Effect of orientational and substitutional disorders on the N 2 vibrational exciton in N 2/CO mixed crystals

Abstract

High resolution Raman spectroscopy was performed on the N 2 vibron bands over a large concentration range at various temperatures in N 2/CO mixed crystals. The Davydov splitting appears for N 2 concentration above 0.2 and linearly increases up to the pure crystal limit. The Raman linewidth exhibits a maximum for a N 2 concentration of 0.15 and decreases towards a residual value in the low concentration limit. Two types of disorder occur in this molecular alloy, namely a substitutional disorder between CO and N 2 molecules and a frozen-in orientational disorder for the asymmetric CO molecules. A Hamiltonian including correlated diagonal and off-diagonal disorder terms was suggested to describe the vibron band. The first moments of the Raman intensity were calculated as a function of the concentration. The first moment accounts for the linear dependence of the Davydov splitting in terms of the concentration. The second moment predicts the residual bandwidth in the low N 2 concentration limit which is proved to result from the orientational disorder of the CO molecules.