Valence Shell Region Research Articles

ChemInform Abstract: Angle‐Resolved Valence Shell Photoelectron Spectroscopy of Neutral Nanosized Molecular Aggregates

AbstractReview: 67 refs.

On Relativity, Bonding, and Valence Electron Distribution

The scalar relativistic contributions to bond or atomization energies of homo- or hetero-polar s−p-bonded atoms, ΔrelE(bond), correlate well with the changes on bond formation of the electron density, integrated throughout the spatial K-shell region of the heavy nucleus, ΔbondρK-region, times Z4, where Z is the nuclear charge. The “bond density changes” in the innermost atomic core regions, however, have no direct simple relationship to the electron density reorganizations in the outer atomic valence shell regions, which determine the nonrelativistic main contribution to the bond energy, ΔnonrelE(bond). Also, scalar relativistic bond energy changes and bond length changes, ΔrelRe, are not directly correlated. Namely, ΔrelE(bond) correlates with the difference ΔbondρK-region, whereas ΔrelRe correlates with the derivative (dρK-region/dR). Reducing the internuclear distance from the separated to the united atom limit through the equilibrium distance value, Re, the density around the nucleus at first decreases in many cases, goes through a minimum, and finally increases again. Therefore (dρK-region/dR) may be positive or negative at Re.

Valence-shell angle-resolved electron energy loss spectra of SF6

Electron-energy-loss spectra of the SF6 molecule have been measured in the valence-shell region (0 - 100 eV) covering an angular range of 1.0° - 15.0°. The spectra have been measured at an impact energy of 1.0 keV. The angular dependence of the spectra showed the presence of three forbidden transitions in the region below 40 eV. Above 40 eV interesting features appear at higher scattering angles, which can be associated to nondipole many-body processes. A new feature was observed at 76 eV. The absolute elastic differential cross section has been obtained through normalization of the relative data to the experimental values found in the literature. The absolute inelastic differential cross sections for the 11.5 eV and 13.2 eV transitions have been determined. The generalized oscillator strength (GOS) for the 11.5 eV peak has been also determined in the 0.05 to 1.3 a.u. K² range. The experimentally determined value for the optical oscillator strength(0.312) for the 11.5 eV peak obtained through extrapolation of the GOS curve showed good agreement with the data found in the literature.

Absolute photoabsorption (6–350 eV) and photoionization (11–80 eV) of methyl chloride using dipole electron scattering and synchrotron radiation spectroscopies

Abstract Absolute oscillator strenghts (cross sections) have been measured for the photoabsorption of CH 3 Cl at low resolution (1 eV fwhm) in the equivalent photon energy range from 6 to 350 eV and at high resolution (0.05 eV fwhm) from 6 to 50 eV using dipole (e,e) spectroscopy. In the valence shell region of CH 3 Cl, from the first ionization threshold to 80 eV, the photoionization efficiency, ionic photofragmentation branching ratios and absolute partial oscillator strengths for molecular and dissociative photoionization have been determined by dipole (e,e+ion) spectroscopy. The major ionic pathways of the dipole induced breakdown scheme of CH 3 Cl are predicted from these results and the partial photoionization oscillator strengths for production of the valence shell electronic states of CH 3 Cl + are estimated. The photoabsorption data have been combined with photoelectron branching ratios for Ch 3 Cl, measured using monochromated synchrotron radiation at photon energies from 21 to 72 eV, to obtain partial photoionization oscillator strengths for production of the electronic states of CH 3 Cl + , which are compared with the partial oscillator strength estimates derived from the (e,e+ion) data.

An experimental and theoretical study of the valence shell photoelectron spectrum of carbon disulphide

The complete valence shell photoelectron spectrum of carbon disulphide has been studied using HeI, HeII and synchrotron radiation. In addition to the photoelectron bands associated with the X 2Πg, A 2Πu, B 2∑u+ and C 2∑g+ single-hole ionic states, several satellite features have been observed which are due to multielectron processes. The high resolution HeI and HeII excited spectra have allowed a detailed analysis to be made of the vibrational structure exhibited in the photoelectron bands corresponding to the single-hole and satellite states. The photoelectron spectra recorded with synchrotron radiation demonstrate that the inner valence region contains much complicated structure, and that distinct features are discernible up to a binding energy of at least 34 eV. Many-body Green's function calculations have been performed to evaluate the ionization energies and pole strengths associated with the main lines and satellite lines distributed throughout the valence shell region, and an interpretation of some of the experimental features is proposed, based upon these predictions.

An experimental and theoretical study of the valence shell photoelectron spectrum of sulphur hexafluoride

Abstract The complete valence shell photoelectron spectrum of sulphur dioxide has been studied using HeI, HeII and synchrotron radiation. The high resolution HeI and HeII excited spectra have allowed a detailed analysis to be made of the vibrational structure exhibited in the photoelectron bands associated with ionisation from the outer valence orbitais. A new satellite band containing vibrational structure has been observed around 19 eV. The photoelectron spectra recorded with synchrotron radiation demonstrate that the inner valence region contains several complicated structures and is dominated by a prominent feature centred at a binding energy around 34 eV. Many-body Green's function calculations have been performed to evaluate the energies and pole strengths associated with the main lines and satellite states distributed throughout the valence shell region, and an interpretation of some of the experimental features is proposed, based upon these predictions.

