Profiles

Michael Gatchell

Michael Gatchell

Postdoktor

Visa sidan på svenska
Works at Department of Physics
Email michael.gatchell@fysik.su.se
Visiting address Roslagstullsbacken 21 C, plan 4, Albano
Room C4:3004
Postal address AlbaNova universitetscentrum, Fysikum 106 91 Stockholm

About me

Postdoctoral researcher at Stockholm University and Leopold-Franzens-Universität Innsbruck in Austria. My areas of research inlcude experiments studying clusters of atomic and molecular systems; collisions between atoms, ions, and molecules; and theoretical studies of these processes with the aid of quantum chemistry calculations and molecular dynamics simulations.

Publications

A selection from Stockholm University publication database
  • 2018. Nathalie de Ruette (et al.). Review of Scientific Instruments 89 (7)

    In this paper, we give a detailed description of an electrospray ion source test bench and a single-pass setup for ion fragmentation studies at the Double ElectroStatic Ion Ring ExpEriment infrastructure at Stockholm University. This arrangement allows for collision-induced dissociation experiments at the center-of-mass energies between 10 eV and 1 keV. Charged fragments are analyzed with respect to their kinetic energies (masses) by means of an electrostatic energy analyzer with a wide angular acceptance and adjustable energy resolution.

  • 2018. Michael Gatchell (et al.). Physical Chemistry, Chemical Physics - PCCP 20 (11), 7739-7745

    We have studied complexes of gold atoms and imidazole (C3N2H4, abbreviated Im) produced in helium nanodroplets. Following the ionization of the doped droplets we detect a broad range of different Au(m)Im(n)(+) complexes, however we find that for specific values of m certain n are magic and thus particularly abundant. Our density functional theory calculations indicate that these abundant clusters sizes are partially the result of particularly stable complexes, e.g. AuIm(2)(+), and partially due to a transition in fragmentation patterns from the loss of neutral imidazole molecules for large systems to the loss of neutral gold atoms for smaller systems.

  • 2018. Alicja Domaracka (et al.). Physical Chemistry, Chemical Physics - PCCP 20 (22), 15052-15060

    Ionization, fragmentation and molecular growth have been studied in collisions of 22.5 keV He2+-or 3 keV Ar+-projectiles with pure loosely bound clusters of coronene (C24H12) molecules or with loosely bound mixed C-60-C24H12 clusters by using mass spectrometry. The heavier and slower Ar+ projectiles induce prompt knockout-fragmentation - C- and/or H-losses - from individual molecules and highly efficient secondary molecular growth reactions before the clusters disintegrate on picosecond timescales. The lighter and faster He2+ projectiles have a higher charge and the main reactions are then ionization by ions that are not penetrating the clusters. This leads mostly to cluster fragmentation without molecular growth. However, here penetrating collisions may also lead to molecular growth but to a much smaller extent than with 3 keV Ar+. Here we present fragmentation and molecular growth mass distributions with 1 mass unit resolution, which reveals that the same numbers of C- and H-atoms often participate in the formation and breaking of covalent bonds inside the clusters. We find that masses close to those with integer numbers of intact coronene molecules, or with integer numbers of both intact coronene and C-60 molecules, are formed where often one or several H-atoms are missing or have been added on. We also find that super-hydrogenated coronene is formed inside the clusters.

  • 2018. Marcelo Goulart (et al.). Physical Chemistry, Chemical Physics - PCCP 20 (14), 9554-9560

    Adducts formed between small gold cluster cations and helium atoms are reported for the first time. These binary ions, Aun+Hem, were produced by electron ionization of helium nanodroplets doped with neutral gold clusters and were detected using mass spectrometry. For a given value of n, the distribution of ions as a function of the number of added helium atoms, m, has been recorded. Peaks with anomalously high intensities, corresponding to so-called magic number ions, are identified and interpreted in terms of the geometric structures of the underlying Au-n(+) ions. These features can be accounted for by planar structures for Au-n(+) ions with n <= 7, with the addition of helium having no significant effect on the structures of the underlying gold cluster ions. According to ion mobility studies and some theoretical predictions, a 3-D structure is expected for Au-8(+). However, the findings for Au-8(+) in this work are more consistent with a planar structure.

