We present measurements of spin-relaxation of Rb atoms in $\mathrm{He}\mathrm{}(98\%)\mathrm{}-{\mathrm{N}}_2\mathrm{}(1\%)\mathrm{}-\mathrm{Xe}\mathrm{}(1\%)\mathrm{}$ mixtures similar to those used in magnetic resonance imaging polarizers. The pressure dependence allows us to separate out contributions from Rb-Xe van der Waals molecules and binary collisions. For the first time, we observe the predicted increase in the molecular contribution at high pressure. Our data suggest that the deduced molecular breakup rate has a strong temperature dependence.

• Received 6 April 2001

DOI:https://doi.org/10.1103/PhysRevA.65.012712