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Title: Photoionization Dynamics in Pure Helium Droplets

Abstract

The photoionization and photoelectron spectroscopy of pure He droplets are investigated at photon energies between 24.6 eV (the ionization energy of He) and 28 eV. Time-of-flight mass spectra and photoelectron images were obtained at a series of molecular beam source temperatures and pressures to assess the effect of droplet size on the photoionization dynamics. At source temperatures below 16 K, the photoelectron images are dominated by fast electrons produced via direct ionization of He atoms, with a small contribution from very slow electrons with kinetic energies below 1 meV arising from an indirect mechanism. The fast photoelectrons have as much as 0.5 eV more kinetic energy than those from atomic He at the same photon energy. This result is interpreted and simulated within the context of a 'dimer model', in which one assumes vertical ionization from two nearest neighbor He atoms to the attractive region of the He2+ potential energy curve. Possible mechanism for the slow electrons, which were also seen at energies below IE(He), are discussed, including vibrational autoionizaton of Rydberg states comprising an electron weakly bound to the surface of a large HeN+ core.

Authors:
; ; ; ;
Publication Date:
Research Org.:
Ernest Orlando Lawrence Berkeley NationalLaboratory, Berkeley, CA (US)
Sponsoring Org.:
USDOE Director, Office of Science
OSTI Identifier:
920170
Report Number(s):
LBNL-62405
Journal ID: ISSN 0022-3654; JPCHAX; TRN: US0805663
DOE Contract Number:
DE-AC02-05CH11231
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of Physical Chemistry; Journal Volume: 111; Journal Issue: 31; Related Information: Journal Publication Date: 2007
Country of Publication:
United States
Language:
English
Subject:
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS; ATOMS; ELECTRONS; HELIUM; IONIZATION; KINETIC ENERGY; KINETICS; MASS SPECTRA; MOLECULAR BEAMS; PHOTOELECTRON SPECTROSCOPY; PHOTOIONIZATION; PHOTONS; POTENTIAL ENERGY; RYDBERG STATES

Citation Formats

Peterka, Darcy S., Kim, Jeong Hyun, Wang, Chia C., Poisson,Lionel, and Neumark, Daniel M. Photoionization Dynamics in Pure Helium Droplets. United States: N. p., 2007. Web. doi:10.1021/jp0710032.
Peterka, Darcy S., Kim, Jeong Hyun, Wang, Chia C., Poisson,Lionel, & Neumark, Daniel M. Photoionization Dynamics in Pure Helium Droplets. United States. doi:10.1021/jp0710032.
Peterka, Darcy S., Kim, Jeong Hyun, Wang, Chia C., Poisson,Lionel, and Neumark, Daniel M. Sun . "Photoionization Dynamics in Pure Helium Droplets". United States. doi:10.1021/jp0710032. https://www.osti.gov/servlets/purl/920170.
@article{osti_920170,
title = {Photoionization Dynamics in Pure Helium Droplets},
author = {Peterka, Darcy S. and Kim, Jeong Hyun and Wang, Chia C. and Poisson,Lionel and Neumark, Daniel M.},
abstractNote = {The photoionization and photoelectron spectroscopy of pure He droplets are investigated at photon energies between 24.6 eV (the ionization energy of He) and 28 eV. Time-of-flight mass spectra and photoelectron images were obtained at a series of molecular beam source temperatures and pressures to assess the effect of droplet size on the photoionization dynamics. At source temperatures below 16 K, the photoelectron images are dominated by fast electrons produced via direct ionization of He atoms, with a small contribution from very slow electrons with kinetic energies below 1 meV arising from an indirect mechanism. The fast photoelectrons have as much as 0.5 eV more kinetic energy than those from atomic He at the same photon energy. This result is interpreted and simulated within the context of a 'dimer model', in which one assumes vertical ionization from two nearest neighbor He atoms to the attractive region of the He2+ potential energy curve. Possible mechanism for the slow electrons, which were also seen at energies below IE(He), are discussed, including vibrational autoionizaton of Rydberg states comprising an electron weakly bound to the surface of a large HeN+ core.},
doi = {10.1021/jp0710032},
journal = {Journal of Physical Chemistry},
number = 31,
volume = 111,
place = {United States},
year = {Sun Feb 04 00:00:00 EST 2007},
month = {Sun Feb 04 00:00:00 EST 2007}
}
  • Relaxation processes following inner-valence ionization of a system can be modified dramatically by embedding this system in a suitable environment. Surprisingly, such an environment can even be composed of helium atoms, the most inert species available. As demonstrated by the examples of Ne and Ca atoms embedded in He droplets, a fast relaxation process [interatomic Coulombic decay (ICD)] takes place merely due to the presence of the He surroundings. We have computed ICD widths for both {sup 4}He{sub N} and {sup 3}He{sub N} droplets doped with Ne and Ca and discuss the findings in some detail. In the case ofmore » Ne, ICD is by far the dominating relaxation pathway. In Ca, atomic Auger decay is also possible but ICD becomes a competitive relaxation pathway in the droplets.« less
  • For temperatures below phase separation of 1000-ppm /sup 3/He in solid /sup 4/He we measure a heat capacity ..gamma..T for a pressure between melting of pure /sup 3/He and /sup 4/He. Together with the confined sample geometry this results in liquid /sup 3/He droplets (phiapprox.10/sup 3/A) either dilute or pure depending on the phase diagram topology which is discussed theoretically. In the case of a pure /sup 3/He droplet we find an anomalously high effective mass m*/m=10, which could be explained by paramagnon effects enhanced by the confined geometry.
  • In this Letter we study the formation of metal clusters of silver, indium, and europium embedded in helium droplets and present spectroscopic (UV) evidence that, contrary to common belief, metal atoms can be stable inside helium droplets. Our method provides a beam of metal clusters up to 2000amu with a precisely known temperature of 0.37K, 2 orders of magnitude lower than previously achieved for free metal clusters. This dramatic decrease in temperature is important for high resolution spectroscopic experiments and opens the door to beam studies of superconducting metal clusters. {copyright} {ital 1996 The American Physical Society.}