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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 studied the magnetic properties of Cs atoms trapped in cubic and hexagonal solid 4 He matrices. Observations in the cubic phase support the assumption of a highly symmetrical trapping site (spherical bubble), while zero-field magnetic resonance spectra and magnetic dipole-forbidden transitions observed in the hexagonal phase are interpreted in terms of a quadrupolar distortion of the atomic bubbles. From the perturbed hyperfine interaction in the hexagonal phase we infer a bubble deformation of 6% 7%. [S0031-9007(98)06578-8] PACS numbers: 67.80.Mq, 32.10.Fn, 32.30.Dx, 76.70.Hb Condensed helium has played an outstanding role in the development of physics in this century. This is evidenced by the discovery of intriguing microscopic and macroscopic quantum phenomena such as Bose-Einstein condensation, superfluidity, or quantized vorticity whose understanding has laid the foundations for the development of the theory of superconductivity. The quantum crystal 4 He is a prototype example of an anharmonic crystal and it has been widely used to test structure calculations of such crystals [1]. The structure of pure crystalline 4 He can be modeled using Monte Carlo methods and is now well understood. Questions not addressed so far are the behavior of the crystal in nonuniform fields and its reaction on the perturbation from an impurity. The reasons for this are twofold: From a theoretical point of view the problem is demanding (formulation of a suitable model, computational efforts), while the major experimental difficulty is the probing of the He structure on a nanoscopic scale. In this Letter we show that one can use metal impurities to address this problem. Besides the understanding of these fundamental questions, our practical interest in doped He crystals is the search for T-violating permanent atomic electric dipole moments [2].So far, the defect structures formed by foreign atoms in condensed 4 He matrices have been studied primarily by optical methods, and the experimental observations were consistent with the assumption that atoms in spherically symmetric states ( 1 S 0 , 2 S 1͞2 , 2 P 1͞2 ) reside in spherical cavities (atomic bubbles). The analysis of the optical excitation and emission spectra has allowed one to infer the gross structure, i.e., the size of the atomic trapping sites [3]. In this paper we show that optically detected magnetic resonance (ODMR) spectroscopy is an extremely sensitive tool to study finer details of the bubble structure. Our particular interest focuses on the question to which extent the symmetry of the trapping site reflects the symmetry of the crystalline structure of the He bulk. Because of the extreme softness of the helium matrix, the repulsive interaction between a foreign atom and the surrounding helium atoms dominates the relatively weak interatomic forces in the helium bulk. The bubble shape is therefore expected to reflect the symmetry of the electronic structure of the foreign atom. At low pressures, solid helium crystallizes in an isotropic body-centered cubic ...

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