A strong increase of second-order nonlinear optical signal intensity due to
electromagnetic field enhancement, in combination with femtosecond nonline
ar optical pump-probe techniques facilitates measuring relaxation time cons
tants tau(r) in large Na-n (n approximate to 10(5)-10(7)) clusters, bound t
o 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 lit
hium fluoride are found. The difference is attributed to either morphologic
al changes or the different thermal properties of the substrates. The subpi
cosecond-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.