Theoretical modeling of the C-2 fluorescence spectrum in Comet Hale-Bopp

We report on observations and analysis of the C-2 Swan band Delta nu = 0 se quence in the optical spectrum of Comet Hale-Bopp, One set of observational data was obtained when this comet was at a large heliocentric distance (3. 03 AU), This unusual distance provided the opportunity to enhance our knowl edge of the intercombination transitions in C-2. These transitions are forbidden for electric dipole radiation, but they exi st due to higher order multipole radiation. Although the transition probabi lities are several orders of magnitude smaller than allowed electronic tran sitions, the intercombination bands play a key role in the fluorescence pro cess of the C-2 radical. This role is to provide a cooling path for rotatio nal and vibrational populations seen in the visible spectrum, Because their exact transition probability is not yet completely clear better quantitati ve knowledge of these transitions can help constrain C-2 quantities in come ts. To analyze the data, an equilibrium fluorescence model with 5652 different vibrational levels was created. This model included the triplet and singlet systems of C-2 involved in the fluorescence process. Theoretical spectra c orresponding to different values of the electronic transition moments for t he a(3)Pi(u)-X(1)Sigma(g)(+) and c(3)Sigma(u)(+)- X-1 Sigma(g)(+) transitio ns were computed. These spectra were then compared to the observational dat a. A good fit is obtained for transition moments of 5 x 10(-6) less than or equal to \Da-X\(2) = \Dc-X\(2) less than or equal to 10(-5) atomic units ( a.u.), for spectra obtained far from the nucleus where the fluorescent equi librium is reached. (C) 2000 Academic Press.