| 1. |
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| 2. |
- Campbell, Eleanor E B, 1960, et al.
(författare)
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Ionisation of Fullerenes and Fullerene Clusters using Ultrashort Laser Pulses
- 2006
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Ingår i: Photochem. Photobiol. Sci.. - : Springer Science and Business Media LLC. - 1474-905X. ; 5, s. 1183-1189
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Tidskriftsartikel (refereegranskat)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.
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| 3. |
- Fedor, Juray, 1979, et al.
(författare)
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Nonthermal power law decay of metal dimer anions
- 2005
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Ingår i: Phys. Rev. Lett.. - 0031-9007. ; 94
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Tidskriftsartikel (refereegranskat)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.
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| 4. |
- Hansen, Klavs, 1958, et al.
(författare)
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Absolute cooling rates of freely decaying fullerenes
- 2009
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Ingår i: Physical Review Letters. ; 103:14
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Tidskriftsartikel (refereegranskat)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.
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| 5. |
- Hansen, Klavs, 1958, et al.
(författare)
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Activation energies for evaporation from protonated and deprotonated water clusters from mass spectra
- 2009
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Ingår i: journal of chemical physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 131
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Tidskriftsartikel (refereegranskat)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 $N = 55-56$.
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| 6. |
- Hansen, Klavs, 1958, et al.
(författare)
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Cluster cooling issue - Foreword
- 2006
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Ingår i: Int. J. Mass Spec.. - : Elsevier BV. - 1387-3806. ; 252:2
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)
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| 7. |
- Hansen, Klavs, 1958
(författare)
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Comparison of algorithms for the calculation of molecular vibrational level densities
- 2008
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 128
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Tidskriftsartikel (refereegranskat)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.
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| 8. |
- Hansen, Klavs, 1958, et al.
(författare)
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Competition between fusion and intra-cluster fusion in highly excited fullerene clusters
- 2007
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Ingår i: Israel Journal of Chemistry. - 0021-2148. ; 47
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Tidskriftsartikel (refereegranskat)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.
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| 9. |
- Hansen, Klavs, 1958, et al.
(författare)
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Density of states of helium droplets
- 2007
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Ingår i: Phys. Rev. B. - 1098-0121. ; 76
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Tidskriftsartikel (refereegranskat)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.
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| 10. |
- Hansen, Klavs, 1958, et al.
(författare)
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Dissociation energies of gold clusters Au+N, N=7-27
- 2006
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Ingår i: Phys. Rev. A. - 1050-2947. ; 73
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Tidskriftsartikel (refereegranskat)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.
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| 11. |
- Hansen, Klavs, 1958, et al.
(författare)
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ISSPIC13 proceedings - Preface
- 2007
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Ingår i: Eur.Phys.J D. - 1434-6060. ; 43
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)
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| 12. |
- Hansen, Klavs, 1958, et al.
(författare)
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Laser power dependence in femtosecond ionization of fullerenes
- 2005
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Ingår i: Journal of Physics: Conference Series. - : IOP Publishing. - 1742-6588 .- 1742-6596. ; 42, s. 282-285
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Tidskriftsartikel (refereegranskat)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.
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| 13. |
- Hansen, Klavs, 1958, et al.
(författare)
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Obtaining colder ensembles of free clusters by using evaporation and recoil
- 2005
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Ingår i: Eur. Phys. J. D. - : Springer Science and Business Media LLC. - 1434-6060. ; 32, s. 339-345
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Tidskriftsartikel (refereegranskat)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.
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| 14. |
- Hansen, Klavs, 1958, et al.
(författare)
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Stability and dissociation pathways of doped AunX+ clusters (X=Y,Er,Nb)
- 2008
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Ingår i: Faraday Discussion. ; 138
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Tidskriftsartikel (refereegranskat)abstract
- Size dependent stabilities, fragmentation pathways and dissociation energies of a series of gas phase cationic doped gold clusters, AunX+ (5 < n < 20; X = Y, Er and Nb), and pure Aun+ clusters were investigated in photofragmentation experiments. Size dependent stability patterns were obtained and the branching between monomer and dimer evaporation was studied. For bare gold, the competing neutral monomer and dimer evaporation channels were found in agreement with earlier studies. For doped clusters, monomer evaporation is the most likely fragmentation channel with the exception of Au18Y+ and Au20Y+ for which gold dimer evaporation is also observed. Relations between the evaporative activation energies and both the experimental abundances and the fragment yield were derived based on unimolecular rate constants. The dissociation energies from this analysis show an odd-even staggering and enhanced stabilities for certain cluster sizes, in agreement with simple electronic shell model predictions.
