Optical Absorption Spectra of Alkali Atoms in Rare-Gas Matrices

Abstract

This paper reports studies of the optical absorption spectra of alkali-doped rare-gas crystals. The samples were formed by deposition from a mixed beam of the two constituents onto a cold plate. Eight combinations of Na, K, Rb, and Cs in Ar, Kr, and Xe were studied. In each case the spectrum consisted of two triplets, one of which was centered at approximately the free-atom value and one of which was shifted to the blue by roughly 1400-1800 ${\mathrm{cm}}^{-1\mathrm{}}$. The relative intensity of the two triplets could be changed by varying the deposition conditions; the unshifted triplet became dominant when the sample was deposited directly on a helium-filled Pyrex tube. The interval between the three components of the unshifted triplet was roughly the same for all combinations of alkali-metal atom and rare-gas matrix. In order to understand the nature of the site, a configuration coordinate scheme was invoked to deal with the energy shifts and the linewidths, and a crystalline-field approach was employed to explain the level splittings. The first analysis indicates that the sites are not substitutional and the second that the splitting can be understood in terms of a lattice distortion which is the same in all cases.