Milestones in Research – Stolterfoht professional

Milestones and Highlights in Research within 40 years

(Nikolaus Stolterfoht)

1965-67 Diploma thesis: Neutron dose measurements using a LiF thermoluminescence dosimeter: First study of dependence on LET (Linear Energy Transfer).
1968-70: Realistic computer simulations of the radioactive fallout from a reactor GAU and the following contamination of rivers in Germany. Assistance in a European Expert Forum.
1967-70 PhD Thesis: Construction of a scattering chamber to measure absolute cross sections for electron emission differential in energy and angle. Novel apparatus including a µ-metal shield and continuous angle setting for the electron spectrometer. World-wide first measurements after those of Rudd et al. from Lincoln, Nebraska. 1976: Absolute cross sections for proton impact on helium, published together with results from the groups of E. Rudd and L. Toburen in Atomic and Nuclear Data Tables, became a well-known standard for comparison with theory and experiment over decades.
1971-72: Observations of Fano shapes in He electron spectra produced by protons similarly as previously observed in pioneering experiments using synchrotron radiation.
1971-74: Early work studying satellite lines in K-Auger spectra of nitrogen, neon, and argon due to multiple ionization by proton and helium impact
1972-75: Observation of significant fluorescence yield variations for argon and neon due to multiple ionization by heavy ion-impact. First determination of the K-fluorescence yield of neon as a function its charge state (collaborations with FOM, Amsterdam, Univ. Washington, Seattle, and KSU, Manhattan, Kansas: three Phys. Rev. Letters.)
1974: Pioneering studies of mechanisms for electron production in 30-MeV Oⁿ⁺+O₂ collisions (Phys. Rev. Lett, 1974). Electron spectra were measured for the first time in high-energy collisions after transporting the scattering chamber from Berlin to the Tandem Accelerator of the University of Seattle (The electron spectra are cited in several reviews and books).
1974-80: First gas target experiments of K-shell vacancy sharing in asymmetric ion-atom collisions. The experiments, that avoided problems inherent in corresponding measurements with solid targets, ultimately confirmed the Demkov-Meyerhof Model for inner-shell vacancy sharing.
1976/77: Early observation of electron correlation in two-electron transitions. Evidence for two-electron excitation in slow Li⁺+He Collisions producing a strong enhancement of the vacancy sharing in favor of Li. (Phys. Rev. Lett. 1976). Collaboration in theory with Univ. Paris Sud, Orsay (with V. Sidis and M. Barat).
1977: First measurements of Auger electron emission from neon using highly charged ions of several hundred MeV from the GSI accelerator. Evidence for energetic ions stripping a target to a few-electron system in a single collision. This observation of a “hammer ionization” motivated several follow-up measurements at different laboratories.
1979: Observation of post-collisional Stark mixing in ion-atom collisions. The receding projectile field produces Stark interaction of closely lying autoionization states leading to strongly asymmetric angular distributions of the ejected electrons (theory paper published in Phys. Rev. Lett., 1979)
1981: Pioneering paper on screening and antiscreening by projectile electrons in high-velocity atomic collisions. Leading author was J.H. McGuire. This theory work initiated corresponding studies in different laboratories for more than a decade. “Antiscreening” was frequently criticized as a misleading expression and in our laboratory it was later referred to as “dielectronic”.
1983-1990: Unprecedented high-resolution studies of Auger transitions in fast projectile ions introducing the method of zero-degree Auger spectroscopy. To realize measurements at zero-degree a Tandem electron spectrometer, consisting of two stages, was constructed. This spectrometer allowed high resolution measurements of Auger lines from fast projectiles by strong suppression of Doppler broadening effects. Furthermore, using light target atoms, a small collisional perturbation was achieved so that the number of excited states was limited. Contrary to hammer ionization, the light target atom acts as a needle ionizing the inner shell of the projectile without much affecting the outer shell. The selective excitation of Auger states via “needle ionization” provided the tool for several follow-up studies in various laboratories. The work originated at HMI, Berlin, was continued during a sabbatical year in the group of Sheldon Datz at Oak Ridge, where to a zero-degree spectroscopy apparatus was transported from Berlin.
1986/87: Measurements of transfer excitation in symmetric He⁺ + He collisions motivated by the search for Resonant Transfer Excitation (RTE) previously observed with x-ray spectroscopy. Unambiguous observation of RTE in O⁵⁺ + He collisions using zero-degree Auger spectroscopy with gas targets (Phys. Rev. Lett., 1986). Subsequent observation of strong forward electron emission from overlapping resonance states produced by transfer excitation involving zero field quantum beats (Phys. Rev. Lett, 1987)
1986: Evidence for Correlated Double Capture (CDC) in low-energy collisions of O⁶⁺ with He. Population of non-equivalent electron configurations through significant energy exchange between two electrons during their transfer from target to projectile. The publication (Phys. Rev Lett., 1986) received much attention in the community with more than 180 citations. The studies involved controversial discussion with different laboratories. Today it is acknowledged that the CDC is an important mechanism for highly charged ions, since electrons can be transferred to deeply lying inner shells, when the potential energy gain can be released by transfer to another electron.
1987-1999 The work devoted to correlated double capture was continued for more than 10 years revealing various novel phenomena. In the two-electron transfer not only energy is exchanged but also angular momenta which leads to the occupation of Rydberg states with high angular momenta (Phys. Rev. Lett., 1987). These states are metastable against autoionizing resulting in the stabilization of electrons after electron capture. The stabilization mechanisms at highly charged ions became a topic that was studied in various groups in the early 1990. In addition, the inverse process referred to as dielectronic excitation was observed, where an electron from a high Rydberg state is transferred to a lower level and the released energy is used to remove an electron from an inner shell (Rapid Comm. in Phys. Rev. A). This process, that is unique for hollow atoms, can produce inner-shell vacancies at very low collision energies.
