This book is primarily intended for graduate chemists and chemical
physicists. Indeed, it is based on a graduate course that I give in the
Chemistry Depart- ment of Southampton University. Nowadays undergraduate
chemistry courses usually include an introduction to quantum mechanics
with particular reference to molecular properties and there are a number
of excellent textbooks aimed specifically at undergraduate chemists. In
valence theory and molecular spectroscopy physical concepts are often
encountered that are normally taken on trust. For example, electron spin
and the anomalous magnetic moment of the electron are usually accepted
as postulates, although they are well understood by physicists. In
addition, the advent of new techniques has led to experimental
situations that can only be accounted for adequately by relatively
sophisticated physical theory. Relativis- tic corrections to molecular
orbital energies are needed to explain X-ray photo- electron spectra,
while the use oflasers can give rise to multiphoton transitions, which
are not easy to understand using the classical theory of radiation. Of
course, the relevant equations may be extracted from the literature,
but, if the underlying physics is not understood, this is a practice
that is at best dissatisfy- ing and at worst dangerous. One instance
where great care must be taken is in the use of spectroscopically
determined parameters to test the accuracy of elec- tronic wave
functions.