Momentum distributions of symmetric (H₂⁺) and asymmetric (HeH²⁺) molecular ions in a circularly polarized laser field under different ionization mechanisms

By numerically solving the two-dimensional (2D) time-dependent Schrödinger equation (TDSE), we present photoelectron momentum distributions (PMDs) and photoelectron angular distributions (PADs) of symmetric (H₂⁺) and asymmetric (HeH²⁺) molecular ions in circularly polarized (CP) laser pulses. By adjusting the laser wavelength, two circumstances of resonance excitation and direct ionization were considered. The ionization mechanism of the resonance excitation was mainly investigated. The results show that the PMDs of H₂⁺ and HeH²⁺ in the y-direction gradually increase with increasing intensity, and the number of PMDs lobes is in good agreement with the results predicted by the ultrafast ionization model. In the resonance excitation scenario, the PMDs of H₂⁺ are dominated by two-photon ionization, whereas the PMDs of HeH²⁺ are a combination of three-photon ionization and two-center interference. Furthermore, the PMDs of HeH²⁺ are stronger in the resonance excitation scenario than those of H₂⁺, which can be explained by the time-dependent population of electrons. In addition, the molecular structure was clearly imprinted onto the PMDs.