Information theory and bulk rotational or vibrational relaxation processes

The possible occurrence in bulk relaxation of pure-exponential, near-exponential, and non-exponential decay of the total energy is examined in terms of the normal-mode and information-theoretic approaches. Experimental tests are suggested for the identification of pure-exponential decay caused by adherence to the 'sum rule', and of near-exponential decay. In the case of near-exponential decay (as opposed to pure-exponential decay), it is not possible to derive reliable state-to-state rate constants by invoking approximate adherence to the sum rule.