Parity-mixing interference in laser-assisted photoionization
Photoionization of atoms by high-order harmonics in the presence of a laser may lead to quantum interference from which information about the photoionization dynamics or the light fields can be extracted. Traditionally, this interference arises from two-photon transitions involving the absorption of consecutive harmonics combined with the absorption and stimulated emission of a laser photon. In this process, parity is conserved. Here, we investigate interference between one- and two-photon transitions in helium using high-order harmonics generated by a few-cycle laser and three-dimensional electron detection. In this case, parity is not conserved. We identify four parity-mixing interference pathways, involving two different harmonic fields or a single harmonic, together with absorption or emission of a probe photon.