Thermionic emission from small clusters: direct observation of the kinetic energy distribution of the electrons

Stabilizing hydrogen-mediated sextuple bonds by quintuple superatomic bonding and a bond

Multiple bond orders of four and five are frequently obtained for d-block elements. However, compounds with sextuple bonding (2σ, 2π, and 2δ), for instance, Cr₂, Mo₂ and W₂, are less stable and trapped only in the gas phase or inert matrices, probably resulting from large repulsion of two σ-type (σ_s and ) orbitals within the same zone. Herein, a superatomic bonding model is proposed to describe experimentally synthesized bridging hydride compounds. We theoretically predicted four unprecedented quintuple bridging hydride species with large E_HL values, among which [(Cp)₂Sc₂(μ-H)₅]^- and [(Cp)₂Mn₂(μ-H)₅]^- (Cp = cyclopentadienyl) contain quintuple superatomic bonding (σ, 2π, and 2δ). In particular, two other species, [(Cp)₂Ti₂(μ-H)₅]^- and [(Cp)₂V₂(μ-H)₅]⁺, were found to feature stable hydrogen-mediated sextuple bonding comprising a quintuple superatomic delocalized bond and an extra localized bond. Thereinto, the superatomic σ_s bond disperses to the outer space for better overlap with the orbitals of bridging H atoms, thus leaving the inner regions with low electron densities and providing adequate space for the bond. These two quintuple bridging hydride compounds are verified to have thermal stability by molecular dynamics simulations and expected to provide an effective strategy to synthesize molecules with stable hydrogen-mediated sextuple bonding under normal experimental conditions.

Thermionic Emission of Negative Ions of Molecules and Small Clusters as a Probe of Low-Energy Attachment

We have been studying the thermionic emission of negatively charged molecules and small clusters for more than a decade. The kinetic energy released distribution (KERD) of mass-selected negative ions has been measured with a velocity map imaging spectrometer. A comparison of the experimental KERD to detailed balance models provided information on the reverse process, namely, the electron attachment to the parent. The electron attachment to neutral systems (reverse process of the electron emission from anions) is usually described in a simplified way as a single electron capture in the framework of the classical Langevin model. Our measurements show that this approach is insufficient and that, in addition to the capture step, an intramolecular vibrational redistribution (IVR) step should be included. As far as multiply charged anions are concerned, the electron attachment to anions (reverse process of the electron emission from dianions) is strongly affected by the repulsive Coulomb barrier (RCB). Previous studies assumed a pure over-the-barrier process, which is in disagreement with our study. Indeed, electron emission is measured below the RCB, revealing significant thermal tunneling. In the present review, we summarize these works on singly and doubly charged anions in an attempt to present a unified view of the involved processes. It is worth noting that the detailed measurements of KERDs in the very low kinetic energy region (typically around 0.1 eV) have been made possible thanks to electron imaging methods, without which all of this work could never have been done, with time-resolution capabilities allowing the disentangling of direct and delayed electron emission.

Vibrational Autodetachment from Anionic Nitroalkane Chains: From Molecular Signatures to Thermionic Emission

We report the kinetic energy distributions in electron autodetachment from nitroethane, 1-nitropropane, and 1-nitrobutane anions upon laser excitation of CH stretching modes measured using velocity map electron imaging. In striking contrast to the case of nitromethane, the kinetic energy distributions show almost no distinct vibrational features, and they can be described by thermionic emission, relating the shape of the distributions to the electron capture cross section of the neutral molecule. The data suggest that a classical description is warranted above ca. 20 meV electron kinetic energy. At lower energies, quantum effects suppress the attachment cross section.

Cage-Like Nanoclusters of ZnO Probed by Time-Resolved Photoelectron Spectroscopy and Theory

Zinc oxide nanoclusters have been predicted as promising building blocks for cluster-assembled materials with unprecedented properties. Here, for the first time these clusters are probed by time-resolved photoelectron spectroscopy and characterized in detail by density functional theory. Their validity as building blocks for cluster-assembled materials is confirmed via rigid cage-like structures facilitating three-dimensional aggregation in combination with large band gaps that are nevertheless significantly lower than any known ZnO polymorph. In addition, electron-hole pair localization in the excited state of the cluster anions combined with their structural rigidity leads to extraordinary long-lived states above the band gap virtually independent of the cluster size, defying the rule "every atom counts".

