I urval från Stockholms universitets publikationsdatabas

• Mutual Neutralization of NO plus with O-

  2024. Mathias Poline (et al.). Physical Review Letters 132 (2)

  Artikel

  We have studied the mutual neutralization reaction of vibronically cold NO+ with O- at a collision energy of approximate to 0.1 eV and under single-collision conditions. The reaction is completely dominated by production of three ground-state atomic fragments. We employ product-momentum analysis in the framework of a simple model, which assumes the anion acts only as an electron donor and the product neutral molecule acts as a free rotor, to conclude that the process occurs in a two-step mechanism via an intermediate Rydberg state of NO which subsequently fragments.

  Läs mer om Mutual Neutralization of NO plus with O-

• The mutual neutralization of hydronium and hydroxide

  2024. Alon Bogot (et al.). Science 383 (6680), 285-289

  Artikel

  Mutual neutralization of hydronium (H₃O⁺) and hydroxide (OH⁻) ions is a very fundamental chemical reaction. Yet, there is only limited experimental evidence about the underlying reaction mechanisms. Here, we report three-dimensional imaging of coincident neutral products of mutual-neutralization reactions at low collision energies of cold and isolated ions in the cryogenic double electrostatic ion-beam storage ring (DESIREE). We identified predominant H2O + OH + H and 2OH + H₂ product channels and attributed them to an electron-transfer mechanism, whereas a minor contribution of H₂O + H₂O with high internal excitation was attributed to proton transfer. The reported mechanism-resolved internal product excitation, as well as collision-energy and initial ion-temperature dependence, provide a benchmark for modeling charge-transfer mechanisms. 

  Läs mer om The mutual neutralization of hydronium and hydroxide

• Stability of C₅₉ Knockout Fragments from Femtoseconds to Infinity

  2024. Michael Gatchell (et al.). Astrophysical Journal 966 (2)

  Artikel

  We have studied the stability of C₅₉ anions as a function of time, from their formation on femtosecond timescales to their stabilization on second timescales and beyond, using a combination of theory and experiments. The C^-₅₉ fragments were produced in collisions between C₆₀ fullerene anions and neutral helium gas at a velocity of 90 km s⁻¹ (corresponding to a collision energy of 166 eV in the center-of-mass frame). The fragments were then stored in a cryogenic ion beam storage ring at the DESIREE facility, where they were followed for up to 1 minute. Classical molecular dynamics simulations were used to determine the reaction cross section and the excitation energy distributions of the products formed in these collisions. We find that about 15% of the C^-₅₉ ions initially stored in the ring are intact after about 100 ms and that this population then remains intact indefinitely. This means that C₆₀ fullerenes exposed to energetic atoms and ions, such as stellar winds and shock waves, will produce stable, highly reactive products, like C₅₉, that are fed into interstellar chemical reaction networks.

  Läs mer om Stability of C₅₉ Knockout Fragments from Femtoseconds to Infinity

• Stability and Cooling of the C²⁻₇ Dianion

  2023. P. K. Najeeb (et al.). Physical Review Letters 131 (11)

  Artikel

  We have studied the stability of the smallest long-lived all carbon molecular dianion () in new time domains and with a single ion at a time using a cryogenic electrostatic ion-beam storage ring. We observe spontaneous electron emission from internally excited dianions on millisecond timescales and monitor the survival of single colder molecules on much longer timescales. We find that their intrinsic lifetime exceeds several minutes—6 orders of magnitude longer than established from earlier experiments on . This is consistent with our calculations of vertical electron detachment energies predicting one inherently stable isomer and one isomer which is stable or effectively stable behind a large Coulomb barrier for →+e⁻ separation.

