Photoionization, ionization energies

Time-of-flight mass spectrometers have been used as detectors in a wider variety of experiments tlian any other mass spectrometer. This is especially true of spectroscopic applications, many of which are discussed in this encyclopedia. Unlike the other instruments described in this chapter, the TOP mass spectrometer is usually used for one purpose, to acquire the mass spectrum of a compound. They caimot generally be used for the kinds of ion-molecule chemistry discussed in this chapter, or structural characterization experiments such as collision-induced dissociation. Plowever, they are easily used as detectors for spectroscopic applications such as multi-photoionization (for the spectroscopy of molecular excited states) [38], zero kinetic energy electron spectroscopy [39] (ZEKE, for the precise measurement of ionization energies) and comcidence measurements (such as photoelectron-photoion coincidence spectroscopy [40] for the measurement of ion fragmentation breakdown diagrams). 

The ionization energy of gaseous disulfane has been determined by photoionization efficiency spectroscopy as 9.40 0.02 eV [25] and by photoelectron spectroscopy as 9.41 eV [61]. Recently, XANES spectra of H2S and H2S2 have been reported which show distinct differences [62]. 

Radiation chemistry highlights the importance of the role of the solvent in chemical reactions. When one radiolyzes water in the gas phase, the primary products are H atoms and OH radicals, whereas in solution, the primary species are eaq , OH, and H" [1]. One can vary the temperature and pressure of water so that it is possible to go continuously from the liquid to the gas phase (with supercritical water as a bridge). In such experiments, it was found that the ratio of the yield of the H atom to the hydrated electron (H/eaq ) does indeed go from that in the liquid phase to the gas phase [2]. Similarly, when one photoionizes water, the threshold energy for the ejection of an electron is much lower in the liquid phase than it is in the gas phase. One might suspect that a major difference is that the electron can be transferred to a trap in the solution so that the full ionization energy is not required to transfer the electron from the molecule to the solvent. 

The question whether or not similar effects play a role in the photoionization of phenols, could be studied only in aprotic surroundings (ionization energy around 8 eV). By using photon energies of about 5eV biphotonic ionization but also monophotonic fragmentation of the phenol molecule leading to phenoxyl radicals [6] takes place. To overcome this complication we used sterically hindered phenols where the first excited singlet relaxes by internal conversion only [7]. Results of biphotonic ionization with UV lasers of different pulse durations are reported and interpreted in this paper. 

PHOTOION IZATT ON. This process, which is also called the atomic photoelectric effect, is the ejection of a bound electron from an atom by an incident photon whose entire energy is absorbed by the ejected electron. This statement means that photoionization cannot occur unless tlie energy of the photon is at least equal at the ionization energy of the particular electron in the particular atom any excess of energy in the photon above this value appears as kinetic energy of the ejected electron. 

The complications just described can be minimized if there is greater selectivity in the ionization process, as is sometime possible when photoionization is used as the excitation mechanism. Because the ionization energy can be more precisely controlled, it is possible in selected cases to produce only the desired reactant-ion species, or at least to minimize production of other ions. As already noted in the earlier section on formation of excited ions, it is also possible to populate specific internal-energy states of some reactant ions by using a photoionization source. One of the earliest photoionization mass spectrometers used to study interaction of internally excited ions with neutrals was that constructed by Chupka et al.91 Such apparatuses typically incorporate a photon source (either a line or a continuum source) and an optical monochromator, which are coupled to the reaction chamber. Various types of mass analyzer, including sector type, time-of-flight (TOF), and quadrupole mass filters, have been used with these apparatuses. Chupka has described the basic instrumental configuration in some detail.854 Photoionization mass spectrometers employed to study interactions of excited ions with neutral species have also been constructed in several other laboratories.80,1144,142,143 The apparatus recently developed by LeBreton et al.80 is illustrated schematically in Fig. 7 and is typical of such instrumentation. 

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