Cooperative two-photon absorption

Abstract

It is well known that irradiation with intense laser light can lead to nonlinear absorption processes by individual molecules. However, the mutual interaction between two molecules can lead to cooperative nonlinear processes which result in entirely new features in the absorption spectrum. In this paper, the theory of cooperative two-photon absorption is developed using the principles of quantum electrodynamics, and expressions are derived for the rate of cooperative absorption for transitions which are forbidden in the absence of any such interaction. Several different cases are examined in detail, including crystalline or matrix-isolated species, gaseous and liquid mixtures, and van der Waals molecules. It is shown that the cooperative absorption rate can be substantially increased by choosing materials with suitable energy levels where one of several resonance enhancement mechanisms can be exploited. Finally, methods of observation are discussed, and a specific photochemical example is given for the case of a mixture of formaldehyde and deuterioformaldehyde.

Two-Photon Cooperative Absorption in Colliding Cold Na Atoms

“…This effect was first observed in a mixture of Ba=Tl, where a pair of atoms were simultaneously excited by a cooperative-absorption process [2]. The problem has also been considered theoretically by a few authors [3,4].…”

Synergistic Effects in Two‐Photon Absorption: the Quantum Electrodynamics of Bimolecular Mean‐Frequency Absorption

“…Estimation of the more general significance of many of the results presented in previous sections must therefore proceed from a different basis. As shown in early work on cooperative photoabsorption (Andrews and Harlow 1983), neighboring molecules can in fact be expected to display a synergistic absorption rate approaching the rate of two-photon absorption by individual molecules, a result which is more readily calculated. This can be argued as follows.…”

“…result corresponds to the expression originally derived in Gaussian units by Andrews and Harlow (1983). As it stands, all molecular and radiative parameters in the above expression are referred to a single Cartesian frame of reference, making the result directly applicable only to rigidly oriented molecules or solids.…”

The Quantum Electrodynamics of Synergistic Two-photon Processes

“…The first process, termed cooperative, corresponds to a case where both photons hw l and hw2 interact with a different molecule as shown in figure 2 [9] . The second mechanism, termed distributive, describes a process in which both photon events occur in a single molecule as in figure 3 [10] .…”