The transition from free molecule to collisional flow during vaporization into vacuum by the test-particle method

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Abstract
A test‐particle method was developed to estimate the conditions that lead to collision‐dominated evaporation during pulse heating of a solid surface in vacuum. The method consists of calculating the number density of evaporated molecules above the surface neglecting collisions and then sending a test particle through this cloud. The probability that the test particle escapes from the cloud without making a collision is the measure of the importance of intermolecular collisions during the blow off. The method is applied to heating of a ∼1 cm diam spot on the surface with a heat pulse of ∼1 msec duration. These conditions characterize laboratory laser vaporization experiments intended to measure the vapor pressures of refractory solids at very high temperatures. The principal variable which determines the escape probability is the maximum surface temperature during the transient. When applied to laser heating of uranium dioxide, collisional effects in the vapor are predicted to begin at a maximum surface temperature between 2600 and 2800 K.