Potassium Clusters Research Articles

Physical and chemical probes of metallic clusters

Two different approaches for studying the electronic structure of clusters are presented. The physical probe uses the photoexcitation of size selected alkali-atom clusters. Giant resonances govern the absorption cross-section. It is due to the collective excitation of delocalized valence electrons. The behavior of the resonance up to large sizes, 900-atom clusters for potassium and 1500-atom clusters for lithium puts into evidence the failure of the jellium model for lithium. The chemical approach makes use of the nucleation process in the presence of oxygen. From the analysis of mass spectra of oxidized products, informations are gained on the valence of metallic atom in the cluster oxide. The method is applied to europium and thulium clusters. A comparison is done with the simple case of alkali clusters.

The observation of pseudorotating lithium and potassium clusters Li5 and K7 in an adamantane matrix

The pseudorotating clusters, Li5 and K7, have been identified in an adamantane matrix by EPR spectroscopy. These new clusters were synthesised by depositing the parent alkali metal atoms at 77 K in the hydrocarbon matrix followed by careful annealing to temperatures above 200 K.

Microscopic study of the dipole surface response in large potassium cluster ions

We present microscopic calculations of the response function of large potassium cluster ions to an external dipole field (rY10(θ, φ)) within different approaches, and with the purpose to compare with recent measurements of the photoabsorption cross section of these systems. Our results indicate that the dipole plasmon energy in these large clusters is almost independent of size, displaying a very slight decrease when going from a size ofN=339 to 913 atoms, and thus deviates from the Mie classical result.

Surface-plasmon absorption in singly and doubly charged potassium clusters.

We report the optical-absorption spectra of hot, singly, and doubly charged potassium cluster ions (${\mathrm{K}}_{\mathit{n}}^{+}$, n=9,11,14--21, and ${\mathrm{K}}_{\mathit{n}}^{2+}$, n=odd, 39--53) in the 1.8--2.2 eV range. The cluster ions are produced by a modified liquid-metal ion source. Within experimental uncertainty, the spectra of all clusters, with the exception of ${\mathrm{K}}_{17}^{+}$, consist of a single, broad peak, centered near h\ensuremath{\nu}=2.0 eV. The results are interpreted in terms of the surface-plasmon absorption of small, liquid-metal droplets. The lack of structure in the observed spectra is attributed to thermal broadening of the individual contributions to the plasmon. A model, based on the liquid-drop model, is introduced to account for the thermal shift and broadening of the surface plasmon.

Influence of nonlocal exchange-correlation effects on the response properties of simple metal clusters.

Time-dependent density-functional theory has been used to study the photoabsorption spectrum of closed-shell sodium (${\mathrm{Na}}_{20}$, ${\mathrm{Na}}_{40}$) and potassium (${\mathrm{K}}_{19}^{\mathrm{\ensuremath{-}}}$, ${\mathrm{K}}_{20}$, ${\mathrm{K}}_{91}^{\mathrm{\ensuremath{-}}}$, ${\mathrm{K}}_{92}$, ${\mathrm{K}}_{197}^{\mathrm{\ensuremath{-}}}$, ${\mathrm{K}}_{198}$) clusters within the spherical jellium model. We extend the treatment of exchange and correlation effects beyond the local-density approximation. The description of the exchange-correlation potential (especially its asymptotic part) and the local-field correction becomes improved in the weighted-density approximation that we use. Because of the larger number of bound states, we obtain an increased Landau damping. The position of the surface plasmon is given with errors of only 0.1 eV, and its fragmentation is explained due to the influence of particle-hole transitions. For the large negatively charged potassium clusters we predict the fragmentation of the surface plasmon due to its interference with the ionization threshold.

Collective resonance in large free potassium cluster ions.

The absorption cross-section profile of large charged clusters is measured by a new procedure based on multistep photoabsorption experiments. Comparison is done with the predictions of classical and quantum treatments. In light of these results, size trends in the optical properties of alkali clusters are discussed.

Ferromagnetism of potassium clusters incorporated into zeolite LTA.

