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1.	Adams, Christopher, et al. (author) Vibrational Autodetachment from Anionic Nitroalkane Chains - From Molecular Signatures to Thermionic Emission 2019 In: Journal of Physical Chemistry A. - : American Chemical Society (ACS). - 1089-5639 .- 1520-5215. ; 123:40, s. 8562-8570 Journal article (peer-reviewed)abstract We report the kinetic energy distributions in electron autodetachment from nitroethane, 1-nitropropane, and 1-nitrobutane anions upon laser excitation of CH stretching modes measured using velocity map electron imaging. In striking contrast to the case of nitromethane, the kinetic energy distributions show almost no distinct vibrational features, and they can be described by thermionic emission, relating the shape of the distributions to the electron capture cross section of the neutral molecule. The data suggest that a classical description is warranted above ca. 20 meV electron kinetic energy. At lower energies, quantum effects suppress the attachment cross section.
2.	Anderson, Emma K., et al. (author) Decays of excited silver-cluster anions Agn, n=4 to 7, in the Double ElectroStatic Ion Ring ExpEriment 2018 In: Physical Review A: covering atomic, molecular, and optical physics and quantum information. - 2469-9926 .- 2469-9934. ; 98:2 Journal article (peer-reviewed)abstract Spontaneous decays of small, hot silver-cluster anions Ag-n(-), n = 4-7, have been studied using one of the rings of the Double ElectroStatic Ion Ring ExpEriment (DESIREE). Observation of these decays over very long time scales is possible due to the very low residual gas pressure (similar to 10(-14)) and cryogenic (13 K) operation of DESIREE. The yield of neutral particles from stored beams of Ag-6(-) and Ag-2(-) anions were measured for 100 milliseconds and were found to follow single power-law behavior with millisecond time-scale exponential cutoffs. The Ag-4(-) and Ag-5(-) anions were stored for 60 s and the observed decays show two-component power-law behaviors. We present calculations of the rate constants for electron detachment from and fragmentation of Ag-4(-) and Ag-5(-). In these calculations, we assume that the internal energy distribution of the clusters are flat and with this we reproduce the early steep parts of the experimentally measured decay curves for Ag-4(-) and Ag-5(-) which extends to tens and hundreds of milliseconds, respectively. The fact that the calculations reproduce the early slopes of Ag-4(-) and Ag-5(-), which differ for the two cases, suggests that it is the changes in fragmentation rates with internal cluster energies of Ag-4(-) and Ag-5(-) rather than conditions in the ion source that determine this behavior. Comparisons with the measurements strongly suggest that the neutral particles detected in these time domains originate from Ag-4(-) -> Ag-3(-) + Ag and Ag-5(-) -> Ag-3(-) +Ag-2 fragmentation processes.
3.	Anderson, Emma K., et al. (author) Spontaneous electron emission from hot silver dimer anions: Breakdown of the Born-Oppenheimer approximation 2020 In: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 124 Journal article (peer-reviewed)abstract We report the first experimental evidence of spontaneous electron emission from a homonuclear dimer anion through direct measurements of Ag−2 → Ag2 + 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.
4.	Barillot, T., et al. (author) Angular asymmetry and attosecond time delay from the giant plasmon resonance in C60 photoionization 2015 In: Physical Review A. Atomic, Molecular, and Optical Physics. - 1050-2947 .- 1094-1622. ; 91 Journal article (peer-reviewed)abstract This combined experimental and theoretical study demonstrates that the surface plasmon resonance in C 60 alters the valence photoemission quantum phase, resulting in strong effects in the photoelectron angular distribution and emission time delay. Electron momentum imaging spectroscopy is used to measure the photoelectron angular distribution asymmetry parameter that agrees well with our calculations from the time-dependent local density approximation (TDLDA). Significant structure in the valence photoemission time delay is simultaneously calculated by TDLDA over the plasmon active energies. Results reveal a unified spatial and temporal asymmetry pattern driven by the plasmon resonance and offer a sensitive probe of electron correlation. A semiclassical approach facilitates further insights into this link that can be generalized and applied to other molecular systems and nanometer-sized metallic materials exhibiting plasmon resonances.
