A beam‐gas collision experiment to measure excitation cross sections for translation‐to‐vibration (T‐V) energy‐transfer collision processes that produce short‐wavelength infrared radiation (SWIR) has been assembled and used to obtain the (0,0°,0 → 0,0°,1) excitation cross section for CO2by collision with Ar. A unique modulation technique used in this system involves pulse‐modulating one of the reactants, injecting the pulses into a flowing plasma carrier, supersonically expanding the pulse and carrier, and extracting a beam from the expansion while preserving the modulation of the reactant. Enhanced radiation due to thermalization of excited CO2at sphere pressures greater than single collision conditions, has been measured and a correction factor for this effect obtained. Measurements of the argon velocity and flux in a helium beam, CO2number density in an integrating sphere, and infrared radiation at 4.25 &mgr;m were made to determine the excitation cross section at a collision velocity of 2.3 km s−1. The average of three sets of measurements yields a value of &sgr;*=3.7(±3)×19−19cm2.