Cluster beams of hydrogen and nitrogen analyzed by time-of-flight mass spectrometry

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Abstract
Hydrogen and nitrogen clusters originating from condensing supersonic nozzle flows are studied by a special version of time‐of‐flight mass spectrometry. The molecular weights of the clusters to be investigated range from 103 to more than 106 and are determined from the change of the cluster speed which is effected by a longitudinal electric field after ionization of the clusters by electron impact. Clusters with up to four positive charges can be distinguished, and the probability for multiple ionization as a function of the electron energy is given for hydrogen clusters containing 6.5 × 104 molecules and for nitrogen clusters containing 1.2 × 104 molecules. Formation of singly charged negative cluster ions of both gases is observed for the first time and becomes more probable with increasing cluster size. While neither fragmentation nor evaporation as a result of the ionization process is observed, beams containing two fractions of masses prior to ionization are found at certain conditions of the cluster beam generation. Except for such cases, the distribution of sizes of singly charged clusters can be derived if the measured narrow speed distribution of the cluster beam is assumed to be valid also for each separate cluster size.