5.	Bhattacharyya, Soumen, et al. (author) Mass-selected photodissociation studies of AlPbn+ clusters (n=7-16): Evidence for the extraordinary stability of AlPb10+ and AlPb12 2013 In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121. ; 87:5 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
6.	Bulgakov, A.V., et al. (author) Molecular fusion of fullerene clusters under action of ultrashort laser pulses 2009 In: Nanosized Material Synthesis by Action of High-Power Energy Fluxes on Matter. - Novosibirsk : Institute of Thermophysics SB RAS, Novosibirsk. - 9785890170101 ; , s. 350-367 Book chapter (other academic/artistic)abstract Abstract finns inte
7.	Campbell, Eleanor E B, 1960, et al. (author) Ionisation of Fullerenes and Fullerene Clusters using Ultrashort Laser Pulses 2006 In: Photochem. Photobiol. Sci.. - : Springer Science and Business Media LLC. - 1474-905X. ; 5, s. 1183-1189 Journal article (peer-reviewed)abstract We give a brief review of the literature concerning the ultra-short pulse ionisation of fullerenes in the gas phase. Emphasis is placed on the excitation time dependence of different ionisation regimes as manifested by photoelectron spectroscopy. The ionisation rates are modelled for the intermediate situation where the excitation energy is equilibrated between electronic degrees of freedom but not yet coupled to vibrational degrees of freedom. The model is shown to describe many aspects of the experiments. New results are presented on the intra-cluster molecular fusion of fullerene molecules when van der Waals bound clusters of fullerenes are exposed to ultra-short laser pulses. Pump–probe measurements give a decay time constant for the intra-cluster fusion reaction of 520 ± 55 fs. A comparison with monomer ionisation results suggests that the time window for the fusion reaction is influenced by the coupling of the electronic excitation energy to vibrational degrees of freedom of the molecules in the cluster.
8.	Chen, Fuqiang, et al. (author) Radiative cooling of cationic carbon clusters, CN+, N=8,10,13-16 2019 In: Physical Chemistry Chemical Physics - PCCP. - : Royal Society of Chemistry (RSC). - 1463-9076 .- 1463-9084. ; 21, s. 1587-1596 Journal article (peer-reviewed)abstract The radiative cooling of highly excited carbon cluster cations of sizes N = 8, 10, 13–16 has been studied in an electrostatic storage ring. The cooling rate constants vary with cluster size from a maximum at N = 8 of 2.6 × 104 s−1 and a minimum at N = 13 of 4.4 × 103 s−1. The high rates indicate that photon emission takes place from electronically excited ions, providing a strong stabilizing cooling of the molecules.
9.	Chen, Hongshan, et al. (author) Low temperature heat capacity of water clusters 2014 In: Chemical Physics Letters. - 0009-2614. ; 610-611, s. 369-374 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.
10.	Echt, O., et al. (author) Vibrational Energy Dependence of the Triplet Lifetime in Isolated, Photo-Excited C60 2004 In: J. Phys. Chem. A. - : American Chemical Society (ACS). - 1089-5639. ; 108, s. 6944-6952 Journal article (peer-reviewed)abstract We have measured the lifetime of the lowest triplet state T1 in free C60 by a pump-probe experiment using lasers with nanosecond pulse durations. At low pump fluence the population of T1 decays with a distinct, narrow distribution of lifetimes. depends on the pump wavelength ( = 532, 355, or 266 nm) as well as the temperature of the source from which C60 is vaporized (420 T 510 C); it ranges from 2 s to 0.3 s. At high pump fluence an additional lifetime as short as 40 ns is observed. A consistent correlation of all observed lifetimes with the experimental parameters is found if is assigned to an ensemble of C60 (T1) that has absorbed either one or two pump photons with the excess energy being randomized over all vibrational modes. Thus, = 2 s corresponds to a vibrational energy Evib = Etotal - Etriplet = 4.6 eV (one-photon absorption at = 532 nm, T = 420 C) while = 40 ns corresponds to Evib = 9.6 eV (two-photon absorption at 355 nm, 480 C). This result strongly suggests that delayed electrons that are emitted from highly excited C60 (Evib 10 eV) on the time scale of 10 ns to 1 ms are not affected by long-lived electronically excited states. The frequently questioned description of delayed electron emission from photoexcited C60 as thermionic emission is, therefore, warranted.
