John S. Tse

Experimental and theoretical shakeup studies. III. The 1s shakeup in CH4, NH3, and H2O

The core 1s level x‐ray photoelectron spectra of CH4, NH3, and H2O have been recorded to high statistics. Several satellite peaks are resolved in each spectrum. The shakeup peaks are assigned with the aid of Xα–SW calculations. A comparison of the ten electron hydride MO diagrams with the neon atomic energy levels reveal the Rydberg character of the empty excited state orbitals. The shakeup transitions in the respective molecules can also be interpreted from atomic models constructed from the experimental shakeup spectrum of neon. The 2p contribution to the total relaxation energy is calculated from a spectral weight analysis of the shakeup spectra.

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.

A SCF Xα SW study of the shake-up in TiCl4

Abstract The Ti 2p electron shake-up spectrum in TiCl 4 is calculated using SCF Xα SW method. Results showed that the low-energy satellite can be ascribed to ligand → metal charge-transfer transitions. However, the nature of the intense peak at 9.8 eV cannot be explained within the one-electron model. An anomaly is noted in the eiperimental spectra of the titanium halides.

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.

Experimental and theoretical shakeup studies. IV. Main group Me2M (M=Zn,Cd,Hg) compounds

We report the first observation of shakeup in the core level ESCA of main group metal compounds. By recording the C 1s and metal (Zn 2p, Cd 3d, Hg 4f) level of Me2M (M=Zn, Cd, and Hg) to high statistics, satellite structures on the higher binding energy side of the respective primary photolines are observed. The peaks are assigned to shakeup transitions by theoretical Xα–SW calculations. Good agreement for both energies and intensities is obtained for the metal core level spectra. The calculations yield C 1s intensities which are much smaller than those observed experimentally.

Electronic structure and ESCA shake-up of the UF6 molecule

Abstract Ionization from the inner shells of the UF 6 molecule is studied using quantum-chemical calculations. The non-relativistic and relativistic multiple scattering Xα methods and a charge-self-consistent version of a relativistic extended Huckel method (REX) are applied. At 4f ionization the effective 5f charge increases by ≈1.1 electrons. The change in valence orbital composition is of the right order of magnitude to explain the shake-up satellites at the 4f lines. The REX method correctly predicts one rather strong shake-up satellite at about the right energy separation.

High-resolution HeI and HeII photoelectron spectra of the zinc and cadmium dihalide valence bands

Abstract We have recorded high-resolution HeI and HeII photoelectron spectra of the valence bands of six MX2 compounds (M = Zn, Cd: X = Cl, Br, I) in the gas phase. Transition-state Xα SW calculations on all six compounds confirm the assignments. For the first time, we have clearly resolved the spin-orbit splitting of the IIg and IIu states in the bromide and iodide spectra: and calculated Πu splittings, using a simple MO LCAO model and the Dxh double group, are in very good agreement with the experimental values. The relative intensities of the photoelectron peaks change markedly from HeI to HeII spectra, and Xα SW cross section calculations on ZnCl2 predict these intensity changes rather well.

Theoretical calculation of shakeup intensities using Xα–SW wave functions

The ground and 1s core hole state molecular wave functions of CH4, NH3, H2O, and HF obtained from Xα–SW calculations using the touching spheres (TS) and overlapping spheres (OS) approximations are used to calculate the intensity of shakeup satellites observed in their ls core level photoelectron spectra. The sudden approximation was assumed in the calculation. In case of TS Xα–SW wave functions, the one electron overlap integral inside the intersphere was calculated via Green’s theorem. For OS Xα–SW wave functions, the integration over the awkwardly shaped intersphere region was circumvented by distributing the intersphere charge into the atomic spheres according to the charge partition scheme suggested by Case and Karplus. Our results show that there are no significant differences between the shakeup energies calculated from the TS and OS approximations. However, shakeup intensities calculated from TS Xα–SW wave functions are more reliable and in better numerical agreement with experiment.

Calculation of multiplet structures in shakeup spectra by the Xα method

The energy expression for calculating upper and lower states of shakeup lines in core line photoelectron spectra were derived. Using the SCF–Xα–SW approximation, we have obtained satisfactory results in molecules with strong hole–valence coupling. The results also suggested configuration interaction is important for molecules with low lying excited states.