The spectroscopy and metastability of the carbon dioxide doubly charged ion, the CO(2) (2+) dication, have been studied with photoionization experiments: time-of-flight photoelectron photoelectron coincidence (TOF-PEPECO), threshold photoelectrons coincidence (TPEsCO), and threshold photoelectrons and ion coincidence (TPEsCO ion coincidence) spectroscopies. Vibrational structure is observed in TOF-PEPECO and TPEsCO spectra of the ground and first two excited states. The vibrational structure is dominated by the symmetric stretch except in the TPEsCO spectrum of the ground state where an antisymmetric stretch progression is observed. All three vibrational frequencies are deduced for the ground state and symmetric stretch and bending frequencies are deduced for the first two excited states. Some vibrational structure of higher electronic states is also observed. The threshold for double ionization of carbon dioxide is reported as 37.340+/-0.010 eV. The fragmentation of energy selected CO(2) (2+) ions has been investigated with TPEsCO ion coincidence spectroscopy. A band of metastable states from approximately 38.7 to approximately 41 eV above the ground state of neutral CO(2) has been observed in the experimental time window of approximately 0.1-2.3 mus with a tendency towards shorter lifetimes at higher energies. It is proposed that the metastability is due to slow spin forbidden conversion from bound excited singlet states to unbound continuum states of the triplet ground state. Another result of this investigation is the observation of CO(+)+O(+) formation in indirect dissociative double photoionization below the threshold for formation of CO(2) (2+). The threshold for CO(+)+O(+) formation is found to be 35.56+/-0.10 eV or lower, which is more than 2 eV lower than previous measurements.
A new spectrometer, electron radical interaction chamber, has been developed to study dissociative electron attachment to unstable molecules such as free radicals. It includes a trochoidal electron monochromator and a time-of-flight mass spectrometer. Radicals are generated with a microwave discharge at 2.45 GHz. Preliminary data are presented for radicals formed when a mixture of helium and sulphur dioxide was passed through the microwave discharge. Several new resonances are observed with the discharge on. Resonances at 0 eV (S−), 0.8, 1.2, 3.0 eV (SO−) and 3.7 eV (SO− and S2O−) are assigned to the radical S2O2 and a resonance at 1.6 eV (S−) is assigned to S2O. No new resonances have been assigned to SO, which was also generated in the microwave discharge.
Threshold photoelectron measurements of the ionic 'satellite' states of krypton and xenon have been undertaken at the Daresbury Synchrotron Radiation Source (SRS) using the fieldpenetration technique. The photon energy ranges covered were 27.5-32.3 eV (Kr) and 23.3-34.8 eV (Xe) with a measured energy resolution of 13 meV, which is an order of magnitude improvement over the previous measurements. The ionic states have been catalogued and where resonance enhancement has been observed, the corresponding doubly excited neutral states have also been identified.
The removal of false coincidences from measurements of coincidences
between two photoelectrons and one or two ions formed in molecular
double photoionization is described. False coincidences arise by several
mechanisms; experimental procedures and mathematical formulae required to
remove all the different false coincidence contributions are described.
Sample spectra taken of the double photoionization of carbon dioxide
are presented to illustrate the method of false coincidence subtraction.
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