Roger Stockbauer

Synchrotron radiation studies of H2O adsorption on TiO2(110)

Synchrotron radiation photoemission has been used to study the interaction of H2O with defective and nearly-perfect TiO2(110) surfaces at temperatures between 160 and 300 K. Ti3+ 3d defect sites are implicated in the adsorption process, and by tuning the photon energy to 47 eV we find that a resonant photoernission process gives an enhanced photoermission sensitivity to the 3d defect states. Defects are produced on TiO2(110) by annealing to 1000 K in UHV; subsequent exposure to 104 L O2 produces nearly perfect surfaces, based on the suppressed Ti3d emission. Both nearly perfect and defective surfaces give rise to dissociative adsorption of H2O at 300 K. The saturation coverages are near 0.1 ML, independent of the initial defect concentration; however, the rate of dissociative adsorption (sticking probability) is higher on defective surfaces. The enhanced sensitivity to the Ti3+ defect states has allowed the observation of a surprising effect; the dissociative adsorption of H2O results in increased defect state intensity on the nearly perfect surfaces. This apparent charge-transfer to the substrate implies that a new model for the dissociation process on oxide surfaces is needed. At 160 K H2O adsorbs molecularly on both the nearly-perfect and the defective surfaces. Subsequent annealing experiments allow estimates of the interaction energies involved in the dissociation process.

Electron emission and ion desorption spectroscopy of clean and oxidized Ti(0001)

The electronic structure of Ti(0001) has been investigated by energy loss spectroscopy (ELS), Auger electron spectroscopy (AES), ultraviolet photoemission spectroscopy (UPS) using synchrotron radiation and electron stimulated desorption (ESD). Resonant electron emission due to a direct recombination process involving an atomic 3p - 3d interaction has been observed. Surface oxidation results in the formation of a thin protective Ti02 layer which is stable to 250 deg C. In the oxide, the direct recombination process following 3p excitation gives rise to resonantly enhanced emission from the oxide valence end. The cross section for electron and photon stimulated O+ desorption is shown to be dominated by these atomic resonance effects as well.

A threshold photoelectron—photoion coincidence mass spectrometer for measuring ion kinetic energy release on fragmentation

Abstract A threshold photoelectron—photoion coincidence mass spectrometer has been constructed to measure ion fragmentation and kinetic energy release on fragmentation as a function of the internal energy of the parent ion. The threshold photoelectron analysis uses a drift tube analyzer followed by a 127° cylindrical plate analyzer for a nominal resolution of 20 meV. The ion mass and kinetic energy analysis uses a pulsed time-of-flight analyzer operated in a space focused mode. This insures that the spread in the flight time of ions of given mass is due primarily to the initial kinetic energy of the ion. The mass resolution is ca. 1 in 20. To illustrate the potential of this instrument, results are presented for kinetic energy release as a function of internal energy for the fragmentation of acetone and methane ions.

Photoelectron–photoion coincidence study of the bromobenzene ion

The technique of variable time photoelectron–photoion coincidence mass spectrometry has been applied to the fragmentation of bromobenzene ion producing a phenyl ion. A detailed analysis of the variation of the breakdown curve with parent ion residence time was performed. The results lead to ΔH °f0 (phenyl ion)=270 kcal/mole in close agreement with recalculated results from an earlier study on chlorobenzene. This, combined with other photoionization results leads to ΔH °f0 (phenyl radical)=83±3 kcal/mole, slightly higher than the value 80.9±2 kcal/mole obtained from neutral kinetics. The analysis leads to a rate‐energy dependence for the fragmentation process and an equivalent 1000 K Arrhenius pre‐exponential factor of ∼9.4×1014 sec−1, which may be compared to the value 2×1015 sec−1 for the analogous neutral process. The possible contribution of spin orbit splitting is discussed.