Potassium clusters are incorporated into supercages of zeolite LTA crystals, with the supercages arrayed in a simple cubic structure at a number density of 5.3\ifmmode\times\else\texttimes\fi{}${10}^{20}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$. About four 4s electrons of K atoms are included in each cluster. The ac magnetic susceptibility increases dramatically with decreases in temperature below 8 K, and shows ferromagnetism below about 4 K. The effective magnetic moment per supercage is estimated to be about 0.13${\mathrm{\ensuremath{\mu}}}_{\mathit{B}}$ at 1.7 K. Magnetic properties resemble those of a reentrant spin glass.

Ultraviolet photoelectron spectroscopy and photofragmentation studies of excess electrons in potassium iodide cluster anions

Ultraviolet photoelectron spectroscopy (UPS) and photofragmentation data for (KI)−n, K(KI)−n , and K2(KI)−n (n up to 13) are presented. The excess electrons in these clusters are all loosely bound with vertical detachment energies below 1.6 eV, and the photofragmentation data correlated well with the UPS results. Our results for the stoichiometric clusters are in good agreement with theoretical predictions, and the ground state geometry of these clusters are discussed. For the excess potassium cluster series, we suggest the possible existence of electron spin pairs in some of these cluster anions.

Desorption and emission of potassium Rydberg atoms and clusters from iron oxide catalyst surfaces

The angular dependence of alkali promoter desorption—emission is studied by molecular beam interaction and surface ionization detection from samples of an industrial potassium-promoted iron oxide catalyst, used for styrene production. In this type of system, highly excited Rydberg state atoms are formed at the surface, which has been observed previously by field ionization at a distance from the surface. A molecular beam of potassium may be directed towards the surface at a fixed angle of 45° to the surface normal. The temperature range used in the studies is 550–1000 K, thus covering the operating temperature range for styrene production. Several different types of angular distributions of potassium are found: flat-topped cosine, tilted cosine, bilobular angular distributions and peaks in the normal direction. The tilted cosine distribution is probably due to back-desorption from the porous surface structure. The bilobular distributions are due to the formation of highly excited Rydberg states of potassium. It is argued that Rydberg atoms can easily diffuse over macroscopic distances, due to their long-range interaction with the surface. The observed peaks in the normal direction are due to clusters of potassium formed by the strong interaction between the Rydberg states, which agrees with previous results.

Unimolecular Coulombic fission in doubly charged alkali-metal clusters.

Delayed asymmetric fission in two singly charged products is studied for doubly charged potassium clusters ${\mathrm{K}}_{\mathit{n}}^{2+}$ and sodium-potassium clusters ${\mathrm{NaK}}_{\mathit{n}\mathrm{\ensuremath{-}}1}^{2+}$, in the size range n=19--28. The small size limit coincides with the limit of the observability of doubly charged clusters in mass spectra, the so-called critical size of stability, ${\mathit{n}}_{\mathit{c}}$. Unimolecular-dissociation experiments probe the competition between the Coulombic fission and the neutral-monomer evaporation around the critical size. When they exist, fission channels mainly include the most stable trimer ion. The metallic-drop model including shell effects well reproduces the experimental unimolecular-dissociation channels. Moreover, it shows that the asymmetric character of the Coulombic fission is mainly due to the surface tension of the cluster.

Thermal broadening of plasmon absorption in potassium cluster ions

The optical absorption spectrum around the surface plasmon peak is presented for K21+. Hot, charged potassium clusters are produced by an adapted liquid metal ion source. The plasmon is found to be slightly red shifted from and distinctly broader than the one reported in an experiment on colder clusters by Brechignac et al. [3]. Mechanisms responsible for the temperature dependence of the width and position of the surface plasmon in small metal clusters are discussed.

A new potassium cluster in zeolites X and A

The reaction of potassium vapour with zeolites K-X and K-A gives rise to a new ionic cluster, K32+.

