Coherent spectroscopy of anharmonic molecular vibrations which disobey the alternative forbiddenness

A new variant of coherent spectroscopy is proposed for investigating the excitation of molecular vibration overtones which do not obey the alternative forbiddenness. Estimates obtained for the CH₄ and HCl molecules show that the resonant susceptibility describing an optical process of the type ω_α=2ω+ω_L (ω is the frequency of infrared pumping radiation tuned to the frequency Ω of a molecular vibration and ω_L is the frequency of a probe beam) is considerably greater than the nonresonant "background" susceptibility. A distinguishing feature of this process is the absence (in an isotropic medium) of the competing process of repeated scattering of a probe beam by the fundamental harmonic of the molecular vibration.