Quantitative studies of the photoabsorption, photoionization, and ionic photofragmentation of methyl fluoride at VUV and soft X-ray energies (7–250 eV) using dipole electron scattering and synchrotron radiation

Absolute oscillator strengths (cross sections) for the valence shell photoabsorption of CH3F have been measured using low resolution (1 eV fwhm) dipole (e, e) spectroscopy in the equivalent photon energy range from 7 to 250 eV. In addition, high resolution (0.048 eV fwhm) dipole (e, e) spectroscopy has been used to study the valence shell region from 8 to 50 eV in more detail. The photoionization efficiency, ionic photofragmentation branching ratios and absolute partial oscillator strengths for molecular and dissociative photoionization have been determined for CH3F by dipole (e, e+ion) spectroscopy from the first ionization threshold to 100 eV. The major pathways of the ionic dipole induced breakdown scheme are predicted from these results. The electronic ion state branching ratios for the photoionization of CH3F were determined from photoelectron spectra recorded using monochromated synchrotron radiation at photon energies from 21 to 72 eV. The partial photoionization oscillator strengths for production of the electronic states of CH3F+ derived from the PES electronic ion state branching ratios and the present low resolution photoabsorption measurements are compared with estimates derived from the dipole induced breakdown scheme and the molecular and dissociative photoionization partial oscillator strengths. The two estimates of electronic state partial photoionization oscillator strengths are in good agreement only at higher photon energies. The differences at lower photon energies arise from errors in the photoelectron branching ratios due to the difficulties of assessing the steeply rising non-spectral background in the photoelectron spectra, particularly in the (1a1)−1 many-body region. Suggestions are made for improving the quantitative accuracy of PES measurements.

An experimental and theoretical study of the valence shell photoelectron spectrum of sulphur dioxide

Abstract The complete valence shell photoelectron spectrum of sulphur dioxide has been studied using HeI, HeII and synchrotron radiation. The high resolution HeI and HeII excited spectra have allowed a detailed analysis to be made of the vibrational structure exhibited in the photoelectron bands associated with ionisation from the outer valence orbitais. A new satellite band containing vibrational structure has been observed around 19 eV. The photoelectron spectra recorded with synchrotron radiation demonstrate that the inner valence region contains several complicated structures and is dominated by a prominent feature centred at a binding energy around 34 eV. Many-body Green's function calculations have been performed to evaluate the energies and pole strengths associated with the main lines and satellite states distributed throughout the valence shell region, and an interpretation of some of the experimental features is proposed, based upon these predictions.

Density functional methods and the spatial distribution of electronic charge

The effect of density functional (DFT) methods upon the form of the electronic charge ditribution is analyzed at the molecular and atomic levels by comparison of the perturbation these functionals make to the HF/DZP density distribution with the changes MP2 electron correlation produce. The density functional methods mimic MP2 densities near and within the valence shell regions, contracting charge towards the nucleus of heavy atoms. The outer distribution outside the valence regions, are not well mimicked, with MP2 polarizing charge away from the nucleus while the DFT techniques contract charge towards the nucleus. The DFT methods mimic the ‘MP2-like’ polarizations in the inner regions of atoms by pushing charge into areas with higher gradients of the charge density in order to lower the total energy of the systems as directed by the form of the functionals. In the outer regions, where there is no corresponding energetic motive, charge is not polarized away from the atoms. This would suggest that the DFT procedures would not be as successful as MP2 in predicting properties based upon the outer charge distribution of molecules, e.g., inter-molecular complexes and hyperpolarizabilities.

Valence- and inner-shell (Cl 2p, 2s; C 1s) photoabsorption and photoionization of carbon tetrachloride. Absolute oscillator strength (5–400 eV) and dipole-induced breakdown pathways

Electronic excitation spectra and absolute oscillator strengths (cross sections) have been measured in the vacuum ultraviolet and soft X-ray regions for the valence- and inner-shell (Cl 2p, 2s; C 1s) photoabsorption of CCl 4 from 5.5 to 400 eV, using low-resolution (1 eV fwhm) dipole (e, e) spectroscopy. High-resolution (0.048 eV fwhm) dipole (e, e) spectroscopy has been used to study the valence region from 5.0 to 30 eV in greater detail. These data are compared with previously reported optical measurements in those limited energy regions where such data exist. Dipole (e, e+ion) spectroscopy has been used at a resolution of 1 eV fwhm to measure the photoionization efficiency and ionic photofragmentation branching ratios for CCl 4 from 11.0 to 80.0 eV in the valence-shell region and from 190.5 to 220.5 eV in the vicinity of the Cl 2p inner-shell edge. Absolute partial oscillator strenghts for dissociative photoionization channels of CCl 4 have been derived. Using these and other spectroscopic data, a dipole-induced breakdown scheme for CCl 4 in both the valence-shell and Cl 2p inner-shell regions is proposed.

