On the extremum behaviour of the heat capacity of species with a low-lying excited electronic state and of isomeric mixtures: a solvable model

Abstract

Within a simple two-state model it is shown that there exists a pronounced maximum in the temperature dependence of the contribution to C p o from the low-lying first excited electronic state; the same reasoning also applies to two-isomer mixtures. The position of the maximum is, for equal degeneracy factors of both states, rigorously determined within the model; its height is 3.65 J K −1 mol −1 and does not depend on the nature of the system. The general results are supplemented by examples on systems of current interest (NO(g); Al(g); F(g); Si 2H 4(g)).