title: High resolution multiphoton spectroscopy by a tunable free-electron-laser light.
abstract: Seeded free electron lasers theoretically have the intensity, tunability, and resolution required for multiphoton spectroscopy of atomic and molecular species. Using the seeded free electron laser FERMI and a novel detection scheme, we have revealed the two-photon excitation spectra of dipole-forbidden doubly excited states in helium. The spectral profiles of the lowest (-1,0)(+1) (1)S(e) and (0,1)(0) (1)D(e) resonances display energy shifts in the meV range that depend on the pulse intensity. The results are explained by an effective two-level model based on calculated Rabi frequencies and decay rates.
authors.affiliation: Jožef Stefan Institute, Jamova cesta 39, SI-1000 Ljubljana, Slovenia and Faculty of Mathematics and Physics, University of Ljubljana, Jadranska 19, SI-1000 Ljubljana, Slovenia.
authors.forename: M
authors.initials: M
authors.lastname: Žitnik
authors.affiliation: Jožef Stefan Institute, Jamova cesta 39, SI-1000 Ljubljana, Slovenia.
authors.forename: A
authors.initials: A
authors.lastname: Mihelič
authors.affiliation: Jožef Stefan Institute, Jamova cesta 39, SI-1000 Ljubljana, Slovenia.
authors.forename: K
authors.initials: K
authors.lastname: Bučar
authors.affiliation: Jožef Stefan Institute, Jamova cesta 39, SI-1000 Ljubljana, Slovenia.
authors.forename: M
authors.initials: M
authors.lastname: Kavčič
authors.affiliation: Uppsala University, Department of Physics and Astronomy, P.O. Box 530, S-75121 Uppsala, Sweden.
authors.forename: J-E
authors.initials: JE
authors.lastname: Rubensson
authors.affiliation: Uppsala University, Department of Physics and Astronomy, P.O. Box 530, S-75121 Uppsala, Sweden.
authors.forename: M
authors.initials: M
authors.lastname: Svanquist
authors.affiliation: Uppsala University, Department of Physics and Astronomy, P.O. Box 530, S-75121 Uppsala, Sweden.
authors.forename: J
authors.initials: J
authors.lastname: Söderström
authors.affiliation: Uppsala University, Department of Physics and Astronomy, P.O. Box 530, S-75121 Uppsala, Sweden and University of Gothenburg, Department of Physics, SE-412 96 Gothenburg, Sweden.
authors.forename: R
authors.initials: R
authors.lastname: Feifel
authors.affiliation: MAX IV Laboratory, Lund University, P.O. Box 118, 22100 Lund, Sweden.
authors.forename: C
authors.initials: C
authors.lastname: Såthe
authors.affiliation: Technische Universität Berlin, Institut für Optik und Atomare Physik, Eugene-Wigner-Building Hardenbergstrasse 36, D-10623 Berlin, Germany.
authors.forename: Y
authors.initials: Y
authors.lastname: Ovcharenko
authors.affiliation: Institute of Physics, University of Freiburg, Hermann-Herder Strasse 3a, D-79104 Freiburg, Germany.
authors.forename: V
authors.initials: V
authors.lastname: Lyamayev
authors.affiliation: European XFEL GmbH, Albert-Einstein-Ring 19, 22761 Hamburg, Germany.
authors.forename: T
authors.initials: T
authors.lastname: Mazza
authors.affiliation: European XFEL GmbH, Albert-Einstein-Ring 19, 22761 Hamburg, Germany.
authors.forename: M
authors.initials: M
authors.lastname: Meyer
authors.affiliation: Sorbonne Université, UPMC Univ Paris 06, UMR 7614, LCPMR, 75005 Paris, France and CNRS, UMR 7614, LCPMR, 75005 Paris, France.
authors.forename: M
authors.initials: M
authors.lastname: Simon
authors.affiliation: Sorbonne Université, UPMC Univ Paris 06, UMR 7614, LCPMR, 75005 Paris, France and CNRS, UMR 7614, LCPMR, 75005 Paris, France.
authors.forename: L
authors.initials: L
authors.lastname: Journel
authors.affiliation: Sorbonne Université, UPMC Univ Paris 06, UMR 7614, LCPMR, 75005 Paris, France and CNRS, UMR 7614, LCPMR, 75005 Paris, France.
authors.forename: J
authors.initials: J
authors.lastname: Lüning
authors.affiliation: Elettra-Sincrotrone Trieste, Strada Statale 14-km 163, 5, I-34149 Basovizza, Trieste, Italy.
authors.forename: O
authors.initials: O
authors.lastname: Plekan
authors.affiliation: Elettra-Sincrotrone Trieste, Strada Statale 14-km 163, 5, I-34149 Basovizza, Trieste, Italy.
authors.forename: M
authors.initials: M
authors.lastname: Coreno
authors.affiliation: Elettra-Sincrotrone Trieste, Strada Statale 14-km 163, 5, I-34149 Basovizza, Trieste, Italy.
authors.forename: M
authors.initials: M
authors.lastname: Devetta
authors.affiliation: Department of Physics, University of Trieste, Via Valerio 2, I-34124 Trieste, Italy.
authors.forename: M
authors.initials: M
authors.lastname: Di Fraia
authors.affiliation: Elettra-Sincrotrone Trieste, Strada Statale 14-km 163, 5, I-34149 Basovizza, Trieste, Italy.
authors.forename: P
authors.initials: P
authors.lastname: Finetti
authors.affiliation: Elettra-Sincrotrone Trieste, Strada Statale 14-km 163, 5, I-34149 Basovizza, Trieste, Italy.
authors.forename: R
authors.initials: R
authors.lastname: Richter
authors.affiliation: Elettra-Sincrotrone Trieste, Strada Statale 14-km 163, 5, I-34149 Basovizza, Trieste, Italy.
authors.forename: C
authors.initials: C
authors.lastname: Grazioli
authors.affiliation: Elettra-Sincrotrone Trieste, Strada Statale 14-km 163, 5, I-34149 Basovizza, Trieste, Italy and eChemistry Laboratory, Faculty of Life and Social Sciences, Swinburne University of Technology, Hawthorn, Melbourne, Victoria 3122, Australia.
authors.forename: K C
authors.initials: KC
authors.lastname: Prince
authors.affiliation: Elettra-Sincrotrone Trieste, Strada Statale 14-km 163, 5, I-34149 Basovizza, Trieste, Italy.
authors.forename: C
authors.initials: C
authors.lastname: Callegari