Fully differential studies for two-photon non-sequential double ionization of helium using a reaction microscope at FLASH
Final Report Abstract
Direct two-photon double ionization (TPDI) of helium at photon energy of around 39.4−54.4 eV represents the most fundamental two-photon two-electron non-linear process and detailed studies are indispensable for the understanding of more complex non-linear optics. Besides that, TPDI has been, and still is, an outstanding quantum mechanical problem and several issues remain unsolved. The main objective of the project is to perform fully differential studies for direct TPDI of helium using a reaction microscope (ReMi) at FLASH (the Free electron LASer in Hamburg). This requires the coincident detection of the two emitted electrons and recoil-ion He2+. The data will provide complete information for electron emission and correlation dynamics of TPDI and therefore serve as a benchmark for theoretical approaches. The main challenge arises from the very small cross section (10-52 cm4s) for TPDI of He. Thus, in order to achieve the goal, we improved our experimental setup by using state-of-the-art data acquisition with corresponding software readout, construction of a non-evaporable getter pump to reach a residual gas pressure as low as 5 ⋅10-12 mbar, and by improving the beam focussing. With a multi-layer mirror a sufficiently high intensity (∼10-14 W/cm2, focusing spot size of 10 µm) has been achieved at a photon flux that is about a factor of ten smaller compared to the focussing mirrors available at FLASH. As natural extension we implemented a segmented multi-layer mirror that, in combination with the ReMi, enables XUV pump-probe experiments with femtosecond resolution for time-resolved molecular reactions or photochemistry. A big step towards the proposed goal are first successful measurements of recoil-ion momentum spectra for TPDI of He at 44 eV and 52 eV, which provide the first differential measurements and uncover the electron emission dynamics for direct TPDI and “virtual” sequential ionization, respectively. Unfortunately, mainly due to the limited effective repetition rate of FLASH (about 300 Hz at present) the ultimate goal - fully differential measurements for TPDI of He - could not be reached under the current experimental conditions. However, as a result of recent improvements achieved at FLASH, we are confident that such measurements will become possible in the very near future. Instead of that, other ground-breaking results have been collected. Among those are few-photon induced multi-ionization of Ne and Ar, fully differential measurements for sequential two-photon double ionization of Ne, two-photon double ionization of D2, and multiple ionization of N2 molecules. The very first fully differential measurement for sequential TPDI of Ne shows that the two electrons are correlated via the intermediately populated and polarized Ne+ state. The angular emission characteristics of the first electron depends on the ejection angle of the second one such that it “knows” whether or not the second electron will be ionized questioning the simple picture of a “step-wise”, time-sequenced, sequential ionization and possibly pointing to an angular entanglement of both electrons. Fully differential measurements on N2 at 53 eV are additionally highlighted results (still under analysis). Measuring the emitted electron by coincidence will allow time-dependent “imaging of molecules from within”. As an extension of the original proposal first time-resolved XUV pump-probe measurements have been performed using a novel split-mirror setup. Demonstrated by a proof-of-principle measurement, where we observed the vibrational motion of the nuclear wave packet (22 fs oscillation period) in the D2+ state, the potential possibilities to directly measure absolute cross sections for direct TPDI of D2 have been worked out. Present experimental approach provides powerful application for timeevolution of chemical reactions. Time-evolution for isomerization of acetylene cation, as a benchmark chemical reaction, was explored for the first time. The observed isomerization time of 52 fs for the A2 Σg+ state confirmed experimentally that isomerisation is responsible for the ultra-fast non-radiative A2 Σg+ relaxation dynamics closing a debate over 40 years originally posted by G. Herzberg. Further, extremely surprising is the experimental observation for the formation of H2+ from linear acetylene molecule, which has never been reported before. These first results show the potential possibilities to control and to manipulate photoinduced reactant-to-product dynamics in basic molecular reactions, and they illuminate the perspectives of this prospering field for studies of conical intersections, vibronic couplings, intra-molecular energy flow, vibrational mode readjustments, breaking and making of molecular bonds, and redistribution of electronic energy surfaces involved in the process of molecular rearrangement.
