Optical-pumping transients in rubidium-87 and application to excited-state disorientation cross sections

Detailed studies have been made of the light transmitted through rubidium-87 vapor during the optical-pumping process, both with and without buffer gas present. The observed transients are single exponentials with no buffer gas present, and double exponentials with buffer gas present, over a wide range of pumping intensities and relaxation times. These results are in excellent agreement with the predictions based on phenomenological equations in which nuclear spin is included and a single relaxation time is assumed. From a study of the amplitudes of the two exponential components of the optical-pumping transient, an effective cross section σeff is deduced for the disorientation of rubidium-87 within the 5P122 state as a result of its collision with a buffer-gas atom. It is shown that σeff=σ12+σ32, where σ12 is the cross section for disorientation within the 5P122 level, and σ32 is the cross section for transfer from the 5P122 level to the 5P322 level by means of collisions with the buffer gas. On the basis of recently measured values for σ32, values are deduced for σ12. The cross sections are: σ12(Rb-He)=1.5(0.8)×10-17 cm2, σ12(Rb-Ne)=4.4(2.2)×10-17 cm2, and σ12(Rb-Ar)=3.5(1.8)×10-16 cm2. These cross sections are deduced from a model in which the probabilities for Rb87 to relax from any hyperfine level to any other are all equal. This model gives a better fit with the observed transients than the assumption that the electron spin only is randomized in the P12 state with the nuclear spin unaffected. We show that accurate relaxation-time measurements can be made by measuring the time constants associated with the double exponentials as a function of light intensity, and extrapolating them to zero light intensity. Moreover, since no shutter is required, relaxation times <10-3 sec can be easily observed.