G.M. Bancroft

Resonances in the silicon L core level cross sections of fluoromethylsilane compounds characterized by photoabsorption spectroscopy and MS-Xα calculations

Abstract High-resolution Si 2p and 2s photoabsorption spectra of the five fluoromethylsilane compounds, Si(CH 3 ) x F 4- x ; x =0–4, obtained in the gas phase using monochromatized synchrotron radiation are presented. Transition state MS-Xα calculations for both the pre- and post-edge regions of the Si 2p and 2s edges of all five molecules are also reported. The theoretical results and trends observed in the experimental spectra of the molecular series facilitate the identification and assignment of numerous spectral features. In the Si 2p pre-edge region, absorptions are assigned as transitions to antibonding or Rydberg orbitals. Combined intensities of transitions to Rydberg orbitals are found to be comparable to the intensities of transitions to antibonding orbitals. For these molecules the Si 2s pre-edge regions of the spectra exhibit less structure, but assignments analogous to those made for the Si 2p pre-edge are made. Three continuum resonances are observed above the Si 2p ionization threshold in the photoabsorption spectra of all five molecules, one near threshold, another ≈ 5 eV above threshold and a high energy peak ≈ 20 eV above threshold. Low energy peaks are shape resonances attributed to transitions into SiC antibonding orbitals lying slightly above threshold. The medium energy peaks are assigned as shape resonances resulting from electronic transitions to quasibound states in the continuum supported by the electronegative fluorine ligands. An atomic-like delayed onset phenomenon is used to explain the higher energy peaks.

High resolution molecular gas phase photoelectron spectra of core levels using synchrotron radiation - vibrational and ligand field splittings

Abstract A new high resolution photoelectron spectrometer designed for core level photoelectron studies of gas phase volatile and involatile compounds using synchrotron radiation is briefly described. Xenon NOO Auger spectral linewidths of 0.13 eV indicate that the electron resolution is 4 and ligand field splitting of the Xe 4d core level of XeF 2 .

A comparison of shape resonant behaviour in the inner-shell photoabsorption and valence-level photoelectron spectra of SF6, SF5Cl and SeF6

Abstract Inner-shell photoabsorption and valence-level photoelectron spectra of gaseous SF 6 , SF 5 Cl, and SeF 6 have been obtained using synchrotron radiation spanning a 21.2–215 eV energy range. The resulting total photoabsorption cross sections of sulphur 2p (l II,III ) in SF 6 , sulphur and chlorine 2p (L II,III ) in SF 5 Cl, and selenium 3p (M II,III )) in SeF 6 are reported here. Variations in corresponding valence-level branching ratios are then compared. The well-known intense shape resonances of SF 6 are maintained in the SF 5 Cl spectra, with subtle differences due to the change in symmetry and in one ligand. In contrast, the Se 3p spectrum is dramatically different, with much broader and weaker features. These observations are interpreted in terms of the potential barrier effect.

High-resolution HeI and HeII photoelectron spectra of the Zn 3d and Cd 4d orbitals in the zinc and cadmium dihalides: Ligand field splittings and intensity variations

We have recorded the high-resolution HeI and HeII photoelectron spectra of the Zn 3d and Cd 4d levels in gas-phase MX2 molecules (M = Zn, Cd; X = Cl, Br, I). The d level spectra split into five peaks due to the combined effect of spin-orbit splitting and ligand field splitting on the d9 hole state, and the spectra have been fitted to a crystal field hamiltonian involving the cubic (C40) and non-cubic (C20) parts of the field from the halide ligands. Additional peaks in some spectra are due to vibrational splitting and configuration interaction. The ¦C20¦ value increases substantially from the chloride to the iodide for both Zn and Cd. Calculations of both the crystal field (C2 CF0) and valence (C2 val0) parts of C20 show that the increase in observed C20 is due to the C2 val0 term attributed to the increase in covalency from the chlorides to the iodides. Shifts in the peak position due to the 2Σ12 g and 1Π32 g states from those expected on the ligand field basis. are attributed to slight bonding effects. These effecs cause a large discrepancy between calculated and observed C40 values. The intensities of the five Zn 3d peaks change markedly from HeI- to HeII-excited spectra. The Xα SW method has been used to calculate the intensities of the σ, π and δ 3d orbitals as a function of photon energy. These calculations show dramatic changes in intensity due. for example. to shape resonances. There is usually qualitative agreement between calculated and observed intensities.

