Shell Electron Energy Loss Spectra Research Articles

Generalized oscillator strength profiles for inner shell excitation of CO2 derived from variable angle electron energy loss spectroscopy

GOS profiles for C 1s and O 1s excitation of CO2 have been derived from inner shell electron energy loss spectra of CO2 measured over a wide range of scattering angles (4–62°). The experimental profiles are compared to computed GOS profiles calculated within the first Born approximation, using ab-initio configuration interaction wave functions for the C 1s transitions and ab-initio generalized multi structural wave functions for the O 1s transitions. The theoretical and experimental results are in generally good agreement indicating that the first Born approximation holds to a surprisingly large momentum transfer for the core excitations studied. The computations predict there should be oscillations in the GOS for O 1s→nsσg (536.5, 539 eV) and npσu Rydberg states (∼539 eV) but not for π symmetry states. Measurements with a newly installed parallel detector provide experimental support for the existence of oscillations in the GOS profile for the O 1s excitation of CO2 at 539 eV. These oscillations may arise from interference between core excitations localized on the left or right oxygen atom of CO2.

Photon Stimulated Ion Desorption of Polymer Thin Films Following Core Excitation

We have studied the photon stimulated ion desorption of poly-isopropenyl-acetate (PiPAc) and poly-vinyl-acetate (PVAc) thin films from the viewpoint of site-specific bond scission. PiPAc is the structural isomer of PMMA, which shows site-specific bond scission. The structures of PiPAc and PVAc are different in the substitutional group of the side-chain, a methyl group or hydrogen. H+, CH+, CH2+, CH3+ and COCH3+ are mainly desorbed from PiPAc and PVAc thin films following the oxygen core excitation. The near edge X-ray absorption fine structure (NEXAFS) spectra of these thin films are assigned through a comparison to the inner shell electron energy loss spectra of the related molecules. The features of the partial ion yield (YPi) spectra of CH3+ and COCH3+ ions are different from that of the total electron yield (Ye) spectrum. This indicates that the efficiencies of desorption of these ions depend on the primary excitation and these ions are consequently desorbed site-specifically. The most remarkable difference in the YPi is seen in the YPi(CH3+) and YPi(COCH3+) spectra. In YPi(CH3+) and YPi(COCH3+) spectra, the peak is assigned to the O 1s(C=O) → σ*(C-C) with the help of this result.

Core Excitation Spectroscopy of Phenyl- and Methyl-Substituted Silanol, Disiloxane, and Disilane Compounds: Evidence for π-Delocalization across the Si−CphenylBond

The Si 1s and 2p solid state photoabsorption (total electron yield) spectra of triphenylsilanol, hexaphenyldisiloxane, and hexaphenyldisilane and the Si 1s spectra (total ion yield) of gaseous trimethylsilanol, hexamethyldisiloxane, hexamethyldisilane, and trimethylmethoxysilane have been recorded using synchrotron radiation. These spectra are compared to inner shell electron energy loss spectra of gaseous triphenylsilanol, hexaphenyldisilane, trimethylmethoxysilane, hexamethyldisiloxane, and hexamethyldisilane in the Si 2p and C 1s regions, measured under scattering conditions where electric dipole transitions dominate (2.5 keV residual energy, θ ≤ 2°). Comparison of the Si 1s and Si 2p spectra of the Ph3Si−X and Me3Si−X species shows there are low-lying transitions at Si which occur exclusively in the Ph3Si−X species. These transitions are attributed to (Si 1s-1,π*Si-Ph) and (Si 2p-1,π*Si-Ph) states in which the core excited electron is delocalized across the Si−C(phenyl) bond into the π* levels of the ...

Transition metal 2p excitaton of organometallic compounds studied by electron energy loss spectroscopy

Oscillator strengths for Mn, Fe, and Co 2p excitation of dicyclopentadienyl, carbonyl and mixed ligand complexes have been derived from inner shell electron energy loss spectra (ISEELS) recorded under electric dipole scattering conditions. The spectra are found to be surprisingly sensitive to the identity of the ligands and/or the character of the metal-ligand bonding and apparently insensitive to the molecular symmetry or even the d count of the metal atoms. A simple model in terms of the relative energies of unoccupied orbitals of large 3d character involving in-line (“dσ*”) and off-axis (“dπ*”) interactions with ligand orbitals is proposed. Extended Huckel (EHT) calculations have been used to aid assignment of the observed spectral features. The EHT results suggest that the metal 2p spectra should be more sensitive to the molecular symmetry than is actually observed. The 2p spectra are compared to the metal 3p spectra of the same species and to the metal 2p spectra of the pure metals and various halide and oxide compounds.

