UV Photoelectron Spectra Research Articles

Growth mechanism and angle-resolved photoemission spectra of cobalt oxide (CoO) thin films on Ag(1 0 0)

Using Auger electron spectroscopy we have studied the growth of cobalt oxide on Ag(1 0 0) substrate at different substrate temperatures (25, 170 and 250 °C). The films grown at 170 °C substrate temperature were found to grow closest to a layer-by-layer growth mode. In order to experimentally investigate the electronic structure of the CoO films angle-resolved UV photoelectron spectra were recorded for a 10 ML CoO film that are discussed and compared with available experimental results.

Assignment of the electronic states of pyrazole by ab initio multi-reference configuration interaction calculations

The gas-phase VUV absorption spectrum of pyrazole, which we reported recently, has been further assigned in the light of multi-reference multi-root CI calculations, using basis sets of varying size up to quadruple zeta quality, and containing both valence and Rydberg type functions. A very intense VUV band centred near 7.8 eV appears to arise from the summation of three calculated bands of ππ* character, of which the first and third are the most intense. The window resonance near the band maximum is ascribed to mutual annihilation of a Rydberg state and valence state, and a probable assignment is discussed. The electron energy loss (EEL) spectrum also obtained previously, showed low-lying triplet states at about 3.9 and 5.1 eV, respectively; the present computations suggest that two triplet (3ππ*) states lie within the 3.9 eV band, and identifies the species involved. The assignment of the UV-photoelectron spectrum has been reconsidered, but the identity of the first three IPs as π3<π2<LPN (lone pair) is confirmed; as a consequence most of the excited singlet and triplet states below 10 eV are likely to be of ππ* and πσ* character with both valence and Rydberg types. However, a number of σσ* and σπ* states including those derived from the lone pair electrons were obtained from the CI study, which give predictive values for the onset of such excitations. The highest pair of occupied orbitals (π3 and π2) both interact strongly with the lowest pair of valence VMOs (π4* and π5*) leading to a wide variety of valence states, and as such the molecule behaves much more like a system with degenerate pairs of π-MOs. Evidence is presented that (short lived) anions formed by electron impact are in fact 2Σ(2A′) rather than 2Π(2A″), and that this is likely to be true for the related anions of pyrrole, furan and isoxazole. The calculated vibration frequencies and a number of 1-electron properties of the ground state have been compared with experimental data.

Sterically crowded diphosphinomethane ligands: molecular structures, UV-photoelectron spectroscopy and a convenient general synthesis of tBu2PCH2PtBu2 and related speciesDedicated to Professor Roald Hoffmann on the occasion of his 65th birthday.Electronic supplementary information (ESI) available: photoelectron spectra for compounds 1–6, ORTEP diagrams for compounds 1–3 and 5–9+, calculated minimum structures (UB3LYP/6-31G**) and structural parameters for two forms of the radical cation dmpm.

A series of highly crowded symmetric and unsymmetric diphosphinomethanes R2PCH2PR′2, important ligands in transition metal chemistry and catalysis, namely tBu2PCH2PtBu2 (dtbpm, 1), Cy2PCH2PCy2 (dcpm, 2), tBu2PCH2PCy2 (ctbpm, 3), tBu2PCH2PiPr2 (iptbpm, 4) and tBu2PCH2PPh2 (ptbpm, 5), has been prepared in high yields, using a general and convenient route, which is described in detail for 1. Other than 4, which is a colourless liquid, these compounds are crystalline solids at room temperature. Their molecular structures have been determined by single crystal X-ray diffraction, along with that of the higher homologue of 1, tBu2CH2CH2tBu2 (dtbpe, 6). The solid-state structures of the dioxide of 1, tBu2P(O)CH2P(O)tBu2 (7), and of two phosphonium cations derived from 1, protonated [tBu2P(H)CH2PtBu2]+ (8+) and the chlorophosphonium ion [tBu2P(Cl)CH2PtBu2]+ (9+), are also described and show a distinct structural influence of the tetracoordinate P centres. The gas phase UV-photoelectron spectra of the diphosphines 1–6 have been measured. Their first two ionisation potentials are found to be nearly degenerate and all are in the low energy range from 7.5 to 7.8 eV. Comparison with related mono- and bidentate phosphines demonstrates that 1–6 are excellent σ-donors towards metals, in accord with their known coordination chemistry. Molecular geometries and electronic structures of the diphosphine systems have been studied by quantum chemical calculations and are compared to experiment. Unlike standard semiempirical methods (AM1, PM3, MNDO), which give rather poor minimum structures and seem inadequate for such sterically crowded systems, ab initio calculations (RHF/6-31G**) predict molecular geometries with reasonable accuracy and reflect the observed trends in experimental ionisation potentials.

