Appendix - Jet Theory - Clustering and Condensation

13.1.9 Appendix - Jet Theory - Clustering and Condensation

Clusters in a beam are formed as a result of expansion and cooling of a gas flow from a nozzle. Clustering is very complicated to predict and depends on the model of dimerization used (Ref. 21). An approximate formula to avoid clustering of more than 10% in monoatomic gases is given by:

(160)

Using this formula for NH_3 (although not a monoatomic gas) to get an idea of the significance of clusterization for typical jet operating conditions of T_o=473 K, P_o=80 torr, sigma=2.2 Å, epsilon_r=9, and d=200 mm yields 0.48. Therefore dimer formation can be expected to be on the order of 10-20%, assuming equation (160) holds for NH_3. Note also that equation (160) is a linear function of nozzle pressure P_o.

Smirnov (Ref. 64) suggested that a beam cluster is actually better for deposition than an atomic beam because the specific released energy resulting from sticking of clusters to a surface is less by an order of magnitude than in the case of an atomic beam. His analysis was carried out only for monoatomic gases only though.

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