Threshold Photoionization and Density Functional Theory Investigations of Small Lanthanum Monoxide Clusters, LanO (n = 2-10).

Small lanthanum monoxide clusters, LanO (n = 2-10), have been studied by laser threshold photoionization (PI) spectroscopy and density functional theory (DFT). Ionization energies (IEs) gradually decrease with cluster size with oscillatory behavior at certain sizes at n = 4, 7, and 9, and the IEs are slightly lower than the corresponding bare Lan clusters. Mulliken charge analysis of the neutral and cation clusters indicates that the threshold PI occurs from a delocalized metal-based 5d orbital. The stable geometric structures and IEs for the LanO (n = 2-8) clusters have been determined from DFT. Zero electron kinetic energy and PI spectra have been calculated based on DFT calculations and compared with the experimental PI spectra. On the basis of favorable agreement between the simulated and experimental PI spectra, reliable ground-state geometric structures have been assigned. The O atom is negatively charged, equivalent to one electronic charge mainly transferred from La atoms bonded to it. Chemical bonding between the La and O atoms is ionic, whereas La atoms are mostly covalently bonded. No direct correlation is found between the electronic properties like the IE, electron affinity, and HOMO-LUMO gap and the stability of these clusters.

Ionization Energies and Ground-State Structures of Neutral Lan (n = 2-14) Clusters: A Combined Experimental and Theoretical Investigation.

Neutral lanthanum clusters are studied by photoionization time-of-flight mass spectroscopy, laser threshold photoionization spectroscopy, and density functional theory (DFT). Mass abundance spectra (MS) registered at multiple photoionization wavelengths in the range of 195-230 nm by single photon ionization reveal the production of all sizes, Lan (n ≥ 50), in good abundance, nullifying previously predicted low abundances for certain sizes in the 3-14 size range. Also, the MS do not reveal the extraordinary stability of any specific size, as one would expect, from previous theoretical predictions of 7- and 13-atom clusters as magic. Ionization energies (IEs) are measured for Lan (n = 2-14) clusters. DFT has been used to determine the stable geometric isomers for 2- to 10-atom clusters and to calculate their IEs. The theoretical IEs of 2-7 atom clusters are in decent agreement with their experimental values; however, the theoretical IEs are somewhat lower by ∼0.4 eV for n ≥ 8 than their experimental IEs.

Structures and ionization energies of small lithium doped germanium clusters.

We present a combined theoretical and experimental investigation of neutral and cationic lithium doped germanium clusters, GenLim (n = 5-10; m = 1-4). The vertical ionization energies and ionization thresholds are derived from threshold photoionization efficiency curves in the 4.68-6.24 eV range and are compared with calculated vertical and adiabatic ionization energies for the lowest energy isomers obtained using DFT computations. The agreement between experimental and computed values supports the identification of the ground state structures. Charge population analysis shows that lithium transfers its valence electron to the Gen hosts to form Gen(mδ-)-mLi(δ+) and Gen((mδ(-)+1))-mLi(δ+) complexes. This is also illustrated by the strong correlation between the size dependent lithium adsorption energies in GenLi and the Gen electron affinities. Neutral GenLim clusters are formed by adsorbing lithium atoms on either triangular or rhombic faces of the Gen framework with the lithium atoms tending to avoid each other. The chemical bonding phenomena of clusters are analyzed in detail using the densities of states and molecular orbitals.

Energy linkage between the singlet and triplet manifolds in LaH, and observation of new low-energy states.

Wavelength-resolved fluorescence spectra of jet-cooled LaH were obtained from D1, E1, and 0(+)((3)Σ(-)) states by exciting isolated rotational levels. The observation of a(3)Δ(1) and a(3)Δ(2) states at 1259.5(5) and 1646(1) cm(-1), respectively, established the missing energy link between the singlet and triplet manifolds. The low-energy b(3)Π(0,1) and B(1)Δ(2) states predicted earlier from ab initio studies were also observed for the first time. Vibrational constants ω(e) = 1418(2) cm(-1), ω(e)x(e) = 15.6(7) cm(-1) for the ground and ΔG(1∕2) = 1326.1(7) and 1312(1) cm(-1), respectively, for the a(3)Δ(1) and b(3)Π(1) states were also determined. Vibrational frequencies were found to be in excellent agreement with earlier ab initio values. However, ab initio term energies and spin-orbit separation of (3)Δ(2)-(3)Δ(1) and (3)Π(1)-(3)Π(0) were found to be in poor agreement with the present observations. Also, the (3)Π state that was predicted to be inverted is observed to be regular.

