RESUMEN
In this gas-phase photoelectron spectroscopy study, we present the electron binding energy spectrum and photoelectron angular distributions of NbN(-) by the velocity-map imaging technique. The electron binding energy of NbN(-) is measured to be 1.42 ± 0.02 eV from the X band maximum which defines the 0-0 transition between ground states of anion and neutral. Theoretical binding energies which are the vertical and adiabatic detachment energies are computed by density functional theory to compare them with experiment. The ground state of NbN(-) is assigned to the (2)Δ3/2 state and then the electronic transitions originating from this state into X(3)ΔΩ (Ω = 1-3), a(1)Δ2, A(3)Σ1 (-), and b(1)Σ0 (+) states of NbN are reported to interpret the spectral features. As a prospective study for catalytic materials, spectral features of NbN(-) are compared with those of isovalent ZrO(-) and Pd(-).
RESUMEN
Rare earth elements (REs) consist of a very important group in the periodic table that is vital to many modern technologies. The mining process, however, is extremely damaging to the environment, making them low yield and very expensive. Therefore, mimicking the properties of REs in a superatom framework is especially valuable but at the same time, technically challenging and requiring advanced concepts about manipulating properties of atom/molecular complexes. Herein, by using photoelectron imaging spectroscopy, we provide original idea and direct experimental evidence that chosen boron-doped clusters could mimic the magnetic characteristics of REs. Specifically, the neutral LaB and NdB clusters are found to have similar unpaired electrons and magnetic moments as their isovalent REs (namely Nd and Eu, respectively), opening up the great possibility in accomplishing rare earth mimicry. Extension of the superatom concept into the rare earth group not only further shows the power and advance of this concept but also, will stimulate more efforts to explore new superatomic clusters to mimic the chemistry of these heavy atoms, which will be of great importance in designing novel building blocks in the application of cluster-assembled nanomaterials. Additionally, based on these experimental findings, a novel "magic boron" counting rule is proposed to estimate the numbers of unpaired electrons in diatomic LnB clusters.
RESUMEN
Metal clusters featuring closed supershells or aromatic character usually exhibit remarkably enhanced stability in their cluster series. However, not all stable clusters are subject to these fundamental constraints. Here, by employing photoelectron imaging spectroscopy and ab initio calculations, we present experimental and theoretical evidence on the existence of unexpectedly stable open-shell clusters, which are more stable than their closed-shell and aromatic counterparts. The stabilization of these open-shell Al-Mg clusters is proposed to originate from the S-P molecular orbital coupling, leading to highly stable species with increased HOMO-LUMO gaps, akin to s-p hybridization in an organic carbon atom that is beneficial to form stable species. Introduction of the coupling effect highlighted here not only shows the limitations of the conventional closed-shell model and aromaticity but also provides the possibility to design valuable building blocks.
RESUMEN
The concept of aromaticity has been advanced beyond the framework of organic chemistry, and multiple aromaticity (σ, π, and δ) has been observed to account for the highly symmetric structures or unusual stability of the clusters. In the present study, the electronic structures and chemical bonding of small monolanthanum boride clusters are investigated using photoelectron imaging spectroscopy and first principles electronic structure calculations. Accurate electron affinities of 1.32 ± 0.04 and 1.13 ± 0.06 eV for the neutral LaB2 and LaB3 clusters are obtained by the vibrationally-resolved photoelectron spectra of the LaB2(-) and LaB3(-) clusters, respectively. It is shown that LaB2(-) and LaB3 exhibit enhanced stability in their respective cluster series, as evidenced from the calculated removal energies and HOMO-LUMO gaps. Molecular orbital analysis discloses that these two clusters possess doubly aromatic characters (σ and π), responsible for their enhanced stability. Interestingly, unlike conventional σ-, π-, and δ-aromaticity formed by the delocalization of unhybridized p or d orbitals, the σ and π delocalized molecular orbitals shown here are formed through the effective overlap between the 5d atomic orbital of the La atom and the p orbitals of the remaining boron atoms, representing an intriguing d-p hybridized aromaticity.
RESUMEN
The electronic and structural properties of small monomagnesium oxide clusters, MgO(x)(-) and MgO(x) (x = 1-4), have been investigated using a synergistic approach combining photoelectron imaging spectroscopy and first principles electronic structure calculations. The adiabatic detachment energy (ADE) and vertical detachment energy (VDE) of MgO(x)(-) clusters along with the photoelectron angular distributions (PADs) are determined experimentally. The measured PADs of the clusters are dependent on both the orbital symmetry and electron kinetic energies. Density-functional theory (DFT) calculations were performed to explore the optimized geometries of neutral and anionic MgO(x) clusters. The theoretical ADE and VDE values calculated according to the optimized geometries are in good agreement with our experimental measurements. In addition, MgO(-) and MgO4 clusters are found to have enhanced relative stability in the corresponding anionic and neutral series, based on both theoretical parameters and the experimental cluster distribution.