11.	Fedor, Juray, 1979, et al. (author) Nonthermal power law decay of metal dimer anions 2005 In: Phys. Rev. Lett.. - 0031-9007. ; 94 Journal article (peer-reviewed)abstract The metastable decay of dimer anions of Cu and Ag has been measured in a storage ring. The decay is found to proceed nonexponentially and is well described by a power law with an exponent of -1. This signals the presence of a continuum of decay constants in the ensemble. This quasicontinuum can be provided by the quantum mechanical tunneling decay of high angular momentum states populated in the source. Numerical calculations for dimers of a variety of elements suggest that this decay behavior can be expected for a wide range of species.
12.	Ferrari, Piero, et al. (author) Radiative cooling of size-selected gas phase clusters 2019 In: International reviews in physical chemistry. - 0144-235X .- 1366-591X. ; 38:3-4, s. 405-440 Journal article (peer-reviewed)abstract Predicted almost forty years ago, the radiation from thermally populated excited electronic states has recently been recognised as an important cooling mechanism in free molecules and clusters. It has presently been observed from both inorganic clusters and carbonbased molecules in molecular beams and ion storage devices. Experiments have demonstrated that many of these systems radiate at rates approaching microsecond time scales, and often with a distinct dependence on the precise number of atoms in the system. The radiation acts as a strongly stabilising factor against both unimolecular decay and thermal electron emission. In astrophysical context, radiative cooling provides a mechanism to dissipate internal energy in star-forming processes, and stabilises molecules selectively in the circumstellar medium. The consequences of an active radiative cooling channel for nanoparticle production will likewise favour special sizes in non-equilibrium formation processes. In this review, the radiative cooling of clusters is presented and illustrated with examples of experiments performed on small carbon, metal, and semiconductor clusters, and on PAH molecules. The experimental and theoretical techniques used are discussed, together with the consequences of radiative cooling on size-to-size stability patterns of clusters.
13.	Ferrari, P., et al. (author) Stability of small cationic platinum clusters 2018 In: Physical Chemistry Chemical Physics. - : Royal Society of Chemistry (RSC). - 1463-9076 .- 1463-9084. ; 10:46, s. 29085-29090 Journal article (peer-reviewed)abstract The relative stability of small cationic platinum clusters is investigated by photofragmentation experiments. Mass spectra show a smooth intensity distribution except for a local intensity minimum at Pt-5(+), revealing enhanced stability of the platinum tetramer Pt-4(+). The possibility that radiative cooling competes with statistical fragmentation after photoexcitation is examined and it is shown that clusters in the N = 3-8 size range do not radiate on the time scale of the experiment. In the absence of radiative cooling, the mass spectra of photofragmented clusters can be well explained by dissociation energies computed using density functional theory. The large calculated HOMO-LUMO gap for Pt-4(+) (approximate to 1.2 eV) is attributed to its highly symmetric structure and provides an explanation for the surprisingly low reactivity of this cluster in different gas-phase reactions.
14.	Ferrari, Piero, et al. (author) The effect of radiative cooling on the size-dependent stability of small boron clusters 2018 In: Physical Review A. Atomic, Molecular, and Optical Physics. - 1050-2947. ; 98 Journal article (peer-reviewed)abstract The mass spectrum of cationic boron clusters, BN+ (N = 5 − 20), after photoexcitation demonstrates that radiative cooling is an important, though often neglected, process in determining the relative stability of small and isolated particles. The observed intensities inmass spectra suggest that B5+, B11+, B13+, and B15+ are particularly stable clusters, consistent with density-functional theory calculations. Quantitative agreement, however, is only obtained if radiative cooling is included in the analysis.All clusters are found to radiate onmicrosecond timescales, suggesting recurrent fluorescence as the dominant photon emission process.
15.	Ferrari, Piero, et al. (author) Thermal radiation and fragmentation pathways of photo-excited silicon clusters 2015 In: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 143:22 Journal article (peer-reviewed)abstract The fragmentation of laser heated silicon clusters was studied by time-of-flight mass spectrometry. For Si+n (n = 5–19, 21), the lowest energy fragmentation pathways were identified as the metastable decay channel occurring after the primary acceleration of the ions. The radiative cooling of laser excited Si+n (n = 5–9, 11, and 13) was quantified via its quenching effect on the amount of metastable fragmentation. The quenching varied strongly with cluster size, from no observable amount for Si+7 to a cooling constant of 3 · 10E5 s−1 for Si+13. In addition, based on the observed fragmentation channels, the ionization energies and the relative binding energies of the clusters were partially ordered, and several ionization energies have been bracketed more precisely.
