Abstract
We present an overview of recent photoionization studies of transient hydride species of Groups III–VI, and the conclusions drawn from these studies. The transient species investigated include BH 3, C 2H 3, SiH n( n = 1–3) PnH n (Pn = N, P, As; n = 1,2), and ChH (Ch = O, S, Se). Photodissociative ionization of certain stable species, e.g. B 2H 6, SiH 4, PnH 3 and ChH 2 inevitably become part of the investigation. From the various dissociative and adiabatic ionization thresholds, we determine a variety of useful thermochemical data, which either help to resolve controversies or provide new experimental information. These include (a) the dimerization energy of borane, (b) the C-H bond energy in ethylene, (c) the incremental bond energies in SiH n PnH n and ChH n , and (d) the 3 B 1− 1 A 1 splitting in SiH 2, NH 2 +, PH 2 + and AsH 2 +. These results provide experimental tests of recent ab initio calculations. They also reveal the limitations of the semiempirical Goddard-Harding model for PnH n and ChH n Perhaps the most interesting structural information is a consequence of the interpay between our experiments and ab initio calculations. Such comparisons lead to the conclusion that the most stable structure of B 2H 5 + is a triple hydrogen bridge, that the ground state of vinyl cation has a very different structure from that of vinyl radical, and is probably the non-classical bridged structure, while the excited triplet state has a more classical ethylene-like structure. Also, the ground state of SiH 4 + is highly distorted from a tetrahedral structure. En passant, the methods of preparation of the transient species and the dynamical behavior of the photoionization process in selected cases will be mentioned.
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