Photoelectron Velocity-map Imaging Research Articles

Probing the geometric and electronic structures of the transition metal oxides RhOn–1/0 (n = 1–4) clusters

The photoelectron spectroscopies of RhOn– (n = 1–2) were obtained via using the photoelectron velocity-map imaging (PE-VMI) approach. The experimental values of the adiabatic detachment energy (ADE) and vertical detachment energy (VDE) for RhO– were reported to be 1.58 ± 0.02 eV. The experimental AED and VDE values of RhO2– were reported to be 2.70 ± 0.02 eV and 2.79 ± 0.02 eV, respectively. The vibrational frequencies of RhO– and RhO2– measured from photoelectron spectra (PES) were 817(76) cm−1 and 932(55) cm−1, respectively. Based on the density functional theory (DFT), the RhOn–1/0 (n = 1–4) clusters were investigated. The optimized configurations of corresponding ground states and low-lying clusters were discovered. Meanwhile, the simulated photoelectron spectroscopy (PES) of RhOn– (n = 1–4) and the theoretical ADE and VDE values of RhOn– (n = 1–4) clusters were unveiled to assist future experimental studies of Rhodium oxide clusters. Moreover, the associated molecular orbitals (MOs), natural population analysis (NPA) and bond order analysis have been utilized to investigate the chemical bonding in these groups.

Spectroscopic Characterization of Thermal Methane Activation by Lewis-Acid-Base Pair in a Gas-Phase Metal Nitride Anion Ta2N3.

Activation and transformation of methane is one of the "holy grails" in catalysis. Understanding the nature of active sites and mechanistic details via spectroscopic characterization of the reactive sites and key intermediates is of great challenge but crucial for the development of novel strategies for methane transformation. Herein, by employing photoelectron velocity-map imaging (PEVMI) spectroscopy in conjunction with quantum chemistry calculations, the Lewis acid-base pair (LABP) of [Taδ+-Nδ-] unit in Ta2N3 - acting as an active center to accomplish the heterolytic cleavage of C-H bond in CH4 has been confirmed by direct characterization of the reactant ion Ta2N3 - and the CH4-adduct intermediate Ta2N3CH4 -. Two active vibrational modes for the reactant (Ta2N3 -) and four active vibrational modes for the intermediate (Ta2N3CH4 -) were observed from the vibrationally resolved PEVMI spectra, which unequivocally determined the structure of Ta2N3 - and Ta2N3CH4 -. Upon heating, the LABP intermediate (Ta2N3CH4 -) containing the NH and Ta-CH3 unit can undergo the processes of C-N coupling and dehydrogenation to form the product with an adsorbed HCN molecule.

Structural and Chemical Bonding Properties of AuS2H0/-: A Photoelectron Velocity-Map Imaging Spectroscopic and Theoretical Study.

Mass-selected photoelectron velocity-map imaging spectroscopy was employed to investigate the geometrical and electronic properties of AuS2H-/0. The comprehensive comparison between the experiment and theoretical calculations establishes that the ground-state AuS2H- anion has a mixed-ligand structure [SAuSH]- with an unsymmetrical S-Au-S unit. Further chemical bonding analyses on AuS2H and comparison with the isoelectronic AuS2- suggest that the unique S-Au-S unit in these species features two three-center, three-electron π-bonding, and one three-center, two-electron σ-bonding. The isoelectronic replacement of the extra electron in AuS2- by the H atom can lead to σ bonding evolution from the electron-sharing bond to the dative bond. These findings are conducive to the fundamental understanding of the intrinsic stability of thiolate-protected gold nanoclusters and their delicate ligand design to achieve desirable properties.

Coordination-induced bond weakening in NiC3: An experimental and theoretical investigation.

