Plmr r . ,!?,m~ Sci., Vol. 38, No. 5, pp. 64-652, 1990 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA 003?-0633~YO $3.00+0.00 Printed m Great Britam. c IYYO Pergamon Press plc COLLISIONAL QUENCHING OF O(‘D) BY O(“P) J.-H. YEE Space Physics Research Laboratory, University of Michigan, Ann Arbor, MI 48109, U.S.A zyxwvutsrqponmlkjih STEVEN L. GUBERMAN Institute for Scientific Research, 33 Bedford Street, Lexington, MA 02173, U.S.A. and A. DALGARNO Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138, U.S.A (Received 13 December 1989) Abstract-Metastable 0( ‘D) atoms may be quenched in collisions with ground state O(‘P) atoms by transitions in the avoided crossing regions of the three lowest ‘fl, states of Oz of which the lowest separates to 0(3P) +O(‘P) and the two upper to O(‘P)+O(‘D). Quanta1 calculations of the adiabatic potential energy curves of the ‘fI, states are carried out with particular attention to an avoided crossing region in the lowest two states around a nuclear separation of 3.2~~. Diabatic potential matrix elements are con- structed from the adiabatic curves by imposing the requirement that they be smooth everywhere. A multi- state diabatic formulation is used to describe the scattering and the cross-sections for the collision-induced quenching of 0( ‘D) atoms arc calculated. The rate coefficient for the quenching of 0( ‘D) atoms by O(‘P) atoms is obtained as a function of temperature. At 1000 K, the value is I.0 x IO- I’ cm’ s ‘, ’ accurate probably to a factor of two. The theoretical rate coefficient is consistent with the empirical value inferred from upper atmosphere data. 1. INTRODUCTION Analysis of the altitude profile of the emission at 630 nm of the red line of atomic oxygen suggested that the O(‘D) atoms are quenched by collisions with O(‘P) atoms (Abreu et ul., 1986). The quenching occurs most probably by transitions between ‘IIy states of 02. The adiabatic potential energy curves (Saxon and Liu, 1977 ; Guberman, 1983) indicate the existence of an avoided crossing between the two lowest %,, states near 3.2a,, at which transitions may be probable at low collision energies. By constructing diabatic rep- resentations of the potential energy curves, diabatic coupling matrix elements may be derived. A diabatic formulation of scattering theory (Heil et al., 1981) may then be used to determine the quenching cross- sections. To carry out a precise construction of the diabatic matrix elements, data points are needed at small inter- vals. We calculated them for thelwo lowest ‘&, states, following the procedures of Guberman (1977, 1983). Additional calculations on the third %, state, which also separates to 0( ‘P) + 0( ‘D) did not indicate the presence of any further avoided crossings and we omitted the third state from the scattering formu- lation. However, the adiabatic potential energy curves of Saxon and Liu (1977) indicate not only the avoided crossing between the lower two TIy states, but also a weak avoided crossing with the third state. Accord- ingly in the scattering calculations based on the curves of Saxon and Liu (1977), we employed a three-state formation. 2. ADIABATIC POTENTIAL ENERGY CURVES The adiabatic wave functions and potential energy curves of the two lowest ‘I&, states of 0, were obtained using a [3s, 2p, Id] contracted Gaussian basis set and orbitals determined in earlier multiconfiguration self- consistent field (MCSCF) calculations on the ground state of 0, (Heil et cd., 1981). The ground state MCSCF calculations employed the 14 molecular spin eigenfunctions that result from a full CI in the valence space with the restriction that the IO and 20 orbitals remain fully occupied. With the ground state orbitals, first order configuration interaction (CI) wave func- tions were determined for the two ‘n, states. The CI wave functions were generated from a reference set consisting of the 10 configurations which result from a full CI prescription in the valence space of ‘Ilu sym- metry with the restriction that the la and 20 orbitals remain fully occupied. In the CI calculation only the lo orbitals were so restricted and excitations were 641