Pure rotational lasing in four electronic states of NH: Impulsive to adiabatic collisional pumping

Abstract

Pure rotational chemically pumped lasing transitions are reported for the free radical NH, as formed by flash photolysis of HN3 in a large excess of rare gas. The bulk of excited nascent population occurs in the a 1Δ (v=0) state but smaller concentrations reaching the c 1Π state result in the eventual population of higher vibrational states of X 3Σ−, a 1Δ, b 1Σ+, and A 3Π. These latter states all exhibit pure rotational lasing. A computer simulation model is constructed that details the full relaxation history of vibrationally and rotationally excited NH particles formed from c 1Π transitions to the a 1Δ state. This model incorporates V→R and R→R,T transfer mechanisms and can accommodate collisions from purely impulsive (with the possibility of V→T energy loss) to purely adiabatic (as affected by vibration, rotation, and mass of the collision partner). The model successfully explains the relative intensities of lasing transitions in the a 1Δ state, the relative intensities observed using different rare gases as buffers and the phenomenon of double pulsed lasing in the same transition, again using various rare gases as collision partners. The lasing transitions observed in A 3Π were found to be enhanced by the presence of the heavier rare gases, and are discussed in terms of collision induced intersystem crossing from the c 1Π state.