T. D. Thomas

Near threshold excitation of KVV Auger spectra in carbon monoxide using electron-electron coincidence spectroscopy

Electron–electron coincidence spectroscopy has been used to separate KVV Auger spectra in CO into several component spectra, each arising from different core‐excited initial states. Results are presented for the Auger decay of ions in which either a carbon 1s or oxygen 1s electron has been ionized and for the decay of neutral molecules in which either a carbon 1s or oxygen 1s electron has been excited to the vacant 2π orbital. The spectra from the neutral molecules have been studied and analyzed in some detail. These autoionization spectra can be broken into two parts. The highest kinetic energy part where the 2π electron participates in the decay is easily understood; in this case, the Auger transitions lead to well‐known one‐hole states of CO+. The lower‐energy part arises from deexcitation with the 2π electron remaining as a spectator. This part of the spectrum is similar to the ‘‘normal’’ Auger spectrum shifted approximately 10 eV by the Coulomb interaction with the spectator electron. The similarity ...

Core‐level electron–electron coincidence spectroscopy

Two electron‐spectrometer systems designed for electron–electron coincidence spectroscopy are described. One, based on two hemispherical analyzers and x‐ray excitation, is especially suited for Auger‐photoelectron coincidence spectroscopy (APECS) of solids and surfaces. The other, using a cylindrical mirror analyzer, a hemispherical analyzer, and electron‐beam excitation is designed for near‐dipole (e, 2e) spectroscopy of gaseous samples. Typical results obtained with these instruments are presented.

Satellite structure in the x‐ray photoelectron spectra of gaseous furan, pyrrole, and thiophen

The intensities and energies of satellite peaks in the x‐ray photoelectron spectra of gaseous furan, pyrrole, and thiophen have been measured. Because contributions owing to inelastic scattering of the electrons can be minimized in the gas phase, these investigations give a clearer picture of the satellite structure than do previously reported measurements in the solid phase. CNDO/2 wavefunctions have been used to predict both energies and intensities of the satellite peaks. Although there is qualitative agreement between theory and experiment, quantitative agreement is lacking. The theoretical calculations show that low‐energy satellites are those involving transitions from occupied to unoccupied π orbitals. The calculated wavefunctions provide information on charge rearrangement during photoionization. In particular, it is found that not only is there transfer of charge from the heteroatom to the carbons during shake up (expected from properties of the neutral molecule), but also there is charge transfe...

Lifetime‐vibrational interference in the autoionization of core‐excited O2

The electron–electron coincidence technique has been used to measure the spectrum of autoionizing (Auger) electrons that are emitted following excitation of a core (1s) electron in O2 to the half‐filled 1πg orbital. The highest kinetic energy peak in the deexcitation spectrum corresponds to a transition to the ground state of O+2(X,2Πg). The energy, width, and shape of the observed peak cannot be described by pure Franck–Condon vibrational calculations. Because the lifetime for deexcitation is comparable to a vibrational period, interference between vibrational levels in the core‐excited state must be taken into account in calculating the transition profile. When this is done the agreement between observed and calculated line shapes is excellent.

Kvv Auger Spectrum Of F2 - The Importance Of Hole-Hole Correlation

The KVV Auger spectrum of F2 has been measured and analyzed in terms of theoretical models of different levels of sophistication. In contrast to the corresponding spectra of many small molecules, this spectrum cannot be described in terms of an independent‐particle model. The approach suggested by Thomas and Weightman, which allows in an approximate way for hole–hole interaction, gives noticeably improved results. A more accurate, configuration‐interaction treatment developed by Jennison gives reasonable agreement with experiment. Peak assignments based on comparison of this theory with the experimental spectrum have been made. This spectrum provides a very clear case where hole–hole correlation effects are important. The experimental spectrum is unusual in that the lines are generally narrow, in contrast to many KVV spectra, which often have quite broad lines. The ionization potentials for the 2σg and 2σu orbitals have been measured. The relative cross sections for ionization from these orbitals are equa...

Extra‐atomic relaxation in HCl, ClF, and Cl2 from x‐ray photoelectron spectroscopy

Application of the Manne–Aberg sum rule to the full photoelectron spectrum of core‐ionized HCl, ClF, and Cl2 gives relative extra‐atomic relaxation energy for these molecules. The values obtained are in excellent agreement with those determined from the Auger parameter. These measurements together with the core‐ionization energies confirm that ClF is Clδ+Fδ− rather than Clδ−Fδ+, as had been suggested by measurements of the molecular Zeeman effect.

Valence photoelectron spectroscopy of N2 and CO: recoil-induced rotational excitation, relative intensities, and atomic orbital composition of molecular orbitals.

Recoil-induced rotational excitation accompanying photoionization has been measured for the X, A, and B states of N(2)(+) and CO(+) over a range of photon energies from 60 to 900 eV. The mean recoil excitation increases linearly with the kinetic energy of the photoelectron, with slopes ranging from 0.73×10(-5) to 1.40×10(-5). These slopes are generally (but not completely) in accord with a simple model that treats the electrons as if they were emitted from isolated atoms. This treatment takes into account the atom from which the electron is emitted, the molecular-frame angular distribution of the electron, and the dependence of the photoelectron cross section on photon energy, on atomic identity, and on the type of atomic orbital from which the electron is ejected. These measurements thus provide a tool for investigating the atomic orbital composition of the molecular orbitals. Additional insight into this composition is obtained from the relative intensities of the various photolines in the spectrum and their variation with photon energy. Although there are some discrepancies between the predictions of the model and the observations, many of these can be understood qualitatively from a comparison of atomic and molecular wavefunctions. A quantum-mechanical treatment of recoil-induced excitation predicts an oscillatory variation with photon energy of the excitation. However, the predicted oscillations are small compared with the uncertainties in the data, and, as a result, the currently available results cannot provide confirmation of the quantum-mechanical theory.

Deexcitation electron spectroscopy of core‐excited NO as a function of excitation energy

Deexcitation electron spectra of core‐excited NO have been measured at several excitation energies in the N 1s→2π and O 1s→2π resonances. The nitrogen spectra exhibit significant variation with excitation energy; the oxygen spectra vary only slightly. Sensitivity to excitation energy occurs because each resonance represents the overlap of three transitions to 2Σ+, 2Δ, and 2Σ− states, and each of these excited states decays to a unique set of levels in the final‐state ion. We have analyzed all spectra by taking into account excitation energy, lifetime‐vibrational interference, and the ordering and splitting of the core‐excited levels. Good agreement between calculated line shapes and experiment occurs if it assumed that the level ordering is 2Δ, 2Σ−, 2Σ+ for core‐excited nitrogen and 2Σ−, 2Δ, 2Σ+ for core‐excited oxygen. Photoexcitation data for oxygen have been analyzed to determine the energies of these states 531.7, 532.7, and 533.7 eV. The deexcitation spectrum from the 2Δ state of nitrogen core‐excite...