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Proceedings of the Physical Society

Radiation damping of degenerate states: III. Fourth-order resonance interaction

Michael R. Philpott

Published Jul 1, 1967• 4 citations
Publisher

Abstract

The calculation of the retarded fourth-order resonance interaction between identical molecules with one shared quantum of excitation energy is described. The interaction is calculated in the electric dipole approximation for molecular transitions with the help of time-ordered graphs and a reduced Hamiltonian which treats molecule-photon and molecule-molecule interactions on the same footing. Interactions involving spins, valence forces and permanent electric or magnetic multipoles are not considered. In the near and wave zones the leading terms of the retarded interaction are proportional to R -6 and R -2 respectively, where R is the intermolecular separation.

DOI
10.1088/0370-1328/91/3/302

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The Journal of Chemical Physics•Jun 1, 1989•7 citations
The calculation of the retarded fourth-order resonance interaction between identical molecules with one shared quantum of excitation energy is described. The interaction is calculated in the electric dipole approximation for molecular transitions with the help of time-ordered graphs and a reduced Hamiltonian which treats molecule-photon and molecule-molecule interactions on the same footing. Interactions involving spins, valence forces and permanent electric or magnetic multipoles are not considered. In the near and wave zones the leading terms of the retarded interaction are proportional to R -6 and R -2 respectively, where R is the intermolecular separation.
The two-photon transitions of a system of two dissimilar molecules are studied in the dipole approximation. Special attention is paid to transitions in which both molecules participate simultaneously. By use of a reduced Hamiltonian the influence of long-range intermolecular forces is also studied. The three main transitions considered occur when: (i) no molecular level is in resonance with a primary photon, (ii) one primary photon excites a molecular level, and (iii) two primary photons excite a molecular level. It is shown that in no case can the mixing of excited states by intermolecular forces be represented as a change to an effective transition dipole moment for one of the molecules. The one-photon decay of a state in which both molecules are excited is also treated.
The interaction of two continuous wave electric fields (lasers) with a two-level molecule, that has a nonzero difference d, between the permanent dipoles of the two states involved in a multiphoton two-color transition, is studied in the rotating wave approximation and by using exact methods. Analytical rotating wave approximation results for the time-dependent populations of the molecular states and for the resonance profiles are derived and used to help discuss the effects of permanent dipole moments in the two-color absorption problem. Several examples of multiphoton, two-color, two-level resonance profiles calculated in the rotating wave approximation and by using exact techniques, are employed for illustrative purposes. They are used to help quantify the conditions for the validity of the two-color rotating wave approximation, which are considerably more restrictive than the analogous conditions for the one-color rotating wave approximation with d≠0, and to illustrate the interpretive and predictive nature of the rotating wave approximation results. For example, there are many multiphoton, two-color transitions that are available if a molecule possesses permanent dipoles (d≠0). The analytic rotating wave approximation expression for the molecule-two laser coupling can often be used to reliably estimate the field parameters of the two lasers that are required to optimize a given transition relative to others.
The theory of the line shape in coherent resonant absorption and emission in the infrared and optical region is developed. The Dicke spin model and the Lorentz oscillator model of a radiating molecule are discussed in detail, and the complications arising for degenerate levels are analyzed. The absorption and emission spectra are expressed in terms of the damped normal modes of the system of equivalent oscillators interacting via the retarded dipole–dipole interaction, and the results are shown to be identical with those derived using Heitler's quantum theory of radiation damping. The theory is applied to small clusters and linear chains of molecules. For a chain that is small compared with the wavelength, the natural line width is proportional to the total number of molecules, whereas for a long chain the width approaches a constant value. The discussion of three-dimensional systems, and the application to the zero-phonon lines in the infrared spectrum of solid hydrogen will be given in a separate paper.
A large number of "molecules," which are taken to be angular momentum oscillators of frequency $\ensuremath{\nu}$, are considered to interact through their dipole moment with two electromagnetic cavity modes of frequencies ${\ensuremath{\omega}}_{1}$ and ${\ensuremath{\omega}}_{2}$, respectively, where $\ensuremath{\nu}={\ensuremath{\omega}}_{1}\ifmmode\pm\else\textpm\fi{}{\ensuremath{\omega}}_{2}$. The behavior of the combined system is analyzed up to fourth order in perturbation theory. Phenomena known as two-photon emission or absorption, parametric amplification, various types of Raman emission, and frequency conversion are exhibited. The analytical method used is applicable to both a quantum-mechanical and a classical description of the modes and molecules; it displays explicitly the resulting differences. Expressions for the rate of energy change of the modes and molecules indicate resonance in second and fourth order. The second-order results are the same for the classical and quantum-mechanical treatments, while the fourth-order results show certain differences, which are examined in detail. The conditions under which each of the above phenomena occurs are discussed in a self-consistent manner, both classically and quantum-mechanically. It is shown that the second-order expression and the most significant part of the fourth-order expression can be derived very simply from a resonance interaction Hamiltonian that consists of a sum of products of three variables---two referring to the two modes, and the third to the molecules. The relationship between this resonance Hamiltonian and the fundamental dipole-moment Hamiltonian is examined, and the approximations by which the resonance Hamiltonian may be derived are considered. Arguments concerning the applicability of perturbation theory to steady-state situations are presented; it is pointed out that the second-order expressions for the energy transfer between molecules and field may be applicable to such a situation, in which case the classical and quantum-mechanical treatments are equivalent.
We numerically investigate optical dephasing from as many as 500 interacting two-level absorbers by making the semiclassical approximation on the radiation–matter interaction which includes retarded dipole–dipole coupling and superradiant coupling. We calculate the superradiant decay rate following single pulse optical excitation, the photon echo decay and the fluorescence. By carrying out an expansion to the second order in time we demonstrate a fundamental difference between the homogeneous free-induction decay and photon echo decay which is due to radiation damping. We show that an echo sequence is capable of reversing superradiant damping in some cases. Specific computations for pentacene doped mixed molecular crystals, where ‘‘domains’’ of resonant impurities are likely to exist, are presented and compared with experimental results.