Photoelectron photoionization technique

To date, by far the best quantitative studies of unimolecular dissociations of ions with a well-defined energy distribution have been those using the photoelectron photoionization technique (PEPICO). These reactions have been discussed in detail elsewhere, most notably by Baer. ... 

The rationalization of mass spectrometric investigations of nitro compounds has benefited significantly from numerous studies applying techniques adopted from photochemistry, such as photodissociation, photoionization and photoelectron photoion coincidence spectroscopy. 

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. 

Figure 29. Relative cross sections for collisional dissociation of CHjBr in selected vibrational-energy states as function of kinetic energy. Data were obtained by photoion photoelectron coincidence technique and were analyzed by assuming only backscattering in center of mass. Maxima of curves were normalized to same relative cross section.86b...

Photoionization always produces two species available for analysis the ion and the electron. By measuring both photoelectrons and photoions in coincidence, the kinetic electron may be assigned to its correlated parent ion partner, which may be identified by mass spectrometry. The extension of the photoelectron-photoion-coincidence (PEPICO) technique to the femtosecond time-resolved domain was shown to be very important for studies of dynamics in clusters [131, 132]. In these experiments, a simple yet efficient permanent magnet design magnetic bottle electron spectrometer was used for photoelectron... 

Combination of PES with mass spectrometry in a coincidence system leads to an extremely powerful technique called photoelectron-photoion coincidence (PEPICO) spectroscopy, which renders it possible to examine dissociations of energy-selected molecular ions. 

Frasinski et al. [15] devised a triple coincidence technique to study double ionization in the single photon regime. In these photoelectron-photoion-photoion coincidence experiments, the photoelectron provides the start pulse to an analogue-to-digital converter and the two ions the stop pulses and it was an easy matter to ensure that there was less than one molecular ion in the interaction region for the period of measurement of the ion energies, about 10 fis. In the multiphoton case, however, may ions are produced per laser pulse and this conventional approach cannot be employed. 

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. 

Several methods exist that allow the determination of the standard enthalpies of formation of the ionic species. The reader is referred to two recent rigorous and detailed chapters by Lias and Bartmess and Ervin. The vast majority of the experimental data reported here are obtained by means of Fourier transform ion cyclotron resonance spectroscopy (FT ICR), high-pressure mass spectrometry (HPMS), selected ion flow tube (SIFT), and pulsed-field ionization (PFI) techniques, particularly pulsed-field ionization photoelectron photoion coincidence (PFI-PEPICO). All these experimental techniques have been examined quite recently, respectively, by Marshall, Kebarle, B6hme," ° Ng" and Baer. These chapters appear in a single (remarkable) issue of the International Journal of Mass Spectrometry. An excellent independent discussion of the thermochemical data of ions, with a careful survey of these and other experimental methods, is given in Ref. 37. 

PFI-PEPICO pulsed field ionization photoelectron-photoion coincidence technique... 

The first ionization potential of ethane was measured by photoionization techniques more than 30 years ago [56-62] and it was found to be between 11.4 and 11.65 eV. Some indication of vibrational fine structure was found by Chupka and Berkowitz [62]. The classic Hel photoelectron spectrum of ethane was recorded by liirner s group (Baker et al. [25, 57] it is reproduced on Figure 3. 

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. ... 

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. ... 

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. 

Since the 1980s, some conceptually new photoionization techniques have appeared in the field of gas-phase investigations whose resolution is better than that of the conventional UPS technique by some orders of magnitude. Two technical novelties preceded the appearance of these techniques the introduction of supersonic jets in photoelectron spectroscopy and the availability of high-energy lasers. 

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