Design and operation of a pulsed Laval nozzle and the characterization of molecular flow through such a nozzle using IR tunable diode laser (TDL) is the central theme of this work. The results here deal with He diluted N2O and CO2 gaseous systems. Boltzmann type plots of the spectral intensity data of both N2O and CO2 show non-linear behaviour. We have attempted to understand this non-linear behaviour of Boltzmann plots in terms of (1) instability in the jet and (2) a two-temperature model for the flowing gas, a cold central core and a hot boundary layer close to the nozzle walls. The model based on jet instability represents the data somewhat poorer than the two-temperature model. The parameters derived from fitting our experimental data to the former model could be used to calculate the thermodynamic parameters only through further approximations. Measured absorption line profile of the P(15) line of the v2 band of N2O as a function of axial distance from the nozzle exit gradually shifts from a Lorentzian to a Gaussian type. Velocity distribution of N2O molecules in a Laval nozzle is determined by differentiating the absorption line profile of the P(15) line (v0=576.235 cm−1) of the v2 band of N2O. Translational temperature of N2O molecules is determined from the observed spectral profiles.