  Läs mer om Stability and Cooling of the C²⁻₇ Dianion

• The largest fullerene

  2023. Michael Gatchell, Henning Zettergren, Klavs Hansen. Physical Chemistry, Chemical Physics - PCCP 25 (25), 16790-16795

  Artikel

  Fullerenes are lowest energy structures for gas phase all-carbon particles for a range of sizes, but graphite remains the lowest energy allotrope of bulk carbon. This implies that the lowest energy structure changes nature from fullerenes to graphite or graphene at some size and therefore, in turn, implies a limit on the size of free fullerenes as ground state structures. We calculate this largest stable single shell fullerene to be of size N = 1 × 10⁴, using the AIREBO effective potential. Above this size fullerene onions are more stable, with an energy per atom that approaches graphite structures. Onions and graphite have very similar ground state energies, raising the intriguing possibility that fullerene onions could be the lowest free energy states of large carbon particles in some temperature range.

  Läs mer om The largest fullerene

• Efficient stabilization of cyanonaphthalene by fast radiative cooling and implications for the resilience of small PAHs in interstellar clouds

  2023. Mark H. Stockett (et al.). Nature Communications 14 (1)

  Artikel

  After decades of searching, astronomers have recently identified specific Polycyclic Aromatic Hydrocarbons (PAHs) in space. Remarkably, the observed abundance of cyanonaphthalene (CNN, C10H7CN) in the Taurus Molecular Cloud (TMC-1) is six orders of magnitude higher than expected from astrophysical modeling. Here, we report unimolecular dissociation and radiative cooling rate coefficients of the 1-CNN isomer in its cationic form. These results are based on measurements of the time-dependent neutral product emission rate and kinetic energy release distributions produced from an ensemble of internally excited 1-CNN+ studied in an environment similar to that in interstellar clouds. We find that Recurrent Fluorescence - radiative relaxation via thermally populated electronic excited states - efficiently stabilizes 1-CNN+, owing to a large enhancement of the electronic transition probability by vibronic coupling. Our results help explain the anomalous abundance of CNN in TMC-1 and challenge the widely accepted picture of rapid destruction of small PAHs in space.

  Läs mer om Efficient stabilization of cyanonaphthalene by fast radiative cooling and implications for the resilience of small PAHs in interstellar clouds

• Efficient radiative cooling of tetracene cations C₁₈H₁₂⁺: absolute recurrent fluorescence rates as a function of internal energy

  2023. Jérôme Bernard (et al.). Physical Chemistry, Chemical Physics - PCCP 25 (15), 10726-10740

  Artikel

  We have measured recurrent fluorescence (RF) cooling rates of internally hot tetracene cations, C₁₈H₁₂⁺, as functions of their storage times and internal energies in two different electrostatic ion-beam storage rings – the cryogenic ring DESIREE with a circumference of 8.6 meters in Stockholm and the much smaller room temperature ring Mini-Ring in Lyon, which has a circumference of 0.71 meters. The RF rates were measured to be as high as 150 to 1000 s⁻¹ for internal energies in the 7 to 9.4 eV energy range, where we have probed the time evolution of the internal energy distribution with nanosecond laser pulses with a 1 kHz repetition rate. These RF rates are found to be significantly higher than those of previously investigated smaller PAHs such as e.g. anthracene and naphthalene, for which the lowest non-forbidden electronic excited state, the D₂ state, is populated with a smaller probability by inverse internal conversion. Furthermore, the D₂–D₀ transition rate is smaller for these smaller molecules than for tetracene. The complementary features of the two storage rings allow for RF rate measurements in a broader internal energy range than has been possible before. The smaller sampling period of about 6 μs in Mini-Ring is ideal to study the cooling dynamics of the hotter ions that decay fast, whereas DESIREE with a sampling period of about 60 μs is better suited to study the colder ions that decay on longer timescales ranging up to hundreds of milliseconds. The excellent agreement between the two series of measurements in the region where they overlap demonstrates the complementarity of the two electrostatic ion-beam storage rings.

  Läs mer om Efficient radiative cooling of tetracene cations C₁₈H₁₂⁺

• Open questions on the interaction dynamics of molecules and clusters in the gas phase

  2022. Michael Gatchell, Henning Zettergren. Communications Chemistry 5

  Artikel

  Emerging experimental techniques combined with theoretical advances allow unprecedented studies of the dynamics of gas phase molecules and clusters induced in interactions with photons, electrons, or heavy particles. Here, the authors highlight recent advances, key open questions, and challenges in this field of research with focus on experimental studies of dynamics of ions stored on millisecond timescales and beyond, and its applications in astrochemistry and astronomy. 