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| 15. |
- Hansen, Klavs, 1958, et al.
(författare)
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The dissociation energy of V13+ and the consequences for radiative cooling
- 2005
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Ingår i: Eur. Phys. J. D. - : Springer Science and Business Media LLC. - 1434-6060. ; 34, s. 67-71
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Tidskriftsartikel (refereegranskat)abstract
- The dissociation energy of V13+ has been determined by comparison of the rates of sequential fragmentation, V13+→V 12+→V 11+, and single-step fragmentation of the first fragment, V12+→V 11+. The dissociation-energy value obtained as D=4.35(13) eV has implications for the amount of radiative cooling of the cluster derived form the data presented earlier [C. Walther et al., Phys. Rev. Lett. 83, 3816 (1999)] and is used to analyze additional results.
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| 16. |
- Hansen, Klavs, 1958, et al.
(författare)
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The frequency factor in statistical fullerene decay
- 2006
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Ingår i: Int. J. Mass Spec.. - : Elsevier BV. - 1387-3806. ; 252, s. 79-95
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Tidskriftsartikel (refereegranskat)abstract
- Experiments on fullerene decay are reviewed and the frequency factor for C2 emission is extracted. The value is also calculated theoretically. Inclusion of a number of previously disregarded degrees of freedom for the products increase the theoretical estimate for C60 to above 1020 s−1, in good agreement with experimental results.
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| 17. |
- Hedén, Martin, 1976, et al.
(författare)
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Molecular Fusion of (C60)N Clusters in the Gas Phase after Femtosecond Laser Irradiation
- 2005
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Ingår i: Phys. Rev. A. - 1050-2947. ; 71
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Tidskriftsartikel (refereegranskat)abstract
- We report observation of molecular fusion of (C60)N+ clusters to form large fullerene molecules after excitation by 800-nm femtosecond laser pulses. The fused species decay by undergoing C2 fragmentation. Such fragmentation behavior for excited clusters of fullerene molecules can be understood in terms of the energetics of molecular fusion as determined previously from fullerene-fullerene collision experiments. Within a certain excitation energy window, there is sufficient time for a substantial atomic rearrangement to take place within the clusters before they dissociate. Below this energy window there is not sufficient internal energy for molecular fusion to occur while above the energy window multiple ionization followed by breakup of the clusters dominates.
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| 18. |
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| 19. |
- Hedén, Martin, 1976, et al.
(författare)
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Thermal radiation from CN+ and La@CN
- 2005
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Ingår i: J. Chem. Phys.. - : AIP Publishing. - 0021-9606. ; 123
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Tidskriftsartikel (refereegranskat)abstract
- The radiative cooling of positively charged fullerene and endohedral fullerene fragments of C60, C70, C84, and La@C82 has been measured in a time-of-flight mass spectrometer. The radiative cooling is measured via its influence on the metastable decay. The emissivity extracted from the data is between 4×10–4 and 13×10–4. These values agree fairly well with the emissivity calculated from considering the low-energy tail of the surface plasmon. No major difference is found in the emission behavior of empty and endohedral fullerenes.
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| 20. |
- Kadhane, U., et al.
