Circular dichroism in photoelectron angular distributions as a probe of atomic and molecular alignment

In this paper we show that circular dichroism in photoelectron angular distributions (CDAD) can be used to probe atomic and molecular alignment in the gas phase. Careful choice of photon (left or right circularly polarized) propagation and photoelectron collection directions breaks the cylindrical symmetry of the target, giving rise to dichroic effects. CDAD exists in the electric dipole approximation. We illustrate the sensitivity of CDAD to alignment by considering photoionization of the A 2Σ+ state of NO. Most of the cases of alignment we consider are created by multiphoton absorption while the others, more general, might be created in fragmentation, desorption, etc. The alignment created by n-photon absorption quickly reaches a classical limit which is reflected in the CDAD spectrum. Finally, we show that CDAD is also a sensitive probe of gas phase atomic state alignment by considering photoionization of the 7P3/2 state of cesium created by single photon absorption from the ground state.