EFFECTS OF BACKGROUND GASES ON THE HOMOGENEOUS NUCLEATION OF VAPORS .2.

Using a high-pressure, upward thermal diffusion cloud chamber, we have observed a strong dependence of the homogeneous nucleation of methanol, ethanol, and 2-propanol vapors on total pressure. In our investigation, this dependence is manifested by increasing values of the measured S-crit (critical supersaturation) for each of the three alcohols with increasing total pressure (P-tot). The increase in total pressure is accomplished by increasing the amount of a background.(or carrier) gas in the diffusion cloud chamber. The observed increase in measured S-crit implies that the homogeneous nucleation rate of each of these alcohols decreases as the total pressure increases. We have observed a significant dependence on the nature of the background gas. The effect of increasing total pressure on the nucleation of each of these alcohols is greater using helium as the background gas than when hydrogen is used. We also observe that the dependence of S-crit on total pressure (as manifested by the slope of the S-crit vs P-tot data at constant temperature) is temperature dependent for each of these alcohols. The measured slope of the S-crit vs P-tot dependence increases as the temperature decreases.