Excitation and ionization of the monohaloethylenes (C 2H 3X, X = F, Cl, Br, I). II. Photoionization by He(I) and He(II) photoelectron spectroscopy and valence-shell excitation by electron energy loss spectroscopy

The He (I) and He (II) photoelectron spectra and the valence-shell electron energy loss spectra for C 2H 3X (X = F, Cl, Br and I) have been recorded. The valence-shell electron energy loss spectra were obtained at an impact energy of 3000 eV and a zero degree scattering angle. In contrast to previous studies, the HOMO orbital in C 2H 3I is reassigned with dominant iodine 5p· character rather than the π(CC) character observed for the HOMO orbitals of C 2H 3F, C 2H 3Cl and C 2H 3Br. This reordering of the assignment in C 2H 3I is based on considerations of peak shape, vibrational profile and relative intensity change between He(I) and He (II) photoelectron spectra as well as observations from the valence shell electron energy loss spectra. The valence shell electron energy loss spectrum of each of the monohaloethylenes is tentatively assigned as being due to transitions from the various occupied valence orbitals to common manifolds of π*(CC), σ-*(CX) and Rydberg final orbitals. A linear correlation between the CX bond strength and the term values of the 2a″→σ* (CX) transition is also demonstrated.

Excitation and ionization of freon molecules. I. Absolute oscillator strengths for the photoabsorption (12–740 eV) and the ionic photofragmentation (15–80 ev) of CF 4

Absolute photoabsorption oscillator strengths (cross sections) for the valence shell of CF 4 have been measured using dipole (e, e) spectroscopy in the equivalent photon energy range 12–200 eV. The present results are in good agreement with optical measurements obtained using synchrotron radiation in the range 12–70 eV and also with earlier reported oscillator strengths derived from electron impact measurements in the range 12–37 eV. Predicted values obtained by extrapolation of the valence shell continuum to high energies are in excellent agreement with published measurements obtained using X-ray characteristic lines and bremsstrahlung radiation. Carbon 1s and fluorine 1s photoabsorption spectra obtained by Bethe-Born conversion of earlier reported inner shell electron energy loss spectra of CF 4 have been placed on an absolute intensity scale by normalizing on atomic cross sections in the respective 1s photoionization continuum regions. The results are compared with bremsstrahlung measurements. The photoionization efficiency and also the ion photofragmentation branching ratios have been determined from time of flight mass spectra measured using dipole (e, e+ion) coincidence spectroscopy at equivalent photon energies ranging from the first ionization threshold up to 80 eV. Absolute partial oscillator strengths for molecular and dissociative photoionization have been derived. The nature of the dipole induced breakdown pathways of CF 4 is investigated by combining the present oscillator strength measurements for molecular and dissociative photoionization with previously published photoelectron data. On the basis of the present work, a revised set of absolute electronic state partial oscillator strengths for CF 4 are presented.

Inner shell and valence shell electronic excitation of ClF 3 by high energy electron impact. An investigation of potential barrier effects

Inner shell (Cl 2p, 2s and F 1s) and valence shell electron energy loss spectra of ClF 3 have been measured under conditions of high impact energy (3–3.7 keV) and zero degree scattering angle. The Coulombic potential barrier model is found to provide an extremely satisfactory understanding of (a) the co-existence in ClF 3 of intense shape resonances and also intense Rydberg transitions plus direct ionization; and (b) the number and symmetry of the continuum shape resonances observed in the ISEELS spectra of ClF 3. The physical significance of the Coulombic potential barrier is further demonstrated in a comparison of the central atom inner shell spectra of SF 6, ClF 3 and HCl.

