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 5²P1/2 state as a result of its collision with a buffer-gas atom. It is shown that σeff=σ1/2+σ3/2, where σ1/2 is the cross section for disorientation within the 5²P1/2 level, and σ3/2 is the cross section for transfer from the 5²P1/2 level to the 5²P3/2 level by means of collisions with the buffer gas. On the basis of recently measured values for σ3/2, values are deduced for σ1/2. The cross sections are: σ1/2(Rb-He)=1.5(0.8)×10^-17 cm², σ1/2(Rb-Ne)=4.4(2.2)×10^-17 cm², and σ1/2(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 P1/2 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.