Probing the structure and aromaticity in coinage Rhenium trimer ${\rm Re}_{3}^{+}$ and its all-metal compounds Re3Li2+, Re3X3+ (X = Be, Mg and Ca) and Re3Y (Y = Al and Ga)

The extensive search for the global minimum structure of [Formula: see text] at the B3LYP/LANL2DZ level of the theory revealed that [Formula: see text] is the ground state. Molecular orbital (MO) analysis reveal that delocalized partial σ and δ bonding in [Formula: see text] makes partial (σ- and δ-) aromaticity. The multiple d-orbital aromaticities are responsible for a three-center metal–metal bond of the triangular Re 3. We also search the pyramidal Re 3 Li 2+, Re 3X3+ ( X = Be , Mg and Ca ) and Re 3 Y ( Y = Al and Ga ) clusters containing [Formula: see text] to reveal that the [Formula: see text] was preserved perfectly in all-metal compounds, which also possess partially σ- and δ-multiple aromatic characters.

Structural and electronic properties of tungsten nanoclusters by DFT and basin-hopping calculations

The structural and electronic properties of small tungsten nanoclusters W_n (n = 2–16) were investigated by density functional theory (DFT) calculations.

Probing the electronic properties of W3O(x)(-/0) (x = 0-2) and W3(2-) clusters: the aromaticity of W3 and W3(2-)

Density functional theory (DFT) calculations are employed to investigate the structural and electronic properties of bare tritungsten clusters (W3, W3(-), W3(2-)) and tritungsten oxide clusters W3Ox(-/0) (x = 1, 2). Generalized Koopmans' theorem is applied to predict the vertical detachment energies and simulate the photoelectron spectra (PES) for W3Ox(-) (x = 0-2) clusters. Extensive DFT calculations are performed in search of the lowest energy structures for both the anions and the neutrals. The bare tritungsten clusters are predicted to be triangular structures with D3h ((3)A1'), C2v ((2)A1) and D3h ((1)A1') symmetry for W3, W3(-) and W3(2-), respectively. For W3O(-) and W3O clusters, the oxygen atom occupies the terminal site, while the next added oxygen atom is found to be a bridging one in both W3O2(-) and W3O2 clusters. Molecular orbital analyses are carried out to elucidate the chemical bonding of these clusters and provide insights into the sequential oxidation from W3(-) to W3O2(-). Partial σ- and δ-aromaticity are revealed in the neutral W3 (D3h, (3)A1'), while the anion W3(2-) (D3h, (1)A1') possesses only δ-aromaticity.

Kinetic energy spectra in thermionic emission from small tungsten cluster anions: evidence for nonclassical electron capture

The delayed electron emission from small mass-selected anionic tungsten clusters W(n)(-) has been studied for sizes in the range 9 < or = n < or = 21. Kinetic energy spectra have been measured for delays of about 100 ns after laser excitation by a velocity-map imaging spectrometer. They are analyzed in the framework of microreversible statistical theories. The low-energy behavior shows some significant deviations with respect to the classical Langevin capture model, which we interpret as possibly due to the influence of quantum dynamical effects such as tunneling through the centrifugal barrier, rather than shape effects. The cluster temperature has been extracted from both the experimental kinetic energy spectrum and the absolute decay rate. Discrepancies between the two approaches suggest that the sticking probability can be as low as a few percent for the smallest clusters.

