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The molecular-frame photoelectron angular distributions (MFPADs) in O 1s photoemission from CO2 molecule were measured. Patterns due to photoelectron diffractions were observed in the MFPADs. The polarization-averaged MFPADs were compared with theoretical calculation and were found to be useful in determining the molecular bond-length, which is a component to determine molecular structures.

The cross sections for the formation of the H(2p) and H(2s) atoms, σ2p and σ2s, respectively, in photoexcitation of C2H2 were obtained in an absolute scale for studying formation and decay of superexcited states in the extreme ultraviolet range. Several superexcited states of C2H2 including multiply excited states were found in the curve of the σ2p cross sections as a function of the incident photon energy. The same states seem to contribute to the variation in the σ2s cross sections as well, which can be ascribed to the non-adiabatic transitions between the 2p and 2s channels. The Σ/Π symmetry-resolved cross sections for the H(2s) atom formation, σ 2 s Σ and σ 2 s Π , were also obtained on an absolute scale. The coupling between the Σ u + 1 and 1Πu states was found to be small. [ABSTRACT FROM AUTHOR]

Site-specific fragmentation caused by Si:1s and 2p core-level photoionizations in F3SiCH2CH2Si(CH3)3 vapor was studied by energy-selected-photoelectron photoion-photoion triple-coincidence spectroscopy. The difference between the chemical shifts of the two Si sites is larger for the 1s ionization than for the 2p (2s) ionization. The fragmentation caused by the Si:1s ionization is more violent than that caused by the Si:2p ionization. The ions and ion pairs showing high site specificity for the Si:1s ionization belong to small fragments compared to those in the Si:2p ionization. Criteria for high site-specificity in fragmentation are discussed in conjunction with the present results. [ABSTRACT FROM AUTHOR]

Inner-shell excitation spectra and fragmentation of small clusters of formic acid have been studied in the oxygen K-edge region by time-of-flight fragment mass spectroscopy. In addition to several fragment cations smaller than the parent molecule, we have identified the production of HCOOH·H+ and H3O+ cations characteristic of proton transfer reactions within the clusters. Cluster-specific excitation spectra have been generated by monitoring the partial ion yields of the product cations. Resonance transitions of O1s(C==O/OH) electrons into πCO* orbital in the preedge region were found to shift in energy upon clusterization. A blueshift of the O1s(C==O)→πCO* transition by ∼0.2 eV and a redshift of the O1s(OH)→πCO* by ∼0.6 eV were observed, indicative of strong hydrogen-bond formation within the clusters. The results have been compared with a recent theoretical calculation, which supports the conclusion that the formic-acid clusters consist of the most stable cyclic dimer and/or trimer units. Specifically labeled formic acid-d, HCOOD, was also used to examine the core-excited fragmentation mechanisms. These deuterium-labeled experiments showed that HDO+ was formed via site-specific migration of a formyl hydrogen within an individual molecule, and that HD2O+ was produced via the subsequent transfer of a deuterium atom from the hydroxyl group of a nearest-neighbor molecule within a cationic cluster. Deuteron (proton) transfer from the hydroxyl site of a hydrogen-bond partner was also found to take place, producing deuteronated HCOOD·D+ (protonated HCOOH·H+) cations within the clusters. [ABSTRACT FROM AUTHOR]

Total kinetic energy released in dissociation from inner-shell excited states of O2 induced by soft x-ray irradiation has been measured using a time-of-flight mass spectrometer operated in a photoion–photoion coincidence mode. The distribution of the total kinetic energy released in the dissociation into O++O+ at a photon energy above the 1s ionization threshold shows three peaks at about 6, 11, and 15 eV and that at an energy of the 1s→2pπg transition has a broad maximum at 11 eV. The total kinetic energy distribution of O++O2+ shows a broad maximum at about 20 eV in all the photon energies. The dissociation pathways are discussed in detail using the kinetic energy distributions obtained. [ABSTRACT FROM AUTHOR]

Fragment ions from N2 in the states of (K)-1(nl)1 have been measured at 54.7° with respect to the polarization direction of the incident photon by using a time-of-flight (TOF) mass spectrometer. Simulation fitting calculation to the measured TOF spectra gave distributions of the kinetic energy for these ions. Ion yield spectra observed showed that no singly charged molecular ion was produced from the states of (K)-1(nl)1 in contrast to that of (K)-1(1πg)1. These results are discussed in connection to the dissociation pathway of N2 in these states. [ABSTRACT FROM AUTHOR]

Dissociation of O2 stimulated by soft x-ray absorption has been studied by using a monochromatized synchrotron radiation and a time-of-flight mass spectrometer. The parent molecular ion O+2 was formed only at 531 eV (excitation of 1s→1πg), and a fragment ion O+ had the highest intensity in all the energies giving core–hole states. The measured time-of-flight spectra were reproduced by a simulation calculation, which provided kinetic energy distributions of O+ and O++. The dissociation pathways from the core–hole states of O2 were discussed using the obtained kinetic energy distribution and ion intensity ratios as well as Auger electron spectra in the literature. [ABSTRACT FROM AUTHOR]

Asymmetry parameters (β) for the fragment ions of N2O following excitation or ionization of the N and O K shell electrons have been determined from photoelectron–photoion coincidence (PEPICO) and photoion–photoion coincidence (PIPICO) spectra measured using monochromatized synchrotron radiation and a rotatable time-of-flight mass spectrometer. Profiles of the β parameters from the N+ ions with low kinetic energies (K.E. <4.5 eV) in the PEPICO spectra were found to be very isotropic (close to 0) around both the N and O K edges. β parameter profiles for the O+ ions with contributions from N+ ions with K.E. >=4.5 eV and N2+ ions overlapped with NO+ ions show structure with negative values at the π* resonances below the N K-edges and positive values at the unresolved Rydberg resonances below the terminal N K edge and O K edge and at the shape resonance above the N K edges. β parameters from the N+–NO+ and N2+–O+ PIPICO signals were found to be considerably more anisotropic than the corresponding values from the PEPICO signals at the π* resonances below the N K edges and the shape resonance above the ionization edges. β parameters from both the PEPICO and PIPICO signals were close to zero at the π* resonance below the O K edge. The higher degree of anisotropy of ionic fragmentation observed in the PIPICO signals highlights the importance of measuring the fragmentation spectra of polyatomic molecules in coincidence in order to obtain accurate β parameters. [ABSTRACT FROM AUTHOR]

Asymmetry parameter values have been determined from the C+ and O+ signals in the time of flight mass spectra of CO 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. Asymmetry parameter values of about -0.8 were obtained from the C+ and O+ ions at the positions of both the C1s-12π* and O1s-12π* resonances below the core edges. Positive values of about 0.5 were obtained at the positions of the σ shape resonances above the C and O K edges. The results are consistent with previous assignments of the symmetries of the final state orbitals for the pre-edge resonances and the symmetries of the outgoing channels which exhibit shape resonances above the C K and O K ionization edges. The results are also consistent with the recently reported symmetry resolved photoionization spectra of CO. Above the ionization edges, the experimental data shows good agreement with the molecular asymmetry parameter values calculated by the relaxed-core Hartree–Fock method. Effects of the relatively intense shake-up transitions accompanying C1s ionization on the asymmetry parameter values are considered. [ABSTRACT FROM AUTHOR]