  • 2018. R. Delaunay (et al.). Carbon 129, 766-774

    We show that the energetic processing of C-60 clusters by slow atomic projectiles leads to ultrafast (< ps) formation of large covalent carbon nanoparticles containing a few hundreds of atoms. The underlying mechanism is found to be due to impulse-driven collisions between the projectile and the nuclei of the molecules. Experimental findings are well reproduced by classical molecular dynamics simulations. The cross sections for molecular growth processes forming covalent systems which contain more than 60 carbon atoms are about 5.10(-14) cm(2) representing more than 70% of the geometrical cross sections. This demonstrates the high efficiency of the underlying processes. The formed carbon nanoparticles contain both aromatic and aliphatic structures which have also been considered as dust components in space.

  • 2017. Marcelo Goulart (et al.). Physical Chemistry, Chemical Physics - PCCP 19 (41), 27968-27973

    Mass spectra of helium nanodroplets doped with H-2 and coronene feature anomalies in the ion abundance that reveal anomalies in the energetics of adsorption sites. The coronene monomer ion strongly adsorbs up to n = 38 H-2 molecules indicating a commensurate solvation shell that preserves the D-6h symmetry of the substrate. No such feature is seen in the abundance of the coronene dimer through tetramer complexed with H-2; this observation rules out a vertical columnar structure. Instead we see evidence for a columnar structure in which adjacent coronenes are displaced in parallel, forming terraces that offer additional strong adsorption sites. The experimental value for the number of adsorption sites per terrace, approximately six, barely depends on the number of coronene molecules. The displacement estimated from this number exceeds the value reported in several theoretical studies of the bare, neutral coronene dimer.

  • 2017. Henning T. Schmidt (et al.). Physical Review Letters 119 (7)

    We apply near-threshold laser photodetachment to characterize the rotational quantum level distribution of OH- ions stored in the cryogenic ion-beam storage ring DESIREE at Stockholm University. We find that the stored ions relax to a rotational temperature of 13.4 +/- 0.2 K with 94.9 +/- 0.3% of the ions in the rotational ground state. This is consistent with the storage ring temperature of 13.5 +/- 0.5 K as measured with eight silicon diodes but in contrast to all earlier studies in cryogenic traps and rings where the rotational temperatures were always much higher than those of the storage devices at their lowest temperatures. Furthermore, we actively modify the rotational distribution through selective photodetachment to produce an OH- beam where 99.1 +/- 0.1% of approximately one million stored ions are in the J = 0 rotational ground state. We measure the intrinsic lifetime of the J = 1 rotational level to be 145 +/- 28 s.

  • 2017. Michael Gatchell (et al.). Physical Chemistry, Chemical Physics - PCCP 19 (30), 19665-19672

    We report on studies of collisions between 3 keV Ar+ projectile ions and neutral targets of isolated 1,3-butadiene (C4H6) molecules and cold, loosely bound clusters of these molecules. We identify molecular growth processes within the molecular clusters that appears to be driven by knockout processes and that could result in the formation of (aromatic) ring structures. These types of reactions are not unique to specific projectile ions and target molecules, but will occur whenever atoms or ions with suitable masses and kinetic energies collide with aggregates of matter, such as carbonaceous grains in the interstellar medium or aerosol nanoparticles in the atmosphere.

