Low-Energy Photoelectron Angular Distributions of Above-Threshold Ionization of Atoms and Molecules in Strong Laser Fields

We present high-resolution fully differential experimental data on low-energy photoelectron angular distributions (PADs) of above-threshold ionization of atoms (Xe), and diatomic (H
$_2$
, O
$_2$
) and triatomic (CO
$_2$
) molecules at the same Keldysh parameter (∼1.4) in near-infrared strong laser fields (25 fs, 795 nm). We observe that the electron zero longitudinal momentum distributions (along laser-polarization direction) show a feature of pronounced “dip” or “peak” for those targets. However, the low-energy PADs are similar, even though the binding energy and electronic core structure are very different. The PADs show that the dominant angular momentum of zero-order above-threshold ionization (E < 0.5 eV) is L = 4 (ng Rydberg states). The experimental observations challenge the current qualitative models that are now only available for atoms. In the end, we discuss the role of Coulomb focusing and the effect of the electron longitudinal momentum on the transverse-momentum distribution.