Negative temperature coefficients of ion-molecule reactions in the ionization chamber of the mass spectrometer: Statistical calculations based on an experimental model for the variation of the microscopic reaction cross-section with relative velocity

A statistical calculation is given of the phenomenological cross section Q of ion-molecule reactions in the ionization chamber of a mass spectrometer, for the case where a sufficienty high repeller field is applied and the microscopic cross section is given by sigma (g) = (g/sub c//g -- 1) sigma , g is the relative velocity, and g/sub c/ is the critical value of this velocity for which sigma becomes zero. It is found that a negative temperature effect may be expected. The magnitude of the effect is calculated for three numerical cases. A graph is given of d(ln Q)/d(ln T) as a function of the dimensionless quantity ( mu /2kT) /sup 1/2/ g, where mu is the reduced mass of the ion and the molecule.