Spectroscopic studies of solid Ar condensed on a gold surface

We have studied condensed solid argon films on a gold surface, measuring its reflected electron energy-loss spectra (REELS) and X-ray photoelectron spectra (XPS) at the temperatures 5 K and 20 K. We have also reproduced theoretically the REELS considering both face-centered cubic (FCC) and hexagonal close-packed (HCP) lattices of solid argon using density-functional theory (DFT), GW approximation, random-phase approximation (RPA) and the Bethe-Salpeter equation (BSE). In the calculations the local-field dielectric screening, quasiparticles, and excitonic effects have been taken into account. The excitonic peaks in the REELS appear at similar to 12 eV whereas the calculated electronic bandgap amounts to 14.3-14.4 eV for both the FCC and HCP lattices. Our results show that a solid argon film is a wide-gap dielectric best suited for implanting Th and studying its lowlying nuclear transition in Th-229 with the energy of 8.2 eV.