Nuclear Spin Relaxation in Dilute Gas Mixtures

Abstract

Measurements of the proton spin relaxation time T1 have been carried out in hydrogen–helium dilute gas mixtures between room temperature and 730 K and in hydrogen–carbon dioxide mixtures between room temperature and 500 K as a function of density and concentration. When the measurements on H2–He are combined with those previously made at lower temperatures, a pronounced minimum in the dependence of T1 on temperature is obtained in the vicinity of 70 K for the limit of 100% helium. The data are analyzed on the assumption that collisions between hydrogen molecules and helium atoms which change the rotational quantum number J of the H2 are much less frequent than those which cause molecular reorientation with no change in J. Under this assumption, the data over the entire temperature range are fitted remarkably well by the theory of Kinsey, Riehl, and Waugh for the velocity dependence of the cross section for molecular reorientation in H2–He collisions using an exponential–6,H2–He potential based on "a priori considerations". The high temperature data are consistent with a reorientation cross section behaving approximately as σ α ν2.4 in the limit of high velocities. The H2–CO2 data below 400 K are consistent with a CO2 quadrupole moment of 4.1 × 10−26 e.s.u. but the behavior of the proton spin relaxation above 400 K is not yet understood.