On the possibility of H2-near-infrared detection in comets with large gas production

Comets with large gas production offer a unique chance to observe a H2-flux of about 105 photon cm-2s-1sr-1 (1 Rayleigh) at wavelengths 8497.4 A, 8560.2 A and 8747.9 A - i.e., where photon counting methods are still applicable. It is shown that population of the vibrational levels, giving rise to these quadrupole overtone transitions, is dominated by photodissociation of methane, and that the emission even of quadrupole lines is not attenuated by collisional quenching. Wavelength scanning by +-1 A is shown to be enough to discriminate between cometary and atmospheric emissions by phase-sensitive subtraction techniques. Solid angle of Ω<10-7 sr has to be used, whence follows that a large ground-based telescope combined with a tilting Fabry-Perot-filter is best suited for detection of the near-infrared H2-emissions at reasonable counting rates and sufficient rejection of the atmospheric background. (Auth.)