Formation of dimers in free jets

A method was developed to calibrate a mass spectrometer for dimer concentration measurements. It is based on determination of total mass flux with a snorkel ionization gauge simultaneously with measurements of monomer and dimer ion currents using a quadrupole mass spectrometer. Measurements of dimer/monomer mole ratio as a function of source pressure were completed at several values of source temperature. Scaling laws for terminal dimer/monomer ion current ratio in free jets were obtained. A sudden-freeze model was used to approximate rate constants for dimer formation in a jet. Dimer formation for each molecular encounter was found to be characterized by an apparent activation energy corresponding to 0.38 times the depth of the intermolecular potential attractive well. A steric factor was also inferred and found to be 0.18, 0.22 and 0.46 for argon, oxygen and carbon dioxide respectively.