Max-Planck-Institut für Kernphysik Heidelberg

Quantum Dynamics and Control

Prof. Dr. Thomas Pfeifer

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Reaction Microscopes: The Cloud Chambers of Atomic, Molecular and Optical Physics

"Reaction Microscopes" developed in 1993 and steadily improved since then are innovative many-particle momentum spectrometers for atomic, molecular and optical physics. They enable the coincident measurement of the vector momenta of several electrons and ions emerging after the break-up of single atoms, molecules or clusters.

Applying combined parallel electric and magnetic fields, all emitted electrons and ions are simultaneously projected on opposite position and time sensitive detectors reaching a detection efficiency of close to 100 %. From the hitting positions and times the particle trajectories are reconstructed and their initial vector momenta are calculated approaching unprecedented momentum resolution for low energy fragments.

Pre-cooled targets are provided either by supersonic expansion or by laser cooling in magneto-optical traps. Large area detectors with ultra-fast, GHz readout are used and continuously further developed


Setup of the Reaction microscope

Areas of Applications:

  • Collisions with charged particles: Ions, electrons, antiprotons and positrons
  • Interaction with strong, short-pulse lasers
  • Interaction with coherent radiation from the Free Electron Laser Attosecond physics
  • Interaction with single photons

A Brief History of Reaction Microscopes

1. Recoil Ion Momentum Spectroscopy: RIMS

The development or RIMS started at Frankfurt University in the group of Prof. H. Schmidt-Böcking: The main research activity of the team in the late seventieth was the investigation of inner shell vacancy production and their decay (quasimolecular x rays) by measuring angular dependent x-ray emission probabilities. Thus, to perform measurements at very small angles in gas targets the detection of the recoiling target ion (transverse kinematics) instead of the slightly scattered projectile was obvious. This idea was discussed between Lew Cocke and Horst Schmidt-Böcking during Lews visit in 1978 in Heidelberg. First experiments on the He target were performed in 1978-9 at the Tandem accelerator at Kansas State University, however, these experiments were not really successful and in principle too difficult applying the simple technique used at KSU and not worth to continue.

In 1982 the Frankfurt group developed a new approach and applied for funding of this project to GSI (but never received neither any answer nor funding). It was the PHD thesis of Joachim Ullrich (1982-1987) to develop and test a new generation of spectrometers to measure at least the transverse momentum of the recoiling target ion and its charge state (Figure 1 and 2). To obtain the (magnetic) charge state separation he developed position sensitive detectors (MCP with wedge and strip anodes). The steady progress of this project was published in the annual reports of the Institut für Kernpyhsik, University Frankfurt, in the early eightieths. At the GSI UNILAC successful experiments could be performed and the first publication on the new recoil ion momentum spectrometer with reliable small angle data appeared in Phys.Lett. A125 (1987) 193 by Ullrich and Schmidt-Böcking. In these experiments in MeV heavy ion rare gas collisions very large impact parameters could be investigated for the first time. During this time Lepera and Ivan Sellin measured with a quite different technique (non coincident method) the momenta of low energetic recoil ions too. They presented first results on the angular dependence at Denton 1986. But they did not continue with this work. Similar work was started in the GANIL group by Jean-Pierre Grandin and first results published in 1988.

In order to test the resolution of the technique the correspondence of recoil and projectile transverse momentum was measured in the low 10-5 rad regime. Surprisingly, the expected correspondence was broken below very small angles and one first concluded that this might be some principle limitation of the recoil-ion momentum spectroscopy technique. However, with the help of CTMC calculations of Ronald Olson and numerous discussions with him (from 1986 -87 RO was Humboldt Award fellow in Frankfurt) it was realized soon, that there was no such fundamental limit, but rather the method was demonstrated to be even sensitive to the momenta of the involved electrons. It was immediately clear that only the thermal motion of the target was the limiting resolution barrier and one had to cool the target gas as much as possible. Thus, measurements beyond the Born-Oppenheimer Approximation were envisioned to become possible and, through a coincidence with the emitted electrons, the correlated motion of nuclei and electrons should become resolvable. This was the birth of the Cold Target Recoil Ion Momentum Spectroscopy COLTRIMS idea.

