MOLECULAR BEAM VELOCITY DISTRIBUTION MEASUREMENTS,
MASSACHUSETTS INST OF TECH CAMBRIDGE FLUID DYNAMICS RESEARCH LAB
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The energy accommodation of molecules during reflection from a solid surface can be measured as a function of angle of incidence, reflection angle, and incident velocity by molecular beam methods described in the report. The velocity distribution of the rebounding beam of molecules is measured, and compared to the velocity distribution of the incident molecular beam. The measurement is made by the time of flight method. A rotating thin disc with a notched edge acts as a shutter to allow a molecular beam to pass only for a short period. The molecular beam density at a distance L downstream of the shutter is sensed by electron beam ionization and ion collection. It is shown that the thermodynamic fluctuation effect limits the signal resolution of the instrument. Preliminary measurements of the incident molecular time of flight distributions show agreement with the Maxwellian theory in the case of a helium beam, and a deficiency of low velocity molecules for nitrogen and argon beams. Preliminary reflected beam measurements using a polycrystalline nickel surface show that at high target temperatures the thermal accommodation coefficient of air is 0.68 and of helium is 0.06.