87Rb hyperfine-transition dephasing in mixed buffer-gas systems

Elucidating the mechanisms of dephasing in the alkali-metal ground state hyperfine transition has remained an unsatisfactorily resolved problem since the mid-1960s, even though its solution has relevance to next-generation atomic clocks. Recently, however, measurements of electronic spin relaxation in strong magnetic fields have resolved a number of outstanding ambiguities, and the situation has greatly improved. Unfortunately, while these studies have illuminated the processes contributing to hyperfine transition dephasing, they only allow one to infer actual dephasing rates, gamma. The direct measurement of dephasing rates remains problematic, primarily as a result of temperature gradient inhomogeneous broadening, which makes it nearly impossible to distinguish systematic from physical effects. Here, we demonstrate that by measuring gamma as a function of mole fraction in mixed buffer-gas systems we can isolate temperature gradient effects, thereby allowing a direct comparison between theory and experiment. In the present work, we examine the linewidth of the Rb-87 hyperfine transition in Ar-N-2 mixed buffer-gas systems. We obtain good agreement between theory and experiment so long as we include the full dephasing contribution from RbAr van der Waals molecules: the spin-rotation contribution, gamma(SR), and the contribution from the change in Rb-87 hyperfine coupling, Gamma(B), which we refer to as the Bouchiat rate. We have been able to measure Gamma(B), obtaining Gamma(B)=87 +/- 6 s(-1) for RbAr.