Theory of Atomic Photon-Particle Coincidence Measurements

Abstract

The theory of measurements in which photons are detected in delayed coincidence with a scattered particle is developed in a form specifically applicable to atomic collisions. Equations are obtained which relate the measured coincidence rate to excitation amplitudes. These equations incorporate the polarization of the radiation, the fine and hyperfine structure of the atomic levels, the coherence of the radiation, and the time dependence of the radiation intensity. A semiclassical model for certain transitions is introduced to illustrate new features of coincidence measurements. The $\mathrm{Ly}-\alpha$ transitions in hydrogen and ${}_{}{}^1{}_{}{}^{}P\to {}_{}{}^1{}_{}{}^{}S$ transitions in He are treated in detail to illustrate the general theory.