Alignment-dependent photoelectron angular distributions in few-photon ionization of molecules by ultra-violet pulses

We used velocity map imaging to investigate the time-dependent ionization dynamics of impulsively-excited rotational wave packets of N₂, CO₂, and C₂H₄ probed by broadband UV pulses (≈ 262 nm). Photoelectron momentum distributions show a strong dependence on alignment, on multiphoton order, and on the electronic and vibrational states of the cation. We show that, without prior knowledge of the photoionization process, partial but substantial information about the molecular-frame photoelectron angular distributions can still be retrieved from the highly-anisotropic laboratory-frame data using a fitting algorithm. We also compare few-photon ionization with single-photon ionization and strong-field ionization. In many cases, we found similarities between the angle-dependent ionization rates by few-photon and by a strong field. On the other hand, the photoelectron angular distributions in few-photon ionization are very sensitive to molecular structure and dynamics, which is similar to single-photon ionization.