Equilibrium Methods for Determining Electron Affinities

The complementary techniques for determining rate constants for thermal electron attachment, detachment, and dissociation are the flowing afterglow, the microwave technique, the ion cyclotron resonance procedures, the swarm and beam procedures, the shock tube techniques, the detailed balancing procedures, the measurement of ion formation and decay, and the high-pressure mass spectrometer procedures. In all cases the measurement of an ion or electron concentration is made as a function of time so that kinetic information is obtained. In the determination of lifetimes for ions, a limiting value of the ion decay rate or k is obtained. 

The electron attachment reactions for inorganic molecules were reviewed in 1974. Those for organic molecules were summarized in 1984 [12, 13]. In many cases activation energies were not measured. If a nominal value for A and A is assumed, the activation energies can be estimated. Recently, the flowing afterglow procedure has been extended to include electron and gas heating so that the dependence of the rate constants on thermal electron attachment can be examined for bulk temperature and electron temperature [14]. 

In the equilibrium methods the electron is treated the same as any other chemical reactant. The measurement of the electron affinity of a molecule involves the measurement of the equilibrium constant for the reaction of thermal electrons with a molecule (AB + e(—) = AB(—)) at some specific temperature or series of temperatures. The equilibrium technique requires (1) a source of thermal electrons, (2) a source of the test species, (3) a method of measuring the equilibrium concentrations of the neutral species, negative ions, and free thermal electrons, and (4) a temperature measurement. The equilibrium constant is directly related to the Ea by this equation  

The three general equilibrium methods are ECD/NIMS, surface ionization, including the magnetron direct capture techniques, and the kinetic or detailed balancing measurement of the rate constants. The ECD method has been applied to about 150 molecules. The NIMS method involves the application of the ECD 

Qan ratio of unity. This was one of the first determinations of a Qan ratio for electron molecule reactions. The ECD values of the excited-state electron affinity, Qlm, and A i and E agree with the swarm values. The low-temperature response of the ECD can be calculated from parameters measured in swarm experiments. 

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