Knowledge Bank

Experimental measurements of line $shapes1$ have shown deviations from the expected Lorentz shape. These deviations can be explained, within experimental error, by the process of multiple absorption and re-emission of photons, some of which eventually find their way to the detector. This has the effect of lessening the measured absorption, especially near line center. Therefore, the integrated absorption and the measured half-width are both less than would ordinarily be expected. A computer has been programmed to synthesize possible photon life histories, using Monte Carlo techniques. Various assumptions have been tested for such parameters as the angular and frequency distribution of the re-emitted photons and the conditions of reflection or absorption at the walls of the absorption cell. The best fit to the experimental curves was obtained by assuming that the photons were occasionally emitted with spatial coherence. This research was supported, in part, by N.A.S.A. Research Grant NsG-539. $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}$ W.G. Herget, et al., J. Opt. Soc. Am. 52, 1108 and 1112 (1962).