Rate Constants for Attachment, Detachment, and Recombination

The specific rate constants of interest to the ECD and NIMS are dissociative and nondissociative electron attachment, electron detachment, unimolecular anion dissociation, and electron and ion recombination. The reactions that have been studied most frequently are electron attachment and electron and ion recombination. To measure recombination coefficients, the electron concentration is measured as a function of time. The values are dependent on the nature of the positive and negative ions and most important on the total pressure in the system. Thus far few experiments have been carried out under the conditions of the NIMS and ECD. However, the values obtained under other conditions suggest that there is a limit to the bimolecular rate constant, just as there is a limit to the value of the rate constant for electron attachment. The bimolecular rate constants for recombination are generally large, on the order of 10 7 to 10-6 cc/molecule-s or 1014 to 1015 1/mole-s at about 1 atm pressure. Since the pseudo-first-order rate constants are approximately 100 to 1,000 s 1, the positive-ion concentrations in the ECD and NIMS are about 109 ions/cc. 

The electron beam and electron swarm experiments [13] can also be used to determine attachment rate constants. However, these are determined as a function of energy and can then be extrapolated to thermal energy. Other techniques used to 

The theoretical methods for calculating the electron affinities of atoms, diatomic molecules, and polyatomic molecules have been summarized and compared with the CURES-EC method for molecules. The density functional calculations of the electron affinities of substituted benzoquinones and scaled ab initio LUMO agree with the evaluated values for nitrobenzenes. 

The rate constants for thermal electron attachment by alternative techniques are compared to those obtained with the ECD. A method of calculating activation energies for rate constants measured at a single temperature is suggested. 

and Goode, G. C. Negative Ions and the Magnetron. New York Wiley-Interscience, 1969. 

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