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Confined helium, density dependence of the inelastic electron and photon scattering cross-sections and the optical oscillator strength

The electron and photon excitation of confined and compressed atoms is discussed. It is shown that the optical oscillator strength, the inelastic X-ray and electron impact cross-sections are all density-dependent. Ab-initio electronic structure calculations are presented for the 1s2→1s2p(1P) transition in helium, which predict that the amplitudes for the inelastic scattering of electrons, optical absorption and the excitation energy are all significantly increased in bulk condensed phases of pure helium compared to those of an isolated atom. The enhancements increase with atomic density but are very similar for bcc and fcc structured bulk phases of the same density. These enhancements need to be incorporated into the analysis of scanning transmission electron microscope studies of helium nano bubbles embedded in different materials. The excitation energy and cross-section enhancements with increasing density arise from the contraction of the 2p orbital on entering a condensed phase. For the bulk phases, but not the bubbles, the close agreement between the experimental and computed values of the energy shifts can be well described by a simple relationship.