RESUMEN
Evolution of electronic properties and the nature of bonding of the 4d-transition metal silicides (ZrSi, NbSi, MoSi and PdSi) are discussed, revealing interesting trends in the transition metal-silicon interactions across the period. The electronic properties of select transition metal silicide diatomics have been determined by anion photoelectron imaging spectroscopy and theoretical methods. The electron binding energy spectra and photoelectron angular distributions obtained by 2.33 eV (532 nm) photons have revealed the distinct features of these diatomics. The theoretical calculations were performed at the density functional theory (DFT) level using the unrestricted B3LYP hybrid functional and at the ab initio unrestricted coupled cluster singles and doubles (triplets) (UCCSD(T)) methods to assign the ground electronic states of the neutral and anionic diatomics. The excited electronic states were calculated by the DFT (TD-DFT)/UB3LYP method. We have observed that the valence molecular orbital configuration of the ZrSi and NbSi anions are significantly different from that of the MoSi and PdSi anions. By combining our experimental and theoretical results, we report that the composition of the highest occupied molecular orbitals shift from a majority of transition metal s- and d-orbital contribution in ZrSi and NbSi, to mainly silicon p-orbital contribution for MoSi and PdSi. We expect these observed atomic scale transition metal-silicon interactions to be of increasing importance with the miniaturization of devices approaching the sub-nanometer size regime.
RESUMEN
We report a combined experimental and theoretical study investigating small zinc oxide clusters. A laser vaporization source and a time-of-flight (TOF) mass spectrometer are employed to produce and identify anionic clusters in the Zn(n)O(m) (n = 1-6, m = 1-7) size regime. The adiabatic detachment energy (ADE) and vertical detachment energy (VDE) of Zn(3)O(3)(-) and Zn(3)O(4)(-) clusters are determined via anion photoelectron spectroscopy. We have utilized density functional theory (DFT) calculations to explore the possible geometries of neutral and anionic Zn(3)O(m) (m = 3-5) clusters, while the theoretical ADE and VDE values are compared with experimental results. The experimentally observed relative abundances among the Zn(3)O(m)(-) (m = 3-5) clusters are investigated through calculations of the detachment energies, dissociation energies, and HOMO-LUMO gaps. We find that the Zn(3)O(3) cluster maintains enhanced stability compared to their oxygen-rich counterparts. Furthermore, by coupling the experimentally obtained photoelectron angular distributions of Zn(3)O(3)(-) and Zn(3)O(4)(-) with electronic structure calculations, the nature of the highest occupied molecular orbitals is discussed, with the goal of aiding the isolation (ligand-capped)/deposition of these building blocks.
RESUMEN
Clusters exhibit an enhancement in ionization rates under intense, ultrafast laser pulses compared to their molecular/atomic counterparts. Studies of ionization enhancement of weakly bound molecules to clusters have not been previously characterized and quantified. We demonstrate that weakly bound ClO to (H(2)O)(n) (n = 1-12) clusters and weakly bound HCl to (H(2)O)(n) (n = 1-12) clusters produce high atomic charge states of chlorine via Coulomb explosion. Density functional theory (DFT) was used to qualitatively compare the interaction energy of ClO with respect to the number of water molecules as well as HCl with respect to the number of water molecules. The chlorine ion signal intensity for each atomic charge state was observed to be dependent on the molecule-cluster bond strength. The observed ionization enhancement was quantified using semiclassical tunneling theory, and it was found that the Cl(3+-5+) and O(2+) charge states are enhanced in ionization. Possible mechanisms of ionization enhancement are explored for weakly bound chlorine species.
RESUMEN
To form cluster-assembled materials, the clusters should have low reactivity and be characterized by a closed-shell electronic configuration with a large gap between the highest occupied and the lowest unoccupied molecular orbitals (HOMO-LUMO). Using spin-polarized density functional theory calculations, we investigate the M-substituted Ti(8)C(12) metallocarbohedrynes to search for less reactive clusters as building blocks for cluster-assembled materials (M = Be, Mg, Ca, Sr, Ba and Sc, Y). The selected atoms in the correct stoichiometry would produce a metallocarbohedryne that is isoelectronic with the Ti(8)C(12)(2+), which has a closed-shell electronic configuration and an enhanced HOMO-LUMO gap of 1.735 eV. According to our results, the HOMO-LUMO gaps of the M-substituted Ti(8)C(12) metallocarbohedrynes are in the range of 0.715-0.979 eV for the case of Be, Mg, Ca, Sr and Ba and in the range of 0.865-1.294 eV for the case of Sc and Y. Among all the M-substituted metallocarbohedrynes we consider here, one of the isomers of Ti(6)Sc(2)C(12) is not only energetically more favorable but also exhibits a larger HOMO-LUMO gap of 1.294 eV. This result indicates that the Ti(6)Sc(2)C(12)(4) metallocarbohedryne should be less reactive than the Ti(8)C(12) metallocarbohedryne which has a narrow HOMO-LUMO gap of 0.146 eV. Moreover, we show that the intercluster interaction between two individual Ti(6)Sc(2)C(12)(4) metallocarbohedrynes is relatively weak compared to the Ti(8)C(12) dimer.
RESUMEN
An analytically expression for the temperature dependence of the signal gain of an erbium-doped fiber amplifier (EDFA) pumped at 1480 nm are theoretically obtained by solving the propagation equations with the amplified spontaneous emission (ASE). It is seen that the temperature dependence of the gain strongly depends on the distribution of population of Er3+-ions in the second level. In addition, the output pump power and the intrinsic saturation power of the signal beam are obtained as a function of the temperature. Numerical calculations are carried out for the temperature range from - 20 to + 60 oC and the various fiber lengths. But the other gain parameters, such as the pump and signal wavelengths and their powers, are taken as constants. It is shown that the gain decreases with increasing temperature within the range of L