Dissociative multi-ionization of NO molecules in strong femtosecond laser field.

Multi-ionization and subsequent Coulomb explosion (CE) of the NO molecule irradiated by a linearly polarized 800 nm laser field is investigated by a reaction microscope, where a number of CE channels of NO with q ≤ 5 for two-body fragmentation and q ≤ 8 for three-body fragmentation were observed. For two-body CE, by analyzing the internuclear separations extracted from kinetic energy releases (KERs), dissociation branching fractions, and laser intensity dependence, interestingly, we found that fragmentation NO → N + NO is produced directly from dissociating NO via non-sequential stairstep ionization, whereas most of the others result from the sequential stairstep ionization. For three-body CE, 25 fragmentation channels of NO (q = 3-8) are distinguished in the present charge-encoded multi-photoion coincidence plot, and the concerted fragmentation mechanism is nominated in a typical Dalitz plot. With the help of the numerical computation with the measured KERs and momentum correlation angles, the geometric structures of molecular ions prior to fragmentation are reconstructed, which display the bending motion and simultaneous two-bond stretching before the CE. Increasing of the bond length for high charged NO indicates the dominating stairstep ionization in the three-body fragmentation.