  Läs mer om Open questions on the interaction dynamics of molecules and clusters in the gas phase

• Clusters of Fullerenes: Structures and Dynamics

  2022. Klavs Hansen, Henning Zettergren. Journal of Physical Chemistry A 126 (44), 8173-8187

  Artikel

  The geometric structures and reaction dynamics of clusters of carbon fullerene molecules are reviewed. The topics on structure cover the elementary building blocks, the interatomic and intermolecular potentials, and the geometric structures of the aggregates. The dynamics part describes the time development after excitation with laser light, in bimolecular collisions, and in collisions with high energy atomic ions. These reactions produce singly or multiply charged clusters, fragmented or intact fullerene monomers, and fusion products. The duration of the interaction time and the transferred energy are found to play determining roles for the nature of the products. Short interaction times and high energy excitation favor intramolecular reactions on excited potential energy surfaces, producing fused products, often with a strong release of the combined excess energy and heat of fusion. Clusters excited at low energies tend to disintegrate into intact fullerene molecules.

  Läs mer om Clusters of Fullerenes

• PDRs4All: A JWST Early Release Science Program on Radiative Feedback from Massive Stars

  2022. Olivier Berne (et al.). Publications of the Astronomical Society of the Pacific 134 (1035)

  Artikel

  Massive stars disrupt their natal molecular cloud material through radiative and mechanical feedback processes. These processes have profound effects on the evolution of interstellar matter in our Galaxy and throughout the universe, from the era of vigorous star formation at redshifts of 1-3 to the present day. The dominant feedback processes can be probed by observations of the Photo-Dissociation Regions (PDRs) where the far-ultraviolet photons of massive stars create warm regions of gas and dust in the neutral atomic and molecular gas. PDR emission provides a unique tool to study in detail the physical and chemical processes that are relevant for most of the mass in inter- and circumstellar media including diffuse clouds, proto-planetary disks, and molecular cloud surfaces, globules, planetary nebulae, and star-forming regions. PDR emission dominates the infrared (IR) spectra of star-forming galaxies. Most of the Galactic and extragalactic observations obtained with the James Webb Space Telescope (JWST) will therefore arise in PDR emission. In this paper we present an Early Release Science program using the MIRI, NIRSpec, and NIRCam instruments dedicated to the observations of an emblematic and nearby PDR: the Orion Bar. These early JWST observations will provide template data sets designed to identify key PDR characteristics in JWST observations. These data will serve to benchmark PDR models and extend them into the JWST era. We also present the Science-Enabling products that we will provide to the community. These template data sets and Science-Enabling products will guide the preparation of future proposals on star-forming regions in our Galaxy and beyond and will facilitate data analysis and interpretation of forthcoming JWST observations.

  Läs mer om PDRs4All

• Survival of polycyclic aromatic hydrocarbon knockout fragments in the interstellar medium

  2021. Michael Gatchell (et al.). Nature Communications 12 (1)

  Artikel

  Laboratory studies play a crucial role in understanding the chemical nature of the interstellar medium (ISM), but the disconnect between experimental timescales and the timescales of reactions in space can make a direct comparison between observations, laboratory, and model results difficult. Here we study the survival of reactive fragments of the polycyclic aromatic hydrocarbon (PAH) coronene, where individual C atoms have been knocked out of the molecules in hard collisions with He atoms at stellar wind and supernova shockwave velocities. Ionic fragments are stored in the DESIREE cryogenic ion-beam storage ring where we investigate their decay for up to one second. After 10 ms the initially hot stored ions have cooled enough so that spontaneous dissociation no longer takes place at a measurable rate; a majority of the fragments remain intact and will continue to do so indefinitely in isolation. Our findings show that defective PAHs formed in energetic collisions with heavy particles may survive at thermal equilibrium in the interstellar medium indefinitely, and could play an important role in the chemistry in there, due to their increased reactivity compared to intact or photo-fragmented PAHs. Ion storage rings allow reactions to be studied over orders of magnitude in time, bridging the gap between typical experimental and astronomical timescales. Here the authors observe that polycyclic aromatic hydrocarbon fragments produced upon collision with He atoms at velocities typical of stellar winds and supernova shockwaves remain intact up to second timescales, thus may play an important role in interstellar chemistry.

