Hongbin Wu

Electronic structure of titanium oxide clusters: TiOy (y = 1−3) and (TiO2)n (n = 1−4)

The electronic structure of two series of small titanium oxide clusters, TiOy (y = 1−3) and (TiO2)n (n = 1−4), is studied using anion photoelectron spectroscopy. Vibrationally resolved spectra are obtained for TiO− and TiO2−. Six low‐lying electronic states for TiO are observed with five of these excited states resulting from multielectron transitions in the photodetachment processes. TiO2 is found to be closed‐shell with a 2 eV highest occupied molecular orbital/lowest unoccupied molecular orbital (HOMO‐LUMO‐) gap. The two lowest triplet and singlet excited states of TiO2 are observed with excitation energies at 1.96 and 2.4 eV, respectively. TiO3 is found to have a very high electron affinity (EA) of 4.2 eV, compared to 1.30 and 1.59 eV for TiO and TiO2, respectively. The larger (TiO2)n clusters are all closed‐shell with HOMO‐LUMO gaps similar to that of TiO2 and with increasing EAs: 2.1 eV for n = 2, 2.9 eV for n = 3, 3.3 eV for n = 4. The small HOMO‐LUMO gaps for the clusters compared to that of bulk ...

A study of the structure and bonding of small aluminum oxide clusters by photoelectron spectroscopy: AlxOy−(x=1–2, y=1–5)

The structure and bonding of aluminum oxide clusters, AlxOy (x=1–2, y=1–5), are studied with anion photoelectron spectroscopy (PES) and are compared with preliminary ab initio calculations. The spectra were obtained at four detachment photon energies: 2.33, 3.49, 4.66, and 6.42 eV. The 6.42 eV spectrum for AlO− reveals the X 2Σ+ ground state and two excited states of AlO. The 6.42 eV spectrum for AlO2− also shows three states for AlO2: X 2Πg ground state and the A 2Πu and B 2Σg+ excited states. The spectra for Al2Oy− clusters show vibrationally resolved ground states which come from Al sp-type orbitals and also high binding energy excited states, which are mainly of oxygen 2p character. Al2O2, which has a D2h rhombus structure, has an electron affinity (EA) of 1.88 eV and its singlet–triplet excitation energy is measured to be 0.49 eV. Much higher EAs are measured for the larger Al2Oy clusters. The PES spectra of Al2O3−, Al2O4−, and Al2O5− show very similar electronic and vibrational structure. Furthermor...

A study of nickel monoxide (NiO), nickel dioxide (ONiO), and Ni(O2) complex by anion photoelectron spectroscopy

We report the first anion photoelectron spectroscopic study of nickel monoxide (NiO), nickel dioxide (ONiO), and nickel-O2 complex, Ni(O2). The adiabatic electron affinity (EA) of NiO is measured to be 1.46 (2) eV. Five low-lying electronic excited states (A 3Π, a 1Δ, B 3Φ, b 1Σ+, c 1Π) are observed for NiO at 0.43 (4), 0.94 (4), 1.24 (3), 1.80 (10), and 2.38 (10) eV above the ground state, respectively. Two isomers are observed for NiO2, i.e., the linear ONiO dioxide and the Ni(O2) complex. The dioxide has a high EA of 3.05 (1) eV while the Ni(O2) complex has a rather low EA of 0.82 (3) eV. Two low-lying excited states are observed for ONiO at 0.40 (2) and 0.77 (3) eV above the ground state, respectively. The vibrational frequency of the ν1 mode of the ground state ONiO (X 3Σg−) is measured to be 750 (30) cm−1. The excited states of the Ni(O2) complex give broad photodetachment features starting at about 1.1 eV above the ground state. Information about the electronic structures of the nickel oxide specie...

A PHOTOELECTRON SPECTROSCOPIC STUDY OF MONOVANADIUM OXIDE ANIONS (VOX-, X = 1-4)

We report on a photoelectron spectroscopic study of monovanadium oxides, VOx− (x=1–4), at four photon energies: 532, 355, 266, and 193 nm. Vibrationally resolved spectra are obtained for VO− at 532 and 355 nm detachment photon energies. Two new low-lying excited states are observed for VO at 5630 and 14 920 cm−1 above the ground state. These states are assigned to two doublet states, 2Σ− and 2Φ, respectively. The 532 and 355 nm spectra of VO2− reveal a single vibrational progression for the ground state with a frequency of 970 cm−1 (ν1). Three electronic excited states are observed for VO2 in the 193 nm spectrum. For VO3−, three surprisingly sharp detachment transitions are observed at 193 nm. The two excited states of VO3 are measured to be 0.59 and 0.79 eV above the ground state. The spectra of VO2− and VO3− are interpreted using the molecular-orbital schemes obtained in a recent ab initio theoretical study [Knight, Jr. et al., J. Chem. Phys. 105, 10237 (1996)], which predicts that both VO2 and VO3 neut...

