Interpretation of collision broadening and shift in atomic spectra

The classical theory of ‘foreign gas’ broadening, due to Lindholm (1945) and Foley (1946) is used to evaluate the constants C6 and C12 of the Lennard-Jones potential C12r-12 - C6r-6 from measurements of collision broadening and shift due to rare gases at pressures of less than one atmosphere. The values of C6 so obtained are of the expected order of magnitude, and where measurements on one spectral line broadened by a number of rare gases are available (Smith 1966), are found to be approximately proportional to the polarizability of the rare gas. This behaviour is also exhibited by the values obtained from elastic-scattering cross sections. The constants C12 for the short range, repulsive part of the potential are discussed, and their values for a number of pairs of atoms in which atleast one member of the pair is a rare gas are found to fit an empirical formula C12 = qR12, where q = (0·9 ± 0·3) x 10-16 erg and R is the sum of the ‘atomic radii’ of the two atoms, where the atomic radius is defined as the distance from the nucleus at which the radial charge density is 0·012 atomic units. The values of C12 extend over more than 8 orders of magnitude, and include estimates made from studies of the equation of state of rare gases, and of elastic scattering cross sections, as well as of collision broadening and shift.