16.	Furukawa, T., et al. (author) Cooling dynamics of photo-excited negative carbon cluster ions stored in an ion storage ring 2015 In: Nuclear Instruments & Methods in Physics Research Section B-Beam Interactions with Materials and Atoms. - : Elsevier BV. - 0168-583X. ; 354, s. 192-196 Journal article (peer-reviewed)abstract A newly found fast radiative cooling process of C-6(-) is studied by simulation based on statistical sharing of the internal energy by the vibrational modes both in the electronic ground and excited states. For comparison, a contrasting slow cooling of C6H- is simulated by the same procedure. The simulation demonstrates a distinct difference in the time evolution of energy distributions between C-6(-) and C6H-, and well reproduces the observed fast cooling of C-6(-) by recurrent electronic transitions and the slow cooling of C6H- by vibrational de-excitation. (C) 2014 Elsevier B.V. All rights reserved.
17.	Gatchell, Michael, et al. (author) Commissioning of the DESIREE storage rings - a new facility for cold ion-ion collisions 2014 In: XXVIII International Conference on Photonic, Electronic and Atomic Collisions (ICPEAC 2013). - : Institute of Physics (IOP). ; 488:1 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.
18.	Goto, Motoshi, et al. (author) Branching ratio between resonant and non-resonant ionization of xenon evaluated from photoelectron angular distributions 2012 In: Physica Scripta. - : IOP Publishing. - 0031-8949 .- 1402-4896. ; 86:3 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.
19.	Goto, Motoshi, et al. (author) Competitive ionization processes of anthracene excited with a femtosecond pulse in the multi-photon ionization regime 2011 In: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 135:21 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.
20.	Goto, Motoshi, et al. (author) Direct observation of internal energy distributions of C5- 2013 In: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 139:5 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.
21.	Goto, Motoshi, et al. (author) Ionization of naphthalene via the Rydberg States using a femtosecond 775 nm pulse 2012 In: Chemical Physics Letters. - 0009-2614. ; 522, s. 23-27 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.
22.	Hansen, Klavs, 1958, et al. (author) Absolute cooling rates of freely decaying fullerenes 2009 In: Physical Review Letters. ; 103:14 Journal article (peer-reviewed)abstract The cooling rates of C$_{60}^-$ have been measured in an electrostatic storage ring between several hundred $\mu$s and several tens of ms with one-photon laser excitation. The absolute energy scale is established by the photon energy and the cooling time interval is derived from the non-exponential decay of the ensemble of hot molecules. The energy decreases due to the combined action of depletion and thermal emission of IR photons with a total energy loss rate that varies inversely proportional to time, 0.9 eV$\cdot 1/t$. The radiative component decreases from a few hundred eV/s at sub ms timescales to several tens of eV/s at 20 ms and confirms that the crossover from depletion to predominantly radiative cooling occurs around 5 ms. The method is applicable to any large molecule or cluster which decays spontaneously, irrespective of the specific decay channel.
23.	Hansen, Klavs, 1958 (author) Action spectroscopy of highly excited molecular ions in molecular beams 2018 In: International Journal of Mass Spectrometry. - : Elsevier BV. - 1387-3806. ; 430, s. 14-21 Journal article (peer-reviewed)abstract The use of action spectroscopy for quantitative comparisons of different parts of a recorded spectrumrequires that the efficiency of the emission process after photon absorption is quantitatively known. Thequantum yields of measured photo-dissociation processes after photo-absorption are derived here formolecular beams with broad energy distributions. The effect of radiative cooling on the yield is included.The quantum yield for both low energy photons and radiative species shows large variations with photonenergy.
24.	Hansen, Klavs, 1958, et al. (author) Activation energies for evaporation from protonated and deprotonated water clusters from mass spectra 2009 In: journal of chemical physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 131 Journal article (peer-reviewed)abstract Experimental mass abundance spectra are used to extract evaporative activation energies (dissociation energies) for protonated water clusters, (H$_2$O)$_N$H$^+$, and deprotonated water clusters, (H$_2$O)$_N$ OH$^-$, in the size range up to hundred molecules. The inversion is achieved by application of the shell correction method adapted from nuclear physics to the abundance spectra. The well known abundance anomaly for protonated clusters which occurs for $N = 20-22$ is found to have the characteristic behavior of a shell closing, whereas other apparent magic numbers are only prominent peaks in the abundance spectra because of the instability of the evaporative precursor. For the deprotonated clusters, we find a similar shell closing for $N = 55-56$.
