Rb-Xe spin relaxation in dilute Xe mixtures

Abstract

We present measurements of spin-relaxation of Rb atoms in He(98%)-N2(1%)-Xe(1%) 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. The realization of magnetic resonance imaging of human subjects using hyperpolarized noble gases @1# has stimulated the intense development of Xe-Rb spin-exchange optical pumping @2# in high pressure buffer gases @3‐5#. The need to minimize the mismatch between the Rb absorption linewidth and the linewidth of inexpensive, high power but broadband diode laser sources requires pressure broadening the Rb resonance lines with many atmospheres of He gas, chosen for its weak spin-relaxation properties @6#. Despite the widespread use of high pressure He for Xe-Rb spin exchange, there exist no systematic studies of spin relaxation and spin exchange rates under such conditions, as most of the pioneering work in Xe spin-relaxation and spin-exchange either had no buffer gas @7# or used N2 buffer gas @8#. Two recent experiments report relaxation rates measured at a single pressure and temperature @4,5#. In this paper we report measurements of spin-relaxation rates at 80 and 150 °C, in