Sequential double ionization of molecules by strong laser fields simulated with time-dependent configuration interaction

A new time-dependent configuration interaction method has been developed for simulating strong field sequential double ionization of molecular systems. Ionization of the neutral is simulated by time-dependent configuration interaction with single excitations (TD-CIS) and an absorbing boundary. At each time step, the ionized part of the wavefunction from the TD-CIS calculation is transferred to a second time-dependent configuration interaction simulation for ionization of the cation to the dication. The second simulation uses a CISD-IP wavefunction that consists of singly ionized configurations and singly excited, singly ionized configurations (TD-CISD-IP). The transfer between the TD-CIS and TD-CISD-IP simulations is accomplished by partitioning the first ionization rate into contributions from individual orbitals or by singular value decomposition of the absorbed wavefunction. Sequential double ionization simulations have been carried out for HBr in five cycle 800 nm linearly polarized pulses and HI (with spin-orbit coupling) in four cycle 800 nm circularly polarized pulses, with intensities chosen so that the population of the neutral was depleted by the mid-pulse. The singular value decomposition of the cation produced by the first ionization is dominated by a single component for the two orientations considered. The population of the cation rises and then falls as it is ionized to the dication. Depending on the pulse shape and field strength, the ionization of the cation to the dication can continue for several half cycles. For HI with circularly polarized light, the rates for both the first and second ionization peak when the electric field is aligned with the p(pi) orbital.

Automated summary

A new time-dependent configuration interaction method has been developed for simulating strong field sequential double ionization of molecular systems. The process involves simulations of neutral ionization and cation to dication ionization using TD-CIS and TD-CISD-IP methods respectively, with transfer between the two achieved through partitioning ionization rates or singular value decomposition. The population of the cation rises and falls as it is ionized, with ionization continuing for several half cycles depending on pulse shape and field strength. The singular value decomposition of the cation dominates for both orientations considered, with ionization rates peaking when the electric field is aligned with the p(pi) orbital.