Mechanical Simulation of the Pressure and the Relaxation to Thermal Equilibrium of a Hot and Dense Rare Gas Cluster

A cold atomic cluster can be very rapidly heated and compressed by a hypersonic impact at a hard surface. The impact can be simulated by computing a classical trajectory for the motion of the atoms. By suddenly confining the hot and dense cluster within a rigid container, it is possible to monitor the time evolution of the force acting on the faces of the container. It is found that the pressure computed this way very rapidly decays to a time-independent value. After a somewhat longer time, this value reproduces the value for the pressure computed as the sum of the kinetic and internal pressures. This agreement is expected for a system in equilibrium. These observations support the conclusion that there is a fast relaxation to thermal equilibrium in these essentially hard-sphere systems. The deviation from equilibrium is primarily due to the propagation of shock waves within the cluster. The equilibrium pressure can reach up to the megabar range.

Electronic and Structural Evolution and Chemical Bonding in Ditungsten Oxide Clusters: W2On- and W2On (n = 1−6)

We report a systematic and comprehensive investigation of the electronic structures and chemical bonding in a series of ditungsten oxide clusters, W2O(n)- and W2O(n) (n = 1-6), using anion photoelectron spectroscopy and density functional theory (DFT) calculations. Well-resolved photoelectron spectra were obtained at several photon energies (2.331, 3.496, 4.661, 6.424, and 7.866 eV), and W 5d-based spectral features were clearly observed and distinguished from O 2p-based features. More complicated spectral features were observed for the oxygen-deficient clusters because of the W 5d electrons. With increasing oxygen content in W2O(n)-, the photoelectron spectra were observed to shift gradually to higher binding energies, accompanied by a decreasing number of W 5d-derived features. A behavior of sequential oxidation as a result of charge transfers from W to O was clearly observed. A large energy gap (2.8 eV) was observed in the spectrum of W2O6-, indicating the high electronic stability of the stoichiometric W2O6 molecule. Extensive DFT calculations were carried out to search for the most stable structures of both the anion and neutral clusters. Time-dependent DFT method was used to compute the vertical detachment energies and compare to the experimental data. Molecular orbitals were used to analyze the chemical bonding in the ditungsten oxide clusters and to elucidate their electronic and structural evolution.

C6− electronic relaxation dynamics probed via time-resolved photoelectron imaging

Anion time-resolved photoelectron imaging has been used to investigate the electronic relaxation dynamics of C(6) (-) following excitation of the C (2)Pi(g)<--X (2)Pi(u) and 2 (2)Pi(g)<--X (2)Pi(u) 0(0) (0) transitions at 607 and 498 nm, respectively. Analysis of evolving photodetachment energy distributions reveals differing relaxation pathways from these prepared states. Specifically, the C (2)Pi(g) 0(0) level relaxes on a time scale of 620+/-30 fs to vibrationally hot ( approximately 2.0 eV) anion ground state both directly and indirectly through vibrationally excited levels of the intermediate-lying A (2)Sigma(g) (+) state that decay with a time scale of 2300+/-200 fs. In contrast, the 2 (2)Pi(g) 0(0) level relaxes much more quickly (<100 fs) to vibrationally hot ( approximately 2.5 eV) anion ground state directly and with transient population accumulation in the A (2)Sigma(g) (+), B (2)Sigma(u) (+), and C (2)Pi(g) electronic levels, as determined by spectral and time-scale analyses. This work also presents the experimental observation of the optically inaccessible B (2)Sigma(u) (+) state, which is found to have an electronic term value of 1.41+/-0.05 eV.

Delayed ionization and fragmentation en route to thermionic emission: statistics and dynamics

Thermionic emission is discussed as a long time (microseconds) decay mode of energy-rich large molecules, metallic and metcar clusters, and fullerenes. We review what is known and consider the many experiments, systems, and theoretical and computational studies that still need to be done. We conclude with a wish list for future work. Particular attention is given to the experimental signatures, such as the dependence on the mode of energy acquisition, and theoretical indications of a not-quite-statistical delayed ionization and to the competition of electron emission with other decay modes, such as fragmentation or radiative cooling. Coupling of the electronic and nuclear modes can be a bottleneck and quite long time-delayed ionization can be observed, as in the decay of high Rydberg states probed by ZEKE spectroscopy, before the onset of complete energy partitioning.