Semiclassical theory of intensities of vibrational fundamentals, overtones, and combination bands

Quantum mechanical and semiclassical results are compared for the spectrum of the Morse oscillator. When an average quantum number [(n + m)/2] trajectory is employed for an n → m transition, good agreement is obtained between the quantum and semiclassical intensities and between the frequencies, for the fundamental and the overtones studied. Spectra of nonrotating OCS in the quasiperiodic and chaotic regions are presented, and the intensities of a number of overtones and combination bands are calculated. Even very low intensity transitions are observed.