Beamline FL 26

Picture of the REMI instrument together will the in-line split&delay and focussing mirror chamber installed at FL26 in the FLASH2 experimental hall ‘Kai Siegbahn’.

The REMI comprises ion and electron time-of-flight spectrometers with various electrodes and a pair of Helmholtz coils for electric and magnetic extraction fields, respectively, to achieve full solid angle detection efficiency. Position-sensitive MCP detectors provide all kinematic information of the ionization fragments.

Two-mirror assembly for split&delay and focusing of the FLASH2 beam: The first mirror is a horizontally slotted mirror which splits the incoming beam and delays the two resulting partial beams with respect to each other by means of piezo-driven translation. The second off-axis ellipsoidal mirror focuses the partial beams in a joint focal spot of about 5 µm in size. Both mirror holders feature 6-D kinematic stages for combined rotation and translation positioning.

With a reaction microscope (REMI) all fragments of a photoionization process can be detected by means of a combination of electron and ion time-of-flight spectrometers and a specific arrangement of electric and magnetic extraction fields [1]. Using a coincident measurement technique, a complete set of all the kinematic properties of the products of the photoionization process can be determined in the experiment. Hence, this device is especially suited to investigate the dynamics of various ionization processes of gas phase and liquid targets.

Over the past years, a number of remarkable scientific results derived from REMI-experiments at FLASH1 have been published by the group of R. Moshammer (MPIK), e.g. regarding the dynamics of inter-molecular coulombic decay processes in weakly-bound rare gas dimers [2] and the dynamics in the dissociation of multiply-charged iodine molecules [3]. With a similar apparatus, the COLTRIMS instrument of the group of R. Dörner of Universität Frankfurt, the existence of so-called Efimov-states in the helium trimer could be proven and very precise measurements of structure and the extremely low binding energies of the He-trimer as well as the He-dimer were successful [4,5].

As a user experimental set-up temporarily installed for a specific beamtime, the complexity of the REMI instrument and especially the demanding vacuum requirements result in long preparation phase before the actual experiment.

Hence, in collaboration with the group of R. Moshammer of MPIK Heidelberg a REMI apparatus has been set-up as a permanent endstation at a dedicated beamline of FLASH2 in order to improve the efficiency of the scientific work with such a powerful as well as complex instrument. It is planned to make this endstation available to all user groups interested in atomic and molecular science.

With respect to time-resolved experiments employing XUV-XUV pump-probe schemes, a mirror chamber has been set-up recently which simultaneously serves as an in-line split&delay stage and a focusing device. The time-delay can be adjusted in the range of +/- 2.7 ps. A focal spot size of 4x5 microns (FWHM) has been achieved in the commissioning. The 30nm carbon mirror coating covers the whole FLASH2 wavelength range and the reflectivity is larger than 75% in the wavelength range between 9 and 41 nm.

In 2018 the new FLASH2 synchronized NIR laser system will become available. In addition, a synchronized HHG system providing VUV photons is planned for FL26, which has been developd in collaboration with the Leibniz Universität Hannover. These sources will enable multi-color pump-probe experiments in the near future.

[1] J. Ullrich et al., Recoil-ion and electron momentum spectroscopy: reaction-microscopes, Rep. Prog. Phys. 66, pp. 1463–1545, (2003)
[2] K. Schnorr et al., Time-Resolved Measurement of Interatomic Coulombic Decay in Ne2 , Phys. Rev. Lett. 111, 093402 (2013)
[3] K. Schnorr et al., Electron Rearrangement Dynamics in Dissociating I2n+ Molecules Accessed by Extreme Ultraviolet Pump-Probe Experiments, Phys. Rev. Lett. 113, 073001 (2014)
[4] S. Zeller et al., Imaging the He2 quantum halo state using a free electron laser, arXiv_1601.03247v1 (2016)
[5] M. Kunitski et al., Observation of the Efimov state of the helium trimer, Science 348, p. 551-555 (2015)

CONTACT FL 26 Beamline:
Markus Braune
E-Mail: Markus Braune
Phone: +49 (0)40 8998 (9)1730
Location: 28k / O2.021