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Soviet Journal of Quantum Electronics

Coherent spectroscopy of anharmonic molecular vibrations which disobey the alternative forbiddenness

Valey F. Kamalov, Nikolai I Koroteev

Published Mar 31, 1978• 1 citation
Publisher

Abstract

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 ωα=2ω+ωL (ω is the frequency of infrared pumping radiation tuned to the frequency Ω of a molecular vibration and ωL 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.

DOI
10.1070/qe1978v008n03abeh010036

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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 ωα=2ω+ωL (ω is the frequency of infrared pumping radiation tuned to the frequency Ω of a molecular vibration and ωL 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.
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We discuss a model of selective absorption of infrared radiation by an anharmonic molecular vibrational mode. The effects of the rotational sublevels and of the energy dissipation mechanisms are described phenomenologically. We show that in the context of our model the absorption process is described by coupled nonlinear equations for the complex vibrational amplitude and for the average excitation quantum number. Two distinct dynamic regimes (coherent and adiabatic limit) can be identified depending on the relative magnitude of the rotational relaxation time and of the laser pulse duration.
An analysis is made of the frequency addition processes in the case of a two-frequency double resonance involving vibrational-rotational transitions and their potential applications in nonlinear spectroscopy. A study is made of the influence of the saturation effect and of the frequency detuning from one- and two-photon resonances on the conversion processes and estimates are obtained for a specific molecular gas. It is shown that in the case of two-frequency interaction under double resonance conditions the efficiency of conversion of a probe wave into a wave at the signal frequency increases at least several hundred times, compared with the case of the one-frequency interaction.
In this paper we present novel and simple pulse sequences for enhancing the intensity of forbidden or highly nonresonant optical transitions. These sequences provide a straightforward approach to circumventing the most serious limitation of optical coherent transient spectroscopy: Available laser intensities are often insufficient to excite a significant fraction of the ground state population into desired excited states, either because of large inhomogeneous broadening or, in the case of multiphoton absorption, large anharmonicities. Optical phase modulation (which can be produced by an acousto-optic modulator) or amplitude modulation (which can be produced by an interferometer) with carefully chosen pulse flip angles and delays can effectively remove even very large energy mismatches, thus permitting essentially complete population inversions arbitrarily far from resonance. Coherent averaging theory and computer calculations are used to derive particularly valuable sequences. Pumping enhancement of 103–105 (depending on individual molecular parameters) for these modulated pulse sequences are predicted. Specific applications to multiphoton pumping of local vibrational modes are discussed.
It was studied the solvent influence over a two level system in the four wave mixing spectroscopy. the solvente effect was incorporated to the model in a stochastic form, through the broadening of the upper molecular level. Two intense waves of the same frequency, ω1, and another wave with minor intensity and ω2 frequency, were considered coupled to the nonlinear system, generating a new signal whose frequency is ω3, and where the wave-vectors follow the relation vector k1 + vector k1 = vector k2 + vector k3. Using the dipolar-electric and the rotating wave approximations, it was calculated the average of the coherence at the ω1, ω2 and ω3 frequencies. From that results, analytical expressions to the susceptibilities at the studied frequencies were obtained. Linear, coherent and coupled processes were identified.