Zero kinetic energy electron

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). 

A successful modification to the technique involves delayed pulsed-field extraction which allows discrimination between zero and near-zero kinetic energy electrons. About 1 ps after the laser pulse has produced photoelectrons, a small voltage pulse is applied. This has the effect of amplifying the differences in fhe velocities of fhe phofoelecfrons and allows easy discrimination befween fhem as a resulf of fhe differenf times of arrival af fhe defector. In fhis way only fhe elections which originally had zero kinetic energy following ionization can be counted to give fhe ZEKE-PE specfmm. 

Next, consider the situation if hv = IE. Here, the electron can just be released, but it cannot travel away from the freshly formed ion. Waiting for some short delay (1 ps) now allows to separate zero kinetic energy electrons from others in... 

The energetics of EC are determined by the electron affinity (EA) of the neutral. The EA is the negative of the enthalpy of reaction of the attachment of a zero kinetic energy electron to a neutral molecule or atom ... 

E. W. Schlag The term zero-kinetic-energy electrons was used already by us in Chem. Phys. Lett. 4, 243 (1969). The new mechanism of redistribution of low-/ into high-/ molecular states is the basis of ZEKE states. 

Figure 10. Schematic energy level diagram of D2 showing the shift of the ionization potential and accompanying energy levels of the neutral D2 under the influence of low-, intermediate-, and high-intensity laser fields. The maximum ponderomotive energy available to a zero kinetic energy electron is shown.

ZEKE Zero-kinetic energy electrons, produced by field ionization of excited Rydberg states just below the ionization limit to a particular internal energy state of the resultant ion. 

Habenicht W, Reiser G and Muller-Dethlefs K (1991) High resolution zero kinetic energy electron spectrum of ammonia. Journal of Chemical Physics 95 4809-4820. 

Miiller-Dethlefs K (1991) Zero kinetic energy electron spectroscopy of molecules - rotational symmetry selection rules and intensities. Journal of Chemical Physics 95 4821-4839. 

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