Photoionization kinetics

Photoionization Kinetic study of allyl radical self-reaction. Selby et al. 

Photoionization, Kinetic study of gas-phase radical reactions et al. [24] Genbai et al. 

Eland JHD (1972) Predissociation of triatomic ions studied by photoelectron photoion coincidence spectroscopy and photoion kinetic energy analysis. International Journal of Mass Spectrometry and Ion Processes 9 397-406. 

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). 

On the other hand, the formation of ethylene was ascribed mainly to the unimolecular decomposition of a neutral excited propane molecule. These interpretations were later confirmed (4) by examining the effect of an applied electrical field on the neutral products in the radiolysis of propane. The yields of those products which were originally ascribed to ion-molecule reactions remained unchanged when the field strength was increased in the saturation current region while the yields of hydrocarbon products, which were ascribed to the decomposition of neutral excited propane molecules, increased several fold because of increased excitation by electron impact. In various recent radiolysis 14,17,18,34) and photoionization studies 26) of hydrocarbons, the origins of products from ion-molecule reactions or neutral excited molecule decompositions have been determined using the applied field technique. However, because of recent advances in vacuum ultraviolet photolysis and ion-molecule reaction kinetics, the technique used in the above studies has become somewhat superfluous. 

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 ). 

The kinetics of electron-ion recombination is well described by the diffusion model both for photoionization and for ionization induced by high-energy irradiation. 

For calculating atomic concentration ratios of the elements, photoionization cross-section by Scofield (4) and apparatus function by VuUi (5) were adopted. Electron escape depth (a) is determined by an experimental equation A =e0 7 (where E is kinetic energy of the electron) proposed by Hirokawa, et. al. (6). 

When a sample maintained in a high vacuum is irradiated with soft X-rays, photoionization occurs, and the kinetic energy of the ejected photoelectrons is measured. Output data and information related to (he number of electrons that arc detected as a function of energy are generated. Interaction of the soft X-ray photon with sample surface results in ionization from the core and valence electron energy levels of the surface elements. 

The simple collision theory for bimolecular gas phase reactions is usually introduced to students in the early stages of their courses in chemical kinetics. They learn that the discrepancy between the rate constants calculated by use of this model and the experimentally determined values may be interpreted in terms of a steric factor, which is defined to be the ratio of the experimental to the calculated rate constants Despite its inherent limitations, the collision theory introduces the idea that molecular orientation (molecular shape) may play a role in chemical reactivity. We now have experimental evidence that molecular orientation plays a crucial role in many collision processes ranging from photoionization to thermal energy chemical reactions. Usually, processes involve a statistical distribution of orientations, and information about orientation requirements must be inferred from indirect experiments. Over the last 25 years, two methods have been developed for orienting molecules prior to collision (1) orientation by state selection in inhomogeneous electric fields, which will be discussed in this chapter, and (2) bmte force orientation of polar molecules in extremely strong electric fields. Several chemical reactions have been studied with one of the reagents oriented prior to collision. ... 

Another approach for the formation of radical anions by LFP has been developed to overcome some of these difficulties. The approach involves the formation of radical anions by trapping a solvated electron produced by photoionization of 4,4 -dimethoxystilbene (DMS) to its cation radical (equations 31 and 32). This photoionization/electron trapping method is quite general for substrates that are transparent where DMS absorbs and that are more easily reduced than dimethoxystilbene. In many ways, this method is similar to pulse radiolysis, another useful approach used to generate radical anions for optical kinetic studies. 

The prompt appearance of these absorption bands within a nanosecond response time of the kinetic spectrometer (Fig. la,b) is a direct indication that intense 308-nm laser pulses induce photoionization of 2AP [10] ... 

In the U.S.S.R. photo-mass spectrometry has been used in a study of the photoionization and photodissociation of methylated benzene derivatives and the aromatic amines,16 and recently the hydrazine derivatives17 (Sec. III). Moreover, for many of the aromatic compounds the kinetic energy distribution of the photoelectrons has been measured in both the gaseous and the solid state (Secs. IV and V). 

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