5 Astrophysical applications

In the diffuse interstellar medium phosphorus, sulfur, and silicon, with ionization potentials of 11.0, 10.36, and 8.15 eV, respectively, exist primarily as ions. The radiative association process may become significant in diffuse clouds and possibly translucent clouds and shocked regions, as the ratio of [H]/[H₂] increases. Other competing processes in the formation of "XH⁺" are

$$X + {\rm H}_3^+ \to X{\rm H}^+ + {\rm H}_2$$ (5)

and

$$X^+ + {\rm H}_2 \to X{\rm H}^+ + {\rm H}.$$ (6)

The latter reaction is usually endothermic.

In Fig. 7 we compare the present calculations of the rate coefficient for formation of SiH⁺ with a prediction for the radiative association of Si⁺ with H₂ (Herbst et al. [1989]), and with equivalent mechanisms for forming the chemically similar molecule CH⁺. At low temperatures, our rate coefficients are a factor of two larger than the estimates of Turner & Dalgarno ([1977]), and are almost an order of magnitude larger than the radiative association rate coefficients of Si⁺ with molecular hydrogen suggesting that the radiative association with atomic hydrogen will be the initiating reaction for the silicon chemistry when [H]/[H₂] >0.1. At low temperatures, the C⁺and Si⁺ radiative association rates appear to be comparable.

Rate coefficients for SH⁺ and PH⁺ from this work are plotted together with other reaction rates for S⁺ and P⁺with H₂ in Fig. 8. For temperatures below about 500 K, radiative association of S⁺ and P⁺ with H appears to be approximately one to three orders of magnitude smaller than that predicted with H₂.

In dense clouds, where almost all the hydrogen exists as H₂, P, S, and Si may be significantly depleted from the gas phase onto grain surfaces. In such regions, the presence of molecules containing these second-row elements may have more to do with grain disruption processes than with gas-phase chemistry.

Acknowledgements

The work of P.C.S. was performed at Oak Ridge National Laboratory which is managed by Lockheed Martin Energy Research Corp. for the U.S. Department of Energy under Contract DE-AC05-96OR22464 and by the ORNL LDRD Seed Money Fund. The Deutsche Forschungsgemeinschaft (grant Bu 450/7-1) supported the work of J. P. G., G. H., and R. J. B.