Thermal energy collisions of K(nd) Rydberg atoms with electron attaching molecules which result in reactions of the following types K(nd) + SF\sb6 → K\sp+ + SF\sb6\sp− K(nd) + CF\sb3I → K\sp+ + I\sp− + CF\sb3 have been investigated at intermediate n. Using the field ionization technique, both rate constants for Rydberg atom destruction and for formation of free negative ions are directly measured for n as low as 12. The data show that, at intermediate n, the rate constants for Rydberg atom destruction start to decrease with decreasing n and to deviate from those predicted on the basis of the simple free electron model. A semi-classical model is presented to explain this decrease. It reveals explicitly the relationship between the rate constant for Rydberg atom destruction and the rate constant for free electron attachment. The data also show that, at intermediate n, only a fraction of the Rydberg electron attachment events leads to formation of free negative ions. Post-attachment interaction between the positive and negative ions formed is investigated. The kinetic energy distribution of the ion pairs in their center-of-mass frame and their separation probability are calculated. The angular dependence of the kinetic energy distribution and the separation probability are also studied. The present experiments, in conjunction with the theoretical calculations, advance understanding of the collisions substantially. Investigation of Zeeman-effect quantum beats observed in the selective field ionization spectra of D state potassium Rydberg atoms in also presented.