T. X. Carroll

Carbon 1s photoelectron spectroscopy of CF4 and CO: Search for chemical effects on the carbon 1s hole-state lifetime

Carbon 1s photoelectron spectra for CF4 and CO have been measured at several photon energies near the carbon 1s threshold. The spectra have been analyzed in terms of the vibrational structure and the natural linewidth. For CO the vibrational structure shows evidence for anharmonicity in both the energy spacing and the intensity. Analysis of the results using an anharmonic model gives an equilibrium bond length for core-ionized CO that is 4.85 pm shorter than that of neutral CO. For CF4, the vibrational structure is very weak, and the analysis shows that the change in equilibrium CF bond length upon ionization is no more than 0.54 pm. Ab initio theoretical calculations give results in accord with these bond-length changes. The unusually small bond-length contraction in CF4 can be understood in terms of nonbonded fluorine–fluorine repulsion. The natural linewidth for core-ionized CO, 95±5 meV, is essentially the same as that of CH4. This result is in contrast with expectations based on the one-center model ...

Xenon N4, 500 Auger spectrum - a useful calibration source

Abstract In the xenon N 4,5 OO Auger spectrum there are 19 prominent lines ranging in kinetic energy from 8 to 36 eV that provide a convenient set of standards for calibrating electron spectrometers. Combining optical data with recent measurements of this spectrum gives energies for these lines that are absolutely accurate to 11 meV. For most lines the relative accuracy is better than 1 meV; for a few it is about 3 meV. The spin–orbit splitting of the xenon 4d lines is determined to be 1979.0±0.5 meV.

Additivity of substituent effects. Core-ionization energies and substituent effects in fluoromethylbenzenes.

Carbon 1s ionization energies have been measured for all of the carbon atoms in eight fluoromethylbenzenes. Enthalpies of protonation have been calculated for protonation at all of the ring carbons in the same molecules. These data together with previously reported data on fluorobenzenes and methylbenzenes provide the basis for studying the additivity of substituent effects and the correlation between enthalpies of protonation with core-ionization energies. Although a linear additivity model accounts reasonably well for both the ionization energies and the enthalpies of protonation, a better description, especially for the enthalpies, is obtained by inclusion of nonlinear terms that account for interactions between two substituents on the same molecule. There are families of nearly parallel correlation lines between enthalpies of protonation and core-ionization energies. The existence of several families can be primarily understood in terms of the linear additivity picture and more completely understood when the nonlinear terms are taken into account. The role of the methyl group as a polarizible pi-electron donor is contrasted with the role of fluorine, which is a substituent of low polarizibility that acts to withdraw electrons from the adjacent carbon and to donate electrons through resonance to the ring. The role of the hydrogen atoms as pi-electron acceptors in the protonated species is illustrated.

Carbon and oxygen kll and sulfur lmm auger spectra of ocs

Abstract Auger spectra of carbon 1s, oxygen 1s and sul fur 2p core-ionized carbonyl sulfide (OCS) are presented and discussed. In each case the Auger decay involves valence electrons, with de-excitation to final states of +2 charge. Because OCS contains three different atoms and the Auger spectrum of each can be measured it is possible to make a number of level assignments for the final states of the +2 ion. These assignments are based on comparisons of the three spectra with each other and with semi-empirical calculations. An three spectra show significant localization effects. The sulfur spectrum exhibits very narrow peaks and allows unambiguous determination of the energies for the ground and first two excited states that derive from the 3π 2 configuration. The energies and splittings of these and other states are compared with other experimental and theoretical findings.

Intensity oscillations in the carbon 1s ionization cross sections of 2-butyne

Carbon 1s photoelectron spectra for 2-butyne (CH3C≡CCH3) measured in the photon energy range from threshold to 150 eV above threshold show oscillations in the intensity ratio C2,3/C1,4. Similar oscillations have been seen in chloroethanes, where the effect has been attributed to EXAFS-type scattering from the substituent chlorine atoms. In 2-butyne, however, there is no high-Z atom to provide a scattering center and, hence, oscillations of the magnitude observed are surprising. The results have been analyzed in terms of two different theoretical models: a density-functional model with B-spline atom-centered functions to represent the continuum electrons and a multiple-scattering model using muffin-tin potentials to represent the scattering centers. Both methods give a reasonable description of the energy dependence of the intensity ratios.

Line shape and lifetime in argon 2p electron spectroscopy

Abstract The argon 2p photoelectron spectrum and the argon L3M23M23 1S0 Auger spectrum have been measured at several photon energies between 6 and 80 eV above the 2p3/2 threshold with an instrumental line width significantly smaller then the natural line width. The spectra are described well by the theory of van der Straten et al. [Z. Phys. D 8 (1988) 35] provided that allowance is made for the instrumental resolution and measurements are made at a sufficiently low pressure. The lifetime (Lorentzian) line width determined from these measurements for the core-ionized atom is 112±3 meV, in good agreement with the line width for the 2p3/2→4s core-excited state, 114±2 meV, indicating that the 4s electron has little influence on the Auger decay rate. Remeasurement of the line width for the carbon 1s hole in carbon dioxide gives values in good agreement with the previous measurement of 99 meV.

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...

New insights into molecular structure and dynamics using soft x-ray electron spectroscopy

The combination of high-resolution electron energy analyzers and 3rd generation synchrotron radiation sources opens up possibilities to study molecular inner-shell photo- and Auger electron emission in a new level of detail. Even weak molecular perturbations of the energy levels and angular emission patterns can be studied. In this report, some examples of such studies are given based on recent experiments at the Advanced Light Source, using a Scienta SES-200 electron spectrometer. Examples of carbon 1s photoemission of ethyne, sulphur 2p photoemission of carbonyl sulphide, and resonant Auger electron emission of carbon dioxide are presented.

High-resolution molecular inner-shell electron spectroscopies

High-resolution inner-shell photoelectron and resonant Auger electron spectroscopies are examined as probes of the local chemical environment of specific atoms within a molecule. The C 1s spectra of CH4 are reported at an experimental resolution better than the natural line width. The spectra were analyzed to extract the basic physical information contained in the line width, vibrational spacings, and vibrational intensities. Spectra of C2H2 were measured at a range of photon energies above the C 1s ionization threshold. The spectra were measured at the highest possible resolution to obtain intensity ratios for the symmetry split C 1s photoelectron lines. A definitive assignment of a shape resonance in the kσu photoionization channel was obtained from these results. Photoelectron spectra of ropyne, HC≡CCH3, were measured at high resolution and a definitive assignment of the three peaks was obtained from spectra of the model compounds ethane and ethyne (CH3CH3 and HC≡CH,) and theoretical calculations of th...