[en] McIntosh has carried out experiments in which electron plasma waves produced in a gas jet target by a CO₂ laser pulse were studied by picosecond-resolution Thomson scattering of a ruby laser pulse. In addition, spectra of the scattered laser light and of the fast electron were obtained. Their analysis of the data was based on the assumption that the plasma waves are created by onset of the stimulated Raman scattering instability. This led to a number of difficulties in the interpretation of the data. We show here that an alternative model of the origin of these plasma waves used previously in analyzing Raman data in several other experiments, removes all these difficulties and is in agreement with the data. In this model, the plasma waves are produced by pulses of fast electrons arising at the quarter-critical surface