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This report was prepared as an eccount of work sponsored by an agency of theunited States Government. Neitherthe United States Government norany agency thereof, nor any of their employees, contractors, subcontractors, or their employees, makes any warranty, express or implied, nor assumes any legal liability or responsibility for any third party'a use or the results of suc11 usa uf ally itlfurrrraliun, apparatus, pMdUCr or process disclosed in this report. nor represents that its use by such third party would not infringe privately owned rights.

Optical pumping methods have been extended to high buffer-gas pressures by replacing the customary resonance lamp by a white-light source. Rb ground-state Zeeman transitions were detected by resonance fluorescence at argon buffer-gas pressures up to 23 atm. Hyperfine-transition frequencies were measured at 4170 Torr (0°C) and at 7390 Torr (0°C) and showed the first nonlinear pressure dependence so far observed.We have optically pumped Rb^^ and Rb®'' atoms in argon buffer gas at pressures up to 30 atm, by replacing the conventional resonance lamp by a high-pressure, 1-kW, xenon arc lamp. In the white light, the optical absorption rate is independent of the pressure-broadened line shape (0.4 A per atm)^ of the Rb atoms, and even the low spectral density of an arc lamp becomes more efficient than a resonance lamp above about 1 atm, the typical upper limit in pressure for previous optical pumping experiments.Although tungsten-halide lamps operate near 3600°K (maximum intensity at the 7948-A D line), the hotter Xe arc (Hanovia type 976 C-1) was found to be more efficient. The spectral width of the pumping light was reduced to about 50 A by a number of filters in the light beam to avoid heating of the sample and instrumental scattering to our detector. With all the apparatus assembled, the short-circuit current of a solar cell in the light beam transmitted through the absorption cell was of the order of 1 mA (5x10^^ photons/sec). Increasing the Rb vapor pressure by heating the absorption cell decreased the absorption by only a few percent, corresponding to the pressure-broadened width of the Rb D line. However, the resonance-fluorescence light detected by a photomultiplier at right angles to the transmitted beam increased in direct proportion to the Rb vapor pressure. All of our signals were detected in the resonance fluorescence, avoiding the 90 % of the light not involved in the pumping process. Since both the detection of atomic polarization and the pumping process itself depend linearly on the optical absorption rate /Q, the signal strength is proportional to IQ^ (at the intersection of the transmitted and detected beams). Maximum signal occurred near 90°C for the cells we used.Well-annealed spherical Pyrex absorption cells 5 cm o.d. with walls roughly 4 mm thick were generally strong enough. Each sample was tested hydraulically at 50 atm before use. To avoid high pressures in the filling process, a large

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