High-dispersion spectra of NH2 in the comet C/1999 S4 (LINEAR): Excitationmechanism of the NH2 molecule

We carried out a high-dispersion spectroscopic observation of the comet C/1 999 S4 (LINEAR) on 2000 July 8.75 (UT). The NH2 (A) over tilde (0, 10, 0)-( X) over tilde (0, 0, 0) and (A) over tilde (0, 9, 0)-(X) over tilde (0, 0, 0) bands were extracted from the observed spectrum and compared with the mo deled spectra. We developed a fluorescence equilibrium model for the NH2, i n which (A) over tilde (0, v(2)', 0)-(X) over tilde (0, 0, 0) transitions a nd the radiative relaxation among the rotational levels in (X) over tilde ( 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 mo deled spectra are consistent with the observed spectra of the (A) over tild e (0, 10, 0) and (A) over tilde (0, 9, 0) bands. Our results show that the fluorescence excitation and the radiative relaxation processes are dominant for NH2 in a coma of the comet at similar to 1 AU from the Sun.