Electronic absorption spectra of solutions of molecular chlorine in molten alkali metal chlorides

Electronic reflection absorption spectroscopy has been used for measuring the electronic spectra of chlorine solutions in molten alkali metal chlorides 2CsCl-NaCl, KCl-NaCl, and CsCl in the range 240–400 nm. Absorption bands of chloride melts are interpreted as electronic transitions in the molecular group Cl₂ and in the triatomic linear group Cl ₃ ^? of symmetry D _∞h and C _∞V .     

Photoionization of alkali metal clusters

The photoionization cross section for spherical alkali metal clusters is predicted to oscillate as a function of the photon wavenumber with a frequency determined by the diameter of the cluster. The oscillations and other principal features of the photo cross section can be worked out analytically using semiclassical techniques. An accurate numerical calculation with different cluster potentials confirms these results qualitatively. Quantitative details depend sensitively on the actual potential. Hence, properties of the true cluster potential can be inferred from the experimental cross section. This might turn out to be useful for improving theoretical cluster potentials.     

Unraveling the absorption spectra of alkali metal atoms attached to helium nanodroplets

The absorption spectra of the first electronic exited state of alkali metal atoms on helium nanodroplets formed of both 4He and 3He isotopes were studied experimentally as well as theoretically. In the experimental part new data on the 2p<--2s transition of lithium on 3He nanodroplets are presented. The absorption spectrum changes drastically when compared to 4He droplets, in contrast to sodium where only marginal differences were observed in former studies. To explain these large differences and to answer some still open questions concerning the interaction of alkali metal atoms with helium nanodroplets, a model calculation was performed. New helium density profiles as well as a refined model allowed us to achieve good agreement with the experimental findings. For the first time the red-shifted intensities in the lithium and sodium spectra are explained in terms of enhanced binding configurations in the excited state displaced spatially from the ground state configurations.     

Fission of ionized alkali metal clusters

We study the symmetric fragmentation of ionized alkali clusters within a liquid-drop type model. The interplay of surface and Coulomb interactions leads to a stability condition against small deformations which depends on the ratioZ ²/N. For systems which are stable against small-amplitude oscillations we consider the possibility of large-amplitude modes eventually leading to fission and give in terms of the same quantity an estimate of the potential barrier for this fission channel.     

Structure of alkali metal ionic clusters

Quantum mechanical calculations show that alkali metal ionic clusters of the type M³⁺₄ have the tetrahedral structure, the stability varying as Na³⁺₄ > K³⁺₄ > Rb³⁺₄. Raman spectroscopy has been employed to establish the tetrahedral structure of Na³⁺₄.     

Vibrational spectra of alkali metal peroxoborates

Raman and i.r. spectra of solid Na₂[B₂(O₂)₂(OH)₄]·6H₂O (normal, ¹⁰B, ¹¹B, and ²H-substituted), Na₂[B₂(O₂)₂(OH)₄]·nH₂O (n=4, 0), Li₂[B₂(O₂)₂(OH)₄] and M^I₂[B₂(O₂)₂(OOH)₂(OH)₂] (M^I= K, Rb, Cs) are reported and the vibrational modes assigned.     

First-principle calculations ofK-edge X-ray absorption spectra for small iron and nickel clusters

In this study we present theoretical X-ray absorption near-edge spectra (XANES) evaluated for Fe₂, Ni₂, and Ni₃ using a modified multiple-scattered wave procedure. The calculations were performed for different internuclear distances of the dimers and by varying the geometry from linear to equilateral triangular for Ni₃. The calculated absorption spectra for both Fe₂ and Ni₂ predict a broad resonant structure at the threshold and weak 1s transitions to unoccupied valence orbitals ?5→0 eV giving a shoulder at the threshold. The resonance structure at the threshold of Ni₃ appeared to be sensitive to the geometry of the clusters.     

Fission of highly charged alkali metal clusters

Highly charged metal clusters can be easily produced using the technique of multi-step photoionization. The critical sizes at which fission occurs are reported here for alkali metal clusters having charges as high as +7. An interpretation within the framework of the liquid droplet model reveals that the fission process is strongly asymmetric and thermally activated.     

Two photon states in alkali metal clusters

We study the problem of two-photon absorption in alkali metal clusters: a) by using a sum rule approach for double dipole excitation operators to have some insight on the nature of the corresponding excited states; b) by using a simple random phase approximation [RPA] model to develope an harmonic model for the excited states of the system which allows an explicit prediction for the probability to excite states in the vibrational band build on the surface plasma resonance.     

Collective excitations in deformed alkali metal clusters

A theoretical study of collective excitations in deformed metal clusters is presented. Sum rules are used to study the splittings of the dipole surface plasma resonance originating from the cluster deformation. The vibrating potential model is developed and used to predict the occurrence of a low lying collective mode of orbital magnetic nature.     

