Electron Spin Resonance of Alkali Atoms in Inert-Gas Matrices

Abstract

The electron spin resonance spectra of alkali atoms (Li, Na, K, Rb, Cs) trapped in inert-gas matrices (Ar, Kr, Xe) at liquid-helium temperature have been observed. The principal hyperfine structure for each alkali atom follows very closely the pattern prescribed by the known hyperfine constants. Around the position of each hyperfine component, there is in most cases a multiple line structure, which is shown experimentally to be associated with multiple trapping sites in the particular matrix. Values of $\frac{\ensuremath{\Delta}A}{{A}_{0}}$ and $\ensuremath{\Delta}{g}_{J}$, tabulated in all cases to describe the matrix effects on the spin resonance, are found to depend upon the nature of the trapping site as well as the character of the matrix.Matrix effects on the electron spin resonance spectra of trapped alkali atoms are treated theoretically by an approach similar to that used in a previous study of trapped hydrogen atoms. In contrast to the latter case, the effect of the repulsive interactions (Coulomb and exchange) is found to be very prominent for alkali atoms in inert-gas matrices. In this treatment, the repulsive energy is approximated by the corresponding part of the Lennard-Jones potential. A detailed calculation of the quantities $\frac{\ensuremath{\Delta}A}{{A}_{0}}$ and $\ensuremath{\Delta}{g}_{J}$ has been carried out for the case of Li in Ar. Theoretical and experimental results for this case are in reasonable agreement assuming that the Li atoms are trapped in substitutional sites. There is as yet no satisfactory explanation for the phenomenon of multiple trapping sites for the alkali atoms.