Synthesis of 1-azaspiro[2.4]hepta-1,4,6-trienes and azaspiroconjugation studied by photoelectron spectroscopy.

1-Azaspiro[2.4]hepta-1,4,6-trienes 3a-c have been prepared by photolysis or thermolysis of 6-azidofulvenes 5a-c, which were accessible by nucleophilic substitution reactions of the precursors 4a,b or by nucleophilic addition of hydrazoic acid to ethenylidene-cyclopentadiene (6c). The UV photoelectron spectrum of 2-methyl-1-azaspiro[2.4]hepta-1,4,6-triene (3c) has been recorded and analyzed by making use of density functional theory (DFT) B3LYP calculations. Substantial homoconjugative interactions have been determined. The lone-pair orbital n(N) of the 2H-azirine nitrogen atom interacts with the pi 1 orbital of the cyclopentadiene ring. The energies of these orbitals are lowered or increased by 0.95 or 0.91 eV with respect to the two parent compounds cyclopentadiene (7) and 3-methyl-2H-azirine (9), respectively. In addition, in compound 3c the pi (C=N) orbital of the three-membered ring interacts with a sigma orbital of the cyclopentadiene unit and is destabilized by 0.47 eV by this effect.

Photoelectron spectroscopy ofAl32000−:Observation of a “Coulomb staircase” in a free cluster

Using a recently developed experimental setup we have measured UV (h ν = 6.42 eV) photoelectron spectra of free size-selected cold aluminum cluster anions with up to 32 000 atoms The spectra exhibit stairlike structures, which result from a superposition of the spectra of different charge states due to the detachment of several electrons from each cluster. The ionization potentials obtained this way allows one to determine the charging energy of the particle as well as the work function of the cluster material.

Electronic structure of coumarins

The electronic structure of coumarin derivatives has been investigated by a combination of UV photoelectron spectra (UPS), semi-empirical MO calculations and comparison with the spectra of related coumarins. The influence of substituents on the S 0 and S 1 electronic states energies is discussed.

Photoelectron Spectra, Electronic Structures, and Conformational Properties of Substituted 2-Styrylpyrroles

The UV photoelectron spectra of three pairs of cis/trans-isomers of 2-(2-methylstyryl)pyrroles (1-6) have been recorded and analysed, making use of density functional theory (DFT) calculations at the B3LYP level. Compounds 1 and 2 have no further substituents in the pyrrole ring. In the other compounds the pyrrole ring is substituted by a methyl group in the 1-(3,4) or 5-position (5,6). Twisted conformations were calculated for 1-6 in which the toluene ring is generally much more distorted from the plane of the central C=C double bond than the pyrrole ring. In the cis-isomers (2, 4, 6) the torsion angle of the toluene groups amounts to about 50 o, while in the trans-isomers (1, 3, 5) the torsional angle is only about half as large. The energy difference delta IP values of the ionization bands associated with the molecular orbitals (MOs) pi7 and pi3 is related to torsion. Calculated delta IP values are generally 5-10 % larger than the experimental values, indicating that torsional vibrations contribute to the apparent distortion of the molecules. The results clearly indicate that PE spectroscopy - in combination with high-level quantum chemical calculations - is a verasatile method to distinguish E/Z isomers of heterocyclic stilbene-like molecules.

Ultraviolet photoelectron spectroscopy of Si 4- to Si 1000-

: Using a new experimental setup we have measured UV (h ν = 6.4 eV) photoelectron spectra of cold silicon cluster anions Si n - in a very broad size range. For sizes up to n = 46 the spectra exhibit rich structures. For larger sizes only smooth spectra have been obtained. No trace of a bandgap has been found even for clusters with more than 1000 atoms.