25.	Hansen, Klavs, 1958, et al. (author) Cluster cooling issue - Foreword 2006 In: Int. J. Mass Spec.. - : Elsevier BV. - 1387-3806. ; 252:2 Journal article (other academic/artistic)
26.	Hansen, Klavs, 1958 (author) Comment on “L. Holmlid, Int. J. Mass Spectrom. 352 (2013) 1” 2016 In: International Journal of Mass Spectrometry. - 1387-3806. ; 399-400, s. 51-52 Journal article (peer-reviewed)
27.	Hansen, Klavs, 1958 (author) Comment on "The dependence of homogeneous nucleation rate on supersaturation" [J. Chem. Phys. 141, 024307 (2014)] 2014 In: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 141 Journal article (peer-reviewed)abstract The role of deviations of cluster binding energies from liquid drop values as extrapolated from bulk values is discussed, with reference to conclusions reached in a recent publication [S. L. Girshick, J. Chem. Phys.141, 024307 (2014)].
28.	Hansen, Klavs, 1958, et al. (author) Comment to Alessandra Candian, Junfeng Zhen, and Alexander G. G. M. Tielens; The Aromatic Universe, Physics Today 71(11), 38 (2018) 2019 In: Physics today. - 0031-9228. ; 72:9 Journal article (other academic/artistic)
29.	Hansen, Klavs, 1958 (author) Comparison of algorithms for the calculation of molecular vibrational level densities 2008 In: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 128 Journal article (peer-reviewed)abstract Level densities of vibrational degrees of freedom are calculated numerically with formulas based on the inversion of the canonical vibrational partition function. The calculated level densities are compared with other approximate equations from literature and with the exact Beyer–Swinehart values, for which a simplified but equivalent version is given. All approximate equations agree at high excitation energies, but our results are vastly superior at low energies for large molecules. The results presented here are therefore of particular relevance for thermal processes of very large molecules, e.g., of biological nature, for which the exact state counting can be prohibitively slow. Furthermore, it is valid for situations where anharmonic motion significantly influences the thermal properties.
30.	Hansen, Klavs, 1958, et al. (author) Competition between fusion and intra-cluster fusion in highly excited fullerene clusters 2007 In: Israel Journal of Chemistry. - 0021-2148. ; 47 Journal article (peer-reviewed)abstract The occurrence of intra-cluster fusion of fullerene molecules inside van der Waals bound fullerene clusters can be observed following femtosecond laser excitation. We show that the fusion reaction competes with the fission of multiply charged clusters. Energetic barriers to the reaction of at least 85 eV have to be overcome, similar to the energetic barriers to molecular fusion observed in fullerene ion–neutral fullerene collision experiments.
31.	Hansen, Klavs, 1958 (author) DECAY DYNAMICS IN MOLECULAR BEAMS 2021 In: Mass Spectrometry Reviews. - : Wiley. - 0277-7037 .- 1098-2787. ; 40:6, s. 725-740 Journal article (peer-reviewed)abstract The phenomenon of power law decays in molecular beams is reviewed. The transition from a canonical to a microcanonical description of the decay is analyzed, and the appearance of the power law decay derived. Deviations from a power law often contain information on parallel competing processes. This is illustrated with examples where thermal radiation or dark unimolecular channels are the competing processes. Also corrections to the power law due to finite heat capacities and from nonideal energy distributions are derived. Finally, the consequences for the interpretation of action spectroscopy data are reviewed. (c) 2020 The Authors. Mass Spectrometry Reviews published by Wiley Periodicals, Inc. Mass Spec Rev
32.	Hansen, Klavs, 1958, et al. (author) Density of states of helium droplets 2007 In: Phys. Rev. B. - 1098-0121. ; 76 Journal article (peer-reviewed)abstract Accurate analytical expressions for the state densities of liquid 4He droplets are derived, incorporating the ripplon and phonon degrees of freedom. The microcanonical temperature and the ripplon angular momentum level density are also evaluated. The approach is based on inversions and systematic expansions of canonical thermodynamic properties.
