Spectroscopy of metal surface states by inverse photoemission

In the past decade inverse photoemission has proved to be a technique well suited to study unoccupied bulk and surface electronic states of solids. This paper presents a brief review focusing on the spectroscopy of surface states on metal surfaces and how these states are influenced by simple adsorbates, surface reconstructions or the presence of a ferromagnetic substrate.

The experimental observations are discussed within the framework of a nearly free electron model of surface states.     

Infrared spectra and electronic structure calculations for the group 2 metal M(OH)2 dihydroxide molecules

Reactions of laser-ablated Mg, Ca, Sr, and Ba atoms with O2 and H2 in excess argon give new absorptions in the O-H and O-M-O stretching regions, which increase together upon UV photolysis and are due to the M(OH)2 molecules (M = Mg, Ca, Sr, and Ba). The same product absorptions are observed in the metal atom reactions with H2O2. The M(OH)2 identifications are supported by isotopic substitution and theoretical calculations (B3LYP and MP2). The O-H stretching frequencies of the alkaline earth metal dihydroxide molecules decrease from 3829.8 to 3784.6 to 3760.6 to 3724.2 cm(-1) in the family series in solid argon, while the base strength of the solid compounds increases. Calculations show that Sr(OH)2 and Ba(OH)2 are bent at the metal center, owing to d orbital involvement in the bonding. Although these molecules are predominantly ionic, the O-H stretching frequencies do not reach the ionic limit of gaseous OH- going down the family group because of cation-anion polarization and p(pi) --> d(pi) interactions.     

Ground states of molecules. 56. MNDO calculations for molecules containing sulfur

MNDO has been extended to sulfur, but without inclusion of 3d AO s. Calculations are reported for heats of formation, geometries, dipole moments, and ionization energies of a variety of sulfur-containing molecules. The average discrepancy between calculated and observed heats of formation is larger than for compounds of other elements, a difference probably due, at least partly, to the lower accuracy of the thermochemical data for sulfur compounds. The calculated dipole moments agree well with experiment as do the calculated ionization energies, except for those corresponding to ionization from sulfur “lone-pair” orbitals which are too high by ca. 1 eV, probably as a result of the neglect in NDDO of interactions between inner and valence shell orbitals. As in the case of other third-period elements, the calculated heats of formation of compounds of sulfur in its higher valence states (S^IV, S^VI) were too positive by large amounts, due presumably to the neglect of 3d AO s.     

Electronic spectra of metal cluster molecules

The electronic absorption spectra (UV-visible-NIR) of a range of molecular metal cluster compounds, including new spectra of Pt₃₀₉(phen*)₃₆O₂₈ in solution and Au₅₅(PPh₃)₁₂Cl₆ in the solid state, are discussed and compared with the spectra of colloidal particles of the corresponding metals. We consider frontier orbital separations, the development of interband absorptions, the possible appearance of molecular plasma resonances, and charge-transfer in the solid state.     

Ground states of molecules. 53.MNDO calculations for molecules containing chlorine

MNDO calculations of heats of formation, dipole moments, ionization potentials, and structures are reported for a wide range of compounds containing chlorine in its characteristic valence state (Cl^I) and one or more of the elements H, B, Be, C, N, O, and F. The calculated errors in the heats of formation and the dipole moments are not significantly greater than those previously reported for compounds containing no chlorine. First vertical ionization potentials were on average 0.95 eV too high. The ordering of higher cationic states was found to be correct, even for species such as Cl₂O, Cl₂, and HOCl, where ab initio–Koopmans' theorem calculations predict the incorrect ordering. The calculated energies and geometries of compounds such as CIF₃ are qualitatively incorrect, probably because of the lack of 3d atomic orbitals in the orbital basis set.     

Random-phase-approximation calculations on triplet spectra of conjugated molecules

Random-phase approximations (RPA) have been applied to the calculation of the triplet π-π* transition spectra of 18 conjugated molecules in the framework of Pariser-Parr-Pople approximations. It is found that the normal RPA (n-RPA) shows the triplet instability for most molecules in the Nishimoto-Mataga approximation of electron-repulsion integrals. However, it is shown that this instability can be circumvented by the use of the renormalized RPA (r-RPA) in which the correlated ground states are calculated by the second-order perturbation theory. It is also shown that even in the n-RPA the suitable parametrization of electron-repulsion integrals removes this instability. It is ascertained that such an increasing order of energies as ω(n-RPA)<ω(Tamm-Dancoff approximation)<ω(r-RPA) holds for most of energy levels.     