  • 2017. Linda Giacomozzi (et al.). Physical Chemistry, Chemical Physics - PCCP 19 (30), 19750-19755

    We have studied collisions between tetraphenylporphyrin cations and He or Ne at center-of-mass energies in the range 50-110 eV. The experimental results were interpreted in view of density functional theory calculations of dissociation energies and classical molecular dynamics simulations of how the molecules respond to the He/Ne impact. We demonstrate that prompt atom knockout strongly contributes to the total destruction cross sections. Such impulse driven processes typically yield highly reactive fragments and are expected to be important for collisions with any molecular system in this collision energy range, but have earlier been very difficult to isolate for biomolecules.

  • 2017. K. Hansen (et al.). Physical Review A 95 (2)

    We have measured the spontaneous neutral particle emission from copper-cluster anions ( Cu-n(-), n = 3-6) stored at cryogenic temperatures in one of the electrostatic ion storage rings of the Double ElectroStatic Ion Ring ExpEriment facility at Stockholm University. The measured rate of emission from the stored Cu-3(-) ions follows a single power-law decay for about 1 ms but then decreases much more rapidly with time. The latter behavior may be due to a decrease in the density of available final states in Cu-3 as the excitation energies of the decaying ions approach the electron detachment threshold. The emissions from Cu-4(-), Cu-5(-), and Cu-6(-) are well described by sums of two power laws that are quenched by radiative cooling of the stored ions with characteristic times between a few and hundreds of milliseconds. We relate these two-component behaviors to populations of stored ions with higher and lower angular momenta. In a separate experiment, we studied the laser-induced decay of Cu-6(-) ions that were excited by 1.13- or 1.45-eV photons after 46 ms of storage.

  • 2016. Michael Gatchell, Henning Zettergren. Journal of Physics B 49 (16)

    Energetic ions lose some of their kinetic energy when interacting with electrons or nuclei in matter. Here, we discuss combined experimental and theoretical studies on such impulse driven reactions in polycyclic aromatic hydrocarbons (PAHs), fullerenes, and pure or mixed clusters of these molecules. These studies show that the nature of excitation is important for how complex molecular systems respond to ion/atom impact. Rutherford-like nuclear scattering processes may lead to prompt atom knockout and formation of highly reactive fragments, while heating of the molecular electron clouds in general lead to formation of more stable and less reactive fragments. In this topical review, we focus on recent studies of knockout driven reactions, and present new calculations of the angular dependent threshold (displacement) energies for such processes in PAHs. The so-formed fragments may efficiently form covalent bonds with neighboring molecules in clusters. These unique molecular growth processes may be important in astrophysical environments such as low velocity shock waves.

  • 2016. S. E. Huber (et al.). Carbon 109, 843-850

    Fullerenes (and clusters composed of them) yield a variety of promising structural, electronic, magnetic and chemical properties, governed by their specific electronic and geometric configuration. These systems have attracted many theoretical and experimental endeavors in order to describe, explain and predict their features. The conclusive description of some specific properties has remained a challenge though, such as a sound physicochemical description of the stability of multiply charged fullerene clusters, which we explore here. We show how simple models based on classical electrostatics allow one to understand the (fragmentation) dynamics of multiply ionized fullerene aggregates without the use of elaborate and time-consuming computational quantum chemical approaches. These models successfully explain why the fullerene pentamer is the smallest dicationic cluster experimentally observed, despite its thermodynamic instability. These predictions are of importance in various fields such as cluster physics, astrochemistry, electrochemistry and solid-state chemistry.

  • 2016. Michael Gatchell (et al.).

    When molecules are excited by photons or energetic particles, they will cool through the emission of photons, electrons, or by fragmenting. Such processes are often thermal as they occur after the excitation energy has been redistributed across all degrees-of-freedom in the system. Collisions with atoms or ions may also lead to ultrafast fragmentation in Rutherford-like scattering processes, where one or several atoms can literally be knocked out of the molecule by the incoming projectile before the energy can be completely redistributed. The resulting fragmentation pathways can in such knockout processes be very different from those in thermal processes.