33.	Hansen, Klavs, 1958 (author) Description of unimolecular reaction rates of Lennard-Jones clusters 2019 In: Journal of Physics B: Atomic, Molecular and Optical Physics. - : IOP Publishing. - 0953-4075 .- 1361-6455. ; 52 Journal article (peer-reviewed)abstract The unimolecular rate constant of Lennard-Jones clusters with N=54−56 atoms has been investigated in constant energy molecular dynamics simulations and compared with the detailed balance predictions. Decays were found to be statistical and absolute rate constants agree quantitatively with detailed balance predictions. The level densities required for the comparison of simulated and calculated rate constants were obtained from separate simulations using the integrated microcanonical temperature. The quantitative agreement between simulations and theory requires that the classical degeneracy of the ground state is included into the level densities.
34.	Hansen, Klavs, 1958, et al. (author) Detection of Recurrent Fluorescence Photons 2016 In: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 117 Journal article (peer-reviewed)abstract We have detected visible photons emitted from the thermally populated electronic excited state, namely recurrent fluorescence (RF), of C−6 stored in an electrostatic ion storage ring. Clear evidence is provided to distinguish RF from normal fluorescence, based on the temporal profile of detected photons synchronized with the revolution of C−6 in the ring, for which the time scale is far longer than the lifetime of the intact photoexcited state. The relaxation (cooling) process via RF is likely to be commonplace for isolated molecular systems and crucial to the stabilization of molecules in interstellar environments.
35.	Hansen, Klavs, 1958, et al. (author) Dissociation energies of gold clusters Au+N, N=7-27 2006 In: Phys. Rev. A. - 1050-2947. ; 73 Journal article (peer-reviewed)abstract Unimolecular decay rates and monomer-dimer branching ratios of gold clusters Au (N=7–27) have been measured as a function of excitation energy in photodissociation experiments on size-selected clusters stored in a Penning trap. Part of the data set has previously been used to extract model-free values of dissociation energies [Vogel et al., Phys. Rev. Lett. 87, 013401 (2001)]. Other parts of the data set do not allow this analysis. We use these data to extract tentative dissociation energies, based on the systematics of deviations between an Arrhenius analysis and the model-free values. The observed systematics also allows an estimate of the true frequency factor which often is much higher than the Arrhenius value but in good agreement with the expected detailed balance value. The data are also reanalyzed including radiative cooling which may explain part of the discrepancy between model-free and Arrhenius dissociation-energy values.
36.	Hansen, Klavs, 1958, et al. (author) Do we know the value of the Gspann parameter? 2004 In: Int.J.Mass Spectrom.. - : Elsevier BV. - 1387-3806. ; 233, s. 215-221 Journal article (peer-reviewed)abstract We reanalyze some of the data used by Klots to establish an experimental basis for the claim by Gspann that clusters have a highest temperature in molecular beams. The value of this temperature is given by the evaporative activation energy and a dimensionless parameter called the Gspann parameter. The value of the Gspann parameter we derive from the data is higher than the value found by Klots, and is consistent with the predictions from detailed balance theory, although a number of uncertainties persist. More recent data on oxygen clusters are analyzed with the same methods and the model dependent uncertainties are shown to be fairly large.
37.	Hansen, Klavs, 1958, et al. (author) Energy flow in peptides after UV photoexcitation of backbone linkages 2017 In: Physical Chemistry Chemical Physics - PCCP. - : Royal Society of Chemistry (RSC). - 1463-9076 .- 1463-9084. ; 19:30, s. 19640-5 Journal article (peer-reviewed)abstract We report on the dissociation channels after UV photoexcitation of peptide cations tagged with 18-crown-6 ether (CE). The model peptides chosen for study were singly protonated (Ala)n–Pro (n = 1, 2, 3) and Pro–Pro (Ala = alanine, Pro = proline) that all contain at least one tertiary amide group with high absorption cross section at 210 nm (5.90 eV). Statistical dissociation was identified from the loss of CE, a process occuring remotely from the initial site of excitation, and therefore requiring flow of energy to the ammonium group where the CE is bound. However, homolytic breakage of the peptide backbone at the site of excitation is competitive, resulting in so-called a radical cations. Density functional theory calculations of dissociation energies were done on the simplest system [Ala–Pro + H+](CE) and found to be 1.87 eV for CE loss and 3.29 eV for the formation of a+(CE) and x. These numbers were used to calculate statistical branching ratios for the dissociation processes based on detailed balance. After the absorption of two 210 nm photons (according to power-dependence measurements), the branching ratio between the two channels is calculated to be less than 10E-4, far below the observed ratio of 0.65. Hence both statistical and non-statistical dissociation contribute to dissociation of these photoexcited peptides.
