Relaxation of nuclear magnetization at low densities

In a recent paper, Tenti and McCourt [J. Chem. Phys. 65, 623 (1976)] presented theoretical evidence for the existence of a minimum in the transverse relaxation time T2 of the nuclear magnetization in dilute gases. Here the transverse relaxation rate is reexamined and it is shown that in the low density regime it is impossible to define a T2. Moreover, the slowing down of the relaxation rate at low densities is due to a decreasing collision rate and not to a dynamical shift as reported by Tenti and McCourt. It is found that the transverse magnetization behaves as a damped oscillator below a certain critical density. Explicit calculations are presented for H2 gas and for this system the T2 description breaks down below 0.02 Torr.