SPECTROSCOPIC CLUSTER-SIZE EFFECTS

In this paper we examine the spectral moments of the absorption line s hapes of aromatic-molecule. (rare gas)n heteroclusters to provide anal ytic expressions for the spectral shifts (deltanu), which are determin ed by the first moment, and for the homogeneous linewidths (GAMMA), wh ich are determined by the central second moment (DELTA) of the line sh ape. Deltanu originates from the cumulative contributions of dispersiv e pair interactions, while DELTA and GAMMA manifest the short-range dy namic nuclear fluctuations. Our analysis elucidates some of the genera l features of these spectroscopic observables, e.g., their dependence on the cluster structure and their size dependence, as well as some of their specific characteristics, e.g., their temperature dependence. W e predict a weak temperature dependence of deltanu (=a+bT with b much less than a/T) and a strong temperature dependence of DELTA(is-proport ