The Gibbs free energy,
G, of ammonia (NH
3) clusters is calculated by using
ab initio molecular orbital methods. The free energy change, Δ
G, for the formation of an NH
3 cluster ion has a minimum near a cluster size of 5, which is the magic number for NH
3 clusters. In ion-induced nucleation of NH
3, the critical cluster size,
n*, is smaller and Δ
G at
n* is lower, compared with homogeneous nucleation of NH
3. The lowered Δ
G results in an increase in the concentration of NH
3 critical nuclei, which is the main cause of the increased nucleation rate in ion-induced nucleation of NH
3.
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