Mass-selected photoelectron velocity-map imaging spectroscopy in conjunction with the density functional theory calculations was employed to investigate the geometrical and chemical bonding properties of NiC3-/0. Both the photoelectron spectrum and photoelectron angular distribution were measured from the spectra, yielding useful geometrical and electronic information about NiC3-/0. The complementary theoretical calculations suggest that the linear and fan-like structures were both populated experimentally in the cluster beam. Further comparative study on the synergistic donor-acceptor interactions in both isomers revealed the side-on coordination-induced bond weakening in the fan-like isomer as compared to the linear isomer. These findings will shed light on the structure-dependent reactivity of transition metal carbides.

Is Pseudohalide CN− a Real Halide? A General Symmetry Consideration

Recently, in light of the significant attention devoted to pseudohalide CN− and cyano radical CN physico-chemical property studies and superhalide behavior exploration in CN−-ligated metal compounds, the photoelectron angular distribution nature of pseudohalide CN− has been directly demonstrated via the photoelectron velocity map imaging technique to be comparable to Cl−. For the halide Cl−, photoelectrons were observed at 266 nm (4.66 eV) to peak, perpendicular to the laser polarization associated with the detachment of p-orbital symmetry. For the analogous pseudohalide CN−, photoelectrons were present at a peak in laser polarization at 266 nm, which can be explained as detachment from mainly atomic s-like orbital symmetry. Although both are often regarded as having the same high electron affinity and similarly strong chemical bonding capabilities to stabilize complexes, their photoelectron angular distributions are distinctly different, which indicates their intrinsically different electronic–structure symmetry (HOMO nature). The approach based on symmetry consideration in this work could be utilized to explain the photoelectron angular distributions of pseudohalide and classic halide ligands via the advanced photoelectron velocity map imaging tool.

Plasmonic Resonant Intercluster Coulombic Decay.

Light-induced energy confinement in nanoclusters via plasmon excitations influences applications in nanophotonics, photocatalysis, and the design of controlled slow electron sources. The resonant decay of these excitations through the cluster's ionization continuum provides a unique probe of the collective electronic behavior. However, the transfer of a part of this decay amplitude to the continuum of a second conjugated cluster may offer control and efficacy in sharing the energy nonlocally to instigate remote collective events. With the example of a spherically nested dimer Na_{20}@C_{240} of two plasmonic systems we find that such a transfer is possible through the resonant intercluster Coulombic decay (RICD) as a fundamental process. This plasmonic RICD signal can be experimentally detected by the photoelectron velocity map imaging technique.

Time-stretched multi-hit 3D velocity map imaging of photoelectrons.

The 2D photoelectron velocity map imaging (VMI) technique is commonly employed in gas-phase molecular spectroscopy and dynamics investigations due to its ability to efficiently extract photoelectron spectra and angular distributions in a single experiment. However, the standard technique is limited to specific light-source polarization geometries. This has led to significant interest in the development of 3D VMI techniques, which are capable of measuring individual electron positions and arrival times, obtaining the full 3D distribution without the need for inversion, forward-convolution, or tomographic reconstruction approaches. Here, we present and demonstrate a novel time-stretched, 13-lens 3D VMI photoelectron spectrometer, which has sub-camera-pixel spatial resolution and 210ps (σ) time-of-flight (TOF) resolution (currently limited by trigger jitter). We employ a kHz CMOS camera to image a standard 40mm diameter microchannel plate (MCP)/phosphor anode detector (providing x and y positions), combined with a digitizer pick-off from the MCP anode to obtain the electron TOF. We present a detailed analysis of time-space correlation under data acquisition conditions which generate multiple electrons per laser shot, and demonstrate a major advantage of this time-stretched 3D VMI approach: that the greater spread in electron TOFs permits for an accurate time- and position-stamping of up to six electrons per laser shot at a 1kHz repetition rate.

