PERTURBATION OF THE VELOCITY DISTRIBUTION IN THE SCATTERING OF MOLECULAR BEAMS. A THERMAL BEAM EFFECT.
MASSACHUSETTS INST OF TECH CAMBRIDGE DEPT OF CHEMISTRY
Pagination or Media Count:
When a thermal detector, in the present case a sensitive radiation thermocouple, is placed in the axis of a beam effusing from a source chamber with which the detector was originally in temperature equilibrium, steady signals are produced which reach values as high as 7.5 microvolts. The effect was reproducibly measured at 300K for Ar, H2, and He over a range of pressure in the source chamber of 5 to 100 mTorr. It is shown that this thermal beam effect results from collisions of the effusing beam particles with molecules in the gas cloud which forms in the exit channel of the beam source. The effect of the scattering is to remove preferentially the slower molecules, which also have the larger scattering cross sections, from the beam and to produce thereby a perturbed speed distribution in the attenuated beam. The experiments have been analyzed in terms of scattering theory for particles with velocity dependent cross sections, to obtain a quantitative measure of the perturbations of the speed distributions for all three systems. Author
- Physical Chemistry