Observation of Atomiclike Electronic Excitations in PureH3eandH

Abstract

The structure of the electronically excited states of 3 He and 4 He clusters is investigated using fluorescence excitation spectroscopy. Distinct bands are observed energetically close to atomic 1s-ns, nd, np transitions and attributed to perturbed excited He atomiclike states with different principle and orbital quantum numbers. The line shifts and widths of the bands of 3 He and 4 He clusters of the same size are different and correlate with the average particle density inside the clusters calculated using the density functional method. DOI: 10.1103/PhysRevLett.87.153403 PACS numbers: 36.20.Kd, 71.35. -y, 73.22. -f, 78.40.Dw Liquid 3 He and 4 He clusters are known to exhibit many fascinating properties such as superfluidity of 4 He clusters [1] or a strongly isotope dependent density. Because of the simple closed-shell-electronic structure of He atoms these extraordinary clusters are also interesting systems to investigate how the electronically excited states evolve from the atom to the bulk.Unfortunately, for 4 He clusters and bulk liquid 4 He our knowledge about the excited levels and the optical properties is fairly limited. Only some experimental and theoretical studies have been performed [2][3][4]. In the case of 3 He comparable investigations do not exist. Usually, in solid or liquid systems isotopic substitution does not have much effect on the optical spectra. However, in He the situation is expected to be different. The density of liquid 3 He is 25% smaller than that of liquid 4 He and according to theoretical work [3,5] it is therefore expected that this will have a considerable effect on the optical absorption. Moreover, in the unique case of He, clusters of different size have also different average particle densities. Surface atoms of He clusters are very loosely bound and have relatively large internuclear separations compared to the bulk. Small He clusters which contain mainly surface atoms have therefore a significantly lower average density. Therefore, with control over the cluster size and the isotopic composition one has two independent parameters to explore the effect of the density on the excited levels. Our study compares the fluorescence excitation spectra of 3 He and 4 He clusters as a function of both the cluster size and the isotopic composition. Moreover, we report the first fluorescence spectra of 3 He and 4 He clusters in the visible and near infrared range (VIS/IR) which are very sensitive to features at higher energies.Photoexcited He clusters emit an intense spectrum in the vacuum-ultraviolet (VUV) region due to transitions to the ground state. To a good approximation, the VUV fluorescence yield is proportional to the photoabsorption, because nonradiative decay to the ground state is inefficient in rare gas clusters [6]. He clusters also emit luminescence in the VIS/IR [7]. This luminescence is due to transitions between electronically excited states of He atoms and molecules formed inside He clusters after photoexcitation.The experimental setup is described in detail else...

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