Zhengbo Qin

Collinear Velocity-map Photoelectron Imaging Spectrometer for Cluster Anions

We describe a collinear velocity-map photoelectron imaging spectrometer, which combines a Wiley-McLaren time-of-flight mass analyzer with a dual-valve laser vaporization source for investigating size-selected cluster and reaction intermediate anions. To generate the reaction anions conveniently, two pulsed valves and a reaction channel are employed instead of premixing carrier gas. The collinear photoelectron imaging spectrometer adopts modified velocity-map electrostatic lens, and provides kinetic energy resolution better than 3%. The performance of the instrument is demonstrated on the photodetachment of Si4− at 532 and 355 nm, and Si3C− at 532 nm, respectively. In both cases, photoelectron spectra and anisotropy parameters are obtained from the images. For Si4−, the spectra show two well-resolved vibrational progressions which correspond to the ground state and the first excited state of the neutral Si4 with peak spacing of 330 and 312 cm−1, respectively. Preliminary results suggest that the apparatus is a powerful tool for characterizing the electronic structure and photodetachment dynamics of cluster anions.

Vibrationally resolved photoelectron imaging of gold hydride cluster anions: AuH− and Au2H−

Photoelectron spectra and angular distributions in photodetachment of gold hydride anions AuH(-) and Au(2)H(-) have been obtained using photoelectron velocity-map imaging. Both the images exhibit vibrationally resolved ground state transitions. The adiabatic electron affinities of AuH and Au(2)H are measured to be 0.758(20) and 3.437(3) eV, respectively. Franck-Condon analyses of the AuH spectra determined that the equilibrium bond length of the ground state of AuH(-) is 1.597(6) A. The photoelectron images of Au(2)H(-) show a vibrational progression of 148(4) cm(-1) assigned to the Au-Au stretching mode at the ground state. Ab initio calculation results are in excellent agreement with the experimental results. For the ground state of Au(2)H, a new bent Au-Au-H structure with the angle of 131 degrees is suggested. Moreover, energy-dependent photoelectron anisotropy parameters are also reported and discussed.

Photoelectron imaging and theoretical studies of group 11 cyanides MCN (M = Cu, Ag, Au).

Photodetachment of group 11 cyanide anions MCN(-) (M = Cu, Ag, Au) has been investigated using photoelectron velocity-map imaging. The electron affinities (EAs) of CuCN (1.468(26)) and AgCN (1.602(22)) are larger, while that of AuCN (2.066(8)) is smaller than those of the free atoms. This intriguing observation was confirmed by theoretical studies and was assigned to the transition between ionic and covalent bond properties. The harmonic frequencies of the extended vibrational progressions in the M-C stretching mode are 460(50), 385(27), and 502(10) cm(-1), respectively, which suggests a stronger bond for Au-CN than for Ag-CN. Electronic structure analysis and model calculations suggest that all M-C bonds in group 11 cyanides are best described as single bonds. A model has been proposed to explain how the relativistic effects influence the Au-C bond strength in AuCN.

Note: A novel dual-channel time-of-flight mass spectrometer for photoelectron imaging spectroscopy

A novel dual-channel time-of-flight mass spectrometer (D-TOFMS) has been designed to select anions in the photoelectron imaging measurements. In this instrument, the radiation laser can be triggered precisely to overlap with the selected ion cloud at the first-order space focusing plane. Compared with that of the conventional single channel TOFMS, the in situ mass selection performance of D-TOFMS is significantly improved. Preliminary experiment results are presented for the mass-selected photodetachment spectrum of F(-) to demonstrate the capability of the instrument.

Octacoordinate Metal Carbonyls of Lanthanum and Cerium: Experimental Observation and Theoretical Calculation

The octacoordinate metal carbonyls La(CO)8(+) and Ce(CO)8(+) were observed in laser vaporization of La and Ce in pure CO gas. The peak intensities in the mass spectra, the infrared photodissociation spectra, and the theoretical calculations indicate that all CO ligands in these two complexes are bonded with the central metal atoms. The CO stretching frequencies in La(CO)8(+) and Ce(CO)8(+) are determined to be 2110 and 2108 cm(-1), respectively. Theoretical studies indicate that the most stable structures for La(CO)8(+) and Ce(CO)8(+) are an Oh geometry at its triplet state and a slightly distorted Oh geometry at its quartet state, respectively. These two complexes represent new octacoordinate metal carbonyls after previously determined U(CO)8(+) and Y(CO)8(+).

Structural Evolution of Homoleptic Heterodinuclear Copper-Nickel Carbonyl Anions Revealed Using Photoelectron Velocity-Map Imaging

The homoleptic heterodinuclear copper-nickel carbonyl anions CuNi(CO)n(-) (n = 2-4) were generated in a pulsed-laser vaporization source and investigated using photoelectron velocity-map imaging spectroscopy. The electron affinities of CuNi(CO)2 (2.15 ± 0.03 eV), CuNi(CO)3 (2.30 ± 0.03 eV), and CuNi(CO)4 (1.90 ± 0.04 eV) were deduced from the photoelectron spectra. Theoretical calculations at the B3LYP level were carried out to elucidate the structures and the electronic properties of CuNi(CO)n(0/1-) (n = 1-4) and to support the experimental observations. Comprehensive comparisons between experiments and calculations suggest that there is a turnover point of the absorption site during the progressive carbonylation process. The carbonyl groups are determined to be preferentially bonded to the nickel atom. When the nickel center satisfies the 18-electron configuration, the copper atom starts to adsorb additional CO molecules. These results will shed light on the bonding mechanisms of the heterometallic carbonyl clusters.