2. Cold Target Recoil Ion Momentum Spectroscopy: COLTRIMS

In 1986 Reinhard Dörner had joined the Frankfurt recoil team headed as a Postdoc by Joachim Ullrich and in his PHD thesis (1986 - 91) cooling of the target (windowless gas cell) to a few Kelvin temperature was implemented into the apparatus (Figures 3 to 4). The two could thus successfully measure the angular dependence of the ionization of He by fast proton impact down to very small angles (below 10-5rad). They used a spectrometer which had no extraction field. During these years the cooperation with Lew Cocke on recoil work started to deepen again and, in the Frankfurt and the KSU labs, recoil ion momentum spectroscopy became to be the major research projects. Information and also manpower was exchanged frequently between both groups. In 1991 Lew Cocke and Horst Schmidt-Böcking together received the Max-Planck Research Award for this collaboration. The KSU group used a diffusive jet with a small recoil ion extraction field. Here Rami Ali and later Vicki Frohne in their PHD works measured successfully transverse momenta, but Rami Ali succeeded for the first time to measure also longitudinal momenta of the recoil ions.

Frankfurt in collaboration with Joachim Ullrich who had changed to GSI in 1989 started in the PhD work of Ottmar Jagutzki (1989- 94) the development of a super sonic jet. Parallel to this work the Caen group of Jean Piere Gardin and Amin Cassimi bought a super sonic jet primarily to use the recoiling target ions produced in fast  heavy-ion atom collisions as a cold ion source, a method that had been pioneered before by Lew Cocke at KSU and Rido Mann at GSI. However, they also realized that one would be able to measure recoil-ion momenta with such as system and started to design a spectrometer in contact with the groups at Frankfurt, GSI and KSU. In parallel, the GSI group began with the development of a spectrometer to be implemented into the experimental storage ring (ESR) at GSI.

Using the experience made in KSU, GSI and GANIL Volker Mergel in Frankfurt succeeded to built between 1993-4 the first high-resolution COLTRIMS system based on a super sonic jet (internal gas temperature a few 10 milli K), where the He gas was pre-cooled to about 15 Kelvin. He measured the capture of electrons from He to excited states achieving the best resolution until then, about a factor of 50 better than everything that has been seen before. At the same time, the GANIL team reported first narrow recoil-ion momentum distributions on a conference, much smaller than the thermal distribution at 300 K.

3. Reaction Microscopes

In parallel, the GSI group now strengthened by Robert Moshammer (since 1992), Martin Unverzagt and later Holger Kollmus as well as Wolfgang Schmidt with strong support from Rido Mann had designed a similar test machine to be implemented into the ESR. Since capture reactions at large velocities as typical for GSI had small cross sections and probably large transverse recoil momenta it turned out in several unsuccessful beam times that COLTRIMS was not applicable in this situation! Moreover, target ionization was very hard to measure as well, since one had to use the direct projectile beam as time trigger - so recoil-ion momentum spectroscopy seemed to be suited for the investigation of quite few, special reaction channels at low velocities only! Nevertheless, first transverse and longitudinal recoil-ion momentum spectra with hitherto unachieved resolution were measured and published for multiple ionization of Neon by heavy-particle impact, essentially in parallel to similar work at GANIL.

In a desperate beam time night and of 1993, again without any success in searching for capture reactions, Robert Moshammer and Joachim Ullrich decided without much hope to really improve things to implement a magnetic projection field that force electrons onto spiral trajectories onto a detector opposite to the ion detector in order to use the electrons as a more efficient trigger (Figure 5). The result was overwhelming: Not only did the coincidence rate increase by orders of magnitude, but also the recoil ion momentum resolution seemed to have improved considerably. Within one week they recognized the reason for the latter (momentum correlation between ionized electrons and ions) and, most importantly, found out that the new method of projecting electrons with combined magnetic and electric fields was indeed a most efficient, rigorously new electron spectrometer, different from all earlier concepts developed over hundred years of electron spectroscopy. Similar as for the ions, from the electron time of flight and position their full momentum vector can be recalculated and is obtained with high resolution, even surpassing the one that was reachable for the ions. Moreover, electron energies down to "zero" velocity became accessible for the first time in angular resolved measurements. The publication in Phys. Rev. Lett. 73 (1994) 3371 reported on the first simultaneous high-resolution measurement of electrons and ions in coincidence.