  Läs mer om Survival of polycyclic aromatic hydrocarbon knockout fragments in the interstellar medium

• Roadmap on dynamics of molecules and clusters in the gas phase

  2021. Henning Zettergren (et al.). European Physical Journal D 75 (5)

  Artikel

  This roadmap article highlights recent advances, challenges and future prospects in studies of the dynamics of molecules and clusters in the gas phase. It comprises nineteen contributions by scientists with leading expertise in complementary experimental and theoretical techniques to probe the dynamics on timescales spanning twenty order of magnitudes, from attoseconds to minutes and beyond, and for systems ranging in complexity from the smallest (diatomic) molecules to clusters and nanoparticles. Combining some of these techniques opens up new avenues to unravel hitherto unexplored reaction pathways and mechanisms, and to establish their significance in, e.g. radiotherapy and radiation damage on the nanoscale, astrophysics, astrochemistry and atmospheric science.

  Läs mer om Roadmap on dynamics of molecules and clusters in the gas phase

• "Smart Decomposition" of Cyclic Alanine-Alanine Dipeptide by VUV Radiation

  2021. Darío Barreiro-Lage (et al.). Journal of Physical Chemistry Letters 12 (30), 7379-7386

  Artikel

  A combined experimental and theoretical study shows how the interaction of VUV radiation with cyclo-(alanine-alanine), one of the 2,5-diketopiperazines (DKPs), produces reactive oxazolidinone intermediates. The theoretical simulations reveal that the interaction of these intermediates with other neutral and charged fragments, released in the molecular decomposition, leads either to the reconstruction of the cyclic dipeptide or to the formation of longer linear peptide chains. These results may explain how DKPs could have, on one hand, survived hostile chemical environments and, on the other, provided the seed for amino acid polymerization. Shedding light on the mechanisms of production of such prebiotic building blocks is of paramount importance to understanding the abiotic synthesis of relevant biologically active compounds.

  Läs mer om "Smart Decomposition" of Cyclic Alanine-Alanine Dipeptide by VUV Radiation

• Experimental and theoretical studies of excited states in Ir-

  2021. Moa K. Kristiansson (et al.). Physical Review A: covering atomic, molecular, and optical physics and quantum information 103 (6)

  Artikel

  The properties of atomic negative ions are to a large extent determined by electron-electron correlation which makes them an ideal testing ground for atomic many-body physics. In this paper, we present a detailed experimental and theoretical study of excited states in the negative ion of iridium. The ions were stored at cryogenic temperatures using the double electrostatic ion ring experiment facility at Stockholm University. Laser photodetachment was used to monitor the relaxation of three bound excited states belonging to the [Xe] 4f(14)5d(8)6s(2) ionic ground configuration. Our measurements show that the first excited state has a lifetime much longer than the ion-beam storage time of 1230 +/- 100 s. The binding energy of this state was measured to be 1.045 +/- 0.002 eV. The lifetimes of the second and third excited states were experimentally determined to be 133 +/- 10 and 172 +/- 35 ms, respectively. Multiconfiguration Dirac-Hartree-Fock calculations were performed in order to extract binding energies and lifetimes. These calculations predict the existence of the third excited bound state that was detected experimentally. The computed lifetimes for the three excited bound states agree well with the experimental results and allow for a clear identification of the detected levels.

