The emission of highly charged ions from Xe clusters exposed to intense extreme ultraviolet laser pulses (λ ∼ 52 nm) from the free electron laser in Japan was investigated using ion momentum spectroscopy. With increasing average cluster size, we observed multiply charged ions Xe z+ up to z = 3. From kinetic energy distributions, we found that multiply charged ions were generated near the cluster surface. Our results suggest that charges are inhomogeneously redistributed in the cluster to lower the total energy stored in the clusters.
We have investigated the high-order above-threshold ionization (HATI) spectrum of formic acid by means of high-resolution electron spectroscopy using 800-nm femtosecond laser light. The resonance-like enhancement structures in the HATI spectrum are shown to be experimentally reproducible and three enhancement-suppression patterns are observed in different energy regimes with increasing laser intensity. We simulated these patterns successfully by computing the exact solution of time-dependent Schrödinger equation based on the simple hydrogen-like atom, subject to a time-dependent high-intensity laser pulse. The agreement between experiment and theory shows that "channel closing" effects play an important role in the observed resonance-like enhancements (RLEs). The existence of the RLEs for formic acid, which lacks any symmetry, shows that molecular symmetry does not affect the RLEs.
We investigate the resonance-like enhancement (RLE) in high-order above-threshold ionization (ATI) spectra of the polyatomic molecules C2H4 and C2H6. In the spectrum-intensity maps, strong and weak RLE areas emerge alternatively for both C2H4 and C2H6 but in di↵erent sequences. Theoretical calculations using the strong field approximation reproduce the experimental observation and analysis shows that the di↵erent characteristics of the two molecules can be attributed to interference e↵ects of molecular orbitals with di↵erent symmetries. For C2H4, the RLE structures are attributed to CC centers of the HOMO orbital. For C2H6, in contrast, the CC centers of the HOMO and HOMO-1 orbitals do not contribute to the RLE due to destructive interference but the hydrogen centers of the bonding HOMO-1 orbital give rise to the multiple RLE regions. Our work, for the first time, reveals the important role of low-lying orbitals and the di↵ering roles of di↵erent atomic centers in the high-order ATI spectrum of molecules.
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