MUTUAL CHARGE NEUTRALIZATION OF GASEOUS IONS (thesis)

The problem of the bimolecular rate constant, alpha , for the mutual charge neutralization reaction (ion-ion recombination) for ions formed by the vacuum ultraviolet photolysis of nitric oxide is considered. The pressure dependence of alpha over a pressure range of 10 to 600 torr for mixtures of a few hundred microns of NO with He, Ar, Kr, Xe, H/sub 2/, D/sub 2/, and N/sub 2/ was measured. From the low-pressure limit of alpha , the rate constant for charge neutralization in the absence of a third body was found to be k/sub o/ = 2.1 plus or minus 0.4 x 10/sup -7/ cm/sup 3// sec. The high-pressure limit of alpha was estimated to be 2.0 plus or minus 0.5 x 10/sup -6/ cm/sup 3//sec. The third-body efficiencies for promoting the charge-neutralization reaction were measured. The results, relative to He as the third-body gas, are H/sub 2/= 1.4 plus or minus 0.4, D/sub 2/= 1.5 plus or minus 0.4, Ar =3.6 plus or minus 0.8, Kr =4.3 plus or min11.0, N/sub 2/ = 5.2 plus or minus 1.1, and Xe = 6.8 plus or minus 1.5. The average ionic mobility in the gas mixtures is estimated, and the mobilities indicate that at least some of the ions must be present as ion clusters. It is shown that the addition of NO/sub 2/ or H/sub 2/O further lowers the mobility. A detailed calculation of the three-body charge-neutralization process was made using a computer. This calculation considers that the rate of charge neutralization is the rate at which ion pairs are deactivated by collision with the neutral gas molecules to form ion pairs that cannot separate to large distances. The potential between the ions and the neutrals is assumed to be an ion-induced dipole potential with a hard-sphere core. The calculation involves an average over the various angles in the collisions. The predicted values of alpha depend on a parameter of the calculation, but over a wide range of this parameter the predicted relative third-body efficiencies are in reasonable agreement with the experimental values. (auth