Atomic Pair Processes and Laser Applications.

Several new mechanisms for the emission and absorption of light by atoms and molecules are described. These processes, which are termed pair emission and pair absorption, respectively, enable two colliding atoms (or molecules, ions, etc.) to simultaneously emit or absorb a single quantum of electromagnetic radiation. The photon energy is equal to the sum of the internal energy changes of the colliding partners. The corresponding wavelength is unrelated to the emission or absorption spectrum of the isolated atoms. Based on the theory of laser induced collisions, explicit expressions are derived for the linear absorption coefficient for pair absorption and the effective Einstein A coefficient for pair emission. Stimulated pair emission and optical gain on pair absorption transitions are also considered. Experimental observations of pair processes are made in several metal vapor systems. Both pair absorption and pair emission are observed and the experimentally measured cross sections are in good agreement with the theoretical predictions. An experiment which demonstrates the use of optically pumped pair absorption transitions in metal vapors for constructing "pair absorption pumped lasers" is described. This technique may be useful for constructing active optical frequency conversion devices. Possible device applications for pair processes such as stimulated pair emission lasers and the use of pair absorption for inverting atomic excited states relative to the ground state are also discussed.