We report an experimental and theoretical study of very low-energy photoelectrons in tunneling ionization process from noble gas atoms interacting with ultrashort intense infrared laser pulses. A universal peak structure with electron energy well below 1 eV in the photoelectron spectrum, corresponding to the double-hump structure in the longitudinal momentum distribution, is identified experimentally for all atomic species. Our quantum and semiclassical analysis reveal the role of long-range Coulomb potential in the production of this very low-energy peak structure.
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.
Providing a possible connection between neutrino emission and gravitational-wave (GW) bursts is important to our understanding of the physical processes that occur when black holes or neutron stars merge. In the Daya Bay experiment, using the data collected from December 2011 to August 2017, a search has been performed for electron-antineutrino signals coinciding with detected GW events, including GW150914, GW151012, GW151226, GW170104, GW170608, GW170814, and GW170817. We used three time windows of ±10 s, ±500 s, and ±1000 s relative to the occurrence of the GW events, and a neutrino energy range of 1.8 to 100 MeV to search for correlated neutrino candidates. The detected electron-antineutrino candidates are consistent with the expected background rates for all the three time windows. Assuming monochromatic spectra, we found upper limits (90% confidence level) on electron-antineutrino fluence of (1.13 − 2.44)×10 11 cm −2 at 5 MeV to 8.0×10 7 cm −2 at 100 MeV for the three time windows. Under the assumption of a Fermi-Dirac spectrum, the upper limits were found to be (5.4 − 7.0)×10 9 cm −2 for the three time windows.
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