VELOCITY DISTRIBUTIONS OF HYDROGEN-ATOMS AND HYDROXYL RADICALS PRODUCED THROUGH SOLAR PHOTODISSOCIATION OF WATER

The calculated velocity distributions of hydrogen atoms and hydroxyl r adicals produced through solar photodissociation of gaseous water mole cules are presented. Under collisionless conditions, the calculation h as been carried out using (1) the most recently available absolute par tial cross sections for the production of H and OH through photodissoc iation of H2O from its absorption onset at 1860 angstrom down to 500 a ngstrom, (2) the newly available vibrational and rotational energy dis tributions of both the excited and ground state OH photofragments, (3) the calculated cross sections for the total dissociation processes, a nd (4) the integrated solar flux in 10 angstrom increments from 500 to 1860 angstrom in the continuum regions and the specific wavelength an d flux at the bright solar fines, e.g., the H Lyalpha, Lybeta, Lygamma , O VI, C III, He I. Because of the lack of data in several dissociati on processes or in certain spectral regions, we can only obtain upper and lower bound velocity distributions. In terms of quantum yields, th e lower bound case is set by assuming that all the undetected neutral products are something other than H and OH fragments and an upper boun d is set by assuming the undetected neutral products am all H and OH f ragments. In terms of internal energy distributions of the OH fragment s the upper bound is set by neglecting them and a lower bound is set b y taking the available or the best estimated values. The calculated re sults show that the H atoms and the OH radicals produced exhibit multi ple velocity groups. Since most of the current cometary modeling uses a single velocity of 20 km/s associated with the photodissociation of H2O, the present results may be uselful in interpreting the many peaks observed in the velocity distributions of the H Lyalpha and Halpha of comets.