Design and performance of a highly efficient mass spectrometer for molecular beams

We present a very efficient 90 degrees sector magnet mass spectrometer detector, based on electron impact ionization, especially adapted for the use with atomic and molecular beams. In this setup, a cylindrically symmetric magnetic field compresses electrons emitted from a large filament into a long, narrow volume containing primarily the molecular beam. This has the advantage of both producing as little as possible background from residual gases and increasing the interaction region between beam and ionizer. Experimental tests using thermal He atoms show that the detector's efficiency and sensitivity are very high, amounting to eta greater than or equal to 0.70 +/- 0.04% and S=0.22 +/- 0.03 A mbar(-1), respectively. The unconventionally long ionization volume, however, may restrict the use of this mass spectrometer to experiments with no, or only moderate timing requirements. (C) 2000 American Institute of Physics. [S0034-6748(00)03705-9].