Please use this identifier to cite or link to this item: http://hdl.handle.net/2381/9617
Title: Size and Isotope Effects of Helium Clusters and Droplets: Identification of Surface and Bulk-Volume Excitations
Authors: von Haeften, Klaus
Laarmann, Tim
Wabnitz, Hubertus
Möller, Thomas
Fink, Karin
First Published: 5-Apr-2011
Publisher: American Chemical Society (ACS)
Citation: Journal of Physical Chemistry, 2011, 115 (25), pp 7316–7326.
Abstract: We report a comprehensive investigation of the electronically excited states of helium clusters and droplets of sizes ranging from a few to several 107 atoms using time-resolved fluorescence excitation spectroscopy and quantum chemical ab initio calculations. We employ various approaches for our analysis considering the lifetime-dependence of the fluorescence intensity, spectral shifts, intensity scaling with cluster size, isotopic dependence, and density-dependence of the calculated electron wave function radii. A unique feature of helium clusters and droplets is their radially varying particle density. Our results show that short-lived fluorescence is sensitive to regions of increased density and probes excitations located in the bulk volume, whereas long-lived fluorescence is sensitive to regions of reduced density such as for small clusters or for the surface of large droplets. Spectra of 3He droplets serve as a reference for low density, but are free from contributions of small clusters. This allows us to distinguish regions of reduced density as these can be due to both surface states or small clusters. Our analysis reveals a picture where spectral features are related to regions of different density due to isotopic composition, cluster size, and surface or bulk volume location of the excitations. The 2s and 2p related excitations appear as blue-shifted wings for small clusters or for excited atoms within the surface layer, whereas in the bulk-volume of large droplets, they appear as distinct bands with large intensities, dominating the entire spectrum. Excitations at energies higher than 23 eV are unambiguously assigned to regions of low and medium density location within the deeper parts of the surface layer but show no relation to the bulk volume. Our findings support the idea that in liquid helium high-lying states and, in particular, Rydberg states are quenched in favor of the 2s and 2p excitations.
DOI Link: 10.1021/jp2008489
ISSN: 1089-5639
eISSN: 1520-5215
Links: http://pubs.acs.org/doi/abs/10.1021/jp2008489
http://hdl.handle.net/2381/9617
Version: Publisher Version
Status: Peer reviewed
Type: Article
Rights: © 2011 American Chemical Society. Deposited with reference to the publisher's archiving policy available from the Journal Publishing Agreement and with the permission of George C. Schatz, Editor-in-Chief.
Appears in Collections:Published Articles, Dept. of Physics and Astronomy

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