38.	Hansen, Klavs, 1958, et al. (author) Enkeltfoton termisk ionisering af store molekyler : Ionisering mellem Einstein og Boltzmann 2017 In: Dansk Kemi. - 0011-6335. ; 98:11/12, s. 22-23 Journal article (other academic/artistic)abstract Ionisering er en fundamental proces i naturen. Den findes i flere varianter. Den sidst opdagede, som beskrives her, forener en-foton absorption med termiske processer.
39.	Hansen, Klavs, 1958, et al. (author) Gold Cluster Electronic Radiative Cooling and Abundances 2017 In: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 121:20, s. 10663-10669 Journal article (peer-reviewed)abstract The effect on abundances of radiative cooling of gold clusters is analyzed quantitatively for cluster sizes n = 8-20 based on known dissociation energies, radiative time Constants and the monomer unimoledilar decay frequency factor, and an assumed dimer loss frequency factor. Radiative cooling is observed to quench the temporal development of the abundance spectra and, in general, to produce larger size-to-size contrasts than observed in the absence of radiation. Also, the emission of photons with large and small energies was compared quantitatively. For small photon energies, the radiative time constant is not given by the photon emission rate constant. An alternative expression for this regime was found to be very accurate. Strong radiative cooling will have consequences both for the interpretation of mass abundance spectra and potentially also for the production of specific mass selected nanoparticles. The present work supplies the conceptual framework for such applications.
40.	Hansen, Klavs, 1958, et al. (author) Influence of thermal radiation on hot cluster decay rates and abundances 2019 In: Chinese Journal of Chemical Physics. - : AIP Publishing. - 1674-0068 .- 2327-2244. ; 32:2, s. 167-174 Journal article (peer-reviewed)abstract The influence of radiative cooling on the unimolecular decay rates of free, hot clusters and molecules with unspecified excitation energies is quantified. Two different regimes, defined by the magnitude of the energy of the photons emitted, are identified and the boundary between them is given. The boundary is determined in terms of the photon emission rate constants and thermal properties of the particles. Also the abundance spectra are calculated for the continuous cooling case, corresponding to small photon energies. The two regimes correspond to continuous cooling and single photon quenching of the unimolecular decay. The radiative effect can be parametrized by a redefinition of the time each individual cluster has available to undergo evaporation, expressed by an effective radiative time constant.
41.	Hansen, Klavs, 1958, et al. (author) Ionisering af store molekyler 2017 In: KVANT. - 0905-8893. ; 28:3, s. 17-20 Journal article (other academic/artistic)abstract Ionisering er en fundamental proces i naturen. Overraskende nok dukker der stadig nye versioner af processen op. Denne artikel beskriver den sidste nye, som blev præsenteret for verden i begyndelsen af 2017.
42.	Hansen, Klavs, 1958, et al. (author) ISSPIC13 proceedings - Preface 2007 In: Eur.Phys.J D. - 1434-6060. ; 43 Journal article (other academic/artistic)
43.	Hansen, Klavs, 1958 (author) Kinetic energy release distributions in unimolecular reactions with spherical potentials 2004 In: Chem. Phys. Lett.. - : Elsevier BV. - 0009-2614. ; 383, s. 270-275 Journal article (peer-reviewed)abstract The distribution of translational kinetic energy releases in unimolecular reactions is discussed with special emphasis on the distributions for reactions with no reverse activation barrier and the role of long-range spherical potentials between the fragments. The distributions are calculated with detailed balance. Angular momentum conservation is included, and total rate constants are given.
44.	Hansen, Klavs, 1958, et al. (author) Laser power dependence in femtosecond ionization of fullerenes 2005 In: Journal of Physics: Conference Series. - : IOP Publishing. - 1742-6588 .- 1742-6596. ; 42, s. 282-285 Journal article (peer-reviewed)abstract Fullerene ionization can under some circumstances be modelled as thermal emission of electrons from a transiently hot electron gas. Application of this idea to experiments involving femtosecond lasers gave theoretical evidence for ion yields that would vary with the pulse energy of the laser to some power, identical to the behavior seen in multi-photon experiments. The reason for this behavior is investigated here. The crucial component is identified as the Poisson statistics for photon absorption and a strong variation of the ion yield with energy.
