Millimeter wave spectroscopic techniques have been used to study the collision induced energy transfer processes that are important for the understanding of the 1.2 mm13CH3F optically pumped FIR laser. In these experiments the microwave diagnostic probe is copropagated with the CO2laser pump beam through a gain cell. The continuous tunability and absolute frequency reference of the experimental technique have made possible a quantitative investigation of the emission/absorption profiles of approximately 25 rotational transitions. From these experimental results and a simple but accurate theoretical model, energy transfer parameters have been calculated by means of a nonlinear least squares fit. These parameters accurately account for all observations and are readily interpretable on the basis of our understanding of fundamental collisional and radiative processes.