| 1. |
- Bhattacharyya, Soumen, et al.
(author)
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Mass-selected photodissociation studies of AlPbn+ clusters (n=7-16): Evidence for the extraordinary stability of AlPb10+ and AlPb12
- 2013
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In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121. ; 87:5
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Journal article (peer-reviewed)abstract
- We report fragmentation pathways and dissociation energies of AlPbn+ (n=7–16) clusters. The clusters are produced with pulsed laser vaporization and studied in a supersonic molecular beam setup. They are mass selected and photodissociated with 532 and 355 nm laser light. Photofragments are thereafter mass separated in a tandem reflectron time-of-flight mass spectrometer. Bare Pbn+ (n = 8–16) clusters preferentially evaporate Pb atoms, with the exception of Pb15+ that fragments by loss of a Pb2 dimer to form the stable Pb13+ cluster. The smallest AlPbn+ (n = 7–11) clusters also show mainly atomic Pb evaporation, whereas the favored fragmentation pathway of the larger clusters (n = 12–16) involves Pb2 and Pb3 fragments. AlPb10+ and AlPb12+ are the most intense fragments of several larger cluster sizes, demonstrating the high stability of these two sizes. Dissociation energies corresponding to the most facile fragmentation channel of AlPbn+ (n = 11–15) are bracketed from the measured laser fluence dependencies of the fragment intensities using constraints imposed by unimolecular reaction rates. ©2013 American Physical Society
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| 2. |
- Chen, Hongshan, et al.
(author)
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Low temperature heat capacity of water clusters
- 2014
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In: Chemical Physics Letters. - 0009-2614. ; 610-611, s. 369-374
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Journal article (peer-reviewed)abstract
- Geometry optimization and vibrational frequency calculation are carried out at the MP2/6-31G(d,p) levelfor 35 low-energy isomers of (H2O)nclusters in the size range n = 6–21. The heat capacities of the clusters are calculated using quantum statistical theories based on the harmonic approximation. The specific heat capacity increases with the cluster size but the difference diminishes gradually with increasing size.The heat capacities divided by the number of intermolecular vibrational modes are very close for all the clusters. The overall picture of the heat capacity of the clusters is bulk-like and it agrees well with the experimental results of size-selected clusters.
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| 3. |
- Gatchell, Michael, et al.
(author)
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Commissioning of the DESIREE storage rings - a new facility for cold ion-ion collisions
- 2014
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In: XXVIII International Conference on Photonic, Electronic and Atomic Collisions (ICPEAC 2013). - : Institute of Physics (IOP). ; 488:1
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Conference paper (peer-reviewed)abstract
- We report on the ongoing commissioning of the Double ElectroStatic Ion Ring ExpEriment, DESIREE, at Stockholm University. Beams of atomic carbon anions (C-) and smaller carbon anion molecules (C-2(-), C-3(-), C-4(-) etc.) have been produced in a sputter ion source, accelerated to 10 keV or 20 keV, and stored successfully in the two electrostatic rings. The rings are enclosed in a common vacuum chamber cooled to below 13 Kelvin. The DESIREE facility allows for studies of internally relaxed single isolated atomic, molecular and cluster ions and for collision experiments between cat-and anions down to very low center-of-mass collision energies (meV scale). The total thermal load of the vacuum chamber at this temperature is measured to be 32 W. The decay rates of stored ion beams have two components: a non-exponential component caused by the space charge of the beam itself which dominates at early times and an exponential term from the neutralization of the beam in collisions with residual gas at later times. The residual gas limited storage lifetime of carbon anions in the symmetric ring is over seven minutes while the 1/e lifetime in the asymmetric ring is measured to be about 30 seconds. Although we aim to improve the storage in the second ring, the number of stored ions are now sufficient for many merged beams experiments with positive and negative ions requiring milliseconds to seconds ion storage.
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| 4. |
- Goto, Motoshi, et al.
(author)
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Branching ratio between resonant and non-resonant ionization of xenon evaluated from photoelectron angular distributions
- 2012
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In: Physica Scripta. - : IOP Publishing. - 0031-8949 .- 1402-4896. ; 86:3
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Journal article (peer-reviewed)abstract
- Photoelectron imaging of xenon gases ionized with a femtosecond 388 nm pulse reveals three ionization processes: resonant and non-resonant multi-photon ionization and autoionization. The relative yield of the first two was evaluated from the deconvolution of their angular distribution for the specific electron energy, which includes the electron production via the 3hv–5p5(2P° 1/2)6s resonance. The non-resonant process gains in importance with increasing laser intensity and accounts for 50% of the ionization yield at 30TWcm−2.
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| 5. |
- Goto, Motoshi, et al.
