A 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 absolute flux density of seeded hyperthermal molecular beams containing reactive and/or condensable species with uncertainties of approximately ±5%. The balance is ultrahigh‐vacuum compatible and can be used in corrosive environments. A specially designed beam stop mounted on the torsion balance lever arm, traps the incoming beam and allows only completely thermalized molecules to exit. The thermalized molecules exit the beam stop in equal numbers per unit time in opposite directions, ensuring that the force exerted on the beam stop by the exiting molecules is zero. The balance was suspended from a 25‐μm‐diam gold‐coated tungsten wire that had a torsion constant of 6.04×10−8 N m/rad and a period of slightly larger than 400 s. Absolute molecular beam flux density measurements were made using both the torsion balance and the effusive method for a variety of pure and seeded molecular beams. The beams were composed of gases that could easily be measured by the effusive method. Measured fluxes ranged from 2.41×1015 mol/s cm2 for a beam of CO seeded in H2 to 1.25×1017 mol/s cm2 for a beam of pure H2. A comparison of the two methods establishes the ability of the torsion balance to accurately measure the flux of molecular beams.