Photoion coincidence techniques

The sensitivity of fragmentation to the nature of resonances was shown for tetramethylsilane by using the photoion photoion coincidence technique (Morin et al. 1986). In this technique, fragmentation channels are identified by the coincidences of pairs of ions in the time-of-flight mass spectrometer. For tetramethylsilane, two resonances were observed in the ion pair yield curves. The lower energy resonance, below the ionization edge, had been shown from electron spectra to decay into a one-hole state, but the production... 

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. 

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

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. 

It is apparent that in both cases energy E is deposited in [AB+ +eej] and that, as in the case of excitation, the photon energy is analogous to the electron energy loss. However, since there are now two electrons sharing the excess energy in electron-impact ionization, it is necessary to use time correlation (coincidence techniques) for the simulation of photoionization... 

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... 

Recently in our laboratory we have initiated a program to study the photoabsorption processes of metal vapors throughout the UV and EUV region. Our research interests are (1) to obtain the absolute cross section measurement of atomic and molecular metal vapors, (2) to study the photoionization processes of molecular metal species, and (3) to study the photodissociation processes of molecular metal ions. Several experimental methods such as the heat-pipe absorption spectroscopy, photoionization mass spectroscopy, and electron-ion coincidence technique, will be used in the study. This report summarizes our first experiment using heat-pipe absorption spectroscopy. 

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. 

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. 

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. ... 

For ionization by cw lasers, quadrupole mass spectrometers are generally used. Their disadvantage is the lower transmittance and the fact that different masses cannot be recorded simultaneously but only sequentially. At sufficiently low ion rates, delayed coincidence techniques in combination with time-of-flight spectrometers can even be utilized for cw ionization if both the photoion and the corresponding photoelectron are detected. The detected electron provides the zero point of the time scale and the ions with different masses are separated by their differences Afa = ion — tQ in arrival times at the ion detector. 

---