SPECIALIZED TORSION BALANCE DESIGNED TO MEASURE THE ABSOLUTE FLUX-DENSITY OF HYPERTHERMAL MOLECULAR-BEAMS CONTAINING REACTIVE SPECIES

A torsion balance is described that has been used to measure the absol ute flux density of seeded hyperthermal molecular beams containing rea ctive and/or condensable species with uncertainties of approximately /-5%. The balance is ultrahigh-vacuum compatible and can be used in co rrosive environments. A specially designed beam stop mounted on the to rsion balance lever arm, traps the incoming beam and allows only compl etely thermalized molecules to exit. The thermalized molecules exit th e beam stop in equal numbers per unit time in opposite directions, ens uring that the force exerted on the beam stop by the exiting molecules is zero. The balance was suspended from a 25-mu m-diam gold-coated tu ngsten wire that had a torsion constant of 6.04X10(-8) N m/rad and a p eriod of slightly larger than 400 s. Absolute molecular beam flux dens ity measurements were made using both the torsion balance and the effu sive method for a variety of pure and seeded molecular beams. The beam s were composed of gases that could easily be measured by the effusive method. Measured fluxes ranged from 2.41x10(15) mol/s cm(2) for a bea m of CO seeded in H-2 to 1.25X10(17) mol/s cm(2) for a beam of pure H- 2. A comparison of the two methods establishes the ability of the tors ion balance to accurately measure the flux of molecular beams. (C) 199 6 American Institute of Physics.