Dieter Gerlich – a passion for molecular physics

On October 15th, 2020, Dieter Gerlich died unexpectedly while on a hike in the Black Forest – one of his favorite pleasure activities. His passion was science, and he was well known for his unique experimental work focusing on collisions relating to astro and ion chemistry. Dieter's scientific life continued to gain momentum even after his retirement in 2009.

Dieter Gerlich was born on September 29, 1944 in Eichenbrück (today's Wągrowiec, Poland). In January 1945 the family fled to Leipzig, where his paternal grandmother lived. The family first settled in 1947 to Braunlage, in 1952 to Badenweiler and finally in 1959 to Freiburg im Breisgau. 1963 Dieter graduated from high school and, after military service in Strub, Berchtesgarden, studied physics and mathematics at the university of Freiburg.

Dieter was an enthusiastic tinkerer and so he not only worked on his cars, but also soon built the first experiments in Christoph Schlier's working group. Along with his supervisor Ernst Teloy, he built the first octopole ion guide, which was the start of a long career of developing new tools for guiding and trapping ions and charged particles, many of which are in broad use internationally in a wide variety of research and analytical instrumentation.

During his doctoral work, Dieter Gerlich initially investigated individual collisions between protons and the hydrogen molecule in a crossed-beam apparatus, which he also set up. These included elastic, inelastic and reactive processes in competition. Through systematic investigations with isotopologues (H₂, HD, D₂), these processes could be distinguished from each other. The question also arose as to whether the collision occurs via an intermediate complex (formally ${\text{H}}_3^{+}$).

He accompanied his experimental work with phase space calculations involving the accessible quantum mechanical states of the collisional system. By counting the states permitted energetically and according to angular momentum conservation, he calculated reaction probabilities. These investigations still today represent a milestone in our understanding of this elementary collision process. The dynamics of H⁺ + H₂ collisions continued to be an interest for the rest of his career. For example, recently he collaborated to establish an ion storage experiment with colleagues at the Max Planck Institute for Extraterrestrial Physics in Garching, to investigate the ortho-to-para conversion of molecular hydrogen in H⁺ + H₂ collisions at low collision energies, relevant to conditions in interstellar molecular clouds.

Questions about the formation and decay of molecules and collision complexes were the core of his doctorate work, focusing on how the desired information can be obtained through experiments. The development of unique experimental tools and methods was a central motivation that ran through Dieter Gerlich's scientific life. For example, he developed methods based on his octopole ion guide to measure both integral and differential cross-sections for ion-molecule collisions. As a postdoctoral researcher, Dieter introduced this so-called Guided Ion Beam (GIB) technology into the group of the later Nobel Prize winner Yuan-Tse Lee at UC Berkeley in 1978/1979. This work was in collaboration with graduate student Scott Anderson, with whom he had a lifelong collaboration and friendship, and resulted in the first measurements of accurate absolute cross-sections for reactions of vibrationally state-selected H₂⁺ with a number of atoms and small molecules. GIB instruments were later established in several laboratories worldwide, and radio-frequency ion guiding quickly became a standard and important component of commercial mass spectrometers, where it is used for ion transport, collision cells, and, most recently, ion mobility for isomer/conformer separations of large bio-related ions.

In addition to the further development of GIB instruments, Dieter Gerlich focused on storing ions in inhomogeneous electric RF fields in order to study collisions between ions and molecules at thermal energies, with reaction products analysed mass spectrometrically, allowing thermal rate coefficients to be measured. Traps with higher-order multipoles were developed to allow more efficient buffer gas cooling, so that reactions could be studied down to temperatures of a few Kelvin, thus creating conditions similar to those of cold interstellar clouds. These ion storage devices have proven to be ideal instruments for simulating cold interstellar ion chemistry, and as with the GIB method, many laboratories around the world took up the technical development. Today these experiments also form a global standard.

