Showing papers by "John D. Bozek published in 1995"
TL;DR: The importance of using mass-resolved ion spectra to determine the {beta} parameters for molecules which undergo a change in geometry upon excitation of a core electron is highlighted by a comparison of these results with the symmetry-resolving ion yield spectra of CO{sub 2.
Abstract: Asymmetry parameter (\ensuremath{\beta}) values have been determined from the ${\mathrm{C}}^{+}$, ${\mathrm{O}}^{+}$, and ${\mathrm{CO}}^{+}$ signals in the time-of-flight (TOF) mass spectra and the ${\mathrm{CO}}^{+}$-${\mathrm{O}}^{+}$ signal in the photoion-photoion coincidence (PIPICO) spectra of ${\mathrm{CO}}_{2}$ excited and/or ionized with monochromatic synchrotron radiation around the C K and O K ionization edges using a rotatable time-of-flight mass spectrometer. Positive \ensuremath{\beta} values of 0.4 were found for the ${\mathrm{C}}^{+}$ ions at the C 1${\mathit{s}}^{\mathrm{\ensuremath{-}}1}$2${\mathrm{\ensuremath{\pi}}}_{\mathit{u}}^{\mathrm{*}}$ and O 1${\mathit{s}}^{\mathrm{\ensuremath{-}}1}$2${\mathrm{\ensuremath{\pi}}}_{\mathit{u}}^{\mathrm{*}}$ resonances, respectively, and negative values between -0.1 and -0.4 were obtained for the other TOF and PIPICO signals at the same photon energies. The difference between these values and the expected value of -1 results from bending the molecule in the excited state before or during its ionic fragmentation. An O-C-O bond angle of 124\ifmmode^\circ\else\textdegree\fi{} was found for ${\mathrm{CO}}_{2}$ in the C 1${\mathit{s}}^{\mathrm{\ensuremath{-}}1}$2${\mathrm{\ensuremath{\pi}}}_{\mathit{u}}^{\mathrm{*}}$ state from the \ensuremath{\beta} value for the ${\mathrm{O}}^{+}$-${\mathrm{CO}}^{+}$ PIPICO signal. The \ensuremath{\beta} values were close to the isotropic value of zero at all other photon energies for the ${\mathrm{C}}^{+}$ ions. \ensuremath{\beta} values for the ${\mathrm{O}}^{+}$ and ${\mathrm{CO}}^{+}$ signals in the TOF spectra and the ${\mathrm{O}}^{+}$-${\mathrm{CO}}^{+}$ signals in the PIPICO spectra were found to be similar.Slightly negative \ensuremath{\beta} values were found below the C K edge, rising to a maximum value of 0.9 at the ${\mathrm{\ensuremath{\sigma}}}_{\mathit{u}}^{\mathrm{*}}$ shape resonance at 312.2 eV. Strongly positive \ensuremath{\beta} values were found just below the O K edge and were assigned to a transition of an O 1s electron to an s-type Rydberg orbital coupled with a shape resonance in the ${\mathrm{\ensuremath{\sigma}}}_{\mathit{g}}$ ionization channel. Two maxima were observed in the \ensuremath{\beta} values above the O K edge and assigned to shape resonances in the ${\mathrm{\ensuremath{\sigma}}}_{\mathit{g}}$ and ${\mathrm{\ensuremath{\sigma}}}_{\mathit{u}}$ continuum channels. The lower energy resonance in the ${\mathrm{\ensuremath{\sigma}}}_{\mathit{g}}$ channel was found at 542.5 eV and the position of the higher energy shape resonance in the ${\mathrm{\ensuremath{\sigma}}}_{\mathit{u}}$ channel was found to be 560 eV. The importance of using mass-resolved ion spectra to determine the \ensuremath{\beta} parameters for molecules which undergo a change in geometry upon excitation of a core electron is highlighted by a comparison of these results with the symmetry-resolved ion yield spectra of ${\mathrm{CO}}_{2}$.
38 citations
TL;DR: In this paper, the yield ratios for ion production were determined from the TOF spectra over the energy range of 44 through 1200 eV, and the partial yield spectra for the individual ions were observed around the Cl L 2,3-edges, the C K-edge and the F K-Edge.
Abstract: Fragment ions produced from CF3Cl have been measured in the soft X-ray region using monochromatized synchrotron radiation and a time-of-flight mass spectrometer. The yield ratios for ion production were determined from the TOF spectra over the energy range of 44 through 1200 eV, and the partial yield spectra for the individual ions were observed around the Cl L2,3-edges, the C K-edge, and the F K-edge. The atomic chlorine ion, Cl+, has the greatest intensity between 100 and 700 eV, and exhibits a steep increase at the Cl L2,3-edges. The yield ratios of F+ and C+ decrease sharply at the Cl L2,3-edges and greatly increase at the C K-edge and F K-edge. Some fragment ions, e. g. CF+, have distinctive yield changes upon excitation of the inner-shell electron to unoccupied molecular orbitals.
14 citations
TL;DR: In this article, the time-of-flight mass spectra and selective photoion-photoion coincidence spectra of CF4 following photoabsorption in the C-edge and F-edge regions have been measured in detail using a monochromatized synchrotron radiation.
Abstract: Time-of-flight mass spectra and selective photoion-photoion coincidence spectra of CF4 following photoabsorption in the C K-edge and F K-edge regions have been measured in detail using a time-of-flight mass spectrometer and monochromatized synchrotron radiation. Kinetic energy distributions (KEDs) of the fragment ions and total KEDs of the fragment ion pairs produced from CF4 have been obtained from the spectra measured. Only CF22+ has been produced at very low kinetic energy. Whereas the total KEDs for ion pairs of C++F+, F++F+, F++CF+ and F++CF2+ do not change against the change in the photon energy, the KEDs for fragment ions change with the photon energy. The KEDs of some ions dissociated from CF4 in the C 1s to 5t2 excited state are much different from those through the F 1s to 5t2 excited state, which indicates that de-excitation processes from these excited states are different.
10 citations
TL;DR: In this article, the soft X-ray region at beamline 9.0.1 at the ALS has been investigated and the performance of the beamline has been shown to be very good.
Abstract: (1995). Ultra‐high resolution in the soft X‐Ray region at beamline 9.0.1 at the ALS. Synchrotron Radiation News: Vol. 8, No. 5, pp. 29-32.
8 citations
TL;DR: In this article, the branching ratios for ion-pair production were obtained from the measured coincidence signals at typical photon energies below and above the Cl L 2,3-edges, the C K-edge, and the F Kedge.
2 citations