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Low-Temperature Ion Trap Studies of N+ (3Pja) + H2 (j) → NH+ + H

Published in The Astrophysical Journal, 2013

Using a cryogenic 22-pole trap, we studied a reaction of nuclear spin isomers of molecular hydrogen with atomic nitrogen ions. We observed something that could be interpreted as a dependence of the reaction rate coefficient on fine structure states of the ion. In order to prove a plausibility of this hypothesis, I constructed a computer model of chemical kinetics and compared the results with the experimental data. This has allowed us to extract state-specific rate coefficients for the reaction. It appears that each of the nitrogen ion’s fine structure states has a significantly different reactivity. This can influence interpretations of some of the emission spectra from interstellar clouds.

Recommended citation: Illia Zymak, Michal Hejduk, Dmytro Mulin, Radek Plašil, Juraj Glosík, Dieter Gerlich, "Low-Temperature Ion Trap Studies of N+ (3Pja) + H2 (j) → NH+ + H". Astrophys. J. 768.1, 86 (2013), p. 86.

Flowing-afterglow study of electron-ion recombination of para–H3+ and ortho–H3+ ions at temperatures from 60 K to 300 K

Published in The Journal of Chemical Physics, 2015

In this paper we describe measurements of electron-ion recombination coefficients for nuclear spin isomers of trihydrogen cations at temperatures down to 60 K. We confirmed that para nuclear spin isomers recombine with a significantly higher rate coefficient than ortho isomers. I developed a method how to modify a population of nuclear spin states of the ions in the existing flowing fterglow apparatus and interpreted the measured data.

Recommended citation: Michal Hejduk, Petr Dohnal, Peter Rubovič, Ábel Kálosi, Radek Plašil, Rainer Johnsen, and Juraj Glosík, "Flowing-afterglow study of electron-ion recombination of para–H3+ and ortho–H3+ ions at temperatures from 60 K to 300 K" J. Chem. Phys. 143(4), 044303 (2015)

Manipulating hydrogen atoms using permanent magnets: Characterisation of a velocity-filtering guide

Published in Review of Scientific Instruments, 2019

In this paper we demonstrate how to pick a narrow band of velocities from a beam of hydrogen atoms. This is important for performing studies on a chemical reactivity between the atoms and ions trapped in a cryogenic ion trap - another device that is being developed by me. We performed extensive molecular dynamics simulations in order to determine a proper design and carried out confirmatory experiments. As a result, we have shown that 52% of particles with v within ± 10 m/s of the target velocity are transmitted for v = 200 m/s, for example. I have built a mathematical model that helped us to interpret results of computer simulations. Without that, we would have struggled to explain some of the observed phenomena.

Recommended citation: Jutta Toscano, Michal Hejduk, Henry G. McGhee, and Brianna R. Heazlewood, "Manipulating hydrogen atoms using permanent magnets: Characterisation of a velocity-filtering guide". Rev. Sci. Instrum. 90.3 (2019), p. 033201.

Evolutionary algorithm optimisation of Zeeman deceleration: Is it worthwhile for longer decelerators?

Published in The Journal of Physical Chemistry A, 2019

We have demonstrated that neutral radicals can be decelerated using relatively short Zeeman decelerators if the switching sequence for electromagnets is optimised using an evolutionary algorithm. It means that investments in long decelerators are not necessary in many cases. Now, several laboratories are considering adapting the same optimisation method to their own experiments. I supervised computer simulations performed by a master degree student and a PhD student.

Recommended citation: Jutta Toscano, Lok Yiu Wu, Michal Hejduk, and Brianna R. Heazlewood, "Evolutionary algorithm optimisation of Zeeman deceleration: Is it worthwhile for longer decelerators?". J. Phys. Chem. A 123 (2019), pp. 5388-5394.