Photoelectron Imaging and Theoretical Calculations of Bimetallic Clusters: AgCu–, AgCu2–, and Ag2Cu–

Bimetallic clusters of AgCu(-), AgCu(2)(-), and Ag(2)Cu(-) are investigated by using photoelectron imaging and theoretical calculations. Their photoelectron spectra have been obtained at the wavelength of 355 nm and that of AgCu(-) is also acquired at 1064 nm. The ground state vertical detachment energies of these three clusters are measured to be 0.96 (1), 2.39 (5), and 2.41 (5) eV. The ground state adiabatic detachment energy of AgCu(-) is measured to be 0.93 (1) eV. Other spectroscopic constants of AgCu(-) including its frequency, bond length, and dissociation energy (relative to the products Ag(-) and Cu) are determined to be 191(15) cm(-1), 2.487(10) Å, and 1.39 eV according to its spectrum at 1064 nm. Only upper limits of the ground state adiabatic detachment energies of AgCu(2)(-) and Ag(2)Cu(-) are estimated by using their spectra at 355 nm. The structures and properties of AgCu(-), AgCu(2)(-), Ag(2)Cu(-), and their neutral counterparts are also computed by using a strategy where the structural optimizations are performed with the PW91PW91 method and the energy calculations are performed with the CCSD (T) method. The calculations are in better agreement with the experiments than most of the previous theoretical work.

Photoelectron Velocity Map Imaging Spectroscopy of Lead Tetracarbonyl-Iron Anion PbFe(CO)4(.).

Joint research of photoelectron velocity map imaging spectroscopy and density functional theory has been performed to probe the geometrical structures and electronic properties for heterodinuclear iron-lead carbonyl cluster PbFe(CO)4(-), which serves as a monomer of the metal-metal bonded oligomer. The photoelectron detachment of PbFe(CO)4(-) is recorded at two different photon energies with rich spectral features. The ground-state transition obtained at 532 nm reveals a broad vibrationally resolved spectral band, which corresponds to the lead-iron stretching, while the 355 nm spectrum displays many more transitions on the higher-energy side, which correspond to the electronic excited states of PbFe(CO)4. Theoretical calculations at the B3LYP level are performed to explore the ground states of both the anionic and neutral PbFe(CO)4 and to support spectral identification of the fine resolved photoelectron spectra. Moreover, the unique chemical bonding between lead and iron in PbFe(CO)4 is discussed with the aid of natural bond orbital analyses.

Octacoordinate metal carbonyls of scandium and yttrium: theoretical calculations and experimental observation

RATIONALE The transition metal carbonyls are among the most important complexes in coordination chemistry. The maximum coordination number in these complexes is seven. Because the cations Sc(+) and Y(+) have empty second outermost d orbital subshells, they can possibly bond eight CO ligands, forming the 18-electron d(10)s(2)p(6) noble gas configuration. The aim of this study is to determine whether the octacoordinate metal carbonyls of Sc(+) and Y(+) exist. METHODS The structures and bonding of M(CO)n(+) (M = Sc and Y, n = 7-9) were studied using Density Functional Theory (DFT) calculations with the functionals of B3LYP and BP86. The cationic complexes from laser ablation of Sc and Y in CO gas were analyzed by time-of-flight mass spectrometry. RESULTS The structures of M(CO)n(+) (M = Sc and Y, n = 7-9) and the bond dissociation energies for the last CO ligand in M(CO)n(+) (M = Sc and Y, n = 8 and 9) were obtained using DFT calculations. The products in the experiment for both metals include the series MO(CO)n(+), MO(H2O)(CO)n(+) and M(CO)n(+) (M = Sc or Y). The intensities of the MO(CO)n(+) and MO(H2O)(CO)n(+) ions change gradually with the number of CO ligands, while most M(CO)n(+) ions are very weak except for three intense ones, Sc(CO)7(+), Sc(CO)8(+) and Y(CO)8(+). CONCLUSIONS Comparisons between the theoretical calculations and the experimental observations indicate that eight CO ligands are chemically bonded on the central atom in the singlet state of Sc(CO)8(+) ((1)A1 state of D(4d) symmetry) and the singlet and triplet states of Y(CO)8(+) ((1)A1 state of D(4d) symmetry and (3)A(1g) state of O(h) symmetry). The (1)A1 states of both Sc(CO)8(+) and Y(CO)8(+) have the 18-electron d(10)s(2)p(6) noble gas configuration. In M(CO)9(+) (M = Sc or Y), the ninth CO is weakly adsorbed on the external shell.

Structure of Au40/−1 in the gas phase: A joint geometry relaxed ab initio calculations and vibrationally resolved photoelectron imaging investigation

We have combined photoelectron velocity-map imaging spectroscopy and high-level ab initio calculations to elucidate the geometries of Au4 (0/-1). Well-resolved ground-state electronic transition was observed in the photoelectron spectrum of Au4 (-) at 446 nm, leading to more accurate electron affinity and vibrational frequencies for the ground state of the neutral Au4 (-). The pure and vibrationally resolved spectra provide definitive experimental evidence for the resolution of the ground-state gold tetramer in the gaseous phase, with the aid of the ab initio calculations and Franck-Condon simulations. The comprehensive comparisons between the experiment and theoretical calculations suggest that the Y-shaped structure is the global minimum for both the neutral and anionic Au4.