Quasiparticle energies in small alkali-metal clusters

The authors have calculated the quasiparticle energies of sodium and potassium clusters using the jellium-sphere-background model for the positive ion cores. The electron self-energy is evaluated in first order in the screened Coulomb interaction with local field effects included. Results show significant improvements over the Kohn-Sham eigenvalues in the local-density-functional theory both for the occupied and for the unoccupied quasiparticle states. The theory is found to give the correct size dependencies for quasiparticle energies in the finite systems.

Cohesive energies of K+n 5<n<200 from photoevaporation experiments

Evaporative cooling of internal energy rich potassium cluster ions K+n (5<n<200) is investigated within two well defined but quite different time windows. One of the time windows starts 1 μs after the photoexcitation of the cluster ions isolates one step in the evaporative cooling cascade. The experimental technique insures the complete determination of the dissociation channels. Tandem time-of-flight mass spectroscopy measures the relative rate of competing dissociation channels from ion fragmentation patterns. The corresponding neutral fragments are unambiguously determined after the reionization. Values for the dissociation energies of K+n (up to n=25) have been deduced from the unimolecular dissociation rates using statistical methods. These values are compared to Hückel calculations. The second time window starting just after the reexcitation of mass selected K+n is used to follow the steps of the photoinduced sequential evaporation from ‘‘hot’’ clusters. The photofragmentation patterns for several photon energies give the atomization energies of ionized clusters up to n=200. Their evolution vs 1/R is compared to the droplet model.

Ab initio many-body perturbation-theoretic study of small potassium clusters.

Accurate ab initio molecular-orbital calculations are performed for small potassium clusters. Many-body perturbation theory has been used to study the effects of electron correlation. Results have been compared with available theoretical and experimental data.

Microclusters confined in zeolite cage

ESR measurements were carried out on the potassium cluster and the rubidium-tellurium cluster confined in the zeolite Y cages. The observed ESR signal consists of a single line and 13 hyperfine lines, which are attributed to conduction electrons and ionic clusters K 4 3+, respectively. Only a single line is observed for the sample with high potassium contents. The signal intensity obeys Curie law from 20K to 300K because the clusters are so small that quantum size effects are important. The addition of tellurium to rubidium clusters decreases the intensity of conduction spin resonance signal.

Infrared study of the reactivity of CO with K 2 and K 3 molecules isolated in solid argon

Cocondensation of K atoms and CO molecules with an excess of argon onto a cold substrate (10 K) leads to the formation of strongly perturbed CO complexes from ground state partners only when a large proportion of K clusters is present. It is established that ground state, isolated K atoms do not react with carbon monoxide as do small potassium clusters. K 2CO, K 3CO and K 2(CO) 2 species are identified by CO stretching motions and are unambiguously characterized using systematic concentration and isotopic studies ( 12C, 13C and 18O) as well as triple doping K/CO/O 2/Ar experiments. K 2(CO) 2 is a very labile species which seems likely to have a C 2v doubly bridged structure. It rearranges into the well known acetylene diolate K … O-CC-O … K species upon exposure to infrared light (0.3–0.45 eV).

Jellium-model calculation for monomer and dimer decays of some potassium clusters

Jellium-model calculation for monomer and dimer decays of some potassium clusters

Collective excitation in closed-shell potassium cluster ions

Photoabsorption spectra of mass-selected potassium cluster ions K 9 + and K 21 + were obtained by photoevaporation spectroscopy in the range 0.6 to 4.7 eV. A giant resonance due to the collective dipole oscillation of the valence electrons dominates the photoabsorption spectra. For these spherical clusters, experimental evidence for the Lorentzian shape of the resonance is obtained. A blue-shift and line narrowing are observed for K 21 +. The dependence of damping on cluster size is discussed.

Quantum size effects in excitations of potassium clusters.

The electronic excitations of potassium clusters embedded in a MgO matrix have been investigated as a function of cluster size and momentum transfer by means of electron-energy-loss spectroscopy. The experimental results show deviations from predictions of classical electrodynamics but they are in qualitative agreement with a calculation for excitations of a jellium sphere made with use of the time-dependent local-density approximation.