The A+Bx condensation reaction: Crossed nozzle beams of Br2 and (Cl2)x or (NH3)x clusters

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Abstract
Nozzle beams of Br2 and Cl2 or NH3 have been crossed in a molecular beam scattering apparatus; the Cl2 or NH3 beam contained (Cl2)x or (NH3)x clusters distributed such that the intensity of a given cluster, Fx, decreased with increasing x for 1⩽x⩽∼50. Mass, angular, and time‐of‐flight spectra of the scattered neutral species all establish that the A+Bx→ABx* bimolecular condensation reaction is being observed. However, working from the data, it is not possible to distinguish between detection of a long‐lived ABx* metastable complex or of a decomposition product formed with low recoil velocity. Product angular distributions are confined to a small region of laboratory scattering angle Θ and peak at small but positive Θ (Θ=0° and 90° defined by cluster and Br2 beam directions, respectively). It is pointed out that this sharp peaking at small Θ is due to a number of experimental factors, including a Jacobian factor varying as sin−2Θ, and should be a universal characteristic of such condensation reactions in crossed beams. In the data it is indicated that there will be a high probability of fragmentation into small daughter ions upon electron bombardment (EB) ionization of an ABx or Bx cluster for the range in x most sensitive to the measurements (∼10⩽x⩽∼50). This in turn implies that the concentration of neutral clusters in the beam can be seriously underestimated if the cluster ion mass spectra produced by EB ionization of the nozzle beam are assigned assuming that fragmentation is inconsequential.

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