Proton spin relaxation due to critical fluctuations in liquid chloroform

Abstract

We report measurements of the proton magnetic spin-lattice relaxation time, T ₁, in liquid chloroform near the critical point for the three resonance frequencies, f _L, 10·0, 20·8 and 56·4 MHz. There is a sharp minimum in T ₁ just above the critical point which is lower and nearer the critical point the lower f _L. If is shown that the results cannot be explained in terms of an approximate theory which includes classical hydrodynamics for the critical density fluctuations. However, Kawasaki's modification for the latter fits the observed behaviour quite well in respect of the value and temperature of the T ₁ minimum and its dependence on frequency, with reasonable critical parameters. It is found that the relaxation time is most sensitive to critical density fluctuations with wave numbers of order 10⁶ cm^-1, which is not readily accessible by the more usual experimental techniques. Away from the critical point the theory does not fit experiment at all well and it is possible that the critical parameters far from the critical point may need to be adjusted, as well as requiring a more refined theory. The effect is expected to be quite general but only observable in favourable liquids.