Interference effects in the R0(0) transition of solid HD.

Abstract

A detailed theoretical analysis of the pure rotational ${R}_{0}$(0) transition in solid HD is given. Although the shape of the zero-phonon line is experimentally observed to be (approximately) symmetric, the integrated absorption coefficient contains contributions not only from the allowed and induced dipole moments, but also from an important interference between these components. Using accurate ab initio values of the dipole moments, the interference is found to be constructive, leading to a theoretical absorption coefficient significantly larger than the reported experimental value. A similar constructive interference has been observed for the ${R}_{0}$(0) transition in the liquid phase and in the gas at 77 K. We conclude that the discrepancy between theory and experiment for the intensity of the ${R}_{0}$(0) line in solid HD cannot, within the context of our present understanding, be reconciled by destructive interference and suggest that in view of the importance of a quantitative understanding of this spectrum, a remeasurement of the intensity of the ${R}_{0}$(0) zero-phonon line and its associated phonon structure be carried out.