Theoretical and experimental studies of nuclear polarization induced by single-modelaser optical pumping are reported. The nuclear polarization is shown to depend on the product of laser intensity and buffer-gas pressure. In 23 Na it approaches one at modest laser intensities (~100 mW/cm 2 ) for pressures as small as a few hundred millitorr.PACS numbers: 32.80. Bx, 29.25.Cy, 29.25.Kf It has recently been emphasized that achieving complete nuclear polarization in dense alkali vapors is important for producing intense polarized light-ion beams by charge exchange 1 and polarized targets for parity-nonconservation studies. 2 The present paper demonstrates that singlemode-laser optical pumping in combination with velocity-changing collisions (vcc) can be used to attain a high degree of nuclear polarization in alkali vapors. General theoretical results are presented for alkalilike systems and confirmed in sodium, where almost complete nuclear polarization is achieved. Several authors have studied vcc processes in laser saturation spectroscopy 3 ' 4 and in laser optical pumping. 5 ' 6 Complete optical pumping has been achieved at high buffer-gas pressures (collision-broadened regime, where vcc processes have no influence), 7 * 8 but the use of laser optical pumping with vcc, at very low buffer pressures, to produce nuclear polarization has not been studied in detail previously.The laser optical-pumping process, in which nuclear polarization is achieved through the coupling between electron and nuclear spins, has been referred to as laser-induced nuclear orienta-tion (LINO). 9 Ordinarily, single-mode laser radiation interacts only with a small fraction of the thermal velocity distribution, of order y/ku ~ 10" 2 , with y and ku the homogeneous and Doppler widths, respectively. This results in poor Doppler coverage, hence incomplete nuclear polarization. Furthermore, in alkali atoms the atomic population is distributed in two groundstate hyperfine (hf) levels, the Doppler-broadened absorption profiles of which overlap in the cases of Li and Na.The present experiments use vcc induced by argon buffer-gas atoms to attain single-mode laser optical pumping of the entire ground-state population. In the vcc regime velocity changes cause every alkali atom to be at the resonant velocity at some time during its diffusion through the laser beam. Furthermore, this spatial diffusion considerably retards wall relaxation and greatly extends the laser-atom interaction time. Because of large collision cross sections high vcc rates can be achieved at low (< 1 Torr) buffer pressures, where collisional broadening is negligible.Consider a gas of three-level atoms with closely spaced or degenerate levels 1 and 2 optically con-236
Using laser-induced nuclear-orientation and optical-saturation techniques, we have produced sub-Doppler changes in the anisotropy of the angular distribution of the 514-keV y rays from the 1-^us isomer 85 Rb w . This permits high-resolution measurements of D\ and D2 hyperfine transitions of atoms of the isomer, which give the isomer's nuclear electric quadrupole moment, -0.73 ±0.17 b, and its nuclear magnetic dipole moment, (6.043 ±0.005)HN. The isomer shift, -113 ±8 MHz relative to the ground state of 85 Rb, corresponds to a change in the mean-square charge radius of +0.174 ±0.008 fm 2 .
The possibility of performing weak interaction experiments using spectroscopic techniques is discussed. In such an experiment, a laser is tuned to an electronic transition of the daughter, and a signal whose strength is proportional to the number of recoil atoms is monitored, such as fluorescence, absorption, or anisotropy induced in radiation emitted in a subsequent nuclear decay. As a specific example of this technique"we consider measuring the angular correlation between the momenta of the electron and the antineutrino emitted in the decay "Kr~"Rb +f3 +v using the laserinduced nuclear orientation technique. Such an experiment is currently under way.
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