Enhanced Stability of Rhombic Dodecahedron Nb15– with Well-Organized Superatomic States

Well-resolved Nbn- clusters are produced and reacted with ethene and propene via a downstream flow tube reactor. Interestingly, the Nbn- clusters readily react with ethene and propene to form dehydrogenation products; however, Nb15- shows up in the mass spectra with prominent mass abundance indicating its inertness to react with olefins. For this cluster, we conduct photoelectron velocity map imaging (VMI) experiments and verify the stability of Nb15- within a highly symmetrical rhombic dodecahedron structure. Theoretical studies show that the stability of the Nb15- cluster is correlated with its superatomic nature pertaining to both geometric and electronic shell closures. Notably, the superatomic 1s orbital is dominated by the 5s electron of the central Nb atom, while the other superatomic orbitals are contributed by s-d hybridization, especially a remarkable contribution of s-dz2 hybridization. Apart from the closed shells, the highly symmetric geometry of Nb15- is associated with a regular polyhedral structure directed by all rhombus facets, embodying a magic number for body-centered dodecahedra, indicative of enhanced stability as a double magic cluster free of olefin adsorption.

Observation of resonances in the transition state region of the F + NH3 reaction using anion photoelectron spectroscopy.

The transition state of a chemical reaction is a dividing surface on the reaction potential energy surface (PES) between reactants and products and is thus of fundamental interest in understanding chemical reactivity. The transient nature of the transition state presents challenges to its experimental characterization. Transition-state spectroscopy experiments based on negative-ion photodetachment can provide a direct probe of this region of the PES, revealing the detailed vibrational structure associated with the transition state. Here we study the F + NH3 → HF + NH2 reaction using slow photoelectron velocity-map imaging spectroscopy of cryogenically cooled FNH3- anions. Reduced-dimensionality quantum dynamical simulations performed on a global PES show excellent agreement with the experimental results, enabling the assignment of spectral structure. Our combined experimental-theoretical study reveals a manifold of vibrational Feshbach resonances in the product well of the F + NH3 PES. At higher energies, the spectra identify features attributed to resonances localized across the transition state and into the reactant complex that may impact the bimolecular reaction dynamics.

Spectroscopic characterization of chain-to-ring structural evolution in platinum carbide clusters

Metal carbides play an important role in catalysis and functional materials. However, the structural characterization of metal carbide clusters has been proven to be a challenging experimental target due to the difficulty in size selection. Here we use the size-specific photoelectron velocity-map imaging spectroscopy to study the structures and properties of platinum carbide clusters. Quantum chemical calculations are carried out to identify the structures and to assign the experimental spectra. The results indicate that the cluster size of the chain-to-ring structural evolution for the PtCn– anions occurs at n = 14, whereas that for the PtCn neutrals at n = 10, revealing a significant effect of charge on the structures of metal carbides. The greatest importance of these building blocks is the strong preference of the Pt atom to expose in the outer side of the chain or ring, exhibiting the active sites for catalyzing potential reactions. These findings provide unique spectroscopic snapshots for the formation and growth of platinum carbide clusters and have important implications in the development of related single-atom catalysts with isolated metal atoms dispersed on supports.

High Resolution Photoelectron Spectroscopy of the Acetyl Anion.

High-resolution photoelectron spectra of cryogenically cooled acetyl anions (CH3CO-) obtained using slow photoelectron velocity-map imaging are reported. The high resolution of the photoelectron spectrum yields a refined electron affinity of 0.4352 ± 0.0012 eV for the acetyl radical as well as the observation of a new vibronic structure that is assigned based on ab initio calculations. Three vibrational frequencies of the neutral radical are measured to be 1047 ± 3 cm-1 (ν6), 834 ± 2 cm-1 (ν7), and 471 ± 1 cm-1 (ν8). This work represents the first experimental measurement of the ν6 frequency of the neutral. The measured electron affinity is used to calculate a refined value of 1641.35 ± 0.42 kJ mol-1 for the gas-phase acidity of acetaldehyde. Analysis of the photoelectron angular distributions provides insight into the character of the highest occupied molecular orbital of the anion, revealing a molecular orbital with strong d-character. Additionally, details of a new centroiding algorithm based on finite differences, which has the potential to decrease data acquisition times by an order of magnitude at no cost to accuracy, are provided.

