arXiv subjects

Klavs Hansen

Publications and source records attributed to Klavs Hansen.

16 recordsLinked to original sources

arXiv2024

Cooling of gold cluster anions, Au$_N^-$, $N=2-13,15$, in a cryogenic ion-beam storage ring

We have measured the spontaneous and photo-induced decays of anionic gold clusters, Au$_N^-$, with sizes ranging from $N = 2$ to 13, and 15. After production in a sputter ion source, the size-selected clusters were stored in the cryogenic electrostatic ion-beam storage ring DESIREE and their neutralization decays were measured for storage times between 0.1 and 100 s. The dimer was observed to decay by electron emission in parallel to neutral atom emission at long times, analogously to the behavior of copper and silver dimers, implying a breakdown of the Born-Oppenheimer approximation. Radiative cooling is observed for all cluster sizes except for the dimer. The decay rates of clusters $N=3,6,8-13,15$ show only a single radiative cooling time. For $N=6-13$ the cooling times have a strong odd-even oscillation with an amplitude that decrease with cluster size, and with the even $N$ having the fast cooling. We compare our results with previous measurements of radiative cooling rates of the corresponding cationic gold clusters, Au$_N^+$, which also show an odd-even effect with a similar oscillation amplitude but at orders of magnitude shorter time scales, and out of phase with the anions.

physics.atom-ph↗

arXiv2024

Size distributions in irreversible particle aggregation

The aggregation of particles in the free molecular regime is determined approximately for situations with a high degree of translational energy equilibration. The mean particle sizes develop linearly in time. Scaling relations are used to derive a linear partial differential equation which is solved to show that the size distributions are close to log-normal asymptotically in time.

physics.atm-clus↗

arXiv2023

The Largest Fullerene

Fullerenes are lowest energy structures for gas phase all-carbon particles for a range of sizes, but graphite remains the lowest energy allotrope of bulk carbon. This implies that the lowest energy structure changes nature from fullerenes to graphite or graphene at some size and therefore, in turn, implies a limit on the size of free fullerenes as ground state structures. We calculate this largest stable single shell fullerene to be of size $N=1\times10^4$, using the AIREBO effective potential. Above this size fullerene onions are more stable, with an energy per atom that approaches graphite structures. Onions and graphite have very similar ground state energies, raising the intriguing possibility that fullerene onions could be the lowest free energy states of large carbon particles in some temperature range.

cond-mat.mes-hall↗

arXiv2022

Cascade infrared thermal photon emission

The time development of the excitation energy of molecules and clusters cooling by emission of thermal vibrational infrared radiation has been studied. The energy distributions and the photon emission rates develop into near-universal functions that can be characterized with only a few parameters, irrespective of the precise vibrational spectra and oscillator strengths of the systems. The photon emission constant and emitted power averaged over all thermally populated states vary linearly with total excitation energy with a small offset. The time developments of ensemble internal energy distributions are calculated with respect to their first two moments. For the derived linear dependence of the emission rate constant, these results are exact.

physics.atm-clus↗

arXiv2022

Isotope enrichment in neon clusters grown in helium nanodroplets

Neon cluster ions Ne$_s^+$ grown in pre-ionized, mass-to-charge selected helium nanodroplets (HNDs) reveal a strong enrichment of the heavy isotope $^{22}$Ne that depends on cluster size s and the experimental conditions. For small sizes the enrichment is much larger than previously reported for bare neon clusters grown in nozzle expansions and subsequently ionized. The enrichment is traced to the massive evaporation of neon atoms in a collision cell that is used to strip helium from the HNDs. We derive a relation between the enrichment of $^{22}$Ne in the cluster ion and its corresponding depletion factor $F$ in the vapor phase. The value thus found for $F$ is in excellent agreement with a theoretical expression that relates isotopic fractionation in two-phase equilibria of atomic gases to the Debye temperature. Furthermore, the difference in zero-point energies between the two isotopes computed from F agrees reasonably well with theoretical studies of neon cluster ions that include nuclear quantum effects in the harmonic approximation. Another fitting parameter provides an estimate for the size s$_i$ of the precursor of the observed Ne$_s^+$. The value is in satisfactory agreement with the size estimated by modeling the growth of Ne$_s^+$, and with lower and upper limits deduced from other experimental data. On the other hand, neon clusters grown in neutral HNDs that are subsequently ionized by electron bombardment exhibit no statistically significant isotope enrichment at all. The finding suggests that the extent of ionization-induced dissociation of clusters embedded in HNDs is considerably smaller than for bare clusters.

physics.atm-clus↗

arXiv2022

Single-photon hot electron ionization of C$_{70}$

Gas phase C$_{70}$ molecules have been ionized with single photons of energies between 16 eV and 70 eV and the electron spectra measured with velocity map imaging in coincidence with the ions. The doubly ionized and unfragmented species was present at photon energies of 22 eV and up, and triply charged ions from 55 eV. The low kinetic energy parts of the spectra are explained with thermal emission of transient hot electrons. Deviations at high photon energies are used to determine a value for the initial electron equilibration time. We propose a generally applicable mechanism, named Resonance Ionization Shadowing, for the creation of hot electrons by absorption of above-threshold energy photons.

physics.atom-ph↗

arXiv2022

Comment on 'Ultradense protium p(0) and deuterium D(0) and their relation to ordinary Rydberg matter: a review' 2019 Physica Scripta 94, 075005

