Fragmentation of Valence and Core–Shell (Cl 2p) Excited C₂Cl₄ Molecule

Abstract

The dynamics of the photofragmentation pathways of tetrachloroethylene with photon energies from 15 up to 250 eV encompassing the Cl 2p edge is presented. In order to distinguish the fragmentation channels, the ionic fragments were separated according to their mass-to-charge ratio, measured in coincidence with the photoelectrons, and collected as a function of the incident photon energy. Distinct minima or maxima are found in the partial ion yield in the region between 40 and 50 eV. These features are believed to be associated with the Cooper minimum which results from a molecular orbital with a strong atomic 3p subshell character. In the shallow core region, some fragmentation patterns are considered in terms of fast fragmentation of the CCl molecule, despite the heavy mass of its fragments. In the present case, the fast fragmentation is favored by the very strong antibonding character of the LUMO, understandable in the frame of the core equivalent model for halogen-containing molecules. In addition, ab initio calculations were performed to obtain states at the Cl 2p edge. Singlet and triplet states at the Cl 2p edge of the CCl molecule, corresponding to the Cl(2p → 9b*) and Cl(2p → 8b*) transitions, were calculated in order to form a basis set of molecular states from which the spin-orbit splitting can be inferred. Multiconfigurational self-consistent field (MCSCF) calculation followed by multireference configuration interaction (MRCI) was the method chosen to establish a set of singlet and triplet states at the 2p excitation edge in addition to the ground state.

Fragmentation of Valence and Carbon Core Excited and Ionized CH2FCF3 Molecule

“…Nearby the C 1s edge, the branching ratios of the C 2 F 3 H 2 + ion, corresponding to the loss of a F atom, presents a peculiar photon energy dependence and is the most intense fragment (∼15−22%) followed by the CFH The energy values were shifted by +4.5 eV to compare with the experimental values. In order to interpret the yields of the measured fragments presented in Figure 7, we performed a series of quantum chemistry calculations based on the inner-shell multiconfigurational self-consistent field (IS-MCSCF) protocol 18 where the active space was formed by the carbon 1s orbital plus the five highest occupied molecular orbitals and the two lower unoccupied molecular orbitals, which consist of an active space formed by eight electrons and six active orbitals (IS-MCSCF (8,6)). The coordinates of the relaxed system calculated in the first excited state of the C(H 2 F) 1s carbon is presented in Figure 8.…”

“…Direct outer or inner-valence photoionization or core-level excitation of molecules constitutes alluring means for the investigation of the fragmentation dynamics. With respect to core-level investigation, the tunability of the photon energy enables one to excite specific resonances neighboring a selected inner-shell edge, supporting selective fragmentation. − In molecules, inner shells are strongly localized on a particular atomic component. − This is the reason why they are usually characterized by adopting atomic labels. The chemical environment of the core vacancies might influence the mass spectra of the photofragments, a phenomenon that is known as site-selective or site-specific photochemistry.…”

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“…Thus, stratospheric ozone models have been refined and the paramount role of chlorine containing has been evaluated. Santos, et al [12] described the valence and Cl 2p excitation and…”

“…Partial ion yield of the Cl 2+ dication after fragmentation of the C 2 Cl 4 molecule as a function of the photon energy[12].…”