Fiber-optic Array Spectroscopy of NH2 In Comet Hale-Bopp: Nature of the rotational energy distributions

Moderate resolution spectra of the 030 band of NH2 near 6300 Å were recorded in Comet Hale-Bopp near 2.6 and 1.0 AU with the HYDRA array spectrometer on the WIYN telescope. The spectra from beyond the collision sphere were examined with respect to the relative rotational energy distributions in NH2. The relative rotational profiles could be consistently fit at both heliocentric positions by a photostationary state model that follows optical pumping and fluorescence in the ÃA1(Pu) B1 band and radiative rotational relaxation in the ground state. The behavior of the spectra with regard to the relative weakness of lines from Ka¢ = even states at large heliocentric distances is explained by this model, where only the solar flux changes. The general heliocentric dependence of the rotational excitation temperature of collision-free NH2 and, to a first-order approximation, polar diatomic radicals is predicted. The form of the heliocentric dependence is coincidentally similar to that of CO in the collision sphere as observed by others. Species that can undergo optical pumping will show a heliocentric dependence of the rotational excitation temperature of the form, Texc(rh) = T1AU/rh 0.5-1.3. The NH2 radical and diatomic hydrides will have flatter rh dependence than the polar first row diatomics. The rh dependence is largely controlled by the ratio of the fluorescence excitation rate and the rate of radiative rotational relaxation.