1987: Experiments at the high-energy accelerator GANIL (France) provided evidence for two-center effects on electron emission in collisions with very fast and highly charged ions. These effects are produced by post-collision deflection of the outgoing electron by the highly charged projectile. The first study of two-center effects received particular attention, since post-collision interaction is unexpected for very fast projectiles of several ten MeV/u. The experimental finding was confirmed by calculations using the Continuum Distorted Wave (CDW) approach in collaboration with a the group of R. Rivarola (Rosario, Argentina). The two-center effects were continued at the cyclotron accelerator in HMI, Berlin.
1989-1992: First observation of Coulomb focusing of autoionization electrons produced in low-energy He⁺+He collisions (Phys. Rev. Lett., 1989). Post-collision effects produce a strong enhancement of autoionization electrons at forward angles, in analogue with the creation of the well-known electron cusp. The experiments were later interpreted quantum mechanically in terms Coulomb „path“ interferences (Phys. Rev. Lett., 1991). Collaboration with the ECR group in Oak Ridge.
1989-1993: Fundamental work about the principles of electron correlation in energetic ion-atom collisions published in various book contributions and reviews (e.g., World Scientific, 1989, Physica Scripta 42 1990, Nucl. Instr. Methods B53, 1991, Physica Scripta T46, 1993). Attempt to provide a basic view of dynamic electron correlation in ion-atom collisions.
1993-95: First experimental verification of a novel concept of time ordering in energetic ion atom collisions. The principles of time ordering were introduced into the field by J. H. McGuire. In a series of 3 publications in Phys. Rev. A, a new formalism was given which was applied to experimental results concerning single and double excitation in fast ion-atom collisions. Subsequent collaboration with theorists from different groups including that of McGuire.
1995-97: Writing a book together with R.D DuBois (Rolla) and R.D. Rivarola (Rosario) about Electron Emission in Heavy Ion-Atom Collisions that appeared in the Springer Series on atoms and Plasmas. The text summarized more than 30 years of research performed in the field of ionization in energetic collisions.
1991-2002: Systematic studies of electron emission caused by interaction of slow highly charged ions with solid surfaces. Formation of hollow atoms was studied by means of Auger electrons spectroscopy. The work at Berlin was focused on hollow atoms moving below the surface. The experimental work was accompanied by calculations modeling the complex processes below the surface. The results were published in several Papers, review articles, and book contributions. Later, the work was focused on the creation of plasmons. For the first time it was shown that highly charged ions can produce plasmons via potential energy transfer (Phys. Rev. Lett., 1998). The work was done in collaboration with several laboratories which were involved in a European Network coordinated from Berlin.
1998-2002: Systematic studies of the three-electron system Li in collisions with high-energy projectiles from the GANIL accelerator. The measurements started with investigations of single ionization of Li by 95 MeV/u Ar¹⁸⁺ ions. Two- and three-body effects were separated by means of theoretical results based on CDW calculations (Phys. Rev. Lett, 1998). The measurements were continued studying three-electron transitions resulting in hollow Li with an empty K shell. Strong electron correlation effects were found to be responsible for double K-shell vacancy production (Phys. Rev. Lett., 1999). The work was performed within an international collaboration including J. Tanis (Kalamazoo), B. Sulik (Debrecen), and J.Y. Chesnel (Caen).
2001-2005: First observation of interference effects in electron emission from H₂ colliding with 60-MeV/u Kr³⁴⁺ ions from the GANIL accelerator (Phys Rev. Lett., 2001). The observed electron emission from the two atomic centers of H₂ is analogue to the fundamental two-slit experiment by Young. Model calculations show that the high incident velocities (about 1/3 of the velocity of light) is necessary to observed the interferences. The first studies have motivated several laboratories to perform experimental and theoretical studies of the same interference effects. Again, the work was performed within the international collaboration including J. Tanis (Kalamazoo), B. Sulik (Debrecen), and J.Y. Chesnel (Caen).
2006 – 2011: Observation of strong isotope effects in theoretical studies of electron transfer into He²⁺ colliding with atomic hydrogen, deuterium, and tritium (Phys. Rev. Lett., 2007). The electron transfer occurs via rotational and radial coupling, whose contributions can be separated, since they occur at significantly different internuclear distances. This separation reveals that rotational coupling produces cross sections that are up three orders of magnitude larger for tritium in comparison with hydrogen.
2002 – 2015: Pioneering experiments studying the transmission of slow highly charged ions through nanocapillaries etched in insulating polymer foils. The capillaries have mesoscopic dimensions of 100 nm diameter and 10 µm length. The experiments indicate that the ions are guided through the capillaries without charge exchange (Phys. Rev. Lett., 2002). A scenario for capillary guiding was proposed in agreement with model calculations. The capillary guiding is produced by self-supporting charge up of the capillary wall, which diminishes when the ions are deflected. The capillary guiding is expected to have promising applications in the field of nanotechnology. Several groups in 17 laboratories started similar measurement.
2011 – 2014: Simulations of ion guiding in capillaries using for the first time a drift model for the removal of the deposited charges. Various interpretations of experimental results.
2012-2016: Writing a review article about Guiding of charged particles through capillaries in insulating materials together with Yasunori Yamazaki, summarizing the work about capillary guiding in a comprehensive manner. The review was published in Physics Reports 629, 1-107 (2016).