Introduction

Abstract

It was difficult to find a suitable title for this discussion, and perhaps I had better explain at the start why this particular title was chosen and what we shall try to discuss. We now know a good deal about the magnitude and nature of interatomic forces, and much of this knowledge has been acquired from the study of molecular spectra. Less is understood about the nature of intermolecular forces, although a number of experimental methods are being used to examine them. Among these methods, molecular spectroscopy again looks promising, probably because it is now possible to obtain a greater precision of measurement as regards the position and intensity of absorption bands or lines, so that small spectral variations which arise in different physical conditions can be detected. In this way, the effect of neighbour molecules upon an absorbing chromophore can be examined, and attempts can be made to interpret the interactions in terms of the appropriate energy levels. This is the leitmotif of our discussion.