A localized-orbital Hartree-Fock description of alkali metal clusters

We propose a new variational method, based on the ab initio Hartree-Fock methods, for the purpose of calculating efficiently both the equilibrium geometry and stability of alkali metal microclusters. Applying this method to neutral and cationic lithium clusters, up to Li₃₆, we find that the calculated stable cluster size corresponds to the observed magic numbers. Our results also indicate that both neutral and cationic clusters exhibit very similar dependence on the number of electrons both in stability and cluster shape, which is quite similar to the one obtained by the shell model. Furthermore, we find that lithium clusters larger than Li₂₆ have an ordered structure while clusters smaller than this do not. This is suggestive of the occurrence of a structural transition to a solid-like phase.     

Stable structures and absorption spectra for SixOy molecular clusters using density functional theory

Structural Chemistry - Calculations are presented of vibrational absorption spectra for energy minimized structures of SixOy molecular clusters using density functional theory (DFT). The size of...     

Competing non-covalent interactions in alkali metal ion-acetonitrile-water clusters

Competitive ion-dipole, ion-water, and water-water interactions were investigated at the molecular level in M+ (CH3CN)n(H2O)m cluster ions for M = Na and K. Different [n,m] combinations for two different n + m cluster sizes were characterized with infrared predissociation spectroscopy in the O-H stretch region and MP2 calculations. In all cases, no differences were observed between the two alkali metal ions. The results showed that at the n + m = 4 cluster size, the solvent molecules interact only with the ion, and that the interaction between the ion and the large dipole moment of CH3CN decreases the ion-water electrostatic interactions. At the n + m = 5 cluster size, at least two different hydrogen-bonded structures were identified. In these structures, the ion-dipole interaction weakens the ability of the ion to polarize the hydrogen bonds and thus decreases the strength of the water-water interactions in the immediate vicinity of the alkali metal ion.     

Reactivity of small transition metal clusters

A new experiment for measuring the reactivity of neutral metal clusters is presented. A low pressure reaction cell is used to measure the sticking ofO ₂ andD ₂ gas on small transition metal clusters ofCu, Fe, Co andNi. The experiment yields absolute values for the sticking, at a controlled number of cluster/gas collisions, facilitating comparison with theoretical calculations and other experiments. The most striking result of these preliminary measurements is the difference between oxygen sticking onCo _N and onCu _N , with the sticking onCu _N showing a clear correlation to the electronic shell model, while the sticking onCo _N only exhibits a sharp increase with size, reaching sticking probability=1.0 forN>25.     

Magnetic properties of free alkali and transition metal clusters

The Stern-Gerlach deflections of small alkali clusters (N<6) and iron clusters (10<N<500) show that the paramagnetic alkali clusters always have a non-deflecting component, while the iron clusters always deflect in the high field direction. Both of these effects appear to be related to spin relaxation however in the case of alkali clusters it is shown that they are in fact caused by avoided level crossing in the Zeeman diagram. For alkali clusters the relatively weak couplings cause reduced magnetic moments where levels cross. For iron clusters however the total spin is strongly coupled to the molecular framework. Consequently this coupling is responsible for avoided level crossings which ultimately cause the total energy of the cluster to decrease with increasing magnetic field so that the iron clusters will deflect in one direction when introduced in an inhomogeneous magnetic field. Experiment and theory are discussed for both cases.     

Application of molecular orbital theory to the interpretation of x-ray absorption spectra of platinum complexes

X-ray L_III-absorption edges of platinum in nine octahedral complexes have been recorded using a bent crystal spectrograph. The edge features of the discontinuities have been interpreted with the help of qualitative molecular orbital diagrams. A correlation between the energy separation of the first two absorption maxima and the spectrochemical series of the ligands has been arrived at.     

Photoabsorption spectra ofs 1-electron metal clusters

Collinear laser/molecular beam photodepletion spectroscopy has developed into a powerful tool for the determination of electronic absorption in neutral alkali metal clusters free of perturbing matrix effects. We briefly discuss the method, present data on selected Na _x and Li _y Na _x?y ,x≤20, contrast measurements with the known optical response of larger colloidal metal clusters, compare microcluster spectra with the predictions of various models and speculate on future experimental and theoretical developments in this field.     

Structure of small molecular hydrogen clusters

The ground state energies and structural properties of small (H₂) _N ,N=2?7, are calculated using the variational Monte Carlo method. These wavefunctions include both short- and long-range correlation effects that are important in the binding of van der Waals clusters. We have investigated these clusters using shadow wavefunctions and found that the coupling to shadow variables raises the energy in all cases, implying that the ground states of these small clusters are properly described as quantum liquids rather than solid structures.