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Real and virtual absorption of weak monochromatic light is analyzed with semiclassical radiation theory. The influence of the light on the atoms is described by an effective ground-state Hamiltonian operator, and the effect of the atoms on the light is described by a dielectric susceptibility operator. These operators are expressed explicitly in terms of familiar ground-state observables, angular factors, oscillator strengths, and plasma dispersion functions. The theory gives a comprehensive description of optical pumping, light shifts, and light modulation due to real and virtual absorption of light, and several new effects are predicted. Repopulation of the atomic ground state from a polarized excited state is not considered in this paper.OPTICAL PUMPING 13

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The propagation of light in an optically pumped vapor is determined semiclassi ally by the expectation value of a dielectric susceptibility operator. We present explicit formulas for the dielectric susceptibility of a vapor of alkali atoms. The qualitative features of the susceptibility are .illustrated for the specific case of Rb '. The theory of this paper is essential for a clear understanding of phenomena such as the birefringence of alkali vapors and the microwave modulation of light by alkali vapors. Some experimental observations of birefringence in rubidium vapor are Interpreted with the semiclassical theory.

The hyperfine separation of tritium has been measured with respect to the hyperfine separation of hydrogen by comparing the frequency of the tritium maser with that of a hydrogen maser. The result is A?(T) = 1 516 701 470.8087±0.0071 Hz, where it is assumed that Av (H) = 1 420 405 751.8 Hz. Wall shifts were measured for tritium and hydrogen for FEP Teflon at 35 °C. For a spherical bulb, d cm in diameter, the fractional wall shift for hydrogen is -(3.61±0.54) X lO" 10 /^ and for tritium it is -(3.14±0.47) X 10~1 0 /d.