Knowledge Bank

Rotationally inelastic cross sections for $HCO+-H2$ collisions at 77 K and 41 K were measured using time-resolved double resonance spectroscopy. The J = 2 level of $HCO+$ was pumped with a gated KVARZ millimerter-wave synthesizer and the $J=3\leftarrow 2$ transition was probed with a klystron based synthesizer. From an analysis of the time dependent millimeter-wave absorption of the probe, rotationally inelastic cross sections were calculated. Since the reactions which from $HCO+$ are highly exothermic, $HCO+$ is not formed in thermal equilibrium. With the ionizing electron beam on, the average rotational and translational temperatures are greater than the temperature of the background gas. These average temperatures are a function of the initial temperature at which the ions are formed, the rate at which the ions are thermalized, and the rate at which the ions are destroyed. We will discuss how we are able to determine the average translational and rotational temperatures of the molecular ions using millimeter-wave time-resolved spectroscopy.