Control of H2 dissociative ionization in the nonlinear regime using vacuum ultraviolet free-electron laser pulses
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