Picosecond response of sodium clusters on dielectric substrates

A strong increase of second-order nonlinear optical signal intensity due to electromagnetic field enhancement, in combination with femtosecond nonlinear optical pump-probe techniques facilitates measuring relaxation time constants tau(r) in large Na-n (n approximate to 10(5)-10(7)) clusters, bound to insulator surfaces. Time constants of tau(r) approximate to 1.8 ps for Na-n clusters on mica and tau(r) approximate to 1.1 ps for Na clusters on lithium fluoride are found. The difference is attributed to either morphological changes or the different thermal properties of the substrates. The subpicosecond-part of the time-resolved response is seen to be dominated by the field-enhancement effect, but is otherwise independent of cluster size for mean radii between 12 and 100 nm. (C) 1999 Elsevier Science B.V. All rights reserved.