In 1992, Dieter Gerlich published a review article which described both the theory and many practical details for ion guides and multipole traps based on ion interactions with inhomogeneous radio-frequency (RF) fields. This became the ‘Bible’ for his students and for many researchers interested in developing instruments based on RF storage techniques (see Figure 1).

The early 1990s were a high point in the art of experimentation in Dieter's laboratory in Freiburg. During this time, he developed a new type of merged beam apparatus, superimposing a beam of low energy guided ions with a supersonic molecular beam, to investigate reactions under single-collision conditions at energies down to the meV range. This enabled the study of elementary processes, such as the reaction of nitrogen ions, N⁺, with hydrogen over a broad energy range, previously unattainable. The formation of NH⁺ by N⁺ + H₂ collisions is the rate-determining step in formation interstellar ammonia (NH₃), and therefore these and other studies were important in understanding the formation of interstellar molecules. In very low-energy isolated collisions, the radiative association is an important mechanism for molecule formation, and Dieter also developed a new approach to measuring such processes using low-temperature ion traps. An important example of this type of study concerned the formation of ${\text{CH}}_2^{+}$. Under the low-pressure conditions in the interstellar medium, the reaction C⁺ + H₂ → ${\text{CH}}_2^{+}$ + hν can only take place if the excess energy in the collision complex is carried away by the emission of a photon (E = hν). This process is very unlikely, however, because the photon emission time scale (∼ms) is much longer than the lifetime of the collision complex (≪ µs), making laboratory detection very challenging. Thousands of collisions occur for every one that results in radiative stabilisation of the ${\text{CH}}_2^{+}$ collison complex. Nonetheless, the storage experiments of the Gerlich group were able to measure the rate coefficients for this important reaction in space for the first time.

In 1993, Dieter accepted a full professorship at Chemnitz University of Technology, where he took responsibility for the application for an Innovation College, which was the East German variant of a collaborative research centre, the largest funding scheme in the German system. Dieter Gerlich's working group used the storage method for non-destructive mass determination of single nanoparticles. The innovation lies in the use of non-destructive mass determination via optical detection of the ‘secular’ or Eigenfrequency for the motion of trapped nanoparticles in a three-dimensional quadrupole (Paul) trap. Mass resolutions (M/ΔM) of up to 10⁵ were achieved, allowing detection of the adsorption and laser desorption of adsorbates on the surface of 500 nm particles to be detected with a sub-monolayer sensitivity. This opened a way to study the interactions of analogues of interstellar dust particles with gas particles, which is another important step towards a better understanding of heterogeneous processes in the interstellar medium. Dieter investigated dust particles with the astrophysicist Thomas Henning in Jena (now at the Max Planck Institute for Astronomy, Heidelberg). They founded the research group for Laboratory Astrophysics, which was funded by the German Science Foundation (DFG) between 1999 and 2005. This was an extremely fruitful collaboration in which Dieter and Thomas brought together around ten working groups in the area. The very lively seminars they held together in Jena and Chemnitz brought together not only the different disciplines but also people with very different life experiences, building a real community. From this initial group, a common national community of laboratory astrophysicists developed, which meets annually at the current locations, Heidelberg, Jena, Cologne, Kassel, Hamburg, and Garching. The initiative by Dieter and Thomas more than 25 years ago is the basis for the international recognition of today's German laboratory astrophysics.

Dieter was not only generous with his knowledge and ideas. He gave many groups entire instruments or parts to help get new research projects started. This spirit first revealed itself in the collaboration with Odile Dutuit (Orsay) and during his postdoctoral period at Berkeley. The last GIB device he built between 2014 and 2019 in Cologne in the laboratories of Stephan Schlemmer together with his former doctoral student Igor Savic. Thanks to Dieter's perseverance, today, this instrument is operated very successfully in Novisad (Serbia), where Igor is now a professor of physics. During his time in Chemnitz, he intensified his collaboration with Dirk Schwalm from the Max Planck Institute for Nuclear Physics in Heidelberg, Mark Smith (University of Arizona, Tucson), and Juraj Glosik (Charles University in Prague). The 22-pole storage device developed into an ‘export hit’. The Prague group eventually inherited the most sophisticated trapping instrument. Many shared works emerged from this more than 20-year collaboration.

