Optical pumping of Cs atoms in solid4He

Abstract

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.

Atomic and molecular defects in solid 4He

“…It was only recently that a careful determination of Cs in HeII was obtained by Furukawa et al [194] who determined a lower bound of T 1 = 2.24 (19) s in a field of 500 μT. This relaxation time is approximately two times longer than the T 1 time in solid 4 He which we have determined to be T 1 ≈ 1 s in external magnetic fields ranging from 10 nT to 100 μT [187]. These relaxation times are longer than the T 1 times observed in atomic vapor cells in which special surface coatings or the addition of inert buffer gases are used to prevent depolarizing collisions with the cell walls.…”

“…We could show that the degree of spin polarization achievable in hcp 4 He depends strongly on the value of the magnetic holding field B 0 , while in bcc it was found to be independent of B 0 [187]. This effect is one of the multiple manifestions of bubble deformations discussed in Sec.…”

“…Except for a single experiment in which we have recorded absorption using a special technique (described in section 9.2 and [187]) all magnetic resonance experiments relied on fluorescence detection.…”

“…Those experiments proved our original assumption that the spin polarization of alkali atoms in solid helium may be very long-lived. A subsequent detailed study of the optical pumping process in the bcc and in the hcp phase [187] revealed that optical pumping in Cs is of the repopulation type, in which the spin polarization in the excited P 1/2 state -populated for a few 10 ns during the pumping cycle -is not affected by the interaction with the helium matrix, while it is known that high pressure He gas completely quenches the excited state polarization. The collision rate of the bubble interface atoms with the Cs atom is so high that the spin-orbit interaction following a collision-induced local deformation of the Cs atom has no time to efficiently disorient the Cs spin before the next collision (from another direction) occurs.…”

“…In Rb, on the other hand, exciplex formation is the main deexcitation channel of the P 1/2 state (cf section 7.4), so that optical pumping of the Rb isotopes is of the depopulation type. As shown in [187] depopulation and repopulation pumping lead to specific redistributions of the sublevel populations in the two hyperfine ground state levels, an effect, which can be used for the experimental distinction of the two mechanisms.…”