Al3Oy (y=0–5) clusters: Sequential oxidation, metal-to-oxide transformation, and photoisomerization

Photoelectron spectra of a series of Al3Oy− clusters (y=0–5) are presented at several photon energies: 532, 355, 266, and 193 nm. The electron affinities and low-lying electronic states of the Al3Oy clusters are reported. The photoelectron spectra clearly reveal a sequential oxidation behavior and how the electronic structure of the clusters evolves from that of a metal cluster at Al3 to that of a complete oxide cluster at Al3O5: Two valence electrons of Al3 are observed to be transferred to each additional O atom until Al3O5, where all the nine valence electrons of Al3 are transferred to the five O atoms. The anion, Al3O5−, which can be viewed as (Al3+)3(O2−)5, is found to be a closed shell cluster, yielding an extremely high electron affinity for Al3O5 (4.92 eV). The electron affinities of the remaining clusters are: 1.90 (Al3), 1.57 eV (Al3O), 2.18 eV (Al3O2), 2.80 eV (Al3O3), and 3.58 eV (Al3O4). An electronic excited state of Al3− is also observed at 0.40 eV above the Al3− ground state. Isomers are o...

TWO ISOMERS OF CUO2 : THE CU(O2) COMPLEX AND THE COPPER DIOXIDE

Photoelectron spectroscopy of CuO−2 has been studied at three detachment wavelengths: 532 nm, 355 nm, and 266 nm. Vibrationally resolved spectra for two distinct isomers are observed: the Cu(O2) complex and the copper dioxide molecule. The Cu(O)2 complex has an electron affinity (EA) of 1.503 (10) eV with a ground state vibrational frequency of 530 (50) cm−1. Its first electronic excited state is 7400 (300) cm−1 above the ground state. The Cu(O2)− anion is also observed to undergo photodissociation to Cu−+O2 at both 532 nm and 355 nm detachment wavelengths. The copper dioxide molecule is found to have a high EA of 3.46 (4) eV. Three low‐lying excited states are observed within 1 eV above the ground state. The first two excited states of the copper dioxide molecule both possess a totally symmetric vibrational frequency of 640 (60) cm−1.

Vibrationally resolved photoelectron spectroscopy of AlO− and AlO2−

Abstract Vibrationally resolved photoelectron spectra of AlO − and AlO 2 − have been obtained at two photon energies, 3.49 and 4.66 eV. Both the ground and first excited states are observed for AlO. The spectrum of AlO 2 − can only be obtained at the higher photon energy due to the high electron affinity (EA) of AlO 2 . The electron affinities of AlO and AlO 2 are measured to be 2.60(1) and 4.23(1) eV, respectively. The rather high EAs for both molecules are consistent with the fact that AlO − and AlO 2 − anions are closed-shell, isoelectronic with SiO and SiO 2 , respectively. The vibrational frequency of the AlO − anion is observed to be 900 (50) cm −1 . The vibrational frequencies and excitation energy obtained for AlO agree well with previous optical measurements. The totally symmetric vibrational frequency is observed to be 750 (40) cm −1 for AlO 2 and 680 (60) cm −1 for AlO 2 − . The AlO 2 molecule, as well as AlO 2 − , is concluded to have a linear OAlO structure from the spectroscopic information obtained.

A combined density functional theoretical and photoelectron spectroscopic study of Ge2O2

We study Ge2O2 and Ge2O−2 with density functional theory (DFT) and photoelectron spectroscopy (PES). We find that Ge2O2 is a rhombus (D2h), closed‐shell molecule with a large HOMO‐LUMO gap. The PES spectrum of Ge2O−2 is obtained at four detachment photon energies: 1064, 532, 355, and 266 nm. Vibrational structure is resolved at the lowest photon energy with a single progression and a frequency of 400 (60) cm−1. The experimental adiabatic electron affinity is obtained to be 0.625 (0.050) eV for Ge2O2. The calculated vertical and adiabatic electron affinities and the HOMO‐LUMO gap are in good agreement with the experimental values. The calculated totally symmetric vibrational mode of Ge2O2 (335 cm−1) is in reasonable agreement with the observed vibration and represents a Ge–Ge breathing motion.

PHOTOELECTRON SPECTROSCOPY OF TRANSITION METAL CLUSTERS

Photoelectron spectroscopy (PES) of size-selected anions is a powerful experimental technique to probe the electronic structure of transition metal clusters. Experiments on V and Ni clusters at several detachment photon energies are presented. Results on V clusters reveal clearly how the bulk band structure is developed as cluster size increases. Experiments on Ni clusters show that the cluster electronic structure changes very little above the 14-atom cluster, in contrast to the magnetic properties of these clusters. The PES spectrum of the 13-atom Ni cluster is observed to be particularly narrow, suggesting that this cluster may possess a high symmetry structure with a high degree of degeneracy. Photon energy-dependent PES spectra are shown to yield information about the nature of the electronic states of the clusters.