Alignment of the hydrogen molecule under intense laser fields.

Alignment of the electronically excited E,F state of the H2 molecule is studied using the velocity mapping imaging technique. Photofragment images of H+ due to the dissociation mechanism that follows the 2-photon excitation into the (E,F; ν = 0, J = 0) electronic state show a strong dependence on laser intensity, which is attributed to the high polarizability anisotropy of the H2 (E,F) state. We observe a marked structure in the angular distribution, which we explain as the interference between the prepared J = 0 and Stark-mixed J = 2 rovibrational states of H2, as the laser intensity increases. Quantification of these effects allows us to extract the polarizability anisotropy of the H2 (E,F J = 0) state yielding a value of 312 ± 82 a.u. (46 Å3). By comparison, CS2 has 10 Å3, I2 has 7 Å3, and hydrochlorothiazide (C7H8ClN3O4S2) has about 25 Å3 meaning that we have created the most easily aligned molecule ever measured, by creating a mixed superposition state that is highly anisotropic in its polarizability.

Probing the Electronic Structure and Chemical Bonding of Mono-Uranium Oxides with Different Oxidation States: UOx(-) and UOx (x = 3-5).

Uranium oxide clusters UOx(-) (x = 3-5) were produced by laser vaporization and characterized by photoelectron spectroscopy and quantum theory. Photoelectron spectra were obtained for UOx(-) at various photon energies with well-resolved detachment transitions and vibrational resolution for x = 3 and 4. The electron affinities of UOx were measured as 1.12, 3.60, and 4.02 eV for x = 3, 4, and 5, respectively. The geometric and electronic structures of both the anions and the corresponding neutrals were investigated by quasi-relativistic electron-correlation quantum theory to interpret the photoelectron spectra and to provide insight into their chemical bonding. For UOx clusters with x ≤ 3, the O atoms appear as divalent closed-shell anions around the U atom, which is in various oxidation states from U(II)(fds)(4) in UO to U(VI)(fds)(0) in UO3. For x > 3, there are no longer sufficient valence electrons from the U atom to fill the O(2p) shell, resulting in fractionally charged and multicenter delocalized valence states for the O ligands as well as η(1)- or η(2)-bonded O2 units, with unusual spin couplings and complicated electron correlations in the unfilled poly oxo shell. The present work expands our understanding of both the bonding capacities of actinide elements with extended spdf valence shells as well as the multitude of oxygen's charge and bonding states.

Manganese-centered tubular boron cluster - MnB16 (-): A new class of transition-metal molecules.

We report the observation of a manganese-centered tubular boron cluster (MnB16 (-)), which is characterized by photoelectron spectroscopy and ab initio calculations. The relatively simple pattern of the photoelectron spectrum indicates the cluster to be highly symmetric. Ab initio calculations show that MnB16 (-) has a Mn-centered tubular structure with C4v symmetry due to first-order Jahn-Teller effect, while neutral MnB16 reduces to C2v symmetry due to second-order Jahn-Teller effect. In MnB16 (-), two unpaired electrons are observed, one on the Mn 3dz(2) orbital and another on the B16 tube, making it an unusual biradical. Strong covalent bonding is found between the Mn 3d orbitals and the B16 tube, which helps to stabilize the tubular structure. The current result suggests that there may exist a whole class of metal-stabilized tubular boron clusters. These metal-doped boron clusters provide a new bonding modality for transition metals, as well as a new avenue to design boron-based nanomaterials.

A combined photoelectron spectroscopy and relativistic ab initio studies of the electronic structures of UFO and UFO(-).

The observation of the gaseous UFO(-) anion is reported, which is investigated using photoelectron spectroscopy and relativisitic ab initio calculations. Two strong photoelectron bands are observed at low binding energies due to electron detachment from the U-7sσ orbital. Numerous weak detachment bands are also observed due to the strongly correlated U-5f electrons. The electron affinity of UFO is measured to be 1.27(3) eV. High-level relativistic quantum chemical calculations have been carried out on the ground state and many low-lying excited states of UFO to help interpret the photoelectron spectra and understand the electronic structure of UFO. The ground state of UFO(-) is linear with an O-U-F structure and a (3)H4 spectral term derived from a U 7sσ(2)5fφ(1)5fδ(1) electron configuration, whereas the ground state of neutral UFO has a (4)H(7/2) spectral term with a U 7sσ(1)5fφ(1)5fδ(1) electron configuration. Strong electron correlation effects are found in both the anionic and neutral electronic configurations. In the UFO neutral, a high density of electronic states with strong configuration mixing is observed in most of the scalar relativistic and spin-orbit coupled states. The strong electron correlation, state mixing, and spin-orbit coupling of the electronic states make the excited states of UFO very challenging for accurate quantum chemical calculations.

The Planar CoB18 (-) Cluster as a Motif for Metallo-Borophenes.

Monolayer-boron (borophene) has been predicted with various atomic arrangements consisting of a triangular boron lattice with hexagonal vacancies. Its viability was confirmed by the observation of a planar hexagonal B36 cluster with a central six-membered ring. Here we report a planar boron cluster doped with a transition-metal atom in the boron network (CoB18 (-) ), suggesting the prospect of forming stable hetero-borophenes. The CoB18 (-) cluster was characterized by photoelectron spectroscopy and quantum chemistry calculations, showing that its most stable structure is planar with the Co atom as an integral part of a triangular boron lattice. Chemical bonding analyses show that the planar CoB18 (-) is aromatic with ten π-electrons and the Co atom has strong covalent interactions with the surrounding boron atoms. The current result suggests that transition metals can be doped into the planes of borophenes to create metallo-borophenes, opening vast opportunities to design hetero-borophenes with tunable chemical, magnetic, and optical properties.

Electronic structure and chemical bonding of a highly stable and aromatic auro-aluminum oxide cluster.

We have produced an auro-aluminum oxide cluster, Au2(AlO)2(-), as a possible model for an Au-alumina interface and investigated its electronic and structural properties using photoelectron spectroscopy and density functional theory. An extremely large energy gap (3.44 eV) is observed between the lowest unoccupied and the highest occupied molecular orbitals of Au2(AlO)2, suggesting its high electronic stability. The global minima of both Au2(AlO)2(-) and Au2(AlO)2 are found to have D2h symmetry with the two Au atoms bonded to the Al atoms of a nearly square-planar (AlO)2 unit. Chemical bonding analyses reveal a strong σ bond between Au and Al, as well as a completely delocalized π bond over the (AlO)2 unit, rendering aromatic character to the Au2(AlO)2 cluster. The high electronic stability and novel chemical bonding uncovered for Au2(AlO)2 suggest that it may be susceptible to chemical syntheses as a stable compound if appropriate ligands can be found.

High resolution photoelectron imaging of UO(-) and UO2(-) and the low-lying electronic states and vibrational frequencies of UO and UO2.

We report a study of the electronic and vibrational structures of the gaseous uranium monoxide and dioxide molecules using high-resolution photoelectron imaging. Vibrationally resolved photoelectron spectra are obtained for both UO(-) and UO2(-). The spectra for UO2(-) are consistent with, but much better resolved than a recent study using a magnetic-bottle photoelectron analyzer [W. L. Li et al., J. Chem. Phys. 140, 094306 (2014)]. The electron affinity (EA) of UO is reported for the first time as 1.1407(7) eV, whereas a much more accurate EA is obtained for UO2 as 1.1688(6) eV. The symmetric stretching modes for the neutral and anionic ground states, and two neutral excited states for UO2 are observed, as well as the bending mode for the neutral ground state. These vibrational frequencies are consistent with previous experimental and theoretical results. The stretching vibrational modes for the ground state and one excited state are observed for UO. The current results for UO and UO2 are compared with previous theoretical calculations including relativistic effects and spin-orbit coupling. The accurate experimental data reported here provide more stringent tests for future theoretical methods for actinide-containing species.

Probing the electronic and vibrational structure of Au2Al2(-) and Au2Al2 using photoelectron spectroscopy and high resolution photoelectron imaging.

The electronic and vibrational structures of Au2Al2(-) and Au2Al2 have been investigated using photoelectron spectroscopy (PES), high-resolution photoelectron imaging, and theoretical calculations. Photoelectron spectra taken at high photon energies with a magnetic-bottle apparatus reveal numerous detachment transitions and a large energy gap for the neutral Au2Al2. Vibrationally resolved PE spectra are obtained using high-resolution photoelectron imaging for the ground state detachment transition of Au2Al2(-) at various photon energies (670.55-843.03 nm). An accurate electron affinity of 1.4438(8) eV is obtained for the Au2Al2 neutral cluster, as well as two vibrational frequencies at 57 ± 8 and 305 ± 13 cm(-1). Hot bands transitions yield two vibrational frequencies for Au2Al2(-) at 57 ± 10 and 144 ± 12 cm(-1). The obtained vibrational and electronic structure information is compared with density functional calculations, unequivocally confirming that both Au2Al2(-) and Au2Al2 possess C2v tetrahedral structures.

