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Soviet Journal of Quantum Electronics

Theory of coherent four-photon spectroscopy of electron resonances in molecular systems

Boris D. Fainberg, I. N. Myakisheva

Published Dec 31, 1987• 16 citations
Publisher

Abstract

A quantum theory of coherent four-photon spectroscopy of electron resonances in molecular systems is developed. A model allowing for real properties of the dispersion of perturbations modulating electron transitions in complex molecules is considered. New relationships are established between the characteristics of spectra of one-photon absorption and the law governing the dispersion of a signal obtained in steady-state four-photon spectroscopy, conditions for excitation of this signal, its dispersion, etc. It is shown that the theory agrees with the available experimental data.

DOI
10.1070/qe1987v017n12abeh011330

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A quantum theory of coherent four-photon spectroscopy of electron resonances in molecular systems is developed. A model allowing for real properties of the dispersion of perturbations modulating electron transitions in complex molecules is considered. New relationships are established between the characteristics of spectra of one-photon absorption and the law governing the dispersion of a signal obtained in steady-state four-photon spectroscopy, conditions for excitation of this signal, its dispersion, etc. It is shown that the theory agrees with the available experimental data.
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Coherent four-photon scattering of light due to excited and autoionizing states of atomic systems is considered. We analyze the influence of one-photon resonances at the frequency of the anti-Stokes signal and at the frequency of the pump wave on the shape of the spectrum of four-photon scattering. We derive analytical expressions for the line shapes of four-photon scattering involving one-photon resonances with allowance for phase-matching conditions and variation in the populations of excited states for limiting regimes of tight focusing and strong absorption in a medium with a cubic nonlinearity. The calculated spectra of four-photon scattering include the interference of resonant and nonresonant components of the third-order nonlinear susceptibility with allowance for one-photon saturation. We develop a method for normalizing experimental spectra of four-photon scattering with one-photon resonances that takes into account distortion of spectral lines due to the influence of phase mismatch and one- photon absorption.
The theory of the coherent pairing of excitons in the presence of a resonant electromagnetic field is generalized to describe the biexciton spectra of molecular crystals, which contain several molecules (atoms) in the unit cell. The excitation spectrum for crystals like anthracene having two molecules per unit cell is discussed in detail. It is found that, at high exciton densities and under resonance of near-resonance conditions and at temperatures below some value T c , the formation of four biexciton states is feasible provided that the dielectric gaps induced by the electromagnetic field are less than those of the corresponding biexciton states. Expressions for the absorption coefficient describing optical transitions to the biexciton states are derived.
Time - dependent behaviors of molecular coherence in resonant multiphotoni onization spectra of molecules are theoretically investigated . An expression for the transition probability of time-resolved two-photon ionization processes via coherently excited resonant intermediate states is derived in the perturbative density matrix formalism. Model calculations of the probability of ionization are performed to predict appearance of quantum beats in pump-probe two-photon ionization experiments. The quantum beat signals in the time-resolved ionization spectra are compared with those in time-resolved total photon emission spectra.
Multiphoton absorption of entangled photons offers ways for obtaining unique information about chemical and biological processes. Measurements with entangled photons may enable sensing biological signatures with high selectivity and at very low light levels to protect against photodamage. In this paper, we present a theoretical and experimental study of the excitation wavelength dependence of the entangled two-photon absorption (ETPA) process in a molecular system, which provides insights into how entanglement affects molecular spectra. We demonstrate that the ETPA excitation spectrum can be different from that of classical TPA as well as that for one-photon resonant absorption (OPA) with photons of doubled frequency. These results are modeled by assuming the ETPA cross-section is governed by a two-photon excited state radiative linewidth rather than by electron-phonon interactions, and this leads to excitation spectra that match the observed results. Further, we find that the two-photon-allowed states with highest TPA and ETPA intensities have high electronic entanglements, with ETPA especially favoring states with the longest radiative lifetimes. These results provide concepts for the development of quantum light-based spectroscopy and microscopy that will lead to much higher efficiency of ETPA sensors and low-intensity detection schemes.
The effects of the nonclassical character for a low-frequency system of optically active vibrations in the steady-state four-photon spectroscopy of electron resonances are studied theoretically under two-frequency ({omega}{sub 1} and {omega}{sub 2}) pumping with the generation of a signal {omega}{sub c}=2{omega}{sub 1}{minus}{omega}{sub 2}. It is shown that the intensity of the signal is decreased more rapidly with a growth of the detuning magnitude {Delta}={omega}{sub 1}{minus}{omega}{sub 2} when {Delta}{gt}0 in comparison with the case {Delta}{lt}0. Such behavior is explained by the predominant contribution to the intensity of the {omega}{sub c} signal for processes of the type single-photon Raman scattering with excitation frequency {omega}{sub 2} and scattering frequency {omega}{sub 1}.