The article by Holmlid and Zeiner-Gundersen (2019, Physica Scripta, vol. 94, 075005) contains a number of claims that explicitly or implicitly contradict fundamental knowledge of modern science. Some can only be true if long held conservation laws are broken. One such is baryon number conservation. A second fatal mistake is the treatment of the structure of molecules that disregard fundamental quantum mechanical aspects, such as the concept of kinetic energy operators and the Heisenberg indeterminacy relations.

physics.chem-ph↗

arXiv2022

Comment on 'Phase transition temperatures of 405-725 K in superfluid ultra-dense hydrogen clusters on metal surfaces' [AIP Advances 6, 045111 (2016)]

The article in \cite{HolmlidAIPA2016} has recently come to our attention. It makes a number of extraordinary claims that each contradict well established facts in several fields, ranging from atomic and molecular physics to superconductivity and superfluidity, with practically no supporting evidence. We think it worthwhile to rectify the literature with this comment.

physics.atm-clus↗

arXiv2022

Thermal radiative cooling of carbon cluster cations C$_N^+$, $N = 9, 11,12, 17-27$

The radiative cooling rates of C$_N^+$ clusters ($N = 9, 11, 12, 17-27$) have been measured in the ultrahigh vacuum of an electrostatic storage ring to values on the order of $10^4$ s$^{-1}$. The rates were measured as a competing channel to unimolecular decay, and the rate constants pertain to the excitation energies where these two channels compete. Such high values can only be explained as photon emission from thermally excited electronic states, a mechanism that has also been seen in polycyclic aromatic hydrocarbon cations. The high rates have a very strong stabilizing effect on the clusters and the underlying mechanism gives a high energy conversion efficiency, with the potential to reach high quantum efficiencies in the emission process. The competing decay of unimolecular fragmentation defines upper limits for photon energies that can be down-converted to lower energy photons. Including previously measured cluster sizes provides the limits for all clusters C$_N^+$, $N=8-27$, of values that vary from 10 to 14.5 eV, with a general increase with size. Clusters absorbing photons of energies below these limits cool down efficiently by emission of photons via electronic transitions and their fragmentation is strongly reduced, increasing their survival in HI regions.

physics.chem-ph↗

arXiv2022

Shells in CO$_2$ Clusters

Abundance spectra of (CO$_2$)$_N$ clusters (up to $N \approx$ 500) acquired under a wide range of adiabatic expansion conditions are analyzed within the evaporative ensemble framework. Analysis reveals that cluster stability functions display strikingly universal patterns for all expansion conditions. These patterns reflect the inherent properties of individual clusters. From this analysis the size-dependent cluster binding energies are determined, shell and subshell closing sizes identified, and cuboctahedral packing ordering for sizes above $N \approx 130$ is confirmed. It is demonstrated that a few percent variation in the dissociation energies translates into significant abundance variations, especially for the larger clusters.

physics.atm-clus↗

arXiv2021

Free energies of triply and quadruply degenerate electron ladder spectra

The electronic free energies in metal clusters are calculated based on a simple ladder spectrum for the level degeneracies three and four, and compared with the known results for degeneracies one and two. The low temperature and the asymptotic high temperature results for free energy differences are generalized to ladder spectra of any degeneracy. Remarkably, free energy differences do not approach zero at high energies for size-independent Fermi energies.

cond-mat.mes-hall↗

arXiv2020

Vibrational angular momentum level densities of linear molecules

While linear molecules in their vibrational ground state cannot carry angular momentum around their symmetry axis, the presence of vibrational excitations can induce deformations away from linearity and therefore also allow angular momentum along the molecular axis. In this work, a recurrence relation is established for the calculation of the vibrational level densities (densities of states) of linear molecules, specified with respect to both energy and angular momentum. The relation is applied to the carbon clusters of sizes $n=4,6,7$ as a case study.

physics.atm-clus↗

arXiv2020

The C$_{60}^-$ thermal electron emission rate

The thermal electron emission rate constant for C$_{60}^-$ has been deduced over a range of 4 eV internal energy from storage ring measurements of the decays of ions reheated with single photons absorption. The thermal radiation from the ions is quantified with respect to continuous cooling and discrete photon quenching.

physics.atm-clus↗

arXiv2011

Slow electron attachment as a probe of cluster evaporation processes

Neutral alkali clusters efficiently capture low-energy electrons with the aid of long-range polarization attraction. Upon attachment, the electron affinity and kinetic energy are 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 ${Na}_N^-$. 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 systematic derivation of cluster evaporation kinetics and internal temperature distributions; a new relation between the dissociation energies of cationic, neutral and anionic metal clusters; and a scenario for inferring the neutral cluster population in the supersonic beam from the cationic mass spectrum.

physics.atm-clus↗

arXiv2007

Density of states of helium droplets

Accurate analytical expressions for the state densities of liquid He-4 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.

physics.atm-clus↗

arXiv2001

Fabrication of mesoscopic Nb wires using conventional e-beam lithographic techniques

Conventional electron beam lithography has been used to fabricate mesoscopic Nb wires with a superconducting transition temperature above 7.0 K. The typical line width and the thickness were 200 nm and 45 nm respectively. Nb was deposited in an ultra-high vacuum evaporation chamber using electron gun heating. All samples exhibited a normal-superconducting transition. The transition temperature decreased with thickness and line width. To demonstrate the feasibility of two angle evaporation techniques we also fabricated small Nb/(Al-)AlO_x/Nb tunnel junctions.

cond-mat.supr-con↗