Off-axis parabolic mirror relay microscope for experiments with ultra-cold matter

Published in Review of Scientific Instuments, 2019

A new optical system is introduced for the imaging of Coulomb crystals held in a cryogenic ion trap where there are space limitations preventing the placement of an objective close to the fluorescing ions. The optical system features an off-axis parabolic (OAP) mirror relay microscope that will serve to acquire images of a lattice of fluorescing ions confined within an ultra-high-vacuum vessel operating at temperatures below 10 K. We report that the OAP mirror relay setup can resolve features smaller than the separation between neighboring ions in Coulomb crystals. The setup presented here consists of two 90-degree OAP mirrors arranged into a relay from which standard microscope optics deliver the image to a camera. This design allows the first element in the imaging setup—an OAP mirror—to be located as close as possible to the ion trap, achieving high resolution without the need for a direct line-of-sight to the trap center or for a view port to be located in close proximity to the ion trap. Such an arrangement would not be possible with a standard microscope objective, which is the approach commonly adopted by the field. OAP mirrors represent a novel solution for delivering polychromatic images with micrometer-scale resolution over extended distances.

Recommended citation: Michal Hejduk and Brianna R. Heazlewood, "Off-axis parabolic mirror relay microscope for experiments with ultra-cold matter". Rev. Sci. Instrum. 90 (2019), p. 123701.

Design and characterization of a cryogenic linear Paul ion trap for ion–neutral reaction studies

Published in Review of Scientific Instruments, 2022

Ultra-high vacuum conditions are ideal for the study of trapped ions. They offer an almost perturbation-free environment, where ions confined in traps can be studied for extended periods of time—facilitating precision measurements and allowing infrequent events to be observed. However, if one wishes to study processes involving molecular ions, it is important to consider the effect of blackbody radiation (BBR). The vast majority of molecular ions interact with BBR. At 300 K, state selection in trapped molecular ions can be rapidly lost (in a matter of seconds). To address this issue, and to maintain state selectivity in trapped molecular ions, a cryogenic ion trap chamber has been constructed and characterized. At the center of the apparatus is a linear Paul ion trap, where Coulomb crystals can be formed for ion–neutral reaction studies. Optical access is provided, for lasers and for imaging of the crystals, alongside ion optics and a flight tube for recording time-of-flight mass spectra. The ion trap region, encased within two nested temperature stages, reaches temperatures below 9 K. To avoid vibrations from the cryocooler impeding laser cooling or imaging of the ions, vibration-damping elements are explicitly included. These components successfully inhibit the coupling of vibrations from the cold head to the ion trap—confirmed by accelerometer measurements and by the resolution of images recorded at the trap center (at 9 and 295 K). These results confirm that the cryogenic ion trap apparatus meets all requirements for studying ion–neutral reactions under cold, controlled conditions.

Recommended citation: Miossec, Chloé, Michal Hejduk, Rahul Pandey, Neville J. A. Coughlan, and Brianna R. Heazlewood. ‘Design and Characterization of a Cryogenic Linear Paul Ion Trap for Ion–Neutral Reaction Studies’. Review of Scientific Instruments 93, no. 3 (2022): 033201.


Thermal properties and dynamics of water cluster anions


This is a presentation about my negative results at Clustertreffen 2015. I studied low temperature phase transition of medium-sized negative water clusters (H2O)n-, n=50. Our hypothesis was that the clusters undergo glass transition. The way how to test this hypothesis was to vary a rate of cooling, i.e. the rate of relaxation of the internal structure. Precooling set-up was built but the project suffered from inability to specify and reliably reproduce conditions of cluster formation in an aggregation source. I believe that if we cannot quantify the generation conditions we do not know which initial structure the cluster has, therefore it is impossible to fix relaxation conditions. The project was abandoned for experimental difficulties.

Glass Transition of Small Water Clusters


This was a presentation I gave at a gigantic recruitment event for a position of Special Post-doctoral Researcher at Riken. I must say that I did not get the job even though I got quite far: I was on a waiting list, so if someone had refused the offer, I would have got it. It is quite pity that I had to throw away the research idea.


Ultracold physics and quantum control (zimní semestr)

Undergraduate course, MFF UK, 2021

V říjnu a listopadu 2021 pořádáme sérii přednášek na téma “Fyzika ultrachladných médií a ovládání kvantových stavů”. Díky finanční podpoře od Fondu strategických partnerství Univerzity Karlovy mají posluchači možnost si vyslechnout tradiční výukové (tj. na studenty orientované) přednášky od vědců z Oxfordské univerzity.