1 Introduction

Molecule formation processes involving second-row elements are of considerable interest, as searches for new molecular species in a variety of interstellar and circumstellar environments continue. Phosphorus-, sulfur- and silicon-containing molecules have been observed and identified, but no diatomic hydrides of these elements, or their associated ions, have been detected to date (Millar & Hobbs [1988]; Magnani & Salzer [1989], [1991]). Other diatomic hydrides, such as OH (which is chemically similar to SH), and diatomic hydride ions, such as CH⁺ (which is chemically similar to SiH⁺) have been observed in considerable abundance. Among the formation mechanisms for SiH⁺, PH⁺, and SH⁺, the process of radiative association should be included.

Using ab initio molecular potential energy curves and transition moments, we have carried out calculations for the radiative association of X⁺ (X = Si, P, or S) with H:

$$X^+ + {\rm H} \to X{\rm H}^+ + \nu .$$ (1)

We present cross sections as a function of collision energy and give rate coefficients over a range of temperatures from 10 to 10 000 K. We compare our rates for radiative association of X⁺ with H to other competing processes for the formation of XH⁺ in interstellar clouds.