Infrared spectra of the group 2 metal dihydroxide molecules

Group 2 metal atoms (Mg, Ca, Sr, and Ba) react on ultraviolet photoexcitation with O(2), H(2) mixtures in solid argon at 10 K to produce new absorptions in the O-H and O-M-O stretching regions. The effect of detailed isotopic substitution on these two absorptions identifies the M(OH)(2) molecules. The stepwise decrease in the O-H stretching modes in this chemical family demonstrates an increase in ionic character, which parallels the increase in base strength for the analogous solid compounds.     

NMR spectra of free-base porphine, porphyrazine, phthalocyanine and naphthalocyanine as well as their metal complexes: density functional calculations

Nuclear magnetic shielding tensors of porphine have been calculated at density functional B3LYP and PBE level using the gauge independent atomic orbital (GIAO) method. The geometries used were optimized using the 6-31G(d) basis set and the NMR calculations were performed using 6-31G(d) and 6-311G(d,p) basis sets, respectively. The calculated NMR shielding tensors and chemical shifts of porphine are compared with previous calculations as well as experimental data and satisfying results are obtained. Further NMR calculations are extended to metal-free and metallo-porphyrazine, -phthalocyanine, and -naphthalocyanine for the first time and the results are compared with experimental data available. The chemical shifts of the atoms in these compounds are assigned according to the experimental data available.     

Infrared spectra and structures of the coinage metal dihydroxide molecules

Laser-ablated Cu, Ag, and Au atoms react with H2O2 and with H2 + O2 molecules during condensation in excess argon to give four new IR absorptions in each system (O-H stretch, M-O-H bend, O-M-O stretch, and M-O-H deformation modes) that are due to the coinage metal M(OH)2 dihydroxide molecules. Isotopic substitution (D2O2, 18O2, 16O18O, D2, and HD) and comparison with frequencies computed by DFT verify these assignments. The calculations converge to 2B(g) ground electronic state structures with C2h symmetry, 111-117 degrees M-O-H bond angles, and substantial covalent character for these new metal dihydroxide molecules, particularly for Au(OH)2. This is probably due to the high electron affinity of gold owing to the effect of relativity.     

Scaling the Coulomb interaction in calculations of electron spectra of transition metal complexes

A simple technique of scaling two-electron integrals in ab initio calculations of the electronically excited states of transition metal complexes is proposed. This technique uses the fact that one-center two-electron integrals depend linearly on the scaling factor when Slater type functions are subjected to scaling transformation. This leads to a linear dependence of the d—d transition energy on the “scale” of Coulomb interaction, which allows one to affect the calculation result by varying the Slater exponential. To test the technique, ab initio configuration interaction and full active space calculations of the low excited states of the CrF ₆ ^3- , MnF ₆ ^2- , and VF ₆ ^3- complexes are performed. For transition elements, a basis of Slater type effective functions chosen from the optical spectra of the atoms and ions of transition elements is used. It is shown that in the STO-6G basis with effective exponentials, experimental transitions are reproduced with an accuracy of about 2000 cm^-1 even with the use of small active space determined by the orbitals localized on the central atom of the complex.     

Vibronic coupling in quasidegenerate electronic states of metal porphyrin molecules

In the absorption spectra of isocyclically substituted metal porphyrins, the inactive second electron transition produces a significant effect on the vibrational structure of the fluorescence spectra due to vibronic borrowing of intensitites. These vibronic interactions involving the second electron level are universal for the spectra of metal porphyrins. Institute of Molecular and Atomic Physics, Belarus Academy of Sciences. Translated fromZhurnal Strukturnoi Khimii, Vol. 36, No. 2, pp. 366–369, March–April, 1995. Translated by L. Smolina     

Higher energy states in the CO dimer: millimeter-wave spectra and rovibrational calculations

New extensive millimeter-wave measurements of the 12C16O dimer have been made, and more than 300 new spectral transitions have been observed in the frequency range 81-135 GHz. A joint analysis of these and previous millimeter-wave data yielded the precise location of 33 new energy levels of A+ symmetry and 20 levels of A- symmetry. These energy levels are located at 8-18 cm(-1) above the zero-point level. Some of them belong to already known stacks, and others make up 9 new stacks of the dimer. Newly determined stacks have K=0, 1, and, for the first time, 2, where K is the projection of the total angular momentum on the intermolecular axis. The energy levels from accompanying rovibrational calculations with the use of a recently developed hybrid CCSD(T)/DFT-SAPT potential are in very good agreement with experiment. Analysis of the calculated wave functions revealed that two new stacks of A+ symmetry with K=2 correspond to overall rotation of the dimer while the other newly observed stacks belong to the geared bend overtone modes. The ground vibrational states of the two "isomers" found are more or less localized at the two minima in the potential surface, whereas all the geared bend excited states show a considerable amount of delocalization.     