45.	Hansen, Klavs, 1958, et al. (author) Magic numbers and stabilities of heavy water clusters,(D2O)ND+, N = 3 −48 2019 In: International Journal of Mass Spectrometry. - : Elsevier BV. - 1387-3806. ; 440, s. 14-19 Journal article (peer-reviewed)abstract The abundance spectrum of deuterated heavy water clusters has been measured, and relative dissociationenergies were extracted from the abundances. The dissociation energies are similar to those of protonatedlight water clusters, except for a slightly reduced excess stability of the N = 21 cluster relative to the N = 22cluster. The difference is interpreted in terms of the special geometry of the N = 21 cluster. The absenceof a similar effect for N = 28/29 suggests that the stability of the N = 28 cluster is not due to a closed shellstructure
46.	Hansen, Klavs, 1958, et al. (author) Non-scrambling of hydrogen in NH 4 + (H 2 O) 3 clusters 2019 In: RSC Advances. - : Royal Society of Chemistry (RSC). - 2046-2069. ; 9:12, s. 6620-6626 Journal article (peer-reviewed)abstract We have measured the metastable decay of protonated, ammonia-doped, deuterated water clusters produced in an electrospray source, dn-NH4+(H2O)3, n = 0-6. The mass spectra show a very strong odd-even effect, consistent with a low degree of scrambling of the hydrogen bound to water and to the ammonia. The relative evaporation rate constant for light water was almost twice the one for heavy water, with the rate for mixed protium-deuterium water molecule intermediate between these two values.
47.	Hansen, Klavs, 1958, et al. (author) Non-scrambling of hydrogen in NH4+(H2O)3 clusters 2019 In: RSC Advances. - : Royal Society of Chemistry (RSC). - 2046-2069. ; 9, s. 6620-6626 Journal article (peer-reviewed)abstract We have measured the metastable decay of protonated, ammonia-doped, deuterated water clusters produced in an electrospray source, dn-NH4 +(H2O)3, n ¼ 0–6. The mass spectra show a very strong odd–even effect, consistent with a low degree of scrambling of the hydrogen bound to water and to the ammonia. The relative evaporation rate constant for light water was almost twice the one for heavy water, with the rate for mixed protium–deuterium water molecule intermediate between these two values.
48.	Hansen, Klavs, 1958, et al. (author) Obtaining colder ensembles of free clusters by using evaporation and recoil 2005 In: Eur. Phys. J. D. - : Springer Science and Business Media LLC. - 1434-6060. ; 32, s. 339-345 Journal article (peer-reviewed)abstract We describe a theoretical model which treats the effects of evaporation-induced recoil on the mass and temperature distributions of a collimated beam of small neutral clusters emitted by a hot-nozzle source. The model incorporates two important consequences of in-flight cluster fragmentation processes. One is the well-known statistical evaporation of atoms and dimers accompanied by cluster size redistribution and cooling, and the other is the accompanying mechanical recoil of the fragments. We predict that the filtering effect introduced by cluster recoil can be used to an advantage by separating out the off-axis cluster population. This fraction will have a significantly narrower and colder distribution of internal temperatures than the on-axis ensemble.
49.	Hansen, Klavs, 1958, et al. (author) Single Photon Thermal Ionization of C60 2017 In: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 118 Journal article (peer-reviewed)abstract We report on experiments which show that C60 can ionize in an indirect, quasithermal boiloff process after absorption of a single photon. The process involves a large number of incoherently excited valence electrons and yields electron spectra with a Boltzmann distribution with temperatures exceeding 104 K. It is expected to be present for other molecules and clusters with a comparatively large number of valence electrons. The astrophysical consequences are briefly discussed.
50.	Hansen, Klavs, 1958, et al. (author) Single photon thermal ionization of large molecules 2017 In: Elettra Highlights. Journal article (other academic/artistic)abstract We have demonstrated that gas phase C60 can ionize in a quasi-thermal process where the absorption of a single photon at energies of several tens of eVs leads to the formation of a transient hot electron gas, while the vibrational degrees of freedom remain cold. The existence of the hot electron gas is signalled by the emission of thermal electrons with Boltzmann-like kinetic energy distributions with temperatures of 10000−20000 K. The ionization mechanism is expected to be general for large molecules and will have, e.g., astrophysical consequences.

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