Refined theoretical study of radiative association: Cross sections and rate constants for the formation of SiN

Abstract

Radiative association of silicon mononitride (SiN) in its two lowest molecular electronic states is studied through quantum and classical dynamics. Special attention is paid to the behavior of the cross section at high collision energies. A modified expression for the semiclassical cross section is presented which excludes transitions to continuum states. This gives improved agreement with quantum mechanical perturbation theory at high energies. The high energy cross section is overestimated if conventional semiclassical theory is used. The modified semiclassical theory should be valid in general for radiative association transitions from an upper to a lower electronic state. We also implement a quantum dynamical optical potential method with the same type of modification. The rate coefficient is calculated using Breit-Wigner theory and the modified semiclassical formula for the resonance and direct contributions, respectively, for temperatures from 10 K to 20,000 K. A rapid decrease in the rate constant for formation of ground state SiN is observed above 2000 K which was not seen previously.

Quadrupole association and dissociation of hydrogen in the early Universe

“…The sharp resonances due to long-lived quasibound states are clearly evident. In virtually all previous work on RA, extremely narrow resonances of this kind have been neglected, either because they are difficult to resolve using a grid-based method or because of an assumed breakdown of perturbation theory [18][19][20]. Recently, it was shown [9] that such resonances increased the LTE rate constant for SiO formation by ∼ 100 times compared to calculations where the resonances were neglected.…”

“…Because the source function (22) is derived for conditions of statistical equilibrium, it does not contribute to the matrix elements in the steady-state equations (19) and (20). These matrices are then obtained from the Planck function and the Maxwell-Boltzmann factor arising from elastic collisions and may be written as…”

Formation of the BeH+ and BeD+ Molecules in Be+ + H/D Collisions Through Radiative Association

“…The fitting parameters are summarized for the formation of BeH+ and BeD + in Table 1 according to different temperature ranges. These rate constants will be made available in the database KIDA (Gustafsson et al, 2012;Wakelam et al, 2012).…”

Dicarbon Formation in Collisions of Two Carbon Atoms

“…where k B is Boltzmann's constant. The complicated resonance structures make it challenging to calculate accurately the rate coefficient using numerical integration (Bennett et al 2003;Gustafsson et al 2012). As a guide, the relationship vσ QM i→f , where v = 2E/µ, is used to designate an effective energy-dependent rate coefficient, where R(E) for a transition from state i to state f is given by,…”