A NOVEL SOURCE DESIGN FOR MOLECULAR-BEAM MACHINES

We describe the performance of a molecular beam machine with a recently introduced wide range turbomolecular pump as a source pump. In comparison to large diffusion pumps the turbo pump has a small pumping speed, which leads to the formation of a weak, partially penetrable barrel shock surrounding the free jet expansion. Collisions of background gas molecules that penetrate the shock with seed gas molecules were found to reduce the density of the seeded molecules in the skimmed molecular beam by about 20%. For many experiments this attenuation can be tolerated, so that full advantage of wide range turbo pumps can be taken.