We have studied both theoretically and experimentally the optical pumping of Cs atoms trapped in ͑bodycentered-cubic and hexagonal-close-packed͒ crystalline 4 He matrices. The theoretical approach is based on rate equations for which time-dependent and asymptotic solutions are obtained in the case of depopulation and repopulation pumping. Comparison with experiments show that repopulation pumping, i.e., a process in which spin polarization in the excited state is not destroyed, is the dominant pumping mechanism in both crystalline phases.

We have observed several new spectral features in the fluorescence of cesium atoms implanted in the hcp phase of solid helium following laser excitation to the 6 2 P states. Based on calculations of the emission spectra using semiempirical Cs-He pair potentials the newly discovered lines can be assigned to the decay of specific Cs * Hen exciplexes: an apple-shaped Cs(AΠ 3/2 )He2 and a dumbbellshaped Cs(AΠ 1/2 )Hen exciplex with a well defined number n of bound helium atoms. While the former has been observed in other enviroments, it was commonly believed that exciplexes with n > 2 might not exist. The calculations suggest Cs(AΠ 1/2 )He6 to be the most probable candidate for that exciplex, in which the helium atoms are arranged on a ring around the waist of the dumbbell shaped electronic density distribution of the cesium atom.PACS numbers: 76.70. Hb,32.80.Wr,32.30.Dx,32.60.+i Alkali atoms and helium atoms in their ground states strongly repel each other by virtue of the Pauli principle. However, an alkali atom excited to one of its P states can exert an attractive potential on a helium atom that can lead to bound states, known as exciplexes. The formation of alkali-helium exciplexes was considered for the first time by Dupont-Roc [1] and Karnorsky et al. [2] as an explanation for the observed quenching of atomic fluorescence from light alkali atoms (Na, Li) embedded in liquid or solid 4 He. In the meantime such molecules have been observed in different environments, such as liquid helium and cold helium gas [3,4] , as well as on the surface of helium nanodroplets [5,6,7,8] . Here we present the first observations of such exciplexes in a solid helium matrix.In earlier experiments [2] we have studied the excitation and fluorescence spectra of atomic cesium implanted into the bcc and hcp phases of solid helium. It was found that the excitation at the D 1 transition (6S 1/2 -6P 1/2 ) results in atomic fluorescence at the same transition, blue shifted (with respect to the free Cs atom) by the interaction with the helium matrix. At the same time, excitation on the D 2 transition (6S 1/2 -6P 3/2 ) produced merely a weak fluorescence on the D 1 emission line, which indicates that the 6P 3/2 atoms are partly quenched into the 6P 1/2 state, while the main relaxation channel remained unknown. Recently, the extension of the spectral range of our detection system allowed us to discover several new emission lines, red shifted with respect to the atomic fluorescence line. We attribute those lines to the formation and decay of Cs * He n molecules.

We have detected by optical means nonfluorescing 85Rb and 87Rb atoms implanted in a body centered cubic 4He crystal. In contrast to cesium the resonance fluorescence of rubidium is strongly quenched by the helium matrix, and the weak resonance absorption of the two Rb isotopes was detected using a double resonance technique. From a comparative study of the (optically detected) magnetic resonance spectra of 85Rb, 87Rb, and 133Cs we infer their effective g(F) factors and conclude that they are not perturbed by the He matrix at a level of 2 x 10(-4). We show further that optical pumping of Rb proceeds via depopulation, whereas for Cs it proceeds via repopulation.