Modeling of nitrogen compounds in cometary atmospheres: Fluorescence models of ammonia (NH₃), hydrogen cyanide (HCN), hydrogen isocyanide (HNC) and cyanoacetylene (HC₃N)

We developed full cascade fluorescence models for NH₃, HCN and HNC, and a new band model for the ν₁ ro-vibrational band of HC₃N. The models are based on ab-initio spectral databases containing millions of spectral lines and also include extremely precise spectral information contained in several high-resolution spectral databases. Using these new models we derive detailed cascade maps for these species, and obtain realistic fluorescence efficiencies applicable to high-resolution infrared spectra. The new models permit accurate synthesis of line-by-line spectra for a wide range of rotational temperatures. We validated the models by comparing simulated emissions of these nitrogen species with measured spectra of comet C/2007 W1 (Boattini) acquired with high-resolution infrared spectrometers at high altitude sites. The new models accurately describe the complex emission spectrum, providing distinct rotational temperatures and production rates at greatly improved accuracy compared with results derived from earlier fluorescence models. In addition, we made use of the completeness and scope of the new databases to investigate possible HCN↔HNC radiative isomerization mechanisms, obtaining estimates of conversion efficiencies under typical cometary conditions.