Experimental Determination of Nucleation Scaling Law for Small Charged Particles

We investigated the nucleation process at the molecular level. Controlled sticking of individual atoms onto mass selected clusters over a wide mass range has been carried out for the first time. We measured the absolute unimolecular nucleation cross sections of cationic sodium clusters ${\mathrm{Na}}_n^{+}$ in the range $n=25–200$ at several collision energies. The widely used hard sphere approximation clearly fails for small sizes: not only should vapor-to-liquid nucleation theories be modified, but also, through the microreversibility principle, cluster decay rate statistical models.

Figure 1

Log-log plot of the SCS as a function of cluster size at two kinetic energies. Experimental results are compared to a Langevin model and a hard sphere model. The calculated mean cross sections in a vapor at 400 K are also shown.

Figure 2

Effective interaction potentials in the harpooning model in the case of the of ${\mathrm{Na}}_{20}^{+}+\mathrm{Na}\to {\mathrm{Na}}_{21}^{+}$ sticking reaction. Dotted lines: diabatic potentials. Solid lines: adiabatic potentials. The coupling is taken as the hopping integral in ${\mathrm{Na}}_2^{+}$ at the distance $r$. The impact parameter is equal here to its critical value $b_{\max }$ (see text).