A 45-degree ion-extraction photoelectron velocity map imaging apparatus coupled to the DUV-IR mass spectrometry and spectroscopy instrument

A photoelectron velocity-map imaging (VMI) apparatus has been developed from the DUV-IR photoionization mass spectrometry and spectroscopy instrument. A 45-degree plate-set is designed and combined with the upright reflection time-of-flight mass spectrometer (Re-TOFMS) to pick up target cluster anions and change their trajectory after the vertical reflection. Following the ion-extraction zone, the mass-selected cluster anions then pass through up-down and left-right trajectory adjustment and beam focus until the arrival at the photoelectron detachment zone to encounter the normal incidence of a pulsed UV laser. The detached electrons are subject to acceleration and detected by a dual micro-channel plate detector and imaging on the phosphor screen. This 45-degree ion-extraction photoelectron VMI setup not only expands the function of our DUV-IR instrument for structural chemistry but also facilitates the compatibility with the conventional upright reflection of TOFMS.

Strong-field ionization of plasmonic nanoparticles

We modeled strong-field ionization of metal nanoparticles by intense infrared laser pulses, accounting for and distinguishing in photoelectron (PE) momentum distributions the effects of PE correlation, PE--residual-charge interactions, PE rescattering and recombination, and transient laser-induced plasmonic fields. Our numerical results for 5-, 30-, and 70-nm-diameter gold nanospheres and peak laser-pulse intensities of $8.0\ifmmode\times\else\texttimes\fi{}{10}^{12}$ and $1.2\ifmmode\times\else\texttimes\fi{}{10}^{13}\phantom{\rule{4.pt}{0ex}}{\text{W/cm}}^{2}$ show how PE velocity-map images are distinctly shaped by PE Coulomb repulsion, residual-charge accumulations, and plasmonic near fields. In contrast to gaseous atomic targets and dielectric nanoparticles, we find very large PE cutoff energies, for both directly emitted and rescattered PEs, that exceed the incident laser-pulse ponderomotive energy by two orders of magnitude.

Photoelectron imaging spectroscopic signatures of CO activation by the heterotrinuclear titanium-nickel clusters

A series of heterotrinuclear Ti2Ni(CO)n– (n = 6–9) carbonyls have been generated via a laser vaporization supersonic cluster source and characterized by mass-selected photoelectron velocity-map imaging spectroscopy. Quantum chemical calculations have been carried out to identify the structures and understand the experimental spectral features. The results indicate that a building block of Ti-Ti-Ni-C four-membered ring with the C atom bonded to Ti, Ti, and Ni is dominated in the n = 6–8 complexes, whereas a structural motif of Ti-Ti-Ni triangle core is preferred in n = 9. These complexes are found to be capable of simultaneously accommodating all the main modes of metal-CO coordination (i.e., terminal, bridging, and side-on modes), where the corresponding mode points to the weak, moderate, high CO bond activation, respectively. The number of CO ligands for a specific bonding mode varies with the cluster size. These findings have important implications for molecular-level understanding of the interaction of CO with alloy surfaces/interfaces and tuning the appropriate CO activation via the selection of different metals.

Hydrogen bonds in mixed-solvent Au-(CH3OH)(H2O) complex: A joint experimental and theoretical study

Compared to the conventional hydrogen bonds, nonconventional hydrogen bonds have gained increasing interest in recent years. In this work, we firstly investigate hydrogen bonds in mixed-solvent Au-(CH3OH)(H2O) complex via gas phase anion photoelectron velocity map imaging associated with quantum chemical calculations. The vertical detachment energy of Au-(CH3OH)(H2O) complex is determined as 3.22 ± 0.02 eV. The calculations reveal these mixed-solvent solvated complexes having two stable low-energy isomers: Iso-a and Iso-b. And the experimental spectrum is ascribed to the contribution from Iso-a isomer. Detailed hydrogen bonding patterns have been revealed for both isomers.

