Multipole moments and trap states in forward scattering of resonance light

We perform nonperturbative analysis of forward scattering of resonance laser light on atoms in magnetic field when the laser drives transitions between states with total angular momenta F=1,2 and study the role of the induced higher-order multipole moments in the forward-scattering signal. It is shown how the multipole moments affect these signals and why not all possible multipoles are revealed for some transitions. The analysis is performed in terms of density-matrix formalism and in terms of coupled and uncoupled (trap) states. Evolution of the trap states in a magnetic field is found to be responsible for specific dependencies of the forward-scattering signals on the multipoles of various ranks. The general theory is applied to the case of the Na D₁ line. We also present a pump-probe method for studying multipoles in systems with states of high angular momenta and with complex transitions overlapping within their Doppler width. {} 1996 The American Physical Society.