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Tamás Csizmadia |
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The reaction microscope end station (REMi-ES), also known as cold-target recoil-ion momentum spectrometer (COLTRIMS), is a device designed to measure the momenta of charged reaction fragments in coincidence.
The setup is mobile, and in principle can be driven by other sources of ELI ALPS as well, but commissioning has only been done for the HR-1 driven GHHG Gas beamline.
The REMi-ES is developed for studying many-particle quantum-dynamics of atoms, molecules and small clusters initiated by the interaction with pulsed, ionising laser radiation. The instrument permits kinematically complete measurements of photoionisation fragments (electrons and ions) over the full solid angle and with high momentum resolution.
The combination of reaction microscopes with lasers and extreme ultraviolet sources opens the way towards addressing a broad range of open questions in atomic, molecular and optical science [1, 2]. The list of research areas and topics include, among others, tunneling ionisation, multi-photon ionisation, above-threshold ionisation, high harmonic generation, etc. Particularly, the most far-reaching applications are manifested in the topics of nonlinearity induced by single- or many-photon absorption, the structural imaging of a single molecule induced by the regime of UV-to-hard-X-ray photons, isomerisation, and ultrafast dynamics in atoms or molecules, to name a few [3—5].
Figure 1: Photo of the REMi-ES assembled in the thin jet configuration.
The REMi system (photographed in Figure 1) consists of a bipolar Spectrometer which simultaneously images the momenta of oppositely charged fragments onto two time and position sensitive Detectors.
After the photoreaction in the interaction region in the centre of the spectrometer, the charged particles are accelerated by electric fields and move towards the two respective detectors. Each detector consists of a pair of multichannel plates followed by two hexagonal delay line anodes. This combination provides sub-ns temporal and sub-mm spatial resolution with a high multi-hit capability. Two coils in the Helmholtz configuration placed outside the vacuum chambers produce a uniform magnetic field that provides additional confinement to higher energy electrons in order to achieve near unit detection efficiency. Due to the high detection efficiency and high-repetition rate laser source, coincidence type single-particle experiments become feasible. when combined with a high-repetition rate laser source. The spectrometer configuration can be adjusted to the specific needs of an experiment. The specifications of the detectors and the spectrometer are demonstrated in Table 1.
The instrument includes a cold particle source derived from a supersonic Gas jet source (Table 2) which can be configured for thin or dense particle beams depending on the type of interaction to be studied. The particle beam intersects the interaction region where it crosses with the optical beam before reaching a Gas dump which effectively removes the beam from the UHV environment of the target chamber.
The signals from the detectors are conditioned by a set of data acquisition (DAQ) electronics including amplifiers, constant fraction discriminators (CFDs) and time-to-digital converters (TDCs). The data is acquired by a PC running the CoboldPC software package.
| Electron detector | Ion detector | |
| Type | RoentDek HEX75L | RoentDek HEX75b |
| Triple layer detector active area | 100 mm | 75 mm |
| Frequency range | up to 500 kHz | up to 500 kHz |
| Dark counts | less than 100 Hz | less than 100 Hz |
| Spatial resolution | better than 120 μm RMS | better than 120 μm RMS |
| Temporal resolution | better than 170 ps RMS | better than 170 ps RMS |
| Detection efficiency | 63% | 63% |
| Dead time | 0 ns if dR > 15 mm | 0 ns if dR > 15 mm |
| Image linearity | 35 μm RMS after software correction | 35 μm RMS after software correction |
Table 1: Specifications of the detectors.
| Electron spectrometer | Ion spectrometer | |
| Energy range | max. 270 eV | max. 270 eV |
| Momentum resolution magnitude | up to 1:300 (depending on focus size) | up to 1:300 (depending on focus size) |
Table 2: Specifications of the spectrometer.
| Helium | |
| Internal beam temperature | down to 0.2 K |
| Beam geometry (size) | adjustable 0.3 mm to 3 mm |
| Particle density | up to 3e12 / cm3 |
| Particle flux | up to 3.5e16 / s |
Table 3: Target beam specifications with helium used as a representative gas
| Vacuum conditions | |
| Target beam at idle | 5e-10 mbar or better (also for thin jet configuration) |
| Target beam in operation | 1e-7 mbar or better (in case of dense jet configuration) |
Table 4: Typical vacuum conditions with and without injecting gas into the interaction region.
The REMi-ES is currently implemented at the second target area of the HR GHHG Gas beamline. The REMi can be used in two modes: in forward focusing mode, where the focusing element is located before the entrance to the instrument and is part of the beamline. In this mode, the XUV beam is focused by a toroidal mirror with 1.2 m exit arm, while the IR light is focused using a holey focusing mirror with 1 m focal length. This option allows online diagnostics on the transmitted optical beam.
In order to increase the applied laser intensity, a back focusing mode can be used, where a focusing mirror is placed just after the interaction region to produce a focal spot upon reflection. Two 1 inch focusing mirrors with 60 and a 70 mm focal lengths are available in-house for the utilisation of this option.
The jet is produced by supersonic expansion from a small nozzle and a set of skimmers, and can be set – by rebuilding the vacuum system – to two configurations: either to provide a dense target for weak optical beams or a thin jet for strongly absorbing interactions. Typical vacuum conditions for each options are summarised in Table 3.
In addition to the spectrometer and particle detectors directly contributing to the measurement of charged particle momenta, further devices can be utilised to support the experiment. A residual gas analyser (Pfeiffer QMG 250 M3 Prisma Pro) is attached to the jet dump for the control/analysis/alignment of the jet and for general vacuum diagnosis. Moreover, the measurement of the IR or XUV pulse energy, or the XUV spectrum (in case of forward focusing mode) is available using diagnostic equipment offered by the HR GHHG Gas beamline.
[1] V. de Jesus, A. Rudenko, B. Feuerstein, K. Zrost, C. Schröter, R. Moshammer, J. Ullrich: “Reaction microscopes applied to study atomic and molecular fragmentation in intense laser fields: Non-sequential double ionization of helium”, J. Electron Spectrosc. Relat. Phenom. 141 (2004) 127–142
https://doi.org/10.1016/j.elspec.2004.06.004
[2] M. Sabbar, S. Heuser, R. Boge, M. Lucchini, L. Gallmann, C. Cirelli, and U. Keller , “Combining attosecond XUV pulses with coincidence spectroscopy”, Review of Scientific Instruments 85 (2014) 103113
https://doi.org/10.1063/1.4898017
[3] R. Dörner, V. Mergel, O. Jagutzki, L. Spielberger, J. Ullrich, R. Moshammer, and H. Schmidt-Bocking: “Cold Target Recoil Ion Momentum Spectroscopy: a ‘momentum microscope’ to view atomic collision dynamics” Phys. Rep. 330 (2000) 95
https://doi.org/10.1016/S0370-1573(99)00109-X
[4] J. Ullrich, R. Moshammer, A. Dorn, R. Dörner, L. Ph. H. Schmidt, H. Schmidt-Böcking: “Recoil-ion and electron momentum spectroscopy: reaction-microscopes”, Rep. Prog. Phys. 66 (2003) 1463
https://doi.org/10.1088/0034-4885/66/9/203
[5] H. Schmidt-Böcking, J. Ullrich, R. Dörner, C. L. Cocke: “The COLTRIMS Reaction Microscope - The Spyhole into the Ultrafast Entangled Dynamics of Atomic and Molecular Systems”, Ann. Phys. (Berlin) 533 (2021) 2100134
https://doi.org/10.1002/andp.202100134