Vibrational absorption spectrum of solid CO in the first harmonic region. Two-phonon transition

Abstract

Infrared absorption spectrum of CO crystal at low temperature, recorded in the Δυ = 2 region, shows a strong Δυ = 2 zero phonon line, a broad phonon side band and a small line which is attributed to the simultaneous vibrational transitions between two molecules. The absorption coefficient and line frequency of this two-phonon line is calculated by a theory taking account both the induced dipole moment between molecular couples and the Fermi resonance between the ν = 2 level and the two-phonon level. It is shown that most of the two-phonon line intensity comes from the Fermi resonance effect. Good agreement with experiment is obtained. The linewidth is given by the convolution product of two 0 → 1 line shapes.