Nonsequential double ionization of aligned molecules by few-cycle laser pulses at low intensity

Abstract

Using the three-dimensional classical ensemble model, nonsequential double ionization (NSDI) of aligned molecules by the few-cycle laser pulse at the low intensity is investigated. Here the two electrons involved in NSDI finally are ionized through a transition doubly excited state induced by the recollision. The results show that the electron correlation behavior in NSDI is strongly dependent on the molecular alignment and the carrier-envelope phase (CEP) of the laser pulse. There are more anti-correlated emissions for the perpendicular molecules than those for the parallel molecules regardless of CEP. The dependence of the electron correlation behavior on molecular alignment can be well explained by the potential energy curves of molecules. That is because the suppressed potential barrier for perpendicular molecules is higher and the electron is more difficult to ionize than for parallel molecules. Thus for perpendicular molecules the ionization of the two electrons has longer time delay, which results in more anticorrelated emissions. Additionally, because the potential barrier for the perpendicular molecules is higher than that for the parallel molecules, the ionization yield of NSDI is about an order of magnitude smaller than that for the parallel molecules. With CEP increasing from 0 to , the anti-correlated emission first increases and then decreases. For parallel alignment, the correlated emission is always dominant at all CEPs. However, for perpendicular alignment, the dominant correlation behavior depends on the CEP of the laser pulse. When the CEP is in a range from 0.3 to 0.7, the anti-correlated emission is dominant. At other CEPs, the correlated emission is dominant. The dependence of the electron correlation behavior on the CEP of the laser pulse is well explained by the dependence of the returning energy of the electron on the CEP of the laser pulse. For different CEPs, the single ionization times resulting in NSDI and the corresponding acceleration electric field are different, which leads to at some CEPs the returning energy of the electron being large and at some other CEPs the returning energy of the electron being small. When those CEPs are available where the returning energy of the electron is larger, the doubly excited state induced by the recollision is more energetic. Thus at those CEPs the emissions of the two electrons from the doubly excited state have smaller time delays and more correlated emissions occur. On the contrary, at those CEPs where the returning energy of the electron is small, more anti-correlated emissions are produced.