/de/research/projects/2.3/topics/Attosecond pump probe/index.html
2.3 Time-resolved XUV-science
Project coordinator(s): A. Rouzée, S. Patchkovskii
Topic: Attosecond XUV pump – XUV probe applications.

A. Rouzee

In the frame of project 2.3, we are currently developing different beam-lines aiming to investigate ultrafast femtosecond and attosecond dynamics in atoms and molecules initiated by a first XUV pump pulse and probe with a second XUV pulse. This topic is separated in three main activities:

  • Femtosecond XUV pump-probe spectroscopy
  • Attosecond pump-probe spectroscopy
  • Multiphoton multiple ionization of atoms and molecules in the XUV and soft X-ray range

Femtosecond XUV pump-probe spectroscopy

Figure: High harmonic generation chamber that includes a 15 cm long cell placed 5 meter away from the focusing optic of the 800 nm driver laser field at the Max born Institute.

Within the first activity, we are actively developing a new 10 meter long beam-line, where we make use of a terrawatt, carrier phase stable, 30 femtosecond laser system to generate high flux, sub-20 fs XUV light by a high order harmonic process. Harmonics at uJ level will be produced by focusing the laser in 5 meter distance. Using a system of silicon mirrors, the fundamental pulse used to generate the XUV pulse will be filtered out and the XUV beam will be splitted in two parts. Both XUV pulses will then be focused into a velocity map imaging spectrometer using a multilayer mirror that selects the 29th harmonic (46 nm). Using photoelectron and photoion spectroscopy, the dynamics induced by the first XUV pulse will be probed by a second XUV pulse. This beamline should be operational at the end of 2012. It will be mainly applied to investigate few-photon ionization of atoms and the different mechanisms leading to the dissociation and fragmentation of a molecule after absorption of one XUV photon.

Attosecond pump-probe spectroscopy

Figure: Beamline for attosecond pump-probe experiments. A removable XUV-IR split mirror (SM1) is used to delay and focus both, the fundamental IR and the attosecond XUV pulse into an experiment (VMI = velocity map imaging spectrometer). Replacing SM1 by an aperture that only transmits the IAP, enables its propagation into an XUV-XUV pump-probe experiment. To this end a set of two toroidal (TM1, TM2) and two flat mirrors (SM2), illuminated at grazing incidence, are used to collimate, split and focus the IAP into an experiment (VMI 2) that is attached on the left.

In the frame of project 4-01, a terawatt-class optical parametric chirped pulse amplifier (OPCPA) is being developed that will deliver 15 mJ, sub-10 fs pulses at 850 nm. This pulse energy is enough to produce isolated attosecond pulses (IAP) by means of high harmonic generation (HHG) which are sufficiently strong to perform first attosecond pump-probe experiments.
For a driver laser of this kind, conversion efficiencies in the generation of IAPs >10-5 are possible, suggesting that already in the initial stages of the project pulse energies of >150 nJ will be achievable, i.e. safely above our estimate of the required pulse energy on target (2 nJ) for a successful attosecond XUV pump-attosecond XUV probe experiment. This beam line will be commissioned in the first half of 2013 which marks the starting point for attosecond XUV pump – attosecond XUV probe experiments.
We anticipate using first rare gases in XUV-XUV auto-correlation measurements, since they offer the possibility to identify multi-electron ionization channels. For example, the role of Auger processes in the production of high charged Xe and Ne ions after 90 eV XUV radiation will be investigated. Thereafter we will investigate the role of the autoionization state in the electron localization mechanism that was observed during two-color dissociative ionization of H2 and D2 by the sequence of an IAP and a few-cycle IR pulse [1]. Using the intense IAPs we envision the possibility of an experiment where an attosecond XUV pump IAP excites the molecule to the first (Q1) auto-ionizing state of H2, and where, at a variable time delay, an attosecond XUV probe IAP ionizes the molecules that have not auto-ionized yet. Since this sequential excitation process brings the molecule to a dramatically different part of the potential energy curves, it is expected that the electrons and/or fragments produced by the second IAP can be readily distinguished from the electrons and/or fragments that are produced by the pump and probe IAPs by themselves.

Multiphoton multiple ionization of atoms and molecules in the XUV and soft X-ray range

Figure: (left) Scheme of doubly resonantly enhanced three-photon double ionization of Ar by a FEL pulse with 21.3 eV photon energy. (right) Measured [(a) and (b)] and calculated (c) angle integrated electron spectra at 21.3 eV photon energy. Theory: Double resonant mechanism including autoionizing state (solid) and without autoionization, multiplied by 100 (dotted).

The last activity within this topic makes use of the high energy, ultrashort XUV and X-ray laser pulse available at free electron laser facilities, like FLASH, the free electron laser of Hamburg, LCLS, the linac coherent light source at Stanford or at the EUV/XFEL facility of Spring-8 in Japan. Since already few years, our team is embedded in experiments aiming to explore the mechanisms behind the multiphoton, multiple ionization of atoms and molecules interacting with a strong XUV pulse. We have for instance investigated in the past the three photon triple-ionization of neon at 90 eV photon energy at FLASH [2], the role of autoionization in triple-photon double ionization of Argon [3] and more recently the interference between resonant and non-resonant two-photon ionization of helium around 20 eV photon energy at the EUV facility of Spring-8 [4]. We are as well involved in the development of two-(three) colors pump-probe experiment at FEL [5-6], where a laser pulse from a table-top laser system is used to excite a particular dynamics insight an atomic or molecular system which is then followed by XUV or X-ray photoionization [7-8].

Collaborations

A closed collaboration exists between project 1.1 and project 4.1 for the development of the terawatt-class OPCPA system and the high harmonic generation cell that will be used to generate intense isolated attosecond pulse.

Activities performed at free electron laser facilities are done in collaboration with external national and international institutes, among them: the MPQ-ASG, CFEL, the Europen XFEL and DESY from Hamburg, Aarhus university, Danemark, Tohoku university, Japan, Lund university, Sweden, Moscow MV Lomonosov state University, Helmholtz-Zentrum Berlin fur Materialien und Energie, Groningen university, The Netherlands, Imperial college London,UK.

Articles

[1] G. Sansone, et al., Nature 465 , 763 (2010).
[2] A. Rouzee, et al., Phys. Rev. A 83 , 031401 (2011).
[3] E. V. Gryzlova, et al., Phys. Rev. A 84 , 063405 (2011) .
[4] R. Ma, et al., submitted to Phys. Rev. Lett. .
[5] P. Johnsson, et al., Opt. Lett., 35, 4163 (2010).
[6] S. Schorb, et al., Appl. Phys. Lett. 100, 121107 (2012).
[7] P. Johnsson, et al., J. Phys. B 42, 134017 (2009).
[8] N. Berrah, et al., J. Mod. Opt. 1362, 1015 (2010).