Tunable diode laser absorption spectroscopy study of CH3CH2OD∕D2O binary condensation in a supersonic Laval nozzle

We have developed a dual-beam tunable diode laser absorption spectroscopy system to follow the cocondensation of water and ethanol in a supersonic Laval nozzle. We determine the D2O monomer concentration in the vapor phase by fitting a Voigt profile to the measured line shape but had to develop a calibration scheme to evaluate the C2H5OD monomer concentration. To measure the temperature of the gas, we seed the flow with CH4 and measure two absorption lines with different lower state energies. These data give a far more detailed picture of binary condensation than axially resolved pressure measurements. In particular, we observe that the C2H5OD monomer starts to be depleted from the gas phase well before D2O begins to condense.