A highlight from this time is the work on collisions of ions with atomic hydrogen. This marks another high point in the art of experimentation in our field. The Chemnitz period was also characterised by conflicts with an administration that provided limited support for research. Dieter Gerlich retired in 2009 so that he could pursue new scientific interests, and he greatly enjoyed life as a scientific ‘vagabond’, devoting his full energy to numerous collaborations. He always enjoyed vigorous scientific discussions with colleagues, because, according to his statements, he urgently needed strong counterparts to sharpen his thoughts. Certainly, anyone who has participated in discussions with Dieter had to sharpen their own thinking, but learned a great deal, got new ideas, and often made a long time friend over a few beers. During ‘retirement’, Dieter greatly enjoyed collaborations with Jana Roithova (Charles University in Prague and now Radboud Universiteit Nijmegen) and with John Maier (University of Basel). In these laboratories, some of his dreams from his early years could be realised, for example, cooling ions to such low temperatures that Helium atoms could be attached to the stored ions. These so-called 'tag' atoms are just weakly bound and change the bonding conditions of the ion only marginally, allowing now numerous groups to record infrared spectra of stored ions.

The so-called action spectra arise from the detection of the predissociated complexes that lose the tag atom upon photon absorption. In this way, the structure of quite complex molecules can be deciphered. This possibility is particularly valued by organic chemists because the structure and reactivity of a molecule often go hand in hand. In this way, John Maier's working group was able to determine the electronic absorption spectrum of gas phase C₆₀⁺ for the first time. It turned out that the absorption bands at 963.2 nm and 957.7 nm exactly match two of the so-called diffuse interstellar bands (DIBs). This discovery has been a scientific sensation, because the first DIBs were observed by Mary Lea Heger in 1934, but the carriers of these absorption bands were previously unknown. In addition to identifying the first molecule that clearly is a carrier of two of the DIBs, this discovery showed that ions as complex as C₆₀⁺ form and survive in space, raising questions regarding their formation and decay. Discoveries, such as C₆₀⁺ as the cause of DIBs, were not so much at the centre of Dieter's interests, which focused more on the fundamental understanding of the processes behind these phenomena. For example, he was not satisfied with his understanding of the elementary reaction H⁺ + H₂ that he studied for his dissertation, and he carried out a fundamental experiment in an ion trap apparatus. In the rotationally inelastic collision of H⁺ + H₂ (J = 1) → H⁺ + H₂ (J = 0) the approximately 170 K of rotational energy is primarily converted into the translation (kinetic energy) of the H⁺. This could be detected by the loss of the fast H⁺ products across the trapping potential barrier. This process is of fundamental interest because a J = 1 → 0 transition in H₂ is usually not possible because of the Pauli exclusion principle. But with the exchange of three identical fermions, new routes of chemical reactions become possible, and this is exactly the kind of fundamental challenge that fascinated Dieter.

Dieter was always full of ideas and full of the spirit to build new instruments to allow experiments that seemed impossible. In September 2020, he turned 76, but he continued to be engaged in new collaborations with the group of Paola Caselli (Max Planck Institute for Extraterrestrial Physics) and with the group by Manfred Kappes (KIT, Karlsruhe) up until his unexpected death in October of that year.

Dieter was not only an extraordinary scientist, but was also a dedicated father to his three children, Anja, Alf, and Meike. With both family and friends, he was a person who loved hiking together in the Black Forest, or sitting around the campfire with a beer, playing his guitar (Figure 2). For his scientific friends, hot discussions over a cold beer were perhaps the miracle formula of fulfilment.

Figure 1. Reproduction of the front page of the seminal 1992 work by Dieter Gerlich on ion guiding and ion trapping. This will remain a heritage for the field of molecular physics.

Figure 2. Dieter at one of his non-scientific pleasures, playing the guitar singing popular songs.