C. I. Ma

Single-event, energy-resolved, Auger--electron, multiple-ion coincidence mass spectroscopy

An experimental method is described that is directed at monitoring the creation and decay of core hole states in molecules from state‐to‐state. Data in the form of a coincidence map are presented and analyzed for the case of nitrous oxide.

Angular dependence of photofragments from K-shell excited N2 and O2 : photoabsorption anisotropy and excited state symmetry

The symmetry of excited states reached by photoexcitation of K‐shell electrons of N2 and O2 is identified from the fragmentation anisotropy. For each molecule, ion yield excitation spectra were obtained at 90°, 54.7°, and 0° relative to the axis of soft x‐ray polarization. The degree of polarization of the synchrotron radiation was determined experimentally. From this determination and the angular dependence of the ion yield, the photoabsorption anisotropy parameter β is obtained as a function of photon energy. Variations in the β spectra are discussed in terms of previously assigned spectral features and are compared with other experimental results. A comparison also is made with theoretical calculations, which are available for the case of N2.

Core‐hole excited states of H2O: Symmetries and relative oscillator strengths

Tunable and polarized soft x‐ray radiation was used to excite core electrons of H2O. Angle‐resolved proton yields were measured by retarding potential analyzers at 0° and 90° with respect to the polarization vector of the radiation. The ability of angle‐resolved detection to reveal hidden resonances in rich molecular spectra is demonstrated, and the anisotropy parameter obtained from the two ion yields identifies the symmetries and the relative oscillator strengths of the core‐hole excited states in the near‐edge region of the spectrum. The analysis of the data substantiates the validity of the axial recoil approximation and provides evidence for ultrafast dissociation. Comparisons with the results of theoretical calculations are made.

Evidence for valence hole localization in the Auger decay and fragmentation of carbon and silicon tetrafluorides

Monochromatic synchrotron radiation was used to excite selectively core electrons of the carbon and fluorine atoms in carbon tetrafluoride and silicon and fluorine in silicon tetrafluoride. The fragmentation processes were examined using time‐of‐flight mass spectroscopy. The mass spectra show the distribution of ions collected in coincidence with low and high energy electrons. Distinct changes in the mass spectra with atomic site of excitation and photon energy are observed. The observation of F2+ ions following fluorine 1s excitation in SiF4 provides significant evidence for a ‘‘valence bond depopulation’’ mechanism involving the formation of a localized, two‐hole final state that persists on the time scale of fragmentation. In contrast, no F2+ was observed for CF4, which indicates that fragmentation for this molecule is more characteristic of a delocalized two‐hole state.

Identification of the core hole excited states of N2: A failure of the equivalent core approximation

Monochromatic soft x‐rays were used to excite the nitrogen molecule at resonances near the K‐ionization edge. The absorption anisotropy parameter β was determined from the simultaneous measurement of the photofragment flux emanating from the x‐ray/molecular‐beam interaction region along two axes, one parallel and the other perpendicular to the electric vector of the polarized synchrotron radiation. The value of the anisotropy parameter identifies the symmetry species of a resonance and allows the core‐hole excited states to be assigned from experimental data. These data force a new assignment of Rydberg states and reveal a failure of the equivalent core approximation.

EXCITATION AND DECAY SPECTRA OF CORE-EXCITED RESONANCES IN ACETONE

Monochromatic synchrotron radiation was used to excite selectively core electrons of the carbon and oxygen atoms in acetone, creating neutral core-excited resonance states. Electron energy spectra associated with Auger decay of three strong resonances were collected. The character of each resonance state was deduced through analysis of its decay spectrum.

Electronic decay of core hole excited states in molecular oxygen

Electron energy spectra characterizing the relaxation of molecular oxygen from different core hole excited states are reported. By using monochromatic synchrotron radiation, spectra are obtained with excitation at the π* and σ* resonances and at various energies above the ionization threshold. The spectra obtained with excitation above threshold are very similar and result from Auger decay. The spectra obtained at the σ* resonance result from spectator autoionization, and those for the π* resonance result from both spectator and participator autoionization. The σ* resonance is broadened by spectral congestion, and the autoionization spectra indicate the presence of three components. Configurations and states produced by the electronic decay are identified, and a correlation between spectator screening and term values is documented.

Dynamical behavior of electronic states of NH32+ prepared from N1s ionization of ammonia

Abstract Several electronic states of NH 3 2+ were prepared by Auger decay following core electron ionization of NH 3 . The ionic products from these states were observed by single-event energy-resolved Auger electron multiple-ion coincidence time-of-flight mass spectrometry. The dependence of the fragmentation channel branching ratios on the electronic state of the dication correlates with the energy threshold for each channel. In general, the kinetic energy released in fragmentation depends only weakly on the electronic state producing the fragments.

Examination of the pathways in the soft X-ray (530.8 eV) induced fragmentation of O2 by electron-ion coincidence measurements

Abstract Monochromatic synchrotron radiation is used to excite the K-shell electrons of oxygen at the π*-resonance. Electronic decay and the concomitant fragmentation are examined using coincident electron energy analysis and time-of-flight mass spectrometry. By the means, the production of specific ionic products, observed in time-of-flight mass spectra, are correlated to specific electronic decay channels, identified in the electron energy spectra. It thereby is possible to relate the dissociation products, their kinetic energies, and their electronic configurations to the electronic configuration of the parent molecular ion produced by electronic decay of the core-hole excited state. Such data provide information regarding the energies of electronic states and the stability and potential energy surfaces of singly and multiply charged molecular ions.

Auger decay and autoionization of core hole excited states in molecular oxygen

Abstract Electron energy spectra characterizing the relaxation of molecular oxygen from different core hole excited states are reported. Spectra are obtained following excitation by monochromatic synchrotron radiation at the π * and σ * resonances and at various energies above the ionization threshold. The spectra obtained with excitation above threshold are very similar and result from Auger decay. The spectra obtained at the σ * resonance result from spectator autoionization and indicate the presence of three unresolved components. The spectra obtained at the π * resonance result from both spectator and participator autoionization.