Second-harmonic generation and shielding effects of alkali clusters on ultrathin organic films

We demonstrate the nonlinear optical activity of large Na and K clusters, g rown on Au metal film-supported alkane thiol monolayers of different chain lengths. Both the total optical second-harmonic signal intensity as well as its dependence on the angle of incidence depend on the surface coverage wi th the alkali metal, thus demonstrating the sensitivity of second-harmonic generation to morphological changes in these complex systems. Simultaneousl y performed extinction and scanning force microscopy measurements under amb ient air conditions allow us to obtain independent information on the surfa ce coverage and cluster morphology. The latter measurements also show that the optical activity of sodium clusters grown on alkane thiols at 150 K and annealed to 300 K is conserved on dosing with oxygen.