Absolute optical oscillator strengths for the photoabsorption of nitric oxide (5–30 eV) at high resolution

High resolution dipole (e, e) spectroscopy has been used to measure absolute optical oscillator strengths for the photoabsorption of nitric oxide in the valence shell region over the equivalent photon energy range 5–30 eV. Absolute optical oscillator strengths for the vibronic transitions of the γ(A 2Σ +-X 2Π), β(B 2Π-X 2Π), δ(C 2Π-X 2Π) and ϵ (D 2Σ +-X 2Π) systems have been obtained. The present results are compared with previously published experimental data and theoretical calculations.

Absolute optical oscillator strengths (11–20 eV) and transition moments for the photoabsorption of molecular hydrogen in the Lyman and Werner bands

The recently developed high resolution dipole (e, e) method has been used to measure the absolute optical oscillator strengths for discrete and continuum transitions for the photoabsorption of molecular hydrogen throughout the valence shell region, in the energy range 11–20 eV. Absolute optical oscillator strengths for the vibrational transitions of the Lyman and Werner bands are determined and compared with available experimental and theoretical data. The variation of the electronic transition moment with internuclear distance is studied for the Lyman and Werner bands. The present oscillator strengths and transition moments are in excellent agreement with the highest levels of theory. Finally the dipole strengths of both bands at the equilibrium internuclear distance (0.741 Å) are also reported.

Inner shell excitation, valence excitation and core ionization in NF 3 studied by electron energy-loss and X-ray photoelectron spectroscopies

Electron energy-loss Spectroscopy (EELS) at impact energies of 2.5–3 keV has been used to obtain the electron excitation spectra for the N 1 s ( K-shell), F 1 s ( K-shell) and valence shell regions of NF 3. The inner shell spectra were recorded using small angle scattering (⩽1° ) while the valence shell spectrum was obtained at zero degree scattering angle. The inner shell excitation spectra show a strongly enhanced 1 s→ δ * type transition and continuum features which are typical for molecules with highly electronegative ligands. One of the peaks in an earlier published photoabsorption study of the N 1 s region has been shown to be due to a N 2 impurity. The valence shell electron energy-loss spectrum shows a number of transitions which are considered to be mainly due to valence-valence type transitions, with also some evidence of Rydberg structure. The X-ray photoelectron spectra (XPS) of the N 1 s and F 1 s electrons along with their associated satellite structures have also been recorded using Al Kα (1486.58 eV) radiation. The vertical ionization potentials for the N 1 s and F 1 s electrons were found to be 414.36 (10) eV and 693.24 (10) eV, respectively. Both spectra exhibit a rich and different satellite structure. These “shake-up” features in the satellite XPS spectra are compared with continuum features of the inner shell electron energy-loss spectra and also with the valence shell spectrum.

Calculation of transition metal compounds using an extension of the CNDO formalism. V. many body effects in the inner valence shell photoionization spectra of free and coordinated carbon monoxide

The inner valence shell region in the photoionization spectra of NiCO, Ni(CO) 4 and Cr(CO) 6 is studied by means of a diagonal two-particle—hole Tamm—Dancoff approximation. Many body effects are found in the energy range of the CO-ligand ionizations. The effects are more pronounced in NiCO and Ni(CO) 4 than in Cr(CO) 6. Since similar results have to be expected for adsorbate systems, the interpretation of photoemission spectra of such systems on the basis of the simple one particle picture should be handled with care.

Breakdown of Koopmans' theorem and strong shake-up bands in the valence shell region of N2 photoelectron spectra

Several ionized states of N2 are studied using ab initio Cl methods. The large corrections to the Koopmans values for the three lowest ionization potentials (IP's) are analyzed by setting up the effective hamiltonian matrix for each ionized state. The most important correction terms to the IP of an orbital, v, result from the type of excitations, ij → k∞ (ij ≠v), where ij → k∞ denotes that an electron in one of the occupied orbitals, either φi or φj, is excited to the vacant orbital, φk and simultaneously an electron in the other orbital is ionized. The excitation ij → k∞ can be shown to be an intramolecular charge-transfer excitation in terms of the valence bond picture. It is this large mixing of the v → ∞ and the ij → k∞ states that is responsible for the strong shake-up bands in the region of 24-32 eV in the photoelectron spectra. The calculated IP's for the three lowest single hole states and for the five shake-up states are in good agreement with experimental values. For comparison, the IP's of CO are also studied.

The molecular energy levels of the azoles: A study by photoelectron spectroscopy and ab initio molecular orbital calculations

HeI photoelectron spectroscopy and ab initio calculations have been applied to the azoles, providing sets of energy levels that correlate well with each other in the upper valence shell region. Observed IPs are assigned to the three π- and to the five o-levels that involve (principally) valence shell p orbitals. The observed vibration structure is not particularly informative as an aid to assignment since both π-and σ-levels give some bands with vibration structure. The calculations provide in addition to eigenvalues (energy levels) a set of eigenvectors, permitting analysis of the bonding characteristics of the levels, and trends apparent within the series.