    This thesis covers extensive studies of collisions between ions/atoms and isolated Polycyclic Aromatic Hydrocarbon (PAH) molecules, isolated fullerene molecules, or clusters of these. The high stabilities and distinct fragmentation channels make these types of molecules excellent test cases for characterizing knockout-driven fragmentation and the reactions that these processes can lead to. I will present experimental measurements for a wide range of energies and compare them with my own molecular dynamics simulations and quantum chemical calculations. In this thesis, I present an in-depth study of the role of knockout in the energetic processing of molecules and clusters. The competition between knockout and thermally driven fragmentation is discussed in detail.

    Knockout-driven fragmentation is shown to result in exotic fragments that are far more reactive than the intact parent molecules or fragments from thermal processes. When such reactive species are formed within molecular clusters efficient molecular growth can take place on sub-picosecond timescales. The cluster environments are crucial here because they protect the newly formed molecules by absorbing excess energy. This is a possible pathway for the growth of large PAHs, fullerenes, and similar carbonaceous complexes found in, for instance, the interstellar medium.

  • 2015. Michael Gatchell (et al.). Physical Review A. Atomic, Molecular, and Optical Physics 92 (5)

    A recent study of soft x-ray absorption in native and hydrogenated coronene cations, C24H12+m + m = 0-7, led to the conclusion that additional hydrogen atoms protect (interstellar) polycyclic aromatic hydrocarbon (PAH) molecules from fragmentation [Reitsma et al., Phys. Rev. Lett. 113, 053002 (2014)]. The present experiment with collisions between fast (30-200 eV) He atoms and pyrene (C16H10+m +, m = 0, 6, and 16) and simulations without reference to the excitation method suggests the opposite. We find that the absolute carbon-backbone fragmentation cross section does not decrease but increases with the degree of hydrogenation for pyrene molecules.

  • 2015. Delaunay Rudy (et al.). Journal of Physical Chemistry Letters 6 (9), 1536-1542

    The present work combines experimental and theoretical studies of the collision between keV ion projectiles and clusters of pyrene, one of the simplest polycyclic aromatic hydrocarbons (PAHs). Intracluster growth processes induced by ion collisions lead to the formation of a wide range of new molecules with masses larger than that of the pyrene molecule. The efficiency of these processes is found to strongly depend on the mass and velocity of the incoming projectile. Classical molecular dynamics simulations of the entire collision process-from the ion impact (nuclear scattering) to the formation of new molecular species-reproduce the essential features of the measured molecular growth process and also yield estimates of the related absolute cross sections. More elaborate density functional tight binding calculations yield the same growth products as the classical simulations. The present results could be relevant to understand the physical chemistry of the PAH-rich upper atmosphere of Saturn’s moon Titan.

  • 2015. Mark H. Stockett (et al.). Journal of Physical Chemistry Letters 6 (22), 4504-4509

    We have measured absolute cross sections for ultrafast (femtosecond) single-carbon knockout from polycyclic aromatic hydrocarbon (PAR) cations as functions of He-PAR center-of-mass collision energy in the 10-200 eV range. Classical molecular dynamics (MD) simulations cover this range and extend up to 105 eV. The shapes of the knockout cross sections are well-described by a simple analytical expression yielding experimental and MD threshold energies of E-th(Exp) = 32.5 +/- 0.4 eV and E-th(MD) = 41.0 +/- 0.3 eV, respectively. These are the first measurements of knockout threshold energies for molecules isolated in vacuo. We further deduce semiempirical (SE) and MD displacement energies, i.e., the energy transfers to the PAH molecules at the threshold energies for knockout, of T-disp(SE) = 23.3 +/- 0.3 eV and T-disp(MD) = 27.0 +/- 0.3 eV. The semiempirical results compare favorably with measured displacement energies for graphene (T-disp = 23.6 eV).

Show all publications by Michael Gatchell at Stockholm University

Last updated: January 8, 2019

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