Alignment dependence of photoelectron angular distributions in the few-photon ionization of molecules by ultraviolet pulses

We probe the time-dependent ionization dynamics of impulsively excited rotational wave packets of N2, CO2, and C2H4 using broadband ultraviolet pulses centered at 262 nm. Photoelectron momentum distributions recorded by velocity-map imaging show a strong dependence on alignment, on multiphoton order, and on the electronic and vibrational states of the cation. We show that substantial information about the molecular-frame photoelectron angular distribution can be obtained from the high-order laboratory-frame asymmetry parameters without any prior knowledge of the photoionization process. We also compare few-photon ionization with one-photon ionization and strong-field ionization.