Cooling and Laser-Induced Fluorescence of Electronically-Excited He$_2$ in a Supersonic Microcavity Plasma Jet

Laser-induced fluorescence (LIF) resulting from transitions between different electronic states of helium dimers generated within a microcavity plasma jet was studied with rotational resolution. In particular, the d$^3\Sigma^+_u$, e$^3\Pi_g$ and f$^3\Sigma^+_u$ states, all having electronic energies above 24 eV, are populated by a microplasma in 4 bar of helium gas and rotationally cooled through supersonic expansion. Analysis of two dimensional maps (spectrograms) of dimer emission spectra as a function of distance from the nozzle orifice indicates collisional coupling during the expansion between the lowest rotational levels of the e$^3\Pi_g$, f$^3\Sigma^+_u$ states and high rotational levels (around N=11) of the d$^3\Sigma^+_u$ state (all of which are in the v = 0 vibrational state). In an attempt to verify the coupling, a scanning dye laser (centered near 596 nm) pumps the b$^3\Pi_g$ $\rightarrow$ f$^3\Sigma^+_u$ transition of the molecule several hundred micrometers downstream of the nozzle. As a result, the emission intensities of relevant rotational lines are observed to be enhanced. This research shows the potential of utilizing microcavity plasma jets as a tool to study and manipulate the collisional dynamics of highly-excited diatomic molecules.