Photoelectron Spectra, Electronic Structures, and Conformational Properties of (E)-Stilbene, Styrylthiophenes, and (Thienylethenyl)pyridines

The UV photoelectron spectra of two isomeric styrylthiophenes (5, 6) and six isomeric (thienylethenyl)pyridines (7−12) have been recorded and analyzed, making use of DFT Becke3LYP calculations. The spectra were compared with those of (E)-stilbene (1e) and the isomeric styrylpyridines (2−4), which are π-isoelectronic with 5−12. Using quantum chemical analyses, planar molecular structures were obtained for all these compounds. However, from the separation ΔIP of the ionization bands associated with the π7 and π3 MOs, it is possible to make a distinction between planar and twisted molecular structures. Accordingly in these compounds, 2-substituted pyridine rings and 3-substituted thiophene rings are nearly untwisted, whereas phenyl rings, 3- and 4-substituted pyridine, and 2-substituted thiophene rings are twisted to an extent similar to that in 1e. The apparent distortion of the molecules is probably caused by torsional vibrations, so that twisted average geometries correspond to planar equilibrium structures. The B3LYP data permit detailed conclusions to be drawn with regard to the conformational preferences of 2- and 3-substituted thiophene and pyridine rings in heterocyclic analogues of 1e, as well as the to relative stabilities of isomers 7−12. The results clearly indicate that PE spectroscopy is a powerful method for analysis of conformational properties of stilbene-like molecules.

UV photoelectron spectroscopy of coumarins

The electronic structure and other properties of 21 coumarin derivatives have been investigated by a combination of experimental and theoretical methods: UV photoelectron spectra (UPS), UV/VIS spectroscopy and semi-empirical MO calculations. The influence of substituents on the properties of coumarin derivatives is discussed and some predictions of unknown properties are made. One of our coumarins has the lowest ionization energy (6.87 eV) yet reported for any coumarin.

Structural chemistry of polycyclic heteroaromatic compounds. Part XI. Photoelectron spectra and electronic structures of tetracyclic hetarenes of the triphenylene type

The UV photoelectron spectra of several tetracyclic heteroaromatic compounds ( 2– 9) which are π-isoelectronic with triphenylene ( 1) have been recorded and analysed making use of semiempirical AM1 and PM3 as well as ab initio/DFT B3LYP calculations. In one series of compounds ( 2– 7), the peripheral benzene rings of 1 are successively substituted by thiophene rings that are either [b]- or [c]-annellated with the central benzene unit. In 2– 7 only marginal shifts are found for most of the IPs of electrons. In the benzotrithiophenes 5– 7, a systematic variation is displayed by IP( π 7). Compared to 1, the π electron system of benzo[c]trithiophene ( 7) is approximately two times as much destabilized as in the isomers 5 and 6 with [b]annellated thiophene rings. The IP[ n(S)] values of the thiophene derivatives 2– 7 indicate that these orbitals are clearly destabilized relative to thiophene. The same holds for the n(O) orbital of the furane derivative 9 in comparison with that of furane. In 9, only the higher π MOs ( π 7– π 9) are destabilized whereas the lower levels ( π 1– π 4) are stabilized, and those in between ( π 5– π 6) remain essentially unshifted. In the pyrrole derivative 8, all π MOs are substantially destabilized by about 0.5–1.6 eV relative to 1.

The split of the electron pairs and the UV photoelectron spectra of open-shell species

Few UV photoelectron spectrum bands observed in the closed shell molecules can be rationalized by molecular orbitals possessing spin and spatial degeneracies, which is in accord with the model of an electron-configuration consisting of electron pairs with doubly occupied orbitals. Numerous UV photoelectron spectrum bands observed in the open-shell species O2 and NO needs to be rationalized by molecular orbitals with spin and spatial degeneracies removed; and thus the formal electron-configuration consisting of doubly occupied orbitals become ambiguous because the electron pairs are split. The electron-pair split not only accounts for the number of the bands observed, but it also accounts for different properties and bonding natures within the ions resulted from ejecting an electron from the open-shell molecule.