Since the implementation of the magnetic field only marginally disturbed the slow ions, electrons and ions both were measurable with essentially 100% solid angle and high resolution, such that a "bubble chamber" of atomic and molecular physics had emerged, named "Reaction Microscope", a general concept applicable for the investigations of any atomic or molecular reaction induced by single photons, laser pulses, electrons and ions at any energy as well as by antiparticles (Figure 6). In a first publication of the GSI group in Nucl. Instr. Meth. B108 (1996) 425 the machine was described in detail and a large number of future investigations, ranging from prescision spectroscopy measurements of inner shell levels of highly charged ions to the investigation of neutrino-electron relative angles or even of the neutrino mass in beta-decay experiments and many more were invisioned.

In 1999 Joachim Ullrich who had changed to Freiburg University in 1997 received the "Leibniz Award", the most prestigious German research prize (3 Mil. DM research funding) for the development of the electron spectrometer and its combination with the recoil-ion detection, in 2006 Robert Moshammer together with him were awarded the "Philip Morris Forschungspreis", both now at the Max-Planck-Institut für Kernpyhsik in Heidelberg.

... until today

Having been developed to explore ion-atom collisions, the COLTRIMS and Reaction Microscope technologies were rapidly applied in various fields of atomic, molecular and optical physics. They were

  • first transferred to investigate photo-ionization processes at HASYLAB in Hamburg (1994), at ALS in Berkeley (1995), later at the ESRF in Grenoble, the APS in Argonne,  KEK in Japan, BESSY in Hamburg, etc,
  • then, since 1998, successfully applied by Alexander Dorn in Freiburg, now in Heidelberg to study electron interactions with atoms and molecules after pioneering work of Robert Moshammer at GSI in 1997,
  • implemented in Aarhus, Groningen, KSU and presently in Heidelberg into Magneto Optical Traps: MOTRIMS,
  • used to investigate collisions with antiprotons at CERN in 1999,
  • extremely successfully applied to explore the interaction of short-pulse high-intensity lasers with atoms and molecules, first in 2000 by the Frankfurt/Giessen and Freiburg/Berlin co-operation now in Heidelberg, Berlin at the MBI, in Kanada, at KSU with new systems being developed at Frankfurt, Zurich, Tokyo and MPQ in Munich,
  • re-designed to be operated in storage rings with first experiments at the CRYRING in Stockholm in 2000, new measurements anticipated for summer 2006 in the ESR at GSI (see Figure 7), further developed to be implemented into the cryogenic storage ring (CSR) planned in Heidelberg as well as in the ultra-low energy storage ring (USR) planned within the FLAIR collaboration at GSI to investigate collisions with ultra-cold molecular ions (CSR) and slow antiprotons (USR), respectively,
  • applied to study basic non-linear processes occurring at the first operating 4th generation free-electron-laser (FEL) light source in Hamburg having started end of 2005 (see Figure 7),
  • equipped with photon detectors in Freiburg to detect the first electron-ion-photon coincidences,
  • operated with two beams, an electron and an overlapping laser beam, to measure the first laser assisted (e,2e) fully differential cross sections,
  • and were finally adapted to envision, for the first time,  to perform fully differential experiments for positron impact, anticipated to be started within 2006 at FRMII in Munic.

With quite some delay, mainly due to the unconventional way to disperse electrons, Reaction Microscopes now have become the state of the art machines in an increasingly large number of laboratories. In the last image at the bottom, one of the latest machines, used at MPI-K in Heidelberg to explore the interaction with ultra-short laser pulses at a base pressure of 10-11 Torr and a target density of only 10-8 Torr is shown, holding the world-record in electron and ion momentum resolution of 5 meV and 500 neV, respectively.