Electron Energy Loss Spectroscopy at 5Å Spatial Resolution

One of the ultimate goals of electron energy loss spectroscopy within the electron microscope is to be able to obtain the electronic structure of interfaces at near atomic resolution. Though this goal has not yet been achieved, there has been considerable effort devoted to elemental composition at high spatial resolution using ELS (eg. References 1-3). In this paper we wish to present initial measurements made across different types of interfaces that show core and valence shell electron energy loss spectra changing within an 8Å spatial scale across the interface.All the measurements have been performed using a modified dedicated STEM (VG Scientific HB-5) equipped with beam blanking facilities, digital control and a wide-gap aberration connected energy loss spectrometer. The details of this instrument have been described elsewhere (4). The main point to be noted is that the incident illumination half angle was 7.5mrad for these experiments and the full width at half maximum of the probe was 4.6Å (measured). With these optical conditions, 90% of all the incident beam current is contained within a diameter of 9.0Å (5). For beam sensitive materials, the recording dose was kept to less than 1/3 the dose for observable sample degradation.

High resolution carbon 1 s and valence shell electronic excitation spectra of trans-1,3-butadiene and allene studied by electron energy loss spectroscopy

The high resolution inner shell electron energy loss spectra of trans-1,3-butadiene and alllene are presented. Also reported are the wide range (5–25 eV) valence shell electron energy loss spectra of these molecules. The inner shell spectra show intense C 1 s→ * transitions well below the Rydberg structure. Transitions to both π * levels in trans-1,3 butadiene are clearly identified. Both molecules show significant structures above the C Is edges which are attributed to σ * shape-resonances as well as double excitations and “shake-up”. Comparisons are made between the inner shell and valence shell spectra. The transitions identified in the inner shell spectra of butadiene and allene help clarify the assignments of the valence shell spectra because of the essential transferability of Rydberg term values. The assignments of the valence shell spectrum of allene are, in turn, helpful in assessing the great variety of published theoretical investigations of this spectrum. In particular, earlier predictions of the π→π * transition in allene at 7.2eV are supported, in disagreement with a more recent calculation.

Inner shell electron energy loss spectra of the methyl amines and ammonia

Electron energy loss Spectroscopy has been used to obtain the inner shell excitation spectra of the methyl amines CH 3NH 2, (CH 3) 2NH and (CH 3) 3N for both the N 1 s and C 1 s regions. A spectrum of the N 1 s region of NH 3 is also presented at higher resolution than previously published data. The C l s spectra are all very similar and the discrete portions may be assigned to Rydberg transitions. However, features attributable to a σ * shape resonance are observed just above the N 1 s and C l s ionization edges. The NH 3 spectrum is ascribed to Rydberg transitions. The N 1 s spectra of the methyl amines, however, become increasingly dominated by a σ* resonance in the continuum with increased methylation. The features in the inner-shell spectra are compared with the reported valence-shell optical absorption spectra and support the Rydberg assignment. The inner-shell spectra of (CH 3) 3N and NH 3 are also compared with previously published inner shell electron energy loss spectra of NF 3 and the third row phosphorus analogues PF 3,P(CH 3) 3andPH 3.

Electronic excitations in phosphorus-containing molecules. I. Inner shell electron energy loss spectra of PH 3, P(CH 3) 3, PF 3 and PCL 3

Electron energy loss Spectroscopy has been used to obtain the inner shell electronic excitation spectra of PH 3, PF 3, PCl 3 and P(CH 3) 3 in the phosphorus L-shell (P 2 p, 2 s) region as well as the respective ligand K -shells (F 1 s, C 1s) and L-shell (Cl 2 p and 2 s) regions. The spectra were obtained under small momentum transfer conditions so that dipole-allowed transitions dominate. An impact energy of 2.5 ke V was used and inelastically scattered electrons were detected at a typical scattering angle of about 1°. A dipoleforbidden transition of unusual character is observed at 135.11 eV in the P 2 p spectrum of PCl 3. Although optically forbidden, as indicated by its absence in a soft X-ray absorption spectrum, the intensity of this transition rises very rapidly with increase in momentum transfer.

Electronic excitation in phosphorus-containing molecules. iii. valence shell electron energy loss spectra of P(CH 3) 3, PCl 3, PF 3, OPCl 3 AND PF 5

The valence shell excitation spectra for P(CH 3) 3, PCl 3, PF 3, OPCl 3 and PF 5 have been obtained using electron energy loss Spectroscopy. The spectra were obtained using an impact energy of 2500 eV and a zero degree scattering angle. The spectra are tentatively assigned and interpreted with the aid of the inner shell excitation spectra and ionization potentials from photoelectron Spectroscopy. The HOMO orbital of PF 5 is confirmed as the 2e'' orbital.