(author)
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Competitive ionization processes of anthracene excited with a femtosecond pulse in the multi-photon ionization regime
- 2011
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In: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 135:21
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Journal article (peer-reviewed)abstract
- To clarify the ionization mechanism of large molecules under multi-photon ionization conditions, photo-electron spectroscopic studies on anthracene have been performed with electron imaging technique. Electron kinetic energy distributions below a few eV reveal that three kinds of ionization channels co-exist, viz. vertical ionization, ionization from Rydberg states, and thermionic hot electron emission. Their relative yield is determined by the characteristic of the laser pulse. The duration in particular influences the ratio between the first two processes, while for higher intensities the last process dominates. Our results provide strong evidence that internal conversion plays an important role for the ionization of the molecule.
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| 6. |
- Goto, Motoshi, et al.
(author)
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Direct observation of internal energy distributions of C5-
- 2013
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In: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 139:5
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Journal article (peer-reviewed)abstract
- Photon induced decay of C5- has been measured in an electrostatic storage ring. The time dependence of the photo-enhanced decay is close to a 1/t decay which indicates a thermal process. The deviation from the expected power of −1 is quantitatively explained by the small heat capacity of the anion. Measurements of the photo-enhanced decay at different storage times and photon energies allow a determination of the radiative cooling rate and the energy distribution of the ions. The average energy content between 15 and 70 ms is found to vary as time to the power −0.72, and at 50 ms the ions contain an average excitation energy of 0.5 eV. The time dependent energy distribution is consistent with cooling by infrared photon emission if published oscillator strengths are reduced by a factor 2.5, in contrast to cooling of larger molecular carbon-based ions where electronic transitions cause a much stronger cooling.
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| 7. |
- Goto, Motoshi, et al.
(author)
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Ionization of naphthalene via the Rydberg States using a femtosecond 775 nm pulse
- 2012
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In: Chemical Physics Letters. - 0009-2614. ; 522, s. 23-27
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Journal article (peer-reviewed)abstract
- Photoelectron spectroscopic studies on naphthalene excited with a femtosecond 775 nm laser pulse reveal two ionization paths via the Rydberg states: a resonance-enhanced, non-relaxed process and one that goes through internal conversion of an undetermined intermediate. Varying the pulse duration with several hundred femtosecond causes only small change in the spectra. Compared to the ionization mechanism of anthracene, the reduced molecular size changes the ionization behavior due to a slower internal conversion rate and the presence of atomic-like Freeman resonances.
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| 8. |
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| 9. |
- Hansen, Klavs, 1958
(author)
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Statistical physics of nanoparticles in the gas phase
- 2013
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Book (other academic/artistic)abstract
- Thermal processes are ubiquitous and an understanding of thermal phenomena is essential for a complete description of the physics of nanoparticles, both for the purpose of modeling the dynamics of the particles and for the correct interpretation of experimental data. This book has the twofold aim to present coherently the relevant results coming from the recent scientific literature and to guide the readers through the process of deriving results, enabling them to explore the limits of the mathematical approximations and test the power of the method. The book is focused on the fundamental properties of nanosystems in the gas phase. For this reason there is a strong emphasis on microcanonical physics. Each chapter is enriched with exercises and 3 Appendices provide additional useful materials.
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| 10. |
- Hansen, Klavs, 1958, et al.
(author)
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Thermal radiation of laser heated niobium clusters Nb_N+, 8 ≤ N ≤ 22
- 2014
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In: Journal of Chemical Physics. - 0021-9606 .- 1089-7690. ; 141
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Journal article (peer-reviewed)abstract
- The thermal radiation from small, laser heated, positively charged niobium clusters has been measured. The emitted power was determined by the quenching effect on the metastable decay, employing two different experimental protocols. The radiative power decreases slightly with cluster size and shows no strong size-to-size variations. The magnitude is 40–50 keV/s at the timescale of several microseconds, which is the measured crossover time from evaporative to radiative cooling.
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| 11. |
- Hartman, Henrik, et al.
(author)
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First storage of ion beams in the Double Electrostatic Ion-Ring Experiment : DESIREE
- 2013
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In: Review of Scientific Instruments. - : American Institute of Physics (AIP). - 0034-6748 .- 1089-7623. ; 84:5
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Journal article (peer-reviewed)abstract
- We report on the first storage of ion beams in the Double ElectroStatic Ion Ring ExpEriment, DESIREE, at Stockholm University. We have produced beams of atomic carbon anions and small carbon anion molecules (Cn-, n = 1, 2, 3, 4) in a sputter ion source. The ion beams were accelerated to 10 keV kinetic energy and stored in an electrostatic ion storage ring enclosed in a vacuum chamber at 13 K. For 10 keV C2- molecular anions we measure the residual-gas limited beam storage lifetime to be 448 s +/- 18 s with two independent detector systems. Using the measured storage lifetimes we estimate that the residual gas pressure is in the 10-14 mbar range. When high current ion beams are injected, the number of stored particles does not follow a single exponential decay law as would be expected for stored particles lost solely due to electron detachment in collision with the residual-gas. Instead, we observe a faster initial decay rate, which we ascribe to the effect of the space charge of the ion beam on the storage capacity.