  Läs mer om Experimental and theoretical studies of excited states in Ir-

• Mutual neutralisation of O⁺ with O⁻

  2021. Mathias Poline (et al.). Physical Chemistry, Chemical Physics - PCCP 23 (43), 24607-24616

  Artikel

  The mutual neutralisation of O⁺ with O⁻ has been studied in a double ion-beam storage ring with combined merged-beams, imaging and timing techniques. Branching ratios were measured at the collision energies of 55, 75 and 170 (± 15) meV, and found to be in good agreement with previous single-pass merged-beams experimental results at 7 meV collision energy. Several previously unidentified spectral features were found to correspond to mutual neutralisation channels of the first metastable state of the cation (O⁺(²D^o), τ ≈ 3.6 hours), while no contributions from the second metastable state (O⁺(²P^o), τ ≈ 5 seconds) were observed. Theoretical calculations were performed using the multi-channel Landau–Zener model combined with the anion centered asymptotic method, and gave good agreement with several experimentally observed channels, but could not describe well observed contributions from the O⁺(²D^o) metastable state as well as channels involving the O(3s ⁵S^o) state.

  Läs mer om Mutual neutralisation of O⁺ with O⁻

• Mutual Neutralization in Li⁺+H⁻/D⁻ and Na⁺+H⁻/D⁻ Collisions

  2021. Paul S. Barklem (et al.). Astrophysical Journal 908 (2)

  Artikel

  Advances in merged-beams instruments have allowed experimental studies of the mutual neutralization (MN) processes in collisions of both Li⁺ and Na⁺ ions with D⁻ at energies below 1 eV. These experimental results place constraints on theoretical predictions of MN processes of Li⁺ and Na⁺ with H⁻, important for non-LTE modeling of Li and Na spectra in late-type stars. We compare experimental results with calculations for methods typically used to calculate MN processes, namely the full quantum (FQ) approach, and asymptotic model approaches based on the linear combination of atomic orbitals (LCAO) and semiempirical (SE) methods for deriving couplings. It is found that FQ calculations compare best overall with the experiments, followed by the LCAO, and the SE approaches. The experimental results together with the theoretical calculations, allow us to investigate the effects on modeled spectra and derived abundances and their uncertainties arising from uncertainties in the MN rates. Numerical experiments in a large grid of 1D model atmospheres, and a smaller set of 3D models, indicate that neglect of MN can lead to abundance errors of up to 0.1 dex (26%) for Li at low metallicity, and 0.2 dex (58%) for Na at high metallicity, while the uncertainties in the relevant MN rates as constrained by experiments correspond to uncertainties in abundances of much less than 0.01 dex (2%). This agreement for simple atoms gives confidence in the FQ, LCAO, and SE model approaches to be able to predict MN with the accuracy required for non-LTE modeling in stellar atmospheres.

  Läs mer om Mutual Neutralization in Li⁺+H⁻/D⁻ and Na⁺+H⁻/D⁻ Collisions

• Final-state-resolved mutual neutralization of Na+ and D-

  2021. Gustav Eklund (et al.). Physical Review A: covering atomic, molecular, and optical physics and quantum information 103 (3)

  Artikel

  The present paper reports on a merged-beam experiment on mutual neutralization between Na+ and D-. For this experiment, we have used the DESIREE ion-beams storage-ring facility. The reaction products are detected using a position- and time-sensitive detector, which ideally allows for determination of the population of each individual quantum state in the final atomic systems. Here, the 4s, 3d, and 4p final states in Na are observed and in all cases the D atom is in its ground state 1s S-2. The respective branching fractions of the states populated in Na are determined by fitting results from a Monte Carlo simulation of the experiment to the measured data. The center-of-mass collision energy is controlled using a set of biased drift tubes, and the branching fractions are measured for energies between 80 meV and 393 meV. The resulting branching fractions are found to agree qualitatively with the only available theoretical calculations for this particular system, which are based on a multichannel Landau-Zener approach using dynamic couplings determined with a linear combination of atomic orbitals model.

  Läs mer om Final-state-resolved mutual neutralization of Na+ and D-

• Spontaneous Electron Emission from Hot Silver Dimer Anions

  2020. Emma K. Anderson (et al.). Physical Review Letters 124 (17)

  Artikel

  We report the first experimental evidence of spontaneous electron emission from a homonuclear dimer anion through direct measurements of Ag-2(-) -> Ag-2 + e(-) decays on milliseconds and seconds timescales. This observation is very surprising as there is no avoided crossing between adiabatic energy curves to mediate such a process. The process is weak, yet dominates the decay signal after 100 ms when ensembles of internally hot Ag-2(-) ions are stored in the cryogenic ion-beam storage ring, DESIREE, for 10 s. The electron emission process is associated with an instantaneous, very large reduction of the vibrational energy of the dimer system. This represents a dramatic deviation from a Born-Oppenheimer description of dimer dynamics.