Photoelectron spectroscopy of natural products. Part 1. Polypodanes

He I UV photoelectron spectra of four naturally occurring polyenes were measured. The spectra were interpreted on the basis of empirical arguments and semiempirical MO calculations. The results show that through-bond interactions between π-orbitals are discernible in spite of the non-rigid molecular geometry. This work shows that natural products have electronic structures which are of interest to a wider circle of chemists than just those who specialize in natural products. Copyright © 1999 John Wiley & Sons, Ltd.

The electronic states of pyrrole studied by optical (VUV) absorption, near-threshold electron energy-loss (EEL) spectroscopy and ab initio multi-reference configuration interaction calculations

A reinvestigation and extension of the observed VUV and EEL spectrum of pyrrole has been carried out, and the spectra assigned by means of high level multi-reference multi-root CI studies. A similar reasssessment of the UV-photoelectron spectrum leads to the conclusion that our previous assignment of IP 3 to ionisation of the inner π-level (1b 1) is correct and that most other assignments to ionisation of a σ-electron are incorrect. This is supported by the calculation of a wide range of Rydberg states derived from the five least bound orbitals, in the IP order: 1a 2 −1 < 2b 1 −1 < 1b 1 −1 < 9a 1 −1 < 6b 2 −1 < 5b 2 −1 < 8a 1 −1 < 7a 1 −1 < 4b 2 −1. It is concluded that each of the two strong absorption bands centred around 6 and 7.5 eV respectively owe their intensity to excitation of two 1ππ ∗ states of symmetry 1A 1 and 1B 2. The range of calculated valence states of ππ ∗ -type has been extended to include those from 1b 1.

Angle-resolved UV photoelectron spectra (UPS) of thin films of perylene-3,4,9,10-tetracarboxylic dianhydride on MoS(2).

Angle-resolved UV photoelectron spectra (ARUPS) were measured for thin films of perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) deposited on cleaved MoS(2) surfaces. The take-off angle (theta) dependence of the photoelectron intensity of the highest pi band showed a sharp maximum at theta = 32-34 degrees. A spectral feature of the binding energy at approximately 8.9 eV, which is believed to originate from a pi state, showed a remarkably different theta dependence from that of the pi band. A quantitative analysis of the observed theta dependencies clearly indicates that (a) the feature at approximately 8.9 eV originates from the oxygen 2p non-bonding states and (b) the molecules lie flat on the substrate surface.

Temperature dependence of photoelectron angular distribution from thin films of chloroaluminum phthalocyanine on MoS(2).

Angle-resolved UV photoelectron spectra were measured for thin films of chloroaluminum phthalocyanine deposited on cleaved MoS(2) surfaces. The take-off angle (theta) dependence of the photoelectron intensity of the highest pi band showed a remarkable sharpening upon cooling the film, indicating that thermal excitation of molecular vibrations gives a considerable broadening of the photoelectron angular distribution. The theta dependence observed at approximately 120 K agrees well with that calculated.

The electronic states and molecular properties of 1,2,3-triazine studied by VUV absorption, near-threshold electron energy-loss spectroscopy and ab initio multi-reference configuration interaction studies

Optical electronic absorption spectra, and electron energy-loss spectra have been obtained for 1,2,3-triazine. The valence and Rydberg excited states have been studied by multi-reference multi-root configuration interaction studies using MRD–CI techniques, and the results related to the experimental spectra. A number of Rydberg states, associated with five ionisation limits, have been identified along with valence states of types 1, 3 nπ * and 1, 3 ππ *. The UV-photoelectron spectrum has been reconsidered by means of MRD–CI, Green's function and the non-diagonal TDA method; the first five IPs are as previously determined. This leads to the ionisation order: 2A 1(11a −1 1)< 2B 2(7b −1 2)< 2B 1(2b −1 1)< 2A 2(1a −1 2)< 2A 1(10a −1 1) . The electronic properties of the molecule have been determined at the equilibrium geometry for a large basis set at both the SCF and MP2 levels.