(författare)
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Near-infrared photoabsorption by C-60 dianions in a storage ring
- 2009
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 131:1
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Tidskriftsartikel (refereegranskat)abstract
- We present a detailed study of the electronic structure and the stability of C-60 dianions in the gas phase. Monoanions were extracted from a plasma source and converted to dianions by electron transfer in a Na vapor cell. The dianions were then stored in an electrostatic ring, and their near-infrared absorption spectrum was measured by observation of laser induced electron detachment. From the time dependence of the detachment after photon absorption, we conclude that the reaction has contributions from both direct electron tunneling to the continuum and vibrationally assisted tunneling after internal conversion. This implies that the height of the Coulomb barrier confining the attached electrons is at least similar to 1.5 eV. For C-60(2-) ions in solution electron spin resonance measurements have indicated a singlet ground state, and from the similarity of the absorption spectra we conclude that also the ground state of isolated C-60(2-) ions is singlet. The observed spectrum corresponds to an electronic transition from a t(1u) lowest unoccupied molecular orbital (LUMO) of C-60 to the t(1g) LUMO+1 level. The electronic levels of the dianion are split due to Jahn-Teller coupling to quadrupole deformations of the molecule, and a main absorption band at 10723 cm(-1) corresponds to a transition between the Jahn-Teller ground states. Also transitions from pseudorotational states with 200 cm(-1) and (probably) 420 cm(-1) excitation are observed. We argue that a very broad absorption band from about 11 500 cm(-1) to 13 500 cm(-1) consists of transitions to so-called cone states, which are Jahn-Teller states on a higher potential-energy surface, stabilized by a pseudorotational angular momentum barrier. A previously observed, high-lying absorption band for C-60(-) may also be a transition to a cone state.
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| 21. |
- Lassesson, Andreas, 1976, et al.
(författare)
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A femtosecond laser study of the endoral fullerenes Li@C60 and La@C82
- 2005
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Ingår i: Eur. Phys. J. D. - : Springer Science and Business Media LLC. - 1434-6060. ; 34, s. 205-209
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Tidskriftsartikel (refereegranskat)abstract
- The endohedral fullerenes La@C82 and Li@C60 have been studied in the gas phase with femtosecond laser excitation. The two molecules show qualitatively the same behaviour with respect to ion yield vs. pulse energy, but markedly different fragmentation patterns, with La@C82 fragmenting via the shrink-wrap mechanism and Li@C60 predominantly losing the metal atom in the first fragmentation step. The ion yields and electron energy distributions of La@C82 agree well with a recently developed model for fs laser ionisation of C60.
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| 22. |
- Podenok, S., et al.
(författare)
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Electric field enhancement factors around a metallic, end-capped cylinder
- 2006
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Ingår i: Nano. - 1793-2920. ; 1, s. 87-93
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Tidskriftsartikel (refereegranskat)abstract
- We have calculated the electric field enhancement factor for a metallic cylinder with a hemispherical end-cap in a plane capacitor geometry paying particular attention to the dependence of the field enhancement factor on the anode distance. In addition, we have investigated the angular dependence of the local field at the end-cap. The numerical results, which cover a range of different ratios of cylinder lengths and anode distances, can be fitted with simple functional expressions which provide a useful scaling for calculations of field emission currents from closed cap carbon nanotubes or nanowires.
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| 23. |
- Prasalovich, Sergei, 1976, et al.
(författare)
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Surface entropy of rare-gas clusters
- 2005
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Ingår i: J. Chem. Phys.. - : AIP Publishing. - 0021-9606. ; 123
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Tidskriftsartikel (refereegranskat)abstract
- Abundances of Ar and Xe clusters produced in a supersonic expansion source are inverted to find relative dissociation energies. The values around the shell and subshell closings at N=55, 71, and 147 differ from theoretical values derived from ground-state energies of Lennard-Jones clusters. A significant part of the difference can be accounted for by the conformational entropies of surface atoms and vacancies.
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| 24. |
- Schweikhard, L., et al.
(författare)
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Cluster studies in ion traps
- 2005
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Ingår i: Proceedings of ISACC 2003. - 1860944957 ; , s. 85-95
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)
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| 25. |
- Schweikhard, L., et al.
(författare)
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Photodissociation of Au30+ clusters stored in a Penning trap
- 2005
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Ingår i: Eur. Phys. J. D. - : Springer Science and Business Media LLC. - 1434-6060. ; 36, s. 179-185
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Tidskriftsartikel (refereegranskat)abstract
- Ion trapping allows detailed studies of atomic clusters with various interactions and over a large range of timescales. An overview of methods at hand is given and a specific example is presented in detail where size-selected Au30+ clusters have been stored in a Penning trap and photofragmented by exposure to the third harmonic of a Nd:YAG laser. The resulting mass spectra were sampled after reaction periods varying from 10μs to 1s. The data are used to extract relative dissociation energies, which agree well with model-free values determined previously by other means, albeit with a slightly larger magnitude of the odd-even effect. Below n≃24 the relative dissociation energies extracted from the abundance spectra develop very little over the five orders of magnitude in time covered in the experiments. This behavior has been predicted, but not tested previously. Above n≃24 both spectra and dissociation energies develop odd-even effects after a storage time of 10–100 ms. Possible reasons for this behavior are discussed.
