Threshold photoelectrons coincidence

Similar values have been obtained for AHffMesSi ) from two independent studies. The bond dissociation enthalpy DHfMeaSi-SiMea) = 332 +12 kJ moC was obtained from a kinetic study on the very low pressure pyrolysis of hexamethyldisilane and the enthalpy of formation of trimethylsilyl ion, AHf (MeaSi ) = 617.3 + 2.3kJmor, was determined using threshold photoelectron-photoion coincidence spectroscopy (TPEPICO). Both data are related to AHf°(Me3Si ). 

Davalos, J. Z. Baer, T. Thermochemistry and Dissociative Photoionization of Si(CH3)4, BrSifCITjlj, ISifCEIjlj, and SijCCElj) Studied by Threshold Photoelectron-Photoion Coincidence Spectroscopy/. Phys. Chem. A 2006, 110, 8572-8579. 

A number of techniques have been used previously for the study of state-selected ion-molecule reactions. In particular, the use of resonance-enhanced multiphoton ionization (REMPI) [21] and threshold photoelectron photoion coincidence (TPEPICO) [22] has allowed the detailed study of effects of vibrational state selection of ions on reaction cross sections. Neither of these methods, however, are intrinsically capable of complete selection of the rotational states of the molecular ions. The TPEPICO technique or related methods do not have sufficient electron energy resolution to achieve this, while REMPI methods are dependent on the selection rules for angular momentum transfer when a well-selected intermediate rotational state is ionized in the most favorable cases only a partial selection of a few ionic rotational states is achieved [23], There can also be problems in REMPI state-selective experiments with vibrational contamination, because the vibrational selectivity is dependent on a combination of energetic restrictions and Franck-Condon factors. 

The translational energy releases reported in the literature for metastable ion decompositions are contained in Tables 1—7. Decompositions of positive ions occurring within an ion source are covered in Table 8 and decompositions of negative ions in an ion source in Table 9. Translational energy releases determined by photoion—photoelectron coincidence (PIPECO) appear, therefore, in Table 8. The results from the extensive series of electron impact (El) measurements [310] at ionizing energies close to threshold appear in Tables 8 and 9. Coverage of dissociations of diatomic ions is not exhaustive. 

TPEPICO threshold photoelectron-photoion coincidence TPES threshold photoelectron spectroscopy... 

The threshold photoelectron-photoion coincidence technique using synchrotron radiation has been employed to study state-selected ion-molecule reactions. We review the experimental procedure and discuss our results on five representative systems [N2Ar]+ [N2H2]+ [ArH2]+ [HeH2]+ and [H2HJ+. Recent theoretical work on these systems is also summarized. 

The study of ion-molecule reactions using state-selected reagents has become a very exciting area of molecular dynamics. We have developed an experimental apparatus in Orsay which utilizes the properties of our tunable synchrotron radiation source at LURE to prepare ions in selected vibronic levels and then to study their reactions. The ions are state-selected using the TPEPICO (threshold-photoelectron/photoion coincidence) method [1]. 

Lago and Baer have studied the photo-ionization of bromine species that play an important role in the catalytic depletion of the Earth s protecting ozone layer by the threshold photoelectron photo-ion coincidence (TPEPICO) technique... 

Photoionization has a number of advantages over electron impact ionization, such as greater sensitivity and thresholds that are easier to define. In coincidence photoelectron techniques (e.g., photoelectron photoion coincidence spectroscopy, PEPICO, and threshold photoelectron photoion coincidence spectroscopy, TPEPICO), the photoelectron and the photoion formed in the ionization process are detected simultaneously, thus providing information on the nature and energies of the ionized products. ... 

Figure 1 Schematic diagram of a threshold photoelectron photoion coincidence experiment. The electron resolution is determined primarily by the length of the electron drift tube and the size of the apertures.

Stockbauer R (1977) A threshold photoelectron photoion coincidence mass spectrometer for measuring ion kinetic energy release on fragmentation. International Journal of Mass Spectrometry and Ion Physics 25 89-101. 

Fig. 15. Angle-integrated photoelectron energy distribution curves of uranium in the region of the giant 5 d -> 5 f resonance (90 eV < hv < 108 eV). The 5 f intensity at Ep is suppressed by more than a factor of 30 at the 5 ds/2 threshold (see the spectra for hv = 92 and 94 eV) and resonantly enhanced above threshold (see, e.g., the spectrum for hv = 99 e V). At an initial energy 2.3eV below Ep a new satellite structure is observed which is resonantly enhanced at the 5 d5/2 and 5 ds onsets. At threshold the satellite coincides with the Auger electron spectrum, which moves to apparently larger initial energies with increasing photon energy (from Ref. 67)...

By comparing threshold and energetic electron detection in coincidence studies, one can conclude that in the former case a 50-500 times higher signals-to-noise ratio is possible, while in the latter case the photon flux of a Hel radiation source is greater by several orders of magnitude. As a final result, coincidence rates are comparable in the two types of experiment. In practice, the threshold approach seems to be preferred because a wider variety of states is available since autoionization aids to excite levels not available in conventional photoelectron spectroscopy. 

The electron-impact spectra of methane have been compared with those of tetrafluoromethane and silane the spectra of all three molecules have been compared with the corresponding photoionization and photoelectron spectra. The relatively new technique of threshold electron-photoion coincidence mass spectrometry has been applied to, inter alia, methane and PH4]-methane. The technique permits the direct determination of the kinetics of dissociation of a molecular ion as a function of that ion. Thus, fragmentation of CHJ and CDJ has been determined as a function of internal energy and the results have been compared to previous photoionization, charge-exchange, and photoelectron-photoion coincidence mass-spectrometric results and to theoretical models. ... 

This article is centred on one of the most important molecular spectroscopic applications of photoionization. First, conventional photoelectron spectrometers based on the pioneering work of Turner, Vilesov and Siegbahn are reviewed, the basic building elements are shown and the principles of operation are discussed. Besides the conventional photoelectron experiment, threshold analysis and photoelectron-photoion coincidence spectroscopy as well as conceptually new techniques related to laser photoionization are discussed briefly. 

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