The Role of Electron Collisions in Lasing in Neutral and Singly Ionized Molecular Nitrogen

Abstract : In this chapter, we will discuss lasing actions in the air that follow the excitation with a short intense laser pulse at 800 nm. We will successively analyze two types of laser actions. The first type is based on the optical transition between the excited triplet states of the neutral nitrogen molecule. Based on the study of the dependence of the lasing signal on the polarization ellipticity of the pump pulse, we unambiguously attribute gain mechanism in this scheme to the electron collisions with neutral nitrogen molecules that result in population inversion. Experimental results on the dynamics of emissions in the forward and backward directions with respect to the direction of the pump pulse are confirmed by numerical simulations based on the Maxwell-Bloch equations. The second type of lasing stems from the transition between the second electronically excited state and the ground state of a singly ionized nitrogen molecule. After reviewing current interpretations of this emission process, which remains to be a controversial issue, we will focus on our interpretation that links stimulated emission in this scheme to superradiance. We will argue that electron recollisions play an essential role in establishing the superradiant gain.