Angular momentum dependence in multiphoton ionization and attosecond time delays

Multiphoton interference measurements in attosecond physics, e.g., the reconstruction of attosecond beating by interference of two-photon transitions (RABITT), are the typical methods of choice to experimentally access the photoionization delay in atoms and molecules. The exact relation between the measurable multiphoton delays and the theoretical single-photon delays is typically modeled by correction terms, continuum-continuum delays, obtained from a high-energy limit of the theory. However, these are unreliable at photoelectron kinetic energies smaller than about 10 eV, and do not have photoemission angular dependence. In this work we develop an accurate analytic alternative that gives accurate correction terms even at very low energies. Our method is computationally very straightforward, predicts correct multiphoton photoelectron angular distributions as well as the expected angular dependence of the continuum-continuum delay. We validate the approach theoretically against state-of-the-art ab initio calculations as well as experimentally with a two-harmonic RABITT setup, which properly separates different higher-order multiphoton pathways and offers a promising way of analyzing congested molecular photoionization spectra.