Kinetic shift in chlorobenzene ion fragmentation and the heat of formation of the phenyl ion

The fragmentation of chlorobenzene ion has been studied by photoelectron–photoion coincidence techniques. By varying the residence time it is possible to obtain breakdown curves as a function of residence time. The parent–daughter transition region shifts to lower energies as the residence time is increased (kinetic shift). The shift is of the order of 0.4 eV in going from 0.7 to 8.9 μs. A systematic analysis of the breakdown curves and residence time effects has been carried out using quasiequilibrium theory. The experimental results and analysis lead to ΔHf00  (phenyl ion)  =275±1 kcal/mol (1151±4 kJ/mol. The systematic analysis shows that this experiment leads to a quite accurate rate‐energy curve in the range of 104–106 s−1. The sensitivity of the QET model has been studied, and the limitations to the determination of activated complex parameters is critically discussed. The parameters obtained in this work are rather similar to those of an analogous neutral process, i.e., thermal decomposition of bro...

Benchmark measurement of iodobenzene ion fragmentation rates

Abstract The unimolecular fragmentation rate of iodobenzene ion has been studied by variable residence time photoelectronphotoion coincidence techniques. The techniques employed variable wavelength with threshold photoelectron detection and fixed (58.4 nm) wavelength with variable energy photoelectron detection, respectively. Residence times of 1.0 = 0.25 or 5.9 ± 0.3 and 21 ± 1 or 57 ± 1 μs were employed. The four sets of measurements were independently analyzed using exact counting of harmonic oscillator states, taking into account the appropriate (and different) apparatus functions and the thermal energy distributions of the parent ions. The resulting rate-energy dependences and fragmentation threshold values were in excellent agreement with one another. The best-fit rate-energy dependence is proposed as a benchmark for calibration of future rate-energy measurements. The resulting Δ H f D 0 (C 6 H 5 + ) = 1133 ± 5 kJ mol is in excellent agreement with earlier results based on a somewhat simpler method of analysis of chlorobenzene and bromobenzene fragmentation rates. Some remaining uncertainties regarding the transition-state model are discussed.

Photoionization and threshold photoelectron—photoion coincidence study of propyne from onset to 20 eV

Abstract The photoionization efficiency curves for allene and its fragments C3H3+, C3H2+ and C3H+ are given from threshold to 20 eV. The threshold photoelectron spectra and breakdown curves are given over the same energy span. The adiabatic ionization potential is determined to be 9.69 ±0.01 eV and the heats of formation of the fragments are derived from the appearance potentials. Autoionization is found to be an important factor in the parent ionization and fragmentation.

Unimolecular kinetics of pyridine ion fragmentation

The fragmentation of the pyridine ion to form C4H4+ and HCN has been studied by means of photoelectron—photoion coincidence mass spectrometry with variable ion-source residence time. A detailed analysis of the time-dependent breakdown curves leads to a T = 0 K fragmentation threshold of 12.15 ± 0.02 eV, ΔHf00(C4H4+) = 1195 ± 12 kJ mol−1, somewhat higher than the value reported by Eland et al. The fragmentation process requires a tight transition state, in agreement with earlier conclusions of Eland et al. There is some disagreement with earlier work on the energy-dependence of the fragmentation rate. It is suggested that this may be due to distortion of the thermal vibrational population distribution of the ion after vertical ionization.

Wavelength and vibrational‐state dependence of photoelectron angular distributions. Resonance effects in 5 σ photoionization of CO

Photoelectron angular distribution were studied for the photoionization of CO for v=0−3 in the range 16–26 eV. (AIP)

A synchrotron radiation study of BaO films on W(001) and their interaction with H2O, CO2, and O2

The interaction of O2, CO2, and H2O with bulk BaO and BaO adlayers adsorbed on W(001) has been examined using ultraviolet photoelectron spectroscopy. H2O reacts with bulk BaO to form Ba(OH)2, while CO2 forms a surface layer of BaCO3. Water and carbon dioxide also react with a (√2×2)R45–BaO monolayer adsorbed on W(001) to produce adsorbed OH and CO3 species bound to the tungsten substrate. The interaction of O2 with W(001) is enhanced by the presence of a BaO monolayer on the substrate. The observations are compared with the results of previous studies.