The rate constant for a reaction involving a positive ion can be modified by applying a Bohm velocity correction, as follows

(4–29)

In Equation 4–29 , the reaction-rate expression is similar to that of Equation 4–14 , used to describe sticking coefficients. As with a sticking coefficient, Bohm reactions allow the user to specify a probability for the reaction to occur, rather than a reaction rate. Also, the Bohm reaction formulation is only valid for reactions where there is exactly one gas-phase ion that is a reactant. The units of the pre-exponential, temperature exponent term, and activation energy correspond to the parameters in a sticking coefficient. The main difference, however, is that the resulting probability is multiplied by the Bohm velocity expression (the term in the square root in Equation 4–29 ), rather than the thermal velocity of the ion species. The Bohm velocity is based on the electron temperature, rather than the thermodynamic temperature of the gas.

The molecular weight in the last term is that of the positive ion. Bohm reactions can be declared through the Pre-processor input via the auxiliary keyword BOHM. An executable can find out which reactions were declared as Bohm reactions by a call to SKIBHM. Use of the BOHM keyword is only allowed for irreversible reactions.