heterjunctions, and reported a series of internal conversion rate constants <<1 s -1 . They go on to say that the resulting rate constants are small entirely due to the large shift in the Gibbs free energy, and that while extraordinarily small these rates are confirmed by weak bi-molecular and trap-assisted recombination experiments. However, as the same method yielded qualitatively appreciable results for a similar system it is more likely that the generalised Mulliken-Hush model was not applicable to the this particular system. In addition, non-radiative rate constants that are small in the Franck-Condon limit are more likely to display either increased anharmonic behaviour or activity in the Herzberg-Teller regime ; analysis of contributions to these regimes may be required.