Condensation of a supersaturated vapor. V. The nucleating effects of ultraviolet light on vapors containing very low concentrations of o-tolualdehyde

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Abstract
The photoinduced nucleation of supersaturated vapors containing low concentrations of o-tolualdehyde has been studied. We have made measurements of the photonucleation spectrum using various host liquids, the steady-state rate of nucleation, the delay time before the start of nucleation, the initial slope of the nucleation rate, the decay of the rate after the light is turned off, the o-tolualdehyde concentration dependence, and the dependence on total pressure and the effect of temperature changes. We show that these measurements are in quantitative agreement with a phenomenological model in which the photoinduced nucleation is initiated by clusters accumulating a small number of photoaffected molecules. Using the vapor concentration, the photon cross section of o-tolualdehyde, and the photon flux, we calculate that a photoaffected o-tolualdehyde concentration as low as 3×107 molecules cm−3 sec−1 (i.e., about 1 part per trillion) is sufficient to cause photoinduced nucleation. This concentration is too small to cause either ordinary heterogeneous nucleation or binary homogeneous nucleation. However, a collision frequency calculation shows that this low concentration of photoaffected molecules is in good agreement with the phenomenological model, since a much smaller number of collisions of photoaffected molecules are then required.