First-principles T-matrix calculations of double-ionization energy spectra of atoms and molecules

Strong electron correlation plays an important role in the determination of double ionization energy, which is required for removing or adding two electrons, particularly in small-sized systems. Starting from the state-of-the-art GW approximation, we evaluate the particle-particle ladder diagrams up to the infinite order by solving the Bethe-Salpeter equation of the T-matrix theory to calculate the double-ionization energy spectra of atoms and molecules (Be, Mg, Ca, Ne, Ar, Kr, CO, C(2)H(2), Li(2), Na(2), and K(2)) from first principles. The ladder diagrams up to the infinite order are significant to calculations of double-ionization energy spectra. The present results are in good agreement with available experimental data as well as the previous calculations using, e.g., the configuration-interaction method.     

Molecular engineering in symmetric end-substituted oligothiophene derivatives: analysis of condensed-phase photoemission spectra using semiempirical Hartree-Fock calculations

The electronic structures of two series of end-capped thiophene oligomers, one set containing the electron-deficient dimesitylboryl end-cap and one containing the electron-rich triaryl amine end-cap, have been modeled using semiempirical quantum chemical calculations and the results used to assign features in the photoemission spectra of the materials in the condensed phase. For the thiophene oligomers end-capped with the electron-deficient dimesitylboryl moieties, the energy of the occupied frontier orbitals is largely governed by pi-type orbitals of the thiophene repeat units in the oligothiophene main chain. Conversely, in oligomers end-capped with electron-rich triarylamine moieties, the occupied frontier orbital energies are largely governed by orbital states of heavily mixed character associated with thiophene pi-type systems and the low-lying nitrogen lone pairs of end capping groups.     

Enhanced raman spectra of molecules adsorbed on small metal particles

The Raman spectra of benzene adsorbed on small nickel and platinum particles have been measured. The spectra are strongly enhanced, compared with those wavelengths, where no enhancement has been observed. The spectra of benzene adsorbed on platinum and nickel particles are rather similar. The most intensive vibrational band is the totally symmetric breathing mode, which dominates the spectrum. Besides the Raman active vibrations, some other weak vibrational modes are observed, which may be assigned to normally infrared and inactive vibrational transitions. The enhanced Raman effect seems to be dependent on the particle size and shape. Therefore it is important to investigate the behaviour of small metallic particles. First results of pyridine bonded to Ag₂ and Ag₃ are reported.     

The electronic structure of heteroaromatic molecules; non-empirical calculations and photoelectron spectra for the isomeric-thiazoles and -thiadiazoles

The He(I) and MgKα photoelectron spectra are reported for the isomeric thiazoles, and He(I) spectra for the isomeric-thiadiazoles. Marked fine structure is apparent on some of the spectra. Comparison with the earlier spectra for thiophen and the azoles, and with ab initio calculations for the seven compounds, has led to assignment of the spectra. The calculations suggest that 1,3,4-thiadiazole is less aromatic than the other title compounds. As expected, the role of the 3d_s orbitals is found to be that of polarisation functions for all of the compounds, but inclusion of these orbitals leads to much better agreement with the observed dipole moments, both in magnitude and direction.     

On the theories of photoemission processes in diatomic molecules

It is shown how various approximations can be introduced in the general expression for angular distribution of electrons ejected in photoionization/photodetachment of diatomic molecules. Thus all the main theories currently in vogue for studying molecular photoemission processes have been unified by deriving the corresponding angular distributions from a single expression. As a prerequisite to this demonstration, a new, exact, and completely ab initio derivation which, unlike a previous formulation, contains explicit form of various operators and wavefunctions involved in matrix elements relevant to a molecular photoemission process, has been given too. This general formulation takes into account also the rotation-vibration-electron coupling. It can, therefore, be used for studying rotational, vibrational, and/or electronic autoionization/ autodetachment in molecules.     

Excited states and photochemistry of saturated molecules. ab initio calculations on methane

STO 4G calculations are carried out on methane in both tetrahedral and Jahn—Teller distorted C_2v and C_3v symmetries. Large increases in CH bond lengths are predicted in all states, and the ^IB₁ state in C_2v symmetry is predicted to dissociate to ^IB₁ methylene and ¹A₁ H₂ width no energy barrier. THe results are compared with earlier INDO calculations, and the latter are found to qualitatively consistent with the ab initio results.