The Photodissociation of Formaldehyde in Comets

Observations of comets in the 905–1180 Å spectral band made with the Far Ultraviolet Spectroscopic Explorer in 2001 and 2004 show unusual features in the fluorescent emissions of CO and H 2 . These include emission from a non-thermal high- J rotational population of CO and solar Ly α induced fluorescence from excited vibrational levels of H 2 , both of which are attributed to the photodissociation of formaldehyde. In this paper we model the large number of observed H 2 lines and demonstrate the dependence of the pumping on the heliocentric velocity of the comet and the solar line profiles. We also derive the rotational and vibrational populations of H 2 and show that they are consistent with the results of laboratory studies of the photodissociation of H 2 CO. In addition to the principal series of H i and O i , the residual spectrum is found to consist mainly of the Rydberg series of C i multiplets from which we derive the mean carbon column abundance in the coma. Fluorescent emissions from N i and N 2 are also searched for.