Diagnostic Properties of Two-Photon-Pumped Stimulated Emission in Atmospheric Species

Abstract

Two-photon excitation provides access to atoms and molecules with absorption resonances in the vacuum ultraviolet (VUV), i.e., to the species otherwise inaccessible for probing under atmospheric conditions, since the atmosphere strongly absorbs VUV radiation. In addition, for high enough laser intensities, two-photon pumping may create population inversion between the pumped energy state and a lower-lying intermediate state, resulting in stimulated emission. In this chapter such an emission is discussed in terms of its diagnostic capacity. The method has primarily been investigated for the detection of a number of atomic species, such as oxygen and nitrogen, and a few small molecules, for example, CO and NH₃. The major benefits of the technique are that the signal propagates in a laser-like beam, the backward-directed beam allows single-ended diagnostics, strong signals allow trace-level detection, and the optical setup is relatively simple. The main disadvantages are the poor and sometimes ambiguous spatial resolution and the difficulties with modeling the process due to the nonlinear dependence of the signal on the concentration of the active species and the integrative-growth nature of the signal. Besides exploring the potential for diagnostics, early works on two-photon-induced stimulated emission have been imperative for today’s development of air-lasing concepts based on backward-directed stimulated emission. This chapter is devoted to a review and summary of these pioneering studies.