# Resonant and Two-Step Processes

Chapter

## Abstract

Consider the angular distribution of photons emitted from an atomic discrete or autoionizing state excited by photon absorption (see Figure 4.1 for the scheme of the process under consideration): The processes in Figure 4.1 are often lumped together as

$$A({\alpha _0}{J_0}) + {\gamma _0} \to {A^ * }(\alpha J) + \gamma $$

*resonance scattering of photons (resonance fluorescence).*The second and the third ones, displayed in Figures 4.1(b) and 4.1(c), respectively, are processes of resonance inelastic scattering. In molecular optics, resonance inelastic scattering is known as*Raman scattering:*Figure 4.1(b) corresponds to emission of the red component, while Figure 4.1(c) corresponds to emission of the violet component in the Raman spectra. A rigorous treatment of the resonance scattering of photons is performed in the second-order perturbation theory of quantum electrodynamics. In our approach, the phenomenon is treated as a pure two-step process. Within the dipole approximation the tensor structure of the transition operator for both steps, excitation and decay, is the simplest, and all angular momenta involved in the phenomenon are fixed in an ideal case. As a result, the angular distributions and polarization can be exhaustively described only by methods of the angular momentum algebra without any dynamical evaluations.## Keywords

Angular Distribution Auger Electron Target Atom Stokes Parameter Scattered Photon## Preview

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© Springer Science+Business Media New York 2000