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This Viewpoint relates to an article by J Ullrich (1997 J. Phys. B: At. Mol. Opt. Phys. 30 2917–74) and was published as part of a series of Viewpoints celebrating 50 of the most influential papers published in the Journal of Physics series, which is celebrating its 50th anniversary.
In the summary of the Topical Review on 'Recoil-ion momentum spectroscopy' [1], it was envisaged that '...the experimental results reviewed in this paper can certainly be considered as just being the fascinating starting point of a large series of kinematically complete experiments to be performed in the near future'.
How true this statement was! Shortly thereafter in 2000 and 2003 two further reviews on the same topic appeared [2, 3], all three together presently achieving about 100 citations per year with a growing tendency. The techniques described, namely (cold target) recoil-ion momentum spectroscopy, (COLT)RIMS, or 'reaction microscopes' (REMI), historically different but often used as synonyms, are now operated in an estimated one hundred laboratories, at least, around the world delivering an overwhelming variety of top-level results.
What is it all about? The REMI multi-particle imaging technology proved to be essential to experimentally advance our basic understanding of the dynamics of correlated atomic and molecular few-particle quantum systems. It was in 1929 when Dirac wrote: 'The general theory of quantum mechanics is now almost complete...The underlying physical laws necessary for the mathematical theory of a large part of physics and the whole of chemistry are thus completely known, and the difficulty is only that the exact application of these laws lead to equations much too complicated to be soluble' [4]. This is true up to the present day such that we need approximate methods. Even though there is tremendous progress in theory as computational abilities grow, experimental benchmarks are indispensable.
This is where REMIs come into play. These instruments allow imaging of the momentum vectors of several atomic or molecular fragments, ions and electrons with high resolution, a large dynamic range and often covering the entire solid angle, thus recording a large part of the final many-particle momentum state. Therefore, they have been dubbed the 'bubble chambers' of atomic and molecular physics. Technically, this is achieved by the preparation of cold target species via ultrasonic expansion (or in magneto-optical traps) and projecting all charged fragments that emerge from the interaction of any kind of electron, ion, or photon beam with the target atoms, molecules or clusters by combined electric and magnetic fields onto time- and position-sensitive detectors. From the times-of-flight and hitting positions, the momentum vectors can be retrieved in coincidence.
RIMS had been developed over more than ten years at the time when the review was written and sophisticated 'reaction microscopes' that represented the decisive step forward had just been invented [5, 6]: the review dealt with describing the basic technology and kinematics, and highlighted the most prominent achievements until then. Among those were the first kinematically complete experiments on ion-impact (multiple) charge transfer (plus ionization) reactions as well as on single and double ionization. Photo double ionization had just been explored in unprecedented comprehension, the discrimination between photo-absorption and Compton-scattering contributions to He double ionization had recently been reported, and first differential measurements on multiple ionization by ion impact as well as on single ionization in electron collisions were reviewed.
Since then, a huge variety of basic quantum dynamical reactions were investigated and only a few can be highlighted. Thus, REMIs were proven to be decisive in tackling long-standing enigmas on the correlated motion of electrons driven by strong laser fields leading to single and multiple ionization [3, 7]. Wave-packet motion in atoms (electronic) [8] and molecules (nuclear) [9], localisation of electrons [10], and the correlated motion of electrons crossing a barrier on ∼100 as time scales have been investigated [11], partly in pump-probe experiments. The 'recollision' mechanism was unambiguously identified [12, 13], being at the heart of attosecond pulse generation and, thus, of 'attosecond physics' [14]. Using mid-IR laser diffraction of recolliding electrons led to the determination of bond lengths of polyatomic molecules with a resolution of <0.1 Å [15], a technology that holds the potential to image nuclear motion with Å position and fs time resolution in chemical reactions.
A series of experiments at various synchrotrons convincingly identified a ubiquitous de-excitation mechanism, the 'interatomic Coulombic decay' (ICD) and its various realisations in rare-gas and water dimers [16]. At the free-electron laser (FEL) in Hamburg (FLASH), its time-dependence was investigated via VUV–VUV pump–probe experiments [17]. REMIs were decisive for exploring multi-photon processes [18, 19], ultra-fast charge migration [20] and isomerization dynamics in ethylene [21] at various FELs, the FLASH, the LCLS (USA) and SACLA (Japan).
Combining a REMI with large-area, position-sensitive x-ray detectors within the CAMP instrument at the LCLS [22] enabled photon imaging experiments on the energy transfer to clusters [23], their structure and structural changes [24], and allowed pioneering serial femtosecond imaging of biological nano-crystals [25], the imaging of single viruses [26] as well as the visualization and characterization of vortices in helium droplets [27]. Permanent end stations based on the REMI technology are available or under construction at all FELs worldwide.
Moreover, fundamental quantum mechanics was explored. Tunnelling times through a laser-induced barrier on attosecond time scales were extracted using the 'attosecond clock' [28], photo-emission time delay [29] and core–hole localisation in homo-nuclear diatomics were investigated [30], the Einstein–Bohr 'Gedanken experiment' was realized using H2 molecules as quantum slit [31], the structure and dynamics of He dimers and trimers was explored [32] and, very recently, bound three-particle Efimov states were imaged and characterized [33] for the very first time.
Again, as 19 years earlier when the review was written, new developments nourish the expectation that the golden age is still ahead of us. The main reason for that vision is that REMI multi-coincidence experiments decisively depend on the availability of high-repetition-rate pulsed beams. Here, tremendous progress is expected to emerge with the European XFEL and LCLS II exhibiting MHz repetition rates of intense x-ray pulses with few- to sub-femtosecond pulse length. Similarly, a recent breakthrough in fiber laser technology yielded ~MHz few-cycle strong laser pulses [34] as well as VUV high-harmonic radiation [35] having the potential to revolutionize femto- and attosecond physics with REMIs. Finally, beams of cold charged clusters or molecular ions ranging from simple species to complicated ones as DNA, all at temperatures of a few kelvin, and highly charged ions are becoming available in a cryogenic storage ring, the CSR [36], which will be equipped with a REMI, offering a wealth of opportunities to investigate collisions of such systems with atoms and molecules as well as their interaction with intense few-cycle infrared laser pulses or VUV high-harmonic radiation.