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| 26. |
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| 27. |
- Sundén, Erika, 1970, et al.
(författare)
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Heat capacities of freely evaporating charged water clusters
- 2009
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 130:22
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Tidskriftsartikel (refereegranskat)abstract
- We report on evaporation studies on positively charged water clusters (H$^+$(H$_2$O)$_N$) and negatively charged mixed clusters (X$^-$(H$_2$O)$_N$) with a small core ion $X$ ($X$=O$_2$,CO$_3$ or NO$_3$), in the size range $N=5$ to $300$. The clusters were produced by corona discharge in ambient air, accelerated to 50 keV and mass selected by an electromagnet. The loss of monomers during the subsequent 3.4 m free flight was recorded. The average losses are proportional to the clusters' heat capacities and this allowed the determination of size-dependent heat capacities. The values are found to increase almost linearly with clusters size for both species, with a rate of 6-8 $k_B$ per added molecule. For clusters with $N<21$ the heat capacities per molecule are lower but the incremental increase higher. For $N>21$ the values are intermediate between the bulk liquid and the solid water 0 $^o$C values.
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| 28. |
- Sveningsson, Martin, 1977, et al.
(författare)
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Quantifying temperature-enhanced electron field emission from individual carbon nanotubes
- 2005
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Ingår i: Physical Review B. - : American Physical Society. ; 72:8
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Tidskriftsartikel (refereegranskat)abstract
- The electron field emission properties of individual multiwalled carbon nanotubes have been examined using a combined STM-TEM microscope. The measured electron emission, for low emission currents, can be fitted with a standard Fowler-Nordheim model. For higher electron emission, above 10 µA for an individual carbon nanotube, we observe a significantly increased emission current leading to a nonlinear Fowler-Nordheim plot. The nonlinearity is caused by thermally enhanced electron emission due to Ohmic heating of the carbon nanotube. This is verified by modeling the electron field emission current. In addition to the influence of radiative cooling and the temperature dependence of the nanotube resistivity, we clearly show that a consideration of the temperature change due to the electron emission process itself, known as the Nottingham effect, is crucial to obtain good agreement with the experimental data.
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| 29. |
- Tomita, S., et al.
(författare)
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Dynamic Jahn-Teller effects in isolated C60- studied by near-infrared spectroscopy in a storage ring
- 2005
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Ingår i: Phys. Rev. Lett.. - 0031-9007. ; 94
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Tidskriftsartikel (refereegranskat)abstract
- We have measured the near-infrared absorption spectrum for isolated C ions at room temperature. Two bands, at 9145 cm-1 and 10460 cm-1, have been identified in addition to the main absorption band at 9382 cm-1, seen also at low temperature in a matrix. An interpretation based on the theory of dynamic Jahn-Teller effects is proposed.
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| 30. |
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| 31. |
- Tomita, S., et al.
(författare)
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Lifetimes of C602- and C702- dianions in a storage ring
- 2006
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Ingår i: J. Chem. Phys.. - : AIP Publishing. - 0021-9606. ; 124
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Tidskriftsartikel (refereegranskat)abstract
- C602- and C702- dianions have been produced by electrospray of the monoanions and subsequent electron pickup in a Na vapor cell. The dianions were stored in an electrostatic ring and their decay by electron emission was measured up to 1 s after injection. While C ions are stable on this time scale, except for a small fraction of the ions which have been excited by gas collisions, most of the C ions decay on a millisecond time scale, with a lifetime depending strongly on their internal temperature. The results can be modeled as decay by electron tunneling through a Coulomb barrier, mainly from thermally populated triplet states about 120 meV above a singlet ground state. At times longer than about 100 ms, the absorption of blackbody radiation plays an important role for the decay of initially cold ions. The tunneling rates obtained from the modeling, combined with WKB estimates of the barrier penetration, give a ground-state energy 200±30 meV above the energy of the monoanion plus a free electron and a ground-state lifetime of the order of 20 s.
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