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Journal of Applied Physics

Effect of dephasing on the molecular absorption in a two-frequency field

Munir Hasan Nayfeh, Kevin King, J. M. Shobaki

Published Oct 1, 1984• 1 citation
Publisher

Abstract

The absorption of a molecule in the field of a fixed frequency coherent wave is studied using a simple, classical one-dimensional anharmonic oscillator model. The absorption is found to develop structure in the presence of a second mode of varying frequency. The structure is found to be sensitive to the intensity and wavelength of the tunable field and to wash out in the presence of collisions.

DOI
10.1063/1.334239

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The absorption of a molecule in the field of a fixed frequency coherent wave is studied using a simple, classical one-dimensional anharmonic oscillator model. The absorption is found to develop structure in the presence of a second mode of varying frequency. The structure is found to be sensitive to the intensity and wavelength of the tunable field and to wash out in the presence of collisions.
We examine the third-overtone absorption of diatomic molecules in the presence of a tunable field in near-third-overtone resonance using a one-dimensional anharmonic-oscillator model. We find that the tunable mode causes modulation of the absorption as a result of an intensity-induced quenching effect. At high intensities the absorption diminishes accompanied with the disappearance of the modulation. Our investigations also show that these effects are insensitive to the dephasing of the interaction.
Various characteristics of molecules have been clarified by spectroscopic methods in which the interactions of molecules with the radiation field were investigated. A stationary response of a molecule excited by a radiation field appears as a spectral line. Information on molecules may be obtained from position (resonance frequency), intensity, and shape and width of the spectral line. So far the interaction of molecules with the radiation field has been studied primarily in a “frequency domain.” Although molecular spectroscopy is one of the research fields in modern science which has the longest history, a variety of important aspects have been demonstrated very recently in this field as radiation sources have been improved. When the radiation field is “weak,” the photon nature of the radiation field is dominant in the interaction with a molecule, such as in the cases of emission, absorption, and scattering of photons. When the radiation field is “strong,” in the sense that many photons exist in a proper mode of the radiation field, the wave nature of the field plays an important role in the interaction. In this case the radiation may be described in the form of the classical field, say, a sinusoidal function of the time and spatial coordinates, coherently interacting with the molecule, which is described quantum mechanically. Here nonlinear properties would appear in the interactions: saturation in absorption, induced scattering, multiphoton processes, coherent transients, etc.
The indirect dephasing model for a vibrating molecule in a dense medium is investigated by using the cumulant perturbation expansion method. The perturbation is taken as a sum of the anharmonic coupling between the high- and low-frequency modes and the interaction between the low-frequency mode and molecular surrounding. All the states of the low-frequency mode is taken into account in the calculation. Our model is available, only when the dephasing time of the high-frequency mode is shorter than the life-time of the low-frequency mode. This condition is satisfied in usual organic molecular crystals.
The resonance absorption and fluorescence of a bi-chromatic electromagnetic field by a single molecule, coupled to a one-dimensional quantum molecular aggregate is studied. Depending on the values of the coupling of the single molecule and its own frequency, with respect to the own frequency of the aggregated molecules, and the strength of the aggregation, the absorption and the fluorescence of the molecule show the contribution from the exciton band, or/and the ones from the local level(s) caused by the single molecule. The linear polarized component of the electromagnetic field produces additional resonances, compared to the only circular polarized component, due to the coherent interference. The strength of the absorption and the fluorescence, caused by those additional resonances, depends on the ratio of the magnitude and the frequency of the linear polarized component. For additional resonances the magnitude of the absorption and the frequency is the decaying oscillating function of the mentioned ratio. Copyright (C) EPLA, 2017
This review emphasizes the physical principles underlying the dephasing of nonstationary vibrational states of molecules with multiple vibrational degrees of freedom. The motion of atoms within molecules can be described to a very good approximation by a many-level system of coupled anharmonic quantized oscillators. The separation of electronic and nuclear timescales, combined with weak symmetry breaking inherent in molecular structures, guarantees that the Hamiltonian can always be cast in a form that is local in the quantum state space of the molecular vibrations. The resulting nonexponential dephasing dynamics can be controlled with external fields to stabilize nonstationary quantum states, and both quantum–classical and fully quantum control formalisms are described. Coupled molecular vibrations interacting with external fields also offer prospects for quantum computing because vibrational level spacings can be made very large compared to thermal noise, and relaxation mechanisms such as infrared fluorescence are many orders of magnitude slower than the timescales required for coherence transfer. Finally, a toy model that provides insights into the interaction of vibrational degrees of freedom with 'solvent' modes is also discussed, and exhibits nonexponential dynamics in certain regimes.