  Läs mer om Spontaneous Electron Emission from Hot Silver Dimer Anions

• Radiative cooling of carbon cluster anions C-2n+1(-) (n=3-5)

  2020. Mark H. Stockett (et al.). European Physical Journal D 74 (7)

  Artikel

  Radiative cooling of carbon cluster anions C-2n+1(-)(n = 3-5) is investigated using the cryogenic electrostatic ion storage ring DESIREE. Two different strategies are applied to infer infrared emission on slow (milliseconds to seconds) and ultraslow (seconds to minutes) timescales. Initial cooling of the ions over the millisecond timescale is probed indirectly by monitoring the decay in the yield of spontaneous neutralization by thermionic emission. The observed cooling rates are consistent with a statistical model of thermionic electron emission in competition with infrared photon emission due to vibrational de-excitation. Slower cooling over the seconds to minutes timescale associated with infrared emission from low-frequency vibrational modes is probed using time-dependent action spectroscopy. For C(9)(-)and C-11(-), cooling is evidenced by the time-evolution of the yield of photo-induced neutralization following resonant excitation of electronic transitions near the detachment threshold. The cross-section for resonant photo-excitation is at least two orders of magnitude greater than for direct photodetachment. In contrast, C(7)(-)lacks electronic transitions near the detachment threshold.

  Läs mer om Radiative cooling of carbon cluster anions C-2n+1(-) (n=3-5)

• Unimolecular fragmentation and radiative cooling of isolated PAH ions

  2020. Mark H. Stockett (et al.). Journal of Chemical Physics 153 (15)

  Artikel

  Time-resolved spontaneous and laser-induced unimolecular fragmentation of perylene cations (C20H12+) has been measured on timescales up to 2 s in a cryogenic electrostatic ion beam storage ring. We elaborate a quantitative model, which includes fragmentation in competition with radiative cooling via both vibrational and electronic (recurrent fluorescence) de-excitation. Excellent agreement with experimental results is found when sequential fragmentation of daughter ions co-stored with the parent perylene ions is included in the model. Based on the comparison of the model to experiment, we constrain the oscillator strength of the D-1 -> D-0 emissive electronic transition in perylene (f(RF) = 0.055 +/- 0.011), as well as the absolute absorption cross section of the D-5 <- D-0 excitation transition (sigma (abs) > 670 Mb). The former transition is responsible for the laser-induced and recurrent fluorescence of perylene, and the latter is the most prominent in the absorption spectrum. The vibrational cooling rate is found to be consistent with the simple harmonic cascade approximation. Quantitative experimental benchmarks of unimolecular processes in polycyclic aromatic hydrocarbon ions like perylene are important for refining astrochemical models.

  Läs mer om Unimolecular fragmentation and radiative cooling of isolated PAH ions

• Decay pathways for protonated and deprotonated adenine molecules

  2019. Linda Giacomozzi (et al.). Journal of Chemical Physics 151 (4)

  Artikel

  We have measured fragment mass spectra and total destruction cross sections for protonated and deprotonated adenine following collisions with He at center-of-mass energies in the 20-240 eV range. Classical and ab initio molecular dynamics simulations are used to provide detailed information on the fragmentation pathways and suggest a range of alternative routes compared to those reported in earlier studies. These new pathways involve, for instance, losses of HNC molecules from protonated adenine and losses of NH2 or C3H2N2 from deprotonated adenine. The present results may be important to advance the understanding of how biomolecules may be formed and processed in various astrophysical environments.

