The effect on the spectrum of a molecule of the environment in which it is located depends upon the changes which the surroundings produce in the electronic, vibrational, rotational and nuclear energies of the upper and lower states of the molecule. Studies of the influence of environment in the gaseous, liquid or solid states can thus be made by any of the appropriate techniques listed in table 1, and it is clearly desirable in studying any one system to use as many different techniques as possible. A basic difference between the effect of environment on electronic and vibrational energy levels arises from the very much greater overlap of electron density with the environment that results from electronic excitation. Hence while for the consideration of changes which arise in the vibrational spectrum it is adequate to consider only the distortion of the curve relating the interaction energy to the intermolecular distance in the electronic ground state, for electronic spectra, however, the changes in the potential curves in both upper and lower states must clearly be taken into account. Collisions between molecules in gases lead to the broadening of rotational energy levels, and much useful information on intermolecular force fields has resulted from observations on pressure broadening of pure rotational lines in the microwave region. Both self-broadening and broadening by different foreign gases have been studied as well as the dependence of line halfwidth Δ v 1/2 on the rotational quantum numbers J and K (Townes & Schawlow 1955).