Calibration of mass spectrometers for van der Waals dimers

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Abstract
A stream tube from a small supersonic free jet in vacuo is passed successively through the ion source of a quadrupole mass spectrometer and into an enclosed ionization gauge which indicates total mass flux. By simultaneous measurements of dimer and monomer ion currents as well as total mass flux we are able to arrive at absolute values of the relative concentration of neutral monomer and dimer in the free jet. Results obtained with argon, carbon dioxide, and oxygen indicate substantial fragmentation of dimers in the ion source. Consequently, populations of neutral dimers in supersonic free jets of the kind widely used as beam sources may be much larger than previous studies have indicated.

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