31 research outputs found
Threshold photoelectron photoion coincidence spectroscopy of trichloroethene and tetrachloroethene
Get PDFThe threshold photoelectron, the threshold photoelectron photoion coincidence and ion breakdown spectra of trichloroethene and tetrachloroethene have been recorded from 9 – 22 eV. Comparisons with the equivalent data for the three dichloroethene molecules and theoretical calculations highlight the nature of the orbitals involved during photoionisation in this energy range. The ground electronic state of CHCl (CCl) is bound, with excited valence states dissociating to CHCl (CCl) and CHCl (CCl). Appearance energies suggest that CHCl forms from CHCl by loss of two chlorine atoms, whereas CCl forms from CCl by loss of a Cl molecule. The translational kinetic energy release into CHCl (CCl) + Cl is determined as a function of energy. In both cases, the fraction of the available energy released into translational energy of the two products decreases as the photon energy increases
Vacuum-UV negative photoion spectroscopy of CH4
Get PDFUsing synchrotron radiation in the range 12-35 eV, negative ions are detected by mass spectrometry following vacuum-UV photoexcitation of methane. Ion yields for H, CH and CH are recorded, the spectra of CH and CH for the first time. All ions display a linear dependence of signal with pressure, showing that they arise from unimolecular ion-pair dissociation. Cross sections for ion-pair formation are put onto an absolute scale by calibrating the signal strengths with those of F from SF and CF. Following normalisation to total vacuum-UV absorption cross sections, quantum yields for anion production are reported. There is a major discrepancy in the H cross section with an earlier measurement, which remains unresolved. The anions arise from both direct and indirect ion-pair mechanisms. For a generic polyatomic molecule AB, the former is defined as AB A + B (+ neutrals), the latter as the predissociative crossing of an initially-excited Rydberg state of AB by an ion-pair state. In a separate experiment, the threshold photoelectron spectrum of the second valence band of CH, ionisation to CH Aï€ A at 22.4 eV, is recorded with an instrumental resolution of 0.004 eV; many of the Rydberg states observed in indirect ion-pair formation converge to this state. The widths of the peaks are lifetime limited, increasing with increasing in the (a) vibrational ladder. They are the first direct measurement of an upper value to the dissociation rate of these levels into fragment ions
The photoionization dynamics of the three structural isomers of dichloroethene
Get PDFUsing tunable vacuum-UV radiation from a synchrotron, the threshold photoelectron spectrum, threshold photoelectron photoion coincidence spectrum and ion breakdown diagram of the 1,1, cis-1,2 and trans-1,2 isomers of CHCl have been recorded in the range 9-23 eV. The energies of the peaks in the threshold photoelectron spectrum are in good agreement with outer-valence Greens function caculations. The major difference between the isomers, both predicted and observed experimentally is that the Fï€ and Gï€ states of CHCl are approximately degenerate for 1,1 and trans-1,2, but well separated for the cis-1,2 isomer. The ground and low-lying valence states of CHCl are bound, with higher-lying states dissociating to CHCl or CH. The translational kinetic energy release into CHCl + Cl is determined as a function of energy. Isolated-state behaviour for the low-lying electronic states of CHCl becomes more statistical as the energy increases
Proton-induced reactions of [2.2]-paracyclophane in the gas phase. A study by FT-ICR-spectrometry and DFT calculation
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