creates a
photoelectron. In photoionization, the initial absorption takes place under aegis of the
Franck-Condon principle. If the ionization is direct and the electron escapes without strong
subsequent interactions, the photoelectron spectrum explicitly contains information about the
initial state of the neutral cluster and the nascent ion. If the ionization is indirect, the electron
kinetic energy may be decoupled from the initial excitation and the electron energy and
angular distributions modified, with both measurements sensitive to the interaction between
the droplet atoms and the escaping electron.
Our group13 previously used photoelectron imaging to examine the photoionization of pure
helium droplets below the atomic helium ionization energy, IE(He)=24.59 eV. We found
that ionization was the result of an indirect autoionization process and that the emitted
electrons had very little kinetic energy, typically < 1 meV, for photon energies ranging from
23.0-24.5 eV. Although no definitive energy loss mechanism was assigned, it appeared that
the emitted electron interacted strongly with the helium environment before escape. This
result contrasted markedly with later experiments on doped helium droplets. Multiphoton
ionization experiments by Radcliffe et al.14 on silver clusters inside helium droplets showed
fast photoelectrons, with little apparent modification to their kinetic energy. The only change
attributable to the droplet environment was a slight asymmetry in the photoabsorption peaks,
and what appeared to be a fast (< ns) relaxation in the intermediate state. Loginov et al.15
measured photoelectron images from multiphoton ionization of aniline inside a helium
droplet, finding photoelectrons that were both faster and slower than those seen from bare
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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/m1/4/: accessed April 19, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.