  Läs mer om Decay pathways for protonated and deprotonated adenine molecules

• Storage time dependent photodissociation action spectroscopy of polycyclic aromatic hydrocarbon cations in the cryogenic electrostatic storage ring DESIREE

  2019. Mark H. Stockett (et al.). Faraday discussions (Online) 217, 126-137

  Artikel

  The multi-photon photodissociation action spectrum of the coronene cation (C24H12+) has been measured in the cryogenic electrostatic storage ring DESIREE (Double ElectroStatic Ion Ring ExpEriment) as a function of storage time. These measurements reveal not only the intrinsic absorption profile of isolated coronene cations, but also the rate at which hot-band absorptions are quenched by radiative cooling. Just after injection, the action spectrum is severely reddened by hot-band absorptions. These hot bands fade with a time constant of 200 ms, which is consistent with radiative cooling via infrared emission from vibrational transitions. A comparison of the present results to those obtained in cryogenic ion trap experiments is discussed at length.

  Läs mer om Storage time dependent photodissociation action spectroscopy of polycyclic aromatic hydrocarbon cations in the cryogenic electrostatic storage ring DESIREE

• Roadmap on photonic, electronic and atomic collision physics

  2019. Friedrich Aumayr (et al.). Journal of Physics B 52 (17)

  Artikel

  We publish three Roadmaps on photonic, electronic and atomic collision physics in order to celebrate the 60th anniversary of the ICPEAC conference. Roadmap III focusses on heavy particles: with zero to relativistic speeds. Modern theoretical and experimental approaches provide detailed insight into the wide range of many-body interactions involving projectiles and targets of varying complexity ranging from simple atoms, through molecules and clusters, complex biomolecules and nanoparticles to surfaces and crystals. These developments have been driven by technological progress and future developments will expand the horizon of the systems that can be studied. This Roadmap aims at looking back along the road, explaining the evolution of the field, and looking forward, collecting nineteen contributions from leading scientists in the field.

  Läs mer om Roadmap on photonic, electronic and atomic collision physics

• Ultraslow radiative cooling of Cn- (n=3-5)

  2019. James N. Bull (et al.). Journal of Chemical Physics 151 (11)

  Artikel

  Ultraslow radiative cooling lifetimes and adiabatic detachment energies for three astrochemically relevant anions, Cn- (n = 3-5), are measured using the Double ElectroStatic Ion Ring ExpEriment (DESIREE) infrastructure at Stockholm University. DESIREE maintains a background pressure of approximate to 10(-14) mbar and temperature of approximate to 13 K, allowing storage of mass-selected ions for hours and providing conditions coined a molecular cloud in a box. Here, we construct two-dimensional (2D) photodetachment spectra for the target anions by recording photodetachment signal as a function of irradiation wavelength and ion storage time (seconds to minute time scale). Ion cooling lifetimes, which are associated with infrared radiative emission, are extracted from the 2D photodetachment spectrum for each ion by tracking the disappearance of vibrational hot-band signal with ion storage time, giving 1e cooling lifetimes of 3.1 +/- 0.1 s (C3-), 6.8 +/- 0.5 s (C4-), and 24 +/- 5 s (C5-). Fits of the photodetachment spectra for cold ions, i.e., those stored for at least 30 s, provide adiabatic detachment energies in good agreement with values from laser photoelectron spectroscopy on jet-cooled anions, confirming that radiative cooling has occurred in DESIREE. Ion cooling lifetimes are simulated using a simple harmonic cascade model, finding good agreement with experiment and providing a mode-by-mode understanding of the radiative cooling properties. The 2D photodetachment strategy and radiative cooling modeling developed in this study could be applied to investigate the ultraslow cooling dynamics of a wide range of molecular anions.

  Läs mer om Ultraslow radiative cooling of Cn- (n=3-5)

• Shock-driven formation of covalently bound carbon nanoparticles from ion collisions with clusters of C-60 fullerenes

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

  Artikel

  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.

