Abstract
We report a comprehensive investigation of the electronically excited states of helium clusters and droplets of sizes ranging from a few to several 10(7) 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 (3)He 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.
Highlights
The helium atom is the simplest example of a many-electron atom
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
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
Summary
The helium atom is the simplest example of a many-electron atom. In light of the simplicity and model character of helium atoms one would naturally be interested to learn how the electronically excited states become modified when an excited helium atom is placed near one, two, or more helium atoms. The VUV photons were converted to the spectral sensitivity range of the photomultiplier using a plate vacuum coated with a thin film of sodium salicylate, just before a window and the photomultiplier This technique is simple and it was found that the set up was insensitive to visible photons emitted from the helium clusters.[8] Absorption of a photon leads to a good approximation to fluorescence of precisely one VUV photon whose registration can be taken as a measure for photo absorption.[14] Limitations exist for high excitation energies where autoionization is possible and for very large helium droplets where more than one excitation per synchrotron light pulse may occur. The frozen-core calculations allowed us to deduce the average electronic wave function radius Æræ
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