Ionization energies and structures of lithium doped silicon clusters.

We report on a combined experimental and theoretical study of the ionization energies and structures of small lithium doped silicon clusters, SinLim with n = 5-11 and m = 3-6. Photoionization efficiency curves are measured in the 4.68-6.24 eV range and subsequently compared with calculated values of both vertical and adiabatic ionization energies for the lowest energy isomers obtained using density functional theory at the B3LYP/6-311+G(d) level. The evolution of the cluster geometries and ionization energies is studied as a function of the number of silicon and lithium atoms along the SinLi3 (n = 5-11) and Si8Lim (m = 1-6) series, respectively. For most studied sizes good agreement is found between the experimental and the calculated ionization energies for the lowest-energy isomer. In the SinLi3 (n = 5-11) series, positively charged lithium atoms surround a negatively charged silicon framework and mainly act as electron donors. Upon sequential lithium addition along the Si8Lim (m = 1-6) series, the Si8 framework deforms from a rhombic (m = 0, 1) over a tetracapped tetragon (m = 1-4) to a square antiprism (m = 4-6) structure. Subsequent addition of lithium implies the addition of excess electrons to the silicon framework, which is reflected in a decrease of the ionization energy with increasing lithium content. This decrease is non-monotonous and odd-even alternations, reflecting the higher stability of clusters with an even number of electrons, are observed. In addition, post-threshold features in the experimental photoionization efficiency curves of SinLi3 (n = 8-11) may be related to the computed density of states of the corresponding lowest energy isomers.

Singly and doubly lithium doped silicon clusters: geometrical and electronic structures and ionization energies.

The geometric structures of neutral and cationic Si(n)Li(m)(0/+) clusters with n = 2-11 and m = 1, 2 are investigated using combined experimental and computational methods. The adiabatic ionization energy and vertical ionization energy (VIE) of Si(n)Li(m) clusters are determined using quantum chemical methods (B3LYP/6-311+G(d), G3B3, and CCSD(T)/aug-cc-pVxZ with x = D,T), whereas experimental values are derived from threshold photoionization experiments in the 4.68-6.24 eV range. Among the investigated cluster sizes, only Si(6)Li(2), Si(7)Li, Si(10)Li, and Si(11)Li have ionization thresholds below 6.24 eV and could be measured accurately. The ionization threshold and VIE obtained from the experimental photoionization efficiency curves agree well with the computed values. The growth mechanism of the lithium doped silicon clusters follows some simple rules: (1) neutral singly doped Si(n)Li clusters favor the Li atom addition on an edge or a face of the structure of the corresponding Si(n)(-) anion, while the cationic Si(n)Li(+) binds with one Si atom of the bare Si(n) cluster or adds on one of its edges, and (2) for doubly doped Si(n)Li(2)(0/+) clusters, the neutrals have the shape of the Si(n+1) counterparts with an additional Li atom added on an edge or a face of it, while the cations have both Li atoms added on edges or faces of the Si(n)(-) clusters.

Electron distribution in partially reduced mixed metal oxide systems: infrared spectroscopy of CemVnOo(+) gas-phase clusters.

Vibrational predissociation spectra of rare-gas-tagged [(CeO(2))(VO(2))(1-2)](+) and [(Ce(2)O(3))(VO(2))](+) clusters are measured in the 400-1200 cm(-1) region. Density functional theory (DFT) is used to determine the geometric and electronic structure of low-energy isomers of the partially reduced clusters. Comparison of experimental and simulated spectra provides evidence for the larger stability of Ce(+3)/V(+5) compared to that of Ce(+4)/V(+4), which confirms that the exceptionally high reducibility of Ce(+4) accounts for the promoting role of ceria in supported vanadium oxide catalysts.