Early Studies Of Systems Not In Thermal Equilibrium

This essay is a brief review the synergistic intertwining of physical concepts that were proposed about a century ago. They focused on consideration of systems that departed from Boltzmann distributions among molecular states, that is, systems to which no unique temperatures could be assigned, that eventually led to the preparation of lasing media, in particular, to chemical lasers. The contribution of each of the threads to the evolution of lasers only became apparent decades after its origination. The goal, which was achieved in many ways, was the development of a variety of techniques for generating high concentrations of selected rotationally, vibrationally, or electronically excited atomic/molecular states that had moderate relaxation times. This is a brief summary of scientific advances that were initiated at the turn of the 20th century that ultimately led to the construction of chemically energized lasing systems. Formulation of the quantum theory for radiation was a prerequisite to the laser concept.