Two-photon resonant excitation of vibrational levels of diatomic molecules by laser radiation

Abstract

A theoretical investigation is made of the two-photon resonant excitation of vibrational levels of diatomic molecules by single-frequency laser radiation. Equations are derived for the calculation of the probability of an allowed two-photon resonant transition in the fundamental vibrational band of a molecule. It is shown that the probability of a two-photon transition in the P and R branches of molecules with a nonzero projection of the electron momentum (Λ≠0) onto the axis joining nuclei is several orders of magnitude higher than the corresponding probability of a transition in S, Q, and O branches of molecules with Λ=0 and Λ≠0. Carbon dioxide laser frequencies coinciding to within ~0.04 cm–1 with the frequencies of allowed two-photon transitions in molecular CO and NO gases are identified. A numerical calculation is given of the two-photon absorption coefficients κ2 of these gases: at a pressure of 100 mm Hg and for a pumping power density of ~109 W/cm2 these coefficients are 8×10–9 and 10–6 cm–1 for CO and NO, respectively. The probability of an allowed two-photon resonant transition is evaluated for the case when a quasiresonant intermediate level is present. This is illustrated by a numerical calculation of the two-photon absorption coefficient of the first vibrational harmonic of the HCl molecule. For an HC1 pressure of ~1 mm Hg, a pumping power density of 107 W/cm2, and a detuning of 10 cm–1 between the frequencies of the laser and the quasiresonant intermediate transition, the two-photon absorption coefficient of HCl is ~10–4 cm–1.