  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 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.
