| 1. |
- Bhattacharyya, Soumen, et al.
(författare)
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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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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121. ; 87:5
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Tidskriftsartikel (refereegranskat)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. |
- Ferrari, Piero, et al.
(författare)
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Radiative cooling of size-selected gas phase clusters
- 2019
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Ingår i: International reviews in physical chemistry. - 0144-235X .- 1366-591X. ; 38:3-4, s. 405-440
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Tidskriftsartikel (refereegranskat)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.
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| 3. |
- Ferrari, Piero, et al.
(författare)
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The effect of radiative cooling on the size-dependent stability of small boron clusters
- 2018
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Ingår i: Physical Review A. Atomic, Molecular, and Optical Physics. - 1050-2947. ; 98
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Tidskriftsartikel (refereegranskat)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.
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| 4. |
- Ferrari, Piero, et al.
(författare)
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Thermal radiation and fragmentation pathways of photo-excited silicon clusters
- 2015
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 143:22
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Tidskriftsartikel (refereegranskat)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.
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| 5. |
- 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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| 6. |
- Hansen, Klavs, 1958, et al.
(författare)
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Thermal radiation of gold clusters on microsecond time scales
- 2017
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Ingår i: Physical Review A. Atomic, Molecular, and Optical Physics. - 1050-2947. ; 96
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Tidskriftsartikel (refereegranskat)abstract
- Small positively charged gold clusters have been found to emit thermal radiation at a very high rate, with time constants ranging from one to 35μs for Aun + (n = 6–13,15). For sizes n = 14,16–20 the radiation occurs on much longer time scales. Strong thermal suppression of the population of higher-lying states puts constraints on the possible energies of excited states that can contribute to the radiation. Taking that into account, an evaluation of the experimentally determined rate constants shows that the strong radiation originates from thermally excited low-lying electronic states hitherto not observed. The origin of these states is discussed and two possibilities are suggested: one is related to electron correlation and electron pairing, and the other results from thermal shape fluctuations.
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| 7. |
- Hansen, Klavs, 1958, et al.
(författare)
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Thermal radiation of laser heated niobium clusters Nb_N+, 8 ≤ N ≤ 22
- 2014
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Ingår i: Journal of Chemical Physics. - 0021-9606 .- 1089-7690. ; 141
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Tidskriftsartikel (refereegranskat)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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