Publications
- “Recoil-Ion Momentum Distributions for Two-Photon Double Ionization of He and Ne by 44 eV Free-Electron Laser Radiation”, Phys. Rev. Lett. 101, 073003 (2008)
A. Rudenko L. Foucar, M. Kurka, Th. Ergler, K. U. Kühnel, Y. H. Jiang, A. Voitkiv, B. Najjari, A. Kheifets, S. Lüdemann, T. Havermeier, M. Smolarski, S. Schössler, K. Cole, M. Schöffler, R. Dörner, S. Düsterer, W. Li, B. Keitel, R.Treusch, M. Gensch, C. D. Schröter, R. Moshammer and J. Ullrich
- “Few-Photon Multiple Ionization of N2 by EUV Free-Electron Laser Radiation”, Phys. Rev. Lett. 102,123002 (2009)
Y. H. Jiang, A. Rudenko, M. Kurka, K. U. Kühnel, Th. Ergler, L. Foucar, M. Schöffler, S. Schössler, T. Havermeier, M.Smolarski, K. Cole, R. Dörner, S. Düsterer, R. Treusch4, M. Gensch, C. D. Schröter R. Moshammer, J. Ullrich
- “Differential cross sections for non-sequential double ionization of He by 52 eV photons from the Free Electron Laser in Hamburg, FLASH” New J. Phys. B 12, 073035 (2010)
M Kurka, J. Feist, D.A. Horner, A. Rudenko, Y.H. Jiang, K.U. Kühnel, L. Foucar, T. N. Rescigno, C.W. McCurdy, R. Pazourek, S. Nagele, M. Schulz, O. Herrwerth, M. Lezius, M.F. Kling, M. Schöffler, A. Belkacem, S. Düsterer, R. Treusch, B.I. Schneider, L.A. Collins, J. Burgdörfer, C.D. Schröter, R. Moshammer and J. Ullrich
- “Tracing direct and sequential two-photon double ionization of D2 in femtosecond EUV laser pulses“, Phys. Rev. A 81 021401(R) (2010)
Y.H. Jiang, A. Rudenko, J. Perez, L. Foucar, M. Kurka K.U. Kühnel, Th. Ergler, J. F. Pérez- Torres, F. Martín, O. Herrwerth, M. Lezius, M.F. Kling, J. Titze, T. Jahnke, R. Dörner, J. L. Sanz-Vicario, M. Schöffler, J. van Tilborg, A. Belkacem, K. Ueda, T. J. M. Zouros, S. Düsterer, R. Treusch, C.D. Schröter, R. Moshammer, and J. Ullrich
- ”Investigating two-photon double ionization of D2 by XUV-pump-XUV-probe experiments”, Phys. Rev. A 81, 051402(R) (2010)
Y.H. Jiang, A. Rudenko, J. F. Pérez-Torres, O. Herrwerth, L. Foucar, M. Kurka, K.U. Kühnel, M. Toppin, E. Plésiat, F. Morales, F. Martín, M. Lezius, M.F. Kling, T. Jahnke, R. Dörner, J. L. Sanz-Vicario, J. van Tilborg, A. Belkacem, M. Schulz, K. Ueda, T. J. M. Zouros, S. Düsterer, R. Treusch, C.D. Schröter, R. Moshammer, and J. Ullrich
- ”Ultrafast Extreme Ultraviolet Induced Isomerization of Acetylene Cations“, Phys. Rev. Lett. 105, 263002 (2010)
Y.H. Jiang, A. Rudenko, O. Herrwerth, L. Foucar, M. Kurka, K.U. Kühnel, M. Lezius, M.F. Kling, J. van Tilborg, A. Belkacem, K. Ueda, S. Düsterer, R. Treusch, C.D. Schröter, R. Moshammer, and J. Ullrich
