A review of the fundamental concepts and models which underpin the spectroscopic study of gases and liquids, first published in 1994.Spectroscopic studies reveal a wealth of information about the rotational and vibrational behaviour of the constituent molecules of gases.Spectroscopic studies reveal a wealth of information about the rotational and vibrational behaviour of the constituent molecules of gases.Spectroscopic studies can reveal a wealth of information about the rotational and vibrational behavior of the constituent molecules of gases and liquids. This book reviews the fundamental concepts and models that underpin such studies. Throughout, discussion of the various quantum mechanical and semiclassical theories is interwoven with analysis of experimental results.Introduction; 1. Rotational relaxation; 2. Orientational relaxation in dense media; 3. Transformation of isotropic scattering spectra; 4. Quantum theory of spectral collapse; 5. Rotational relaxation. Kinetic and spectral manifestations; 6. Impact theory of orientational relaxation; 7. Rotation and libration in a fluctuating cell; Appendices; References. ...an excellent monograph for research workers in one of the more abstruse and difficult areas of physical chemistry. Robert R. Sharp, Journal of the American Chemical Society