Suchergebnisse

The role of the nuclear degrees of freedom in nonlinear two-photon single ionization of H2 molecules interacting with short and intense vacuum ultraviolet pulses is investigated, both experimentally and theoretically, by selecting single resonant vibronic intermediate neutral states. This high selectivity relies on the narrow bandwidth and tunability of the pulses generated at the FERMI free-electron laser. A sustained enhancement of dissociative ionization, which even exceeds nondissociative ionization, is observed and controlled as one selects progressively higher vibronic states. With the help of ab initio calculations for increasing pulse durations, the photoelectron and ion energy spectra obtained with velocity map imaging allow us to identify new photoionization pathways. With pulses of the order of 100 fs, the experiment probes a timescale that lies between that of ultrafast dynamical processes and that of steady state excitations ; This research was supported by "Investissements d'Avenir" LabEx PALM (ANR-10-LABX-0039-PALM) and EquipEx ATTOLAB (ANR-11-EQPX-0005- ATTOLAB), as well as by the EU-H2020 Laserlab- Europe 654148. This work is supported by the ERC advanced Grant No. 290853—XCHEM—within the seventh framework program of the European Union. We also acknowledge the financial support from the MINECO Project No. FIS2016-77889-R and the European COST Action XLIC CM1204 and the computer time from the CCC-UAM and Marenostrum Supercomputer. A. P. acknowledges a Ramón y Cajal contract from the Ministerio de Economía y Competitividad (Spain). F. M. acknowledges support from the "Severo Ochoa" Program for Centres of Excellence in R&D (MINECO, Grant No. SEV-2016-0686) and the "María de Maeztu" Program for Units of Excellence in R&D (MDM-2014- 0377). D. D. and M. H. acknowledge support by Institut de Physique (CNRS). M. M. acknowledges support by the Deutsche Forschungsgemeinschaft (DFG) under Grants No. SFB925/A3 and CUI, No. DFG-EXC1074. We gratefully acknowledge the members of the FERMI team at Elettra-Sincrotrone Trieste whose work made this experiment possible

The toolbox for imaging molecules is well-equipped today. Some techniques visualize the geometrical structure, others the electron density or electron orbitals. Molecules are many-body systems for which the correlation between the constituents is decisive and the spatial and the momentum distribution of one electron depends on those of the other electrons and the nuclei. Such correlations have escaped direct observation by imaging techniques so far. Here, we implement an imaging scheme which visualizes correlations between electrons by coincident detection of the reaction fragments after high energy photofragmentation. With this technique, we examine the H2two-electron wave function in which electron-electron correlation beyond the mean-field level is prominent. We visualize the dependence of the wave function on the internuclear distance. High energy photoelectrons are shown to be a powerful tool for molecular imaging. Our study paves the way for future time resolved correlation imaging at FELs and laser based X-ray sources ; This work was funded by the Deutsche Forschungsgemeinschaft, the BMBF, the European Research Council under the European Union Seventh Framework Programme (FP7/2007-2013)/ERC grant agreement 290853 XCHEM, the MINECO projects FIS2013-42002-R and FIS2016-77889-R, and the European COST Action XLIC CM1204. All calculations were performed at the CCC-UAM and Mare Nostrum Supercomputer Centers. We are grateful to the staff of PETRA III for excellent support during the beam time. K.M. and M.M. would like to thank the DFG for support via SFB925/A3. A.K. and V.S. thank the Wilhelm und Else Heraeus-Foundation for support. J.L. would like to thank the DFG for support. S.K. acknowledges support from the European Cluster of Advanced Laser Light Sources (EUCALL) project which has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 654220. T.W. was supported by the U.S. Department of Energy Basic Energy Sciences under Contract No. DE-AC02-05CH11231. A.P. acknowledges a Ramón y Cajal contract from the Ministerio de Economa y Competitividad