Sr. Cook et al., SPECIALIZED TORSION BALANCE DESIGNED TO MEASURE THE ABSOLUTE FLUX-DENSITY OF HYPERTHERMAL MOLECULAR-BEAMS CONTAINING REACTIVE SPECIES, Review of scientific instruments, 67(5), 1996, pp. 1781-1789
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.