In 2004 Joachim Ullrich was awarded the David Bates Prize of the Division of Atomic, Molecular and Plasma Physics by the Institute of Physics for his outstanding contributions to Atomic and Molecular Physics connected with the Reaction Microscopes.

img/remi1 The heart of the first recoil ion momentum spectrometer build by Ullrich in 1983 for his PhD thesis in order to measure small transverse momenta transfer to the recoiling target ions in fast ion-atom collisions. The ion beam propagates on the axis of the inner cylinder and leaves the spectrometer through the little hole on the top. Inside the cylinder is the target gas at a low pressure. Recoil ions that are created along the ion path drift (at all angles) to the wall of the cylinder and can only leave it through the little mesh covered hole at the left side of the inner cylinder. Their drift time from the axis to the wall is proportional to their transverse momentum. Between inner and outer cylinder, the ions are accelerated and emerge through the somewhat larger mesh-covered hole at the left of the outer cylinder barely visible on the picture.
img/rims5 After leaving the acceleration path, the recoil ions were focussed by an Einzellense, then deflected by CoSm permanent magnet (not visible on the picture and then detected by a two-dimensional position sensitive channelplate detector with wedge&strip readout. Only the detector housing is seen, placed below the ion extraction axis in order to detect the deflected ions.
img/rims3 Having learned that the majority of transferred momenta were in the order of the momenta of the target atoms before the collision due to their thermal motion at 300 K, the inner target cell was cooled down by a copper connection fixed onto the cold head of a cryo pump. Even though some improvement was visible, the effect was not sufficient and further modification was needed.

Now, both cylinders were replaced by a massive copper block, tightly connected to the first stage of the cryo pump cold head, but electrically isolated. The second stage was connected to a cold shield, partly surrounding the inner part. A second cold shield from the top was cooled by liquid nitrogen. The cylinder, not visible any more is inside the block parallel to the bore hole row seen at the side. Entrance and exit holes for the beam were extremely small and the pre-cooled gas was guided via several tubes along the beam path into the cell in order to guarantee an static and homogenous pressure distribution inside the cylinder. Temperatures of a few Kelvin were routinely achieved.

This version became the basis of the PhD thesis of Reinhard Dörner and was used several years as the recoil-ion spectrometer with the highest transverse momentum resolution.

img/remi2 Prototyp of the second Reaction Microscope developed by the GSI group. From the top, the pre-cooled supersonic jet comes in and the ion beam propagates from the right to the left. After the first few successful beamtimes at the end of 1993, it was clear, that different from all other existing designs, it was better to extract ions and electrons along the ion beam symmetry direction. Thus, immediately a longitudinal extraction was realized early in 1994. Part of the small, still hand wounded copper coil to generate the magnetic guiding field for the electrons is seen in the left lower part of the figure. With this machine, several successful experiments leading to at least three publications in Physical Review Letters were performed while a third, improved version was under construction.
The third, rigorously improved version of the GSI Reaction Microscope coming into operation during 2005 with Robert Moshammer, Martin Unverzagt, Wolfgang Schmitt and Joachim Ullrich. A thick but compact two stage supersonic jet was realized. The guiding magnetic field was substantially improved by implementing large, 1.5 m diameter Helmholtz-coils. The background pressure was reduced to a few 10-8 Torr. For the efficient electron detection, a set of three position sensitive 40 mm diameter channelplates, arranged in a "Mercedes Star" configuration, were implemented.
img/remi_esr Design drawing of the Reaction Microscope to be implemented into the ESR storage ring at GSI during summer 2006. In addition to the target electron and recoil ion detection a high-resolution zero degree magnetic electron spectrometer is incorporated to detect fast MeV electrons that are moving in the continuum of the projectile ion beam (design by Siegbert Hagmann). They are either generated by projectile ionization or capture (radiative or kinematically)  into the continuum of the projectile.
img/remi_fel State of the art Reaction Microscope brought into operation in November 2005 at the VUV FEL of DESY Hamburg. Base pressure is 10-11 Torr. 120 mm diameter channelplate detectors are used equipped delay line anodes with a 1 GHz Flash ADC read -out.
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