This paper reports studies of the optical absorption spectrum and the $X$-band electron spin resonance spectrum of rubidium atoms trapped in an argon matrix. The samples were formed by deposition from a mixed beam of the two constituents onto a cold plate. The optical absorption measurements showed two groups of lines. The red group, whose absorption lines are near those of the free atom, has a triplet structure; the blue group, whose absorption lines are shifted roughly 1650 ${\mathrm{cm}}^{-1\mathrm{}}$ to the blue, has four lines. The ESR measurements showed three major trapping sites. A motorized circular polarizer was used to measure the differential absorption of right and left circularly polarized light. By monitoring the differential absorption of right and left circularly polarized light in the presence of a microwave field, a correlation was found between the optical absorption sites and the microwave absorption sites. A crystal-field model is employed in an attempt to fit the data and to understand the nature of the sites.

• Received 7 August 1967

DOI:https://doi.org/10.1103/PhysRev.166.207