Lorentzian Line Crossings in Forward-Scattered Light

The zero-field line crossing of the mercury isotope ${}_{}{}^{204}{}_{}{}^{}\mathrm{Hg}$ was studied in forward-scattered light, with the scattering atoms both in a foreign gas and in vacuum. With sufficient foreign-gas pressure, a Lorentzian shape was obtained, in contrast to the Gaussian shape found for atoms in vacuum. An approximate theoretical treatment gives a field dependence of the forward-scattered intensity in satisfactory agreement with the measurements. Line crossings of Zeeman components of different isotopes, contained in separate cells, are also discussed.