THEORY OF COHERENT RESONANT ABSORPTION AND EMISSION AT INFRARED AND OPTICAL FREQUENCIES

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Abstract
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.
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