This book provides an introduction to the theory of atomic and molecular
collision processes which occur in the interstellar medium. It is
directed towards graduate students and research workers in the fields of
astrophysics and astrochemistry who wish to analyse and interpret
observations of the interstellar gas. The book will also be of interest
to physical chemists who seek a better appreciation of their discipline
to an important and expanding area of astronomy. The text deals
primarily with processes occuring in interstellar molecular clouds,
where the electron density is low and collisions with the abundant
neutral species, H, H2, and He, predominate. As the energies involved
are very small, the theory is necessarily based on the quantum
mechanical method with simpler, classical models being used to interpret
the results wherever possible. The compilation of sources of data
pertaining to the rotational excitation of interstellar molecules will
be helpful to radioastronomers interpreting mm-wave observations of
molecular clouds.