High-Dispersion Spectra of NH2 in the Comet C/1999S4 (LINEAR): Excitation Mechanism of the NH2 Molecule

We carried out a high-dispersion spectroscopic observation of the comet C/1999S4 (LINEAR) on 2000 July 8.75 (UT). The NH₂ Ã(0,10,0)$-$ X̃(0,0,0)$ and $ Ã(0,9,0)$-$ X̃(0,0,0)$ bands were extracted from the observed spectrum and compared with the modeled spectra. We developed a fluorescence equilibrium model for the $ NH_2$, in which $ Ã(0,v_2^{},0)$-$ X̃(0,0,0)$ transitions and the radiative relaxation among the rotational levels in $ X̃(0,0,0)$ were considered. The rotational structure of the spin doublet ($ F_1$ and $ F_2$ levels) and the Swings effect were considered in the model. The modeled spectra are consistent with the observed spectra of the $ Ã(0,10,0)$ and $ Ã(0,9,0)$ bands. Our results show that the fluorescence excitation and the radiative relaxation processes are dominant for $ NH_2$ in a coma of the comet at $ ~$ 1 AU from the Sun.