Synergetic electron donation and back-donation interactions in (Au − CO2)− complex: A joint anionic photoelectron velocity-map imaging spectroscopy and theoretical investigation

The reaction of Au− with CO2 has been investigated by photoelectron velocity-map imaging. The chemisorbed (Au − CO2)− complex is predominant in the experiment. The further reduction of CO2 into CO is theoretically proved to be thermodynamically endothermic and kinetically infeasible. Nevertheless, the bent CO2 moiety and elongated C − O bonds provide the clearest sign for anionic activation of CO2. Chemical bonding analyses indicate that there are electron back-donations from the 5d/6s atomic orbitals of Au− to the anti-bonding π orbitals of CO2, in synergy with unprecedented donation from the delocalized bonding orbitals of CO2 to the metal 6p atomic orbital.

Spectroscopic Characterization of the Synergistic Mechanism of Ruthenium-Lithium Hydrides for Dinitrogen Cleavage.

Elucidating the role of alkali/alkaline earth metal hydrides in dinitrogen activation remains an important and challenging goal for spectroscopic studies of bulk systems, because their spectral signatures are often masked by the collective effects. Herein, mass-selected photoelectron velocity-map imaging spectroscopic and quantum chemical calculation techniques are utilized to explore the promotion mechanism of LiH in the Ru-based catalysts toward N2 activation. The RuHN2- anion is determined to be a N2-tagged complex. In contrast, the RuHN2(LiH)n- (n = 1 and 2) anions are characterized to have N≡N bond-cleaved ring structures. These observations indicate that the complexation of LiH to RuH- significantly facilitates N≡N bond cleavage. Theoretical analyses show that the synergy between Ru and LiH efficiently lowers the energy barrier of N≡N bond cleavage. These findings clarify the pivotal roles played by the LiH species in the transition metal catalysts for N2 activation and have important practical implications for the prospective design of high-performance catalysts via metal tuning strategies.

Observation of unsaturated platinum carbenes Pt2C2n - (n = 1-3) clusters: A photoelectron imaging spectroscopic and theoretical study.

The structural and bonding properties of the Pt2C2n - (n = 1-3) complexes have been investigated by mass-selected photoelectron velocity-map imaging spectroscopy with quantum chemical calculations. The adiabatic detachment energies and vertical detachment energies of Pt2C2n - have been obtained from the measured photoelectron imaging spectra. Theoretical results indicate that the lowest-energy isomers of Pt2C2n - (n = 1-3) possess linear chain-shaped configurations. The binding motif in the most stable isomer of Pt2C2 - has a linear cumulenic structure with a Pt=C=C=Pt configuration, and the structural characteristic persists up to all the lowest-energy isomers of the Pt2C4 - and Pt2C6 - anions. The chemical bonding analyses indicate that the Pt2C2n - (n = 1-3) complexes have multicenter two-electron characteristics.

Regulating and Directionally Controlling Electron Emission from Gold Nanorods with Silica Coatings.

Dielectric coatings offer a versatile means of manipulating hot carrier emission from nanoplasmonic systems for emerging nanocatalysis and photocathode applications, with uniform coatings acting as regulators and nonuniform coatings providing directional photocurrent control. However, the mechanisms for electron emission through dense and mesoporous silica (SiO2) coatings require further examination. Here, we present a systematic investigation of photoemission from single gold nanorods as a function of dense versus mesoporous silica coating thicknesses. Studies with dense coatings on gold nanostructures clarify the short (∼1 nm) attenuation length responsible for severely reduced transmission through the silica conduction band. By contrast, mesoporous silica is much more transmissive, and a simple geometric model quantitatively recapitulates the electron escape probability through nanoscopic porous channels. Finally, photoelectron velocity map imaging (VMI) studies of nanorods with coating defects verify that photoemission occurs preferentially through the thinner regions, illustrating new opportunities for designing photocurrent distributions on the nanoscale.

Photoelectron velocity-map imaging spectroscopy of nickel carbide: examination of the low-lying electronic states

Ground-state NiC is better described as involving one electron-sharing σ bond, one electron-sharing π bond, and one dative π bond.