High Resolution Gas Phase Photoelectron Spectra of Core d Levels Using He II Radiation

Using a newly constructed charged particle oscillator He II discharge lamp, we have obtained high resolution (≤0.03 eV instrumental resolution) spectra of the outermost metal core d levels in alkyl compounds of Cd, In, and Pb. The Cd and In 4d spectra of Me2Cd (Me = CH3), Et2Cd (Et = C2H5) and Me3In are split into five peaks due to the combined effects of spin orbit splitting (5/2 λ) and an asymmetric crystal field (the C20 term which transforms like the electric field gradient). As expected, the spin orbit split Pb 5d levels in the tetrahedral molecule Et4Pb (C20 = 0) show no noticeable crystal field splitting. The new photoelectron effect enables electric field gradients to be obtained in the gas phase. Our derived C20 values are consistent with previously measured nuclear electric field gradients in Cd and In compounds; and the measured C20 value for Me2Cd is in qualitative agreement with that calculated by H. Basch from an ab initio self-consistent field (SCF) calculation. Because the measured linewidths are mainly due to the inherent natural linewidths of the d levels, we can readily obtain natural d widths from these spectra. The high resolution spectra of the Cd 4d levels of Et2Cd enable us to resolve Cd 4d peaks due to Cd atoms and EtCd· radicals. The Cd 4d spectra of Me2Cd and Et2Cd taken at 21.2 eV, 40.8 eV, and 51.0 eV photon energies show marked changes in the relative intensities of the five 4d peaks.

High resolution core level photoelectron spectroscopy using He II radiation: electric field gradient splittings in group IIB and group III compounds

Abstract Using a charged particle oscillator He II discharge lamp, we have obtained high resolution 40.8 cV photoelectron spectra of the 3d core levels of Zn and Ga in Me2Zn and Me3Ga respectively, and the Pb 5d core levels in Et4Pb. The overall instrumental resolution is between 25 meV and 40 meV for all spectra. Inherent core level widths for the Zn 3d (⩽0.025 eV), Ga 3d (⩽0.15 eV) and Pb 5d (⩽0.32 eV) energy levels have been obtained from these spectra. The Me2Zn and Me3Ga spectra consists of five peaks due to the combined effects of spin orbit coupling and large non-cubic ligand fields (the C02 term in the crystal field expansion). In contrast, the Et4Pb 5d spectrum shows no appreciable ligand field splitting, as expected for a tetrahedral molecule.

High-resolution He II photoelectron spectra of high-temperature inorganic vapours

Abstract High-resolution He II spectra are reported for ZnCl 2 and TlCl in the gas phase. Total instrumental resolution of ≈30 meV enables us to resolve new features in the spectra due to ligand-field splitting, vibrational splitting and configuration interaction.

Linewidths and line shapes in solid state ESCA studies: Electric field gradient broadening of Sn 3d lines

Abstract ESCA Sn 3d linewidths of nonconducting organometallic tin solids, prepared by in situ evaporation, are within about 10% of the values for analogous gas phase measurements. The similar line shapes (≈ 35% gaussian, 65% lorentzian) observed for solid state and gas phase spectra show that effects which give increasing gaussian components, such as charge broadening and phonon broadening, are very small in our solid state spectra. The general correlation between ESCA Sn 3d linewidths and Mossbauer quadrupole splittings indicates that electric field gradient splitting of the Sn 3d lines approaches ≈ 0.4 eV.

Shake-up satellites in x-ray photoelectron (ESCA) spectra of 5f0 Th(IV) compounds

Abstract X-ray photoelectron (ESCA) spectra of the Th 4f levels have been recorded in 12 Th compounds. Weak satellites are observed in all the 4f spectra at binding energies 4–6 eV from the primary photolines. On the basis of a multiple scattering calculation in ThO 2 , these satellites are assigned to ligand (e.g. O 2p) to Th 5f shake-up transitions. As with La 3d satellites, the satellite intensities correlate well with the nephelauxetic order of ligands.

CORE LEVEL PHOTOIONIZATION MASS-SPECTROSCOPY OF THE FLUOROMETHYLSILANES, SI(CH3)XF4-X (X=0-4), AROUND THE SI-2P IONIZATION EDGES

Abstract Total ion yield spectra, total electron yield spectra, photoionized mass spectra and mass resolved partial photoion yield spectra of the five fluoromethylsilane compounds, Si(CH 3 ) x F 4- x ( x = 0–4), measured around the Si 2p core ionization edges using monoc synchrotron radiation are reported. Photoionized mass spectra of the five compounds were also measured around the Si 2s and F 1s ionization edges. For each of the fluoromethylsilane compounds, total ion yield and total electron yield spectra are very similar to each other and to the corresponding photoabsorption spectra with the exception of the decreased intensities of the discrete resonances. The photoion mass spectra show that almost all possible molecular fragments are generated by excitation and ionization of the Si 2p, Si 2s and F 1s core electrons. Branching ratios of the photoion mass spectra peak areas indicate that the methyl groups are much more labile than the fluorine atoms at all photon energies, both below and above the core ionization edges, in the mixed fluoromethylsilane compounds. Relative areas of the methyl peaks in the photoion mass spectra are enhanced at the discrete resonances below the ionization threshold with more pronounced enhancements above the ionization threshold. An explanation for the greater lability of the methyl ligands is proposed based on the effect of the electron withdrawing and donating properties of the fluorine atoms and methyl groups which also affect the relative Auger transition rates and core hole lifetimes. Partial ion yields of the SiF + and SiMe + fragment ions also exhibit some specific enhancements at Si 2p→σ* resonances below the ionization edge.