  Läs mer om Shock-driven formation of covalently bound carbon nanoparticles from ion collisions with clusters of C-60 fullerenes

• The threshold displacement energy of buckminsterfullerene C-60 and formation of the endohedral defect fullerene He@C-59

  2018. Mark H. Stockett (et al.). Carbon 139, 906-912

  Artikel

  We have measured the threshold center-of-mass kinetic energy for knocking out a single carbon atom from C-60(-) in collisions with He. Combining this experimental result with classical molecular dynamics simulations, we determine a semi-empirical value of 24.1+0.5 eV for the threshold displacement energy, the energy needed to remove a single carbon atom from the C-60 cage. We report the first observation of an endohedral complex with an odd number of carbon atoms, He@C-59(-), and discuss its formation and decay mechanisms.

  Läs mer om The threshold displacement energy of buckminsterfullerene C-60 and formation of the endohedral defect fullerene He@C-59

• Ion collision-induced chemistry in pure and mixed loosely bound clusters of coronene and C-60 molecules

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

  Artikel

  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.

  Läs mer om Ion collision-induced chemistry in pure and mixed loosely bound clusters of coronene and C-60 molecules

• DESIREE electrospray ion source test bench and setup for collision induced dissociation experiments

  2018. Nathalie de Ruette (et al.). Review of Scientific Instruments 89 (7)

  Artikel

  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.

  Läs mer om DESIREE electrospray ion source test bench and setup for collision induced dissociation experiments

• Interactions of energetic ions with fullerenes, PAHs, and their weakly bound clusters

  2017. Henning Zettergren. Nuclear Instruments and Methods in Physics Research Section B 408, 9-15

  Artikel

  This brief review highlights recent advances in our understanding on how fullerenes, Polycyclic Aromatic Hydrocarbons (PAHs), and their clusters respond to singly and multiply charged keV-ion impact. These studies reveal how the projectile charge, mass, and velocity may be tuned to investigate, e.g., the stabilities of multiply charged monomers and clusters, different monomer and cluster cooling processes, molecular heating by Coulomb explosions of highly charged clusters, and impulse driven molecular growth processes.

  Läs mer om Interactions of energetic ions with fullerenes, PAHs, and their weakly bound clusters

• The structure of coronene cluster ions inferred from H-2 uptake in the gas phase

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

  Artikel

  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.

  Läs mer om The structure of coronene cluster ions inferred from H-2 uptake in the gas phase

• Rotationally Cold OH- Ions in the Cryogenic Electrostatic Ion-Beam Storage Ring DESIREE

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

  Artikel

  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.

  Läs mer om Rotationally Cold OH- Ions in the Cryogenic Electrostatic Ion-Beam Storage Ring DESIREE

• Knockout driven fragmentation of porphyrins

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

  Artikel

  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.

  Läs mer om Knockout driven fragmentation of porphyrins

• Knockout driven reactions in complex molecules and their clusters

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

  Artikel

  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.

  Läs mer om Knockout driven reactions in complex molecules and their clusters

• A precedent of van-der-Waals interactions outmatching Coulomb explosion

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

  Artikel

  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.

  Läs mer om A precedent of van-der-Waals interactions outmatching Coulomb explosion

• Interaction and charge transfer between dielectric spheres

  2016. Fredrik Lindén, Henrik Cederquist, Henning Zettergren. Journal of Chemical Physics 145 (19)

  Artikel

  We present exact analytical solutions for charge transfer reactions between two arbitrarily charged hard dielectric spheres. These solutions, and the corresponding exact ones for sphere-sphere interaction energies, include sums that describe polarization effects to infinite orders in the inverse of the distance between the sphere centers. In addition, we show that these exact solutions may be approximated by much simpler analytical expressions that are useful for many practical applications. This is exemplified through calculations of Langevin type cross sections for forming a compound system of two colliding spheres and through calculations of electron transfer cross sections. We find that it is important to account for dielectric properties and finite sphere sizes in such calculations, which for example may be useful for describing the evolution, growth, and dynamics of nanometer sized dielectric objects such as molecular clusters or dust grains in different environments including astrophysical ones.

  Läs mer om Interaction and charge transfer between dielectric spheres

• Fragmentaion and detachemnt of hot silver and copper dimer anions

  Emma K. Anderson (et al.).

  Läs mer om Fragmentaion and detachemnt of hot silver and copper dimer anions

Visa alla publikationer av Henning Zettergren vid Stockholms universitet