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| 12. |
- Ito, G., et al.
(author)
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Cooling Dynamics of Photoexcited C6 − and C6H−
- 2014
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In: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 112
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Journal article (peer-reviewed)abstract
- We report conclusive evidence of an efficient cooling mechanism via the electronic radiative transitions of hot small molecular anions isolated in vacuum. We stored C6− and C6H− in an ion storage ring and observed laser-induced electron detachment with delays up to several milliseconds. The terminal hydrogen atom caused a drastic change in the decay profiles. The decay of hotoexcited C6H− is slow and nonexponential, which can be explained by depletion cooling, whereas that for C6− occurs extremely fast, on a time scale below 0.1 ms and can only be explained by electronic radiative cooling via low-lying electronic excited states.
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| 13. |
- Ito, Gen, et al.
(author)
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Difference in cooling dynamics between photo-excited C6- and C6H-
- 2014
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In: Journal of Physics, Conference Series. - 1742-6588 .- 1742-6596. ; 488
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Journal article (peer-reviewed)abstract
- Laser-induced electron detachment of C6- and C6H- stored in an ion storage ring was observed respectively, focusing attention onto the delayed processes with the time constants up to several milliseconds. We found the drastic difference in the decay profiles of photo-excited C6- and C6H- that is, C6- decayed much faster than C6H-. The unexpected fast decay of C6- indicates that it is governed by the radiative cooling with electronic transitions rather than vibrational nor depletion cooling.
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| 14. |
- Johansson, J. O., et al.
(author)
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Anisotropic hot electron emission from fullerenes
- 2012
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In: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 136:16
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Journal article (peer-reviewed)abstract
- Photoelectron spectra for fullerenes C-60 and C-70 ionized using 800 nm laser pulses with pulse durations from 120 to 1000 fs show thermal electron kinetic energy distributions but they also exhibit angular anisotropy with respect to the laser light polarization. The effective temperature of electrons, measured along the laser polarization direction, is significantly higher than in the perpendicular direction. We explain this observation by considering that the emission of the thermal electrons is uncorrelated with the phase of the laser pulse, unlike directly ionized electrons, and, depending on the time of emission, they may experience an additional "kick" from the vector potential of the laser field when they are emitted from the molecule. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4704828]
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| 15. |
- Kjellberg, Mikael, 1980, et al.
(author)
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Femtosecond electron spectroscopy of coronene, benzo[GHI]perylene and anthracene
- 2010
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In: Journal of Chemical Physics. - 0021-9606. ; 133:7
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Journal article (peer-reviewed)abstract
- The large polycyclic aromatic hydrocarbon molecules coronene, benzo[GHI]perylene, and anthracene have been ionized with femtosecond laser pulses at low laser intensities and the ionization process studied with velocity map imaging spectroscopy, supplemented with ion yield measurements. The electron spectra of coronene and benzo[GHI]perylene are structureless. Based on fluence and pulse duration dependence measurements, it is shown that the electron spectra are not produced in field ionization processes, and the ionization mechanism is identified to be a quasithermal statistical electron emission, previously suggested for the fullerenes C60 and C70. The anthracene photoelectron spectra are dominated by above threshold ionization features, but with some indication of quasithermal ionization at longer pulses.
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| 16. |
- Kjellberg, Mikael, 1980, et al.
(author)
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Momentum map imaging photoelectron spectroscopy of fullerenes with femtosecond laser pulses
- 2010
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In: Physical Review A. - 1050-2947. ; 81:2
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Journal article (peer-reviewed)abstract
- Photoelectron spectra of C60 and C70, ionized with ultrashort laser pulses, are measured with a momentum-map-imaging electron spectrometer. Above the photon energy, 1.6 eV, the spectra are essentially structureless and well described by Boltzmann distributions, with temperatures on the order of 104 K. This result is similar to previous results for C60 using a time-of-flight electron spectrometer, which are confirmed in this study. Comparisons of electron energy distributions recorded for identical laser intensities but for different pulse durations demonstrate unambiguously that a significant fraction of the electrons are emitted in quasithermal processes and results argue strongly against a field ionization or direct multiphoton ionization mechanism. For electron energies above the photon energy, which account for about half the intensity, the whole signal is consistent with this quasithermal emission.
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| 17. |
- Kresin, Vitaly V., et al.