The interaction of carbon monoxide with the pure and potassium-promoted Cu(332) surface

The adsorption behaviour of the KCO coadsorbate system has been studied at an adsorption temperature of 95 K on the vicinal Cu(332) surface. Thermal desorption spectroscopy experiments show that the K adatoms disturb the electrostatic potential induced by the steps of the vicinal surface, and therefore the CO molecules adsorbed on step sites are the first to be influenced by the coadsorbed alkali. With K present on the surface in a metal-like form, a strong KCO interaction is evidenced by a stabilization of the adlayer up to temperatures >500 K. Work-function change measurements reveal a change in the bonding of CO towards the precovered surface: the results cannot be interpreted using a simple Blyholder model, but imply a direct interaction of potassium with the 1π orbital of CO. The existence of this direct interaction gives rise to an abolition of the (1π + 5σ) degeneration in the CO-induced states in the UV-photoelectron spectra.

Theoretical investigation of UV photoelectron spectra of pyridine compound molecules

A set of pyridine compounds are used to illustrate the possibility of assigning complex UV photoelectron spectra (UPS) by means of independent channel calculations that account for both energies and cross-sections. 2-ethynylpyridine (2EP), 4-ethynylpyridine (4EP), 1,4-bis(2-pyridyl)-1,3-butadine (B2PBD), 1,4-bis(4-pyridyl)-1,3-butadine (B4PBD), and their subunits, pyridine and mono-, di- and tri-acetylene, have thus been investigated using the independent channel direct static exchange method. The calculations show that the cross-section of the 4a 1 (n) channel of pyridine at 21.2 eV photon energy is twice those of the π channels (1a 2 and 2b 2 ), which supports the reversed order of the 2b 2 and 4a 1 (n) Hartree-Fock molecular orbitals (MOs) suggested recently by Moghaddam et al. , (Chem. Phys., 207 (1996) 19). Correlations of occupied orbitals between the subunits and their compound molecules, 2EP, 4EP, B2PBD and B4PBD have been predicted by means of population analysis. A simple relation for correcting the calculated binding energies (BEs) for the σ MOs has been found for pyridine and for the compound molecules in the region (12 eV < 20 eV) where the pyridine character is dominant. Previous assignments for the π bands (BE < 12 eV) have been slightly renewed and assignments for the σ bands (BE > 12 eV, up to 20 eV) have been made for the first time. Significant cross-section variations with photoelectron energy are predicted, leading to He I and He II spectra with quite different intensity distributions.

Conformational and UV photoelectron spectroscopy analysis of the chalcogenobispyridines

It is now possible to review the conformational and electronic behaviours of the isomers of chalcogenobispyridines and related compounds, so providing information on the link between the geometric and electronic properties, and providing some insight into their observed and anticipated physical and biological activities. Moreover, observations can now be made on the effect of the bridging atoms (O, S, Se, Te) and positional isomerism on conformation and electronic distributions; on the balance between resonance and inductive effects; and on the energetics of conformer interconversion. As a result of the non-rigid nature of the chalcogenobispyridines and related compounds, it is necessary to determine the preferred geometries to calculate the ionization energies (IEs). For the entire family of the chalcogenobispyridines, the minimum energy structures predict the pyridine ring planes to be nearly perpendicular, allowing maximum overlap of the chalcogen valence orbitals with the π system of only one of the pyridine rings. Rotational barrier height calculations do not support the fully conjugated planar forms on steric grounds, but suggest that a range of near-isoenergetic conformers surround the minimum structure. Ab initio methods (such as restricted Hartree-Fock (HF) methods) were found to assign incorrectly the UV photoelectron spectra of these pyridine-based compounds. While HF methods were shown to be reasonably successful for the prediction of the IEs of the π-type molecular orbitals (MOs), they failed to position the IEs correctly for the long-pair nitrogen MOs (n N), so requiring the use of computer-intensive configuration interaction (CI) methods. Bands caused by distinct conformers could not be resolved in the UV photoelectron spectra of the chalcogenobispyridines. Conformational analyses predicted low energy pathways for concerted disrotatory motion, permitting a smooth transition between near isoenergetic rotamers. The calculated IEs of the most populated conformers were found to be within the instrumental resolution, so were difficult to resolve. This was supported by the production of ‘synthetic’ spectra. Within the composite-molecule model, correlations with the IEs of pyridine and the methylchalcogenopyridines assisted in the spectral assignment of the chalcogenobispyridines. For each congener, the first IE was assigned to ionization from an anti-bonding π−n x combination in which the contribution of pyridine π character was found to decrease down the group. The spectra recorded with different radiation sources were valuable in identifying bands associated primarily with the chalcogen character.