Chemical shifts of small heterogeneous Ar/Xe clusters

Heterogeneous rare-gas clusters produced by a coexpansion of an argon/xenon mixture have been studied using synchrotron-radiation-based photoelectron spectroscopy. Both valence and Xe $4d_{5/2}$ core-level photoelectron spectra were recorded for three different concentrations of the primary argon/xenon mixture and, for those mixtures, spectra were recorded at several different stagnation conditions. The studied size regime of the mixed clusters ranges from large, similar to those studied in an earlier paper [Phys. Rev. A 69, 031210(R) (2004)], to very small—as reflected in the cluster line shapes and chemical shifts. The chemical shifts obtained from a curve fitting procedure similar to that used in our earlier paper are discussed in terms of the mixed cluster structure which can be expected from equilibrium considerations and the Lennard-Jones parameters of the constituent atoms. Molecular dynamics simulations of the vertical polarization shifts allow more specific assignments of “on-top” sites and interfacial sites.

Figure 1

Different sites on the surface and on top of an icosahedral cluster structure.

Figure 2

Xe 5$p$ and Ar 3$p$ UPS spectra grouped by Xe concentration. The topmost spectrum is from the 2.5$\%$ series. The spectra below are from the 5$\%$ series and, second from the top, the 9$\%$ spectra can be found. The inset below shows pure xenon- and argon-cluster UPS spectra. The atomic Ar $3p$ spin orbit components are marked with connected vertical bars in the pure cluster spectrum; the atomic Xe $5p_{3/2,1/2}$ components at 12.13 and 13.44 eV are also indicated.

Figure 3

X-ray photoelectron spectra of the Xe $4d_{5/2}$ level of pure (boxed at the bottom) and mixed Ar/Xe clusters created in a coexpansion. The dashed lines below each spectrum are the results from a least squares curve fitting procedure (see Table ).