(author)
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Slow electron attachment as a probe of cluster evaporation processes
- 2011
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In: Journal of Physical Chemistry A. - : American Chemical Society (ACS). - 1089-5639 .- 1520-5215. ; 115, s. 6961-6972
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Journal article (peer-reviewed)abstract
- Neutral alkali clusters efficiently capture low-energy electrons with the aid of long-range polarization attraction. Upon attachment, the electron energy is dissipated into vibrations, heating the cluster and triggering evaporation of atoms and dimers. This process offers a novel means to explore nanocluster bonding and evaporation kinetics. The present work investigates the formation of . A crossed-beam experiment reveals that relative anion abundances become strongly and nontrivially restructured with respect to the neutral precursor beam. This restructuring is explained in quantitative detail by an analysis of evaporative cascades initiated by the attachment. The analysis thus furnishes a complete description of the electron attachment process, from initial attraction to final rearrangement of the cluster population. In addition, the paper describes a reconstruction of neutral cluster population in the supersonic beam, a new relation between the dissociation energies of cationic, neutral and anionic metal clusters, and a systematic derivation of cluster evaporation kinetics and internal temperature distributions.
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| 18. |
- Leidlmair, Christian, et al.
(author)
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Structures, Energetics, and Dynamics of Helium Adsorbed on Isolated Fullerene Ions
- 2012
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In: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 108:7
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Journal article (peer-reviewed)abstract
- Helium adsorbed on C60+ and C70+ exhibits phenomena akin to helium on graphite. Mass spectra suggest that commensurate layers form when all carbon hexagons and pentagons are occupied by one He each, but that the solvation shell does not close until 60 He atoms are adsorbed on C60+, or 62 on C70+. Molecular dynamics simulations of C60Hen+ at 4 K show that the commensurate phase is solid. Helium added to C60He32+ will displace some atoms from pentagonal sites, leading to coexistence of a registered layer of immobile atoms interlaced with a nonregistered layer of mobile atoms.
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| 19. |
- Najafian, Kaveh, et al.
(author)
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Radiative cooling of C7-
- 2014
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In: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 140:10
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Journal article (peer-reviewed)abstract
- The spontaneous and photo-induced neutralization of C_7- produced in a laser ablation source was measured in an electrostatic storage ring. The measurements provide three independent determinations of the radiative cooling of the ions, based on the short time spontaneous decay and on the integrated amplitude and the shape of the photo-induced neutralization signal. The amplitude of the photo-induced signal was measured between 0.5 ms and 35 ms and found to depend on photon wavelength and ion storage time. All three signals can be reproduced with identical thermal IR radiative cooling rates with oscillator strengths equal to theoretical predictions. In addition, the measurements provide the excitation energy distribution.
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| 20. |
- Ryding, Mauritz Johan, 1981, et al.
(author)
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X-ray induced fragmentation of size-selected salt cluster-ions stored in an ion trap
- 2014
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In: RSC Advances. - 2046-2069. ; 4:88, s. 47743-47751
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Journal article (peer-reviewed)abstract
- A method for spectroscopic characterization of free ionic clusters and nanoparticles utilizing X-ray synchrotron radiation is presented. We demonstrate that size-selected ammonium bisulphate cluster ions, NH 4+(NH 4HSO4) n, captured in a linear ion trap, exhibit well-defined core-level absorption edges in the reconstructed fragment-ion abundance spectra. In addition to the specific photo-fragmentation pathways observed at the N1s-, O1s- and S2p-edges, dissociation also occurs as a consequence of clusters colliding with helium present as buffer gas in the ion trap. Separate off-beam experiments were conducted to establish the activation kinetics of these collision induced dissociation processes. Furthermore, it is demonstrated that the electrons released upon photoionization of background helium are too few in number to produce multiply charged cluster ions, and thereby induce fragmentation of the salt clusters, to any significant degree. The mechanisms for photon absorption and subsequent cluster fragmentation are analysed and discussed. In addition to its inherent element specificity, the method holds promise for cluster structure elucidation resulting from the sensitivity of the near edge absorption structure to the local chemical environment of the excited atom.
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| 21. |
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| 22. |
- Svanqvist, Mattias, et al.
(author)
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Non-jellium scaling of metal cluster ionization energies and electron affinities
- 2010
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In: EUROPEAN PHYSICAL JOURNAL D. - : Springer Science and Business Media LLC. - 1434-6060 .- 1434-6079. ; 56:2, s. 199-203
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Journal article (peer-reviewed)abstract
- Experimental literature data on ionization energies and electron affinities of metal clusters are reviewed and analyzed in terms of an expansion in the inverse cluster radius. The coefficient of the finite size correction for ionization energies decreases with increasing bulk work function whereas the corresponding coefficient for electron affinities decreases. This sharply contrasts the predictions based both on density functional theory of spherical jellium clusters and on classical electrostatics. A scaling of the coefficient for the ionization potentials with the atomic radius yields a linear behavior which extrapolates to zero around 6 eV.
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