Shifts In Photoelectron Spectroscopy Research Articles

The ESCA molecule—Historical remarks and new results

The C 1s photoelectron spectrum of ethyl trifluoroacetate (CF3COOCH2CH3), also known as the ‘ESCA molecule’, is the most illustrative showcase of chemical shifts in photoelectron spectroscopy. The binding energies of the four carbon atoms of this molecule spread over more than 8eV with energy separations ranging from 1.7 to 3.1eV owing to different chemical environments and hence different charge states of these atoms. The paper discusses history and importance of this spectrum in the field of photoelectron spectroscopy starting from the time of invention of the ESCA technique. The main focus of the paper is a ‘revisit’ of this spectrum using the most modern experimental and computational tools. Large geometrical changes, different for each ionization site, and the presence of two conformers of ethyl trifluoroacetate influence the spectral lineshapes of all four C 1s lines. These effects are carefully modeled by theory and investigated in the experimental spectrum.

Inhomogeneous charge transfer within monolayer zinc phthalocyanine absorbed on TiO2(110)

The d-orbital contribution from the transition metal centers of phthalocyanine brings difficulties to understand the role of the organic ligands and their molecular frontier orbitals when it adsorbs on oxide surfaces. Here we use zinc phthalocyanine (ZnPc)/TiO(2)(110) as a model system where the zinc d-orbitals are located deep below the organic orbitals leaving room for a detailed study of the interaction between the organic ligand and the substrate. A charge depletion from the highest occupied molecular orbital is observed, and a consequent shift of N1s and C1s to higher binding energy in photoelectron spectroscopy (PES). A detailed comparison of peak shifts in PES and near-edge X-ray absorption fine structure spectroscopy illustrates a slightly uneven charge distribution within the molecular plane and an inhomogeneous charge transfer screening between the center and periphery of the organic ligand: faster in the periphery and slower at the center, which is different from other metal phthalocyanine, e.g., FePc/TiO(2). Our results indicate that the metal center can substantially influence the electronic properties of the organic ligand at the interface by introducing an additional charge transfer channel to the inner molecular part.

Time dependence of photovoltaic shifts in photoelectron spectroscopy of semiconductors.

Photovoltage-induced peaks shifts in photoelectron spectroscopy (PES) were recently shown to be important in studies of semiconductor interface formation [M. H. Hecht, Phys Rev. B 41, 7918 (1990)]. In that process, the photons used to probe the sample were shown to cause a reduction in surface band bending, hence shifting the spectrum in energy. In this paper it is shown that the time dependence of the charging and discharging process can have an important influence on experimentally observable phenomena. Since the charging process is very fast, the effective photocurrent in a pulsed-synchrotron-radiation experiment may be orders of magnitude higher than previously assumed. Using a simple model of the discharge profile it is shown that the photovoltage persists on a time scale that is macroscopic under typical PES conditions. Finally, a noncontacting method for determining equilibrium surface band bending is proposed.

Final-state screening and chemical shifts in photoelectron spectroscopy.

Photoelectron spectroscopic measurements of core-electron levels in cations of transition-series insulators commonly show satellite features in the vicinity of the main photoelectron peaks. In this paper, the relationship between main lines and adjacent satellites is systematically examined for 3 d-series insulators. The spectral features are interpreted using a relaxation model that allows for the possibility of different final-state screening conditions of the photoinduced hole. Systematic trends are examined by employing atomic calculations and utilizing transition-state theory to calculate electron removal energies for atoms in appropriately simulated chemical environments. It is demonstrated that the main-line and satellite energies are both dependent on the cation charge environment but with different degrees of sensitivity. The consequence is that sensitivity to chemical change can be monitored in the observed main-line to satellite separation. Model predictions are tested by monitoring the dependence of the satellite separations on the cation valence state and on the electronegativity of both cations and ligated anions. Systematic tests are applied to 3 d-series insulators but more general applicability of the relaxation model is also discussed. 93 references, 19 figures, 1 table.

Abstract: Can chemisorption bonding shifts be separated from relaxation‐energy shifts in photoelectron spectroscopy?

Consider the surface‐molecule limit of chemisorption in which an adatom orbital with eigenvalue εa forms a bond with substrate orbitals of energy εs. The molecular orbital (MO) eigenvalues are (typeset math) where the hopping integral V causes a chemical bonding shift. In a photoemission experiment from such a MO, the electrons ejected from the bond are observed (upon subtraction of the photon energy) at the energy (typeset math) where δε±(rel) is the relaxation energy due to hole creation in the MO state.1 The problem to be addressed is how to determine a value of V, given ε (obs).Demuth and Eastman2 have analyzed photoemission spectra from CO chemisorbed on Ni under the assumption that the relaxation shift of the nonbonding σ orbitals on CO is identical to the relaxation shift of the bonding π orbitals and (implicitly) that the relaxation shifts of the Ni d orbitals are equal to the nonbonding (clean surface) value. In this approximation scheme, the neglect of the MO charge‐cloud distortion due to bonding‐charge pileup between the a and s centers is compensated for by allowing each molecular constituent to have its full uncoupled relaxation shift. To check out the independent atom relaxation energy approximation, the relaxation (or polarization) energy shift for hole creation in a diatomic hydrogen molecule embedded in an electron gas has been calculated3 and the results indicate that the relaxation energy differs by only 2%–9% (depending on electron‐gas density in the range 5≳rs≳2) from the uncoupled atom value. Assuming that this difference can be neglected, we can then replace ε±(obs) of Eq. (2) by ε± of Eq. (1) with εs→εs+ δεs(rel) =εs(obs) and εa→εa+δεa(rel) =εa(obs) where now δεa(rel) is the extra‐atomic or polarization self‐energy.1,4,5 Within this scheme, the observed MO energy is related to the known quantities εs(obs) (from clean surface experiments) and εa (from gas‐phase experiments) and the unknown quantities δεa(rel) and V by (typeset math) Rather than calculate V, we calculate the polarization energy δεa (rel) for a hole created in an atomic orbital state φa(r) on the adatom located a distance s from the effective image plane of the surface. Following Hedin4 and Hodges,5 the polarization energy is given by: (typeset math) with the nonlocal self‐energy (typeset math) the ω component of the potential induced at r′ by a charge at (typeset math) with εq(ω) the electron‐gas dielectric function, r∥ and q lying in the plane of the surface, and G0 the hole Green’s function.Taking εq(ω) ?1−ω2p/ω2 with ωp the bulk plasmon frequency and neglecting dynamic and van der Waals polarization, the system of Eqs.(4) can be reduced to (typeset math) For illustrative purposes take φ:da to be a single, optimal Gaussian, (typeset math) where now, the z origin is taken at the image plane, and the atom– surface separation is s. Equations (5) and (6) can be reduced6 to the single quadratture: (typeset math) with 0.5 ≲δεa(rel)/εim<1 for atomic scale values of s and with εim=e2/4s, the image potential shift.8 As a parenthetical check, we note that the relaxation shifts experienced ty the 5p levels of Xe physisorbed on W, as reported by Waclawski and Herbst,9 fall within this range.Since a well defined procedure for calculating the adatom relaxation energy shifts [Eqs. (4) – (7)] has been given, the only remaining unknown quantity in Eq. (3) is V and thus we can conclude that (within the approximation scheme discussed here) chemisorption bonding shifts can be separated from relaxation energy shifts in photoelectron spectroscopy.