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 $$ \mathrm{NH}_2$$ $$ \tilde{\mathrm{A}}(0,10,0)$$-$$ \tilde{\mathrm{X}}(0,0,0)$$ and $$ \tilde{\mathrm{A}}(0,9,0)$$-$$ \tilde{\mathrm{X}}(0,0,0)$$ bands were extracted from the observed spectrum and compared with the modeled spectra. We developed a fluorescence equilibrium model for the $$ \mathrm{NH}_2$$, in which $$ \tilde{\mathrm{A}}(0,v_2^{\prime},0)$$-$$ \tilde{\mathrm{X}}(0,0,0)$$ transitions and the radiative relaxation among the rotational levels in $$ \tilde{\mathrm{X}}(0,0,0)$$ 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 $$ \tilde{\mathrm{A}}(0,10,0)$$ and $$ \tilde{\mathrm{A}}(0,9,0)$$ bands. Our results show that the fluorescence excitation and the radiative relaxation processes are dominant for $$ \mathrm{NH}_2$$ in a coma of the comet at $$ \sim$$ 1 AU from the Sun.