Strong electron correlation in UO2(-): a photoelectron spectroscopy and relativistic quantum chemistry study.

The electronic structures of actinide systems are extremely complicated and pose considerable challenges both experimentally and theoretically because of significant electron correlation and relativistic effects. Here we report an investigation of the electronic structure and chemical bonding of uranium dioxides, UO2(-) and UO2, using photoelectron spectroscopy and relativistic quantum chemistry. The electron affinity of UO2 is measured to be 1.159(20) eV. Intense detachment bands are observed from the UO2(-) low-lying (7sσg)(2)(5fϕu)(1) orbitals and the more deeply bound O2p-based molecular orbitals which are separated by a large energy gap from the U-based orbitals. Surprisingly, numerous weak photodetachment transitions are observed in the gap region due to extensive two-electron transitions, suggesting strong electron correlations among the (7sσg)(2)(5fϕu)(1) electrons in UO2(-) and the (7sσg)(1)(5fϕu)(1) electrons in UO2. These observations are interpreted using multi-reference ab initio calculations with inclusion of spin-orbit coupling. The strong electron correlations and spin-orbit couplings generate orders-of-magnitude more detachment transitions from UO2(-) than expected on the basis of the Koopmans' theorem. The current experimental data on UO2(-) provide a long-sought opportunity to arbitrating various relativistic quantum chemistry methods aimed at handling systems with strong electron correlations.

Temperature-dependent, nitrogen-perturbed line shape measurements in the ν1 + ν3 band of acetylene using a diode laser referenced to a frequency comb.

The P(11) line of the ν1 + ν3 combination band of C2H2 was studied using an extended cavity diode laser locked to a frequency comb. Line shapes were measured for acetylene and nitrogen gas mixtures at a series of temperatures between 125 and 296 K and total pressures up to 1 atm. The data were fit to two speed-dependent line shape models and the results were compared. Line shape parameters were determined by simultaneously fitting data for all temperatures and pressures in a single multispectrum analysis. Earlier pure acetylene measurements [Cich et al. Appl. Phys. B 2012, 109, 373-38] were incorporated to account for self-perturbation. The resulting parameters reproduce the observed line shapes for the acetylene-nitrogen system over the range of temperatures and pressures studied with average root-mean-square observed-calculated errors of individual line measurement fits of approximately 0.01% of maximum transmission, close to the experimental signal-to-noise ratios. Errors in the pressure measurements constitute the major systematic errors in these measurements, and a statistical method is developed to quantify their effects on the line shape parameters for the present system.

Probing the electronic structures of low oxidation-state uranium fluoride molecules UF(x)- (x = 2-4).

We report the experimental observation of gaseous UF(x)(-) (x = 2-4) anions, which are investigated using photoelectron spectroscopy and relativistic quantum chemistry. Vibrationally resolved photoelectron spectra are obtained for all three species and the electron affinities of UF(x) (x = 2-4) are measured to be 1.16(3), 1.09(3), and 1.58(3) eV, respectively. Significant multi-electron transitions are observed in the photoelectron spectra of U(5f(3)7s(2))F2(-), as a result of strong electron correlation effects of the two 7s electrons. The U-F symmetric stretching vibrational modes are resolved for the ground states of all UF(x) (x = 2-4) neutrals. Theoretical calculations are performed to qualitatively understand the photoelectron spectra. The entire UF(x)(-) and UF(x) (x = 1-6) series are considered theoretically to examine the trends of U-F bonding and the electron affinities as a function of fluorine coordination. The increased U-F bond lengths and decreased bond orders from UF2(-) to UF4(-) indicate that the U-F bonding becomes weaker as the oxidation state of U increases from I to III.

What is the best DFT functional for vibronic calculations? A comparison of the calculated vibronic structure of the S1-S0 transition of phenylacetylene with cavity ringdown band intensities.

The sensitivity of vibronic calculations to electronic structure methods and basis sets is explored and compared to accurate relative intensities of the vibrational bands of phenylacetylene in the S(1)(A(1)B(2)) ← S(0)(X(1)A(1)) transition. To provide a better measure of vibrational band intensities, the spectrum was recorded by cavity ringdown absorption spectroscopy up to energies of 2000 cm(-1) above the band origin in a slit jet sample. The sample rotational temperature was estimated to be about 30 K, but the vibrational temperature was higher, permitting the assignment of many vibrational hot bands. The vibronic structure of the electronic transition was simulated using a combination of time-dependent density functional theory (TD-DFT) electronic structure codes, Franck-Condon integral calculations, and a second-order vibronic model developed previously [Johnson, P. M.; Xu, H. F.; Sears, T. J. J. Chem. Phys. 2006, 125, 164331]. The density functional theory (DFT) functionals B3LYP, CAM-B3LYP, and LC-BLYP were explored. The long-range-corrected functionals, CAM-B3LYP and LC-BLYP, produced better values for the equilibrium geometry transition moment, but overemphasized the vibronic coupling for some normal modes, while B3LYP provided better-balanced vibronic coupling but a poor equilibrium transition moment. Enlarging the basis set made very little difference. The cavity ringdown measurements show that earlier intensities derived from resonance-enhanced multiphoton ionization (REMPI) spectra have relative intensity errors.

Nb2©Au6: a molecular wheel with a short Nb[triple bond, length as m-dash]Nb triple bond coordinated by an Au6 ring and reinforced by σ aromaticity.

We report a photoelectron spectroscopy and high-resolution photoelectron imaging study of a bimetallic Nb2Au6 - cluster. Theoretical calculations, in conjunction with the experimental data, reveal that Nb2Au6 -/0 possess high-symmetry D 6h structures featuring a Nb-Nb axis coordinated equatorially by an Au6 ring. Chemical bonding analyses show that there are two π bonds and one σ bond in the Nb2 moiety in Nb2©Au6, as well as five totally delocalized σ bonds. The Nb[triple bond, length as m-dash]Nb triple bond is strengthened significantly by the delocalized σ bonds, resulting in an extremely short Nb-Nb bond length comparable to the quintuple bond in gaseous Nb2. The totally delocalized σ bonding in Nb2©Au6 is reminiscent of σ aromaticity, representing a new bonding mode in metal-ligand systems. The unusually short Nb-Nb bond length in Nb2©Au6 shows that the Au6 ring can serve as a bridging ligand to facilitate multiple bonding in transition metal dimers via delocalized σ bonding.

Photoelectron Spectroscopy of BiAu(-) and BiBO(-): Further Evidence of the Analogy between Au and Boronyl.

Boronyl (BO) is a monovalent σ radical with a B≡O triple bond. Its chemistry has remained relatively unknown, though analogy has been established for BO with monovalent atoms, such as H or Au. Here we report a photoelectron spectroscopic study of BiAu(-) and BiBO(-), showing further evidence of the analogy between Au and BO. The photoelectron spectra of BiAu(-) and BiBO(-) are found to be similar, suggesting that they possess similar electronic structure and chemical bonding. The electron affinities of BiAu and BiBO are measured to be 1.38(4) and 1.84(3) eV, respectively. The ground states of both BiAu and BiBO are shown to be a triplet (X (3)Σ(-)). In addition, vibrational structures are resolved in the spectra of BiBO(-). Two vibrational frequencies at 320(30) and 1860(50) cm(-1) are measured for the ground state of BiBO, corresponding to the Bi-B and B-O stretching modes, respectively. The low-lying electronic excited states of BiAu and BiBO are also found to be similar: their first four excited states are A (1)Δ, B (1)Σ(+), C (3)Π, and D (1)Π, with excitation energies at 0.71, 1.29, 2.54, and 2.67 eV for BiAu and 0.63, 1.26, 3.68, and 3.82 eV for BiBO, respectively, above the (3)Σ(-) ground state. Weak photoelectron features related to two-electron detachment transitions are also observed for both anions because of strong electron correlation effects in the ground state of the anions.

Probing the structures of gold-aluminum alloy clusters AuxAly(-): a joint experimental and theoretical study.

Besides the size and structure, compositions can also dramatically affect the properties of alloy nanoclusters. Due to the added degrees of freedom, determination of the global minimum structures for multi-component nanoclusters poses even greater challenges, both experimentally and theoretically. Here we report a systematic and joint experimental/theoretical study of a series of gold-aluminum alloy clusters, AuxAly(-) (x + y = 7,8), with various compositions (x = 1-3; y = 4-7). Well-resolved photoelectron spectra have been obtained for these clusters at different photon energies. Basin-hopping global searches, coupled with density functional theory calculations, are used to identify low-lying structures of the bimetallic clusters. By comparing computed electronic densities of states of the low-lying isomers with the experimental photoelectron spectra, the global minima are determined. It is found that for y ≥ 6 there is a strong tendency to form the magic-number square bi-pyramid motif of Al6(-) in the AuxAly(-) clusters, suggesting that the Al-Al interaction dominates the Au-Au interaction in the mixed clusters. A closely related trend is that for x > 1, the gold atoms tend to be separated by Al atoms unless only the magic-number Al6(-) square bi-pyramid motif is present, suggesting that in the small-sized mixed clusters, Al and Au components do not completely mix with one another. Overall, the Al component appears to play a more dominant role due to the high robustness of the magic-number Al6(-) square bi-pyramid motif, whereas the Au component tends to be either "adsorbed" onto the Al6(-) square bi-pyramid motif if y ≥ 6, or stays away from one another if x < y < 6.

Competition between drum and quasi-planar structures in RhB18-: motifs for metallo-boronanotubes and metallo-borophenes.

Metal-doped boron clusters provide new opportunities to design nanoclusters with interesting structures and bonding. A cobalt-doped boron cluster, CoB18-, has been observed recently to be planar and can be viewed as a motif for metallo-borophenes, whereas the D9d drum isomer as a motif for metallo-boronanotubes is found to be much higher in energy. Hence, whether larger doped boron drums are possible is still an open question. Here we report that for RhB18- the drum and quasi-planar structures become much closer in energy and co-exist experimentally, revealing a competition between the metallo-boronanotube and metallo-borophene structures. Photoelectron spectroscopy of RhB18- shows a complicated spectral pattern, suggesting the presence of two isomers. Quantum chemistry studies indicate that the D9d drum isomer and a quasi-planar isomer (Cs) compete for the global minimum. The enhanced stability of the drum isomer in RhB18- is due to the less contracted Rh 4d orbitals, which can have favorable interactions with the B18 drum motif. Chemical bonding analyses show that the quasi-planar isomer of RhB18- is aromatic with 10 π electrons, whereas the observed RhB18- drum cluster sets a new record for coordination number of eighteen among metal complexes. The current finding shows that the size of the boron drum can be tuned by appropriate metal dopants, suggesting that even larger boron drums with 5d, 6d transition metal, lanthanide or actinide metal atoms are possible.

Bond-bending isomerism of Au2I3-: competition between covalent bonding and aurophilicity.

We report a joint photoelectron spectroscopy and theoretical investigation of the gaseous Au2I3- cluster, which is found to exhibit two types of isomers due to competition between Au-I covalent bonding and Au-Au aurophilic interactions. The covalent bonding favors a bent IAuIAuI- structure with an obtuse Au-I-Au angle (100.7°), while aurophilic interactions pull the two Au atoms much closer, leading to an acutely bent structure (72.0°) with an Au-Au distance of 3.08 Å. The two isomers are separated by a small barrier and are nearly degenerate with the obtuse isomer being slightly more stable. At low temperature, only the obtuse isomer is observed; distinct experimental evidence is observed for the co-existence of a combination of isomers with both acute and obtuse bending angles at room temperature. The two bond-bending isomers of Au2I3- reveal a unique example of one molecule being able to oscillate between different structures as a result of two competing chemical forces.

Cobalt-centred boron molecular drums with the highest coordination number in the CoB16- cluster.

The electron deficiency and strong bonding capacity of boron have led to a vast variety of molecular structures in chemistry and materials science. Here we report the observation of highly symmetric cobalt-centered boron drum-like structures of CoB16(-), characterized by photoelectron spectroscopy and ab initio calculations. The photoelectron spectra display a relatively simple spectral pattern, suggesting a high symmetry structure. Two nearly degenerate isomers with D8d (I) and C4v (II) symmetries are found computationally to compete for the global minimum. These drum-like structures consist of two B8 rings sandwiching a cobalt atom, which has the highest coordination number known heretofore in chemistry. We show that doping of boron clusters with a transition metal atom induces an earlier two-dimensional to three-dimensional structural transition. The CoB16(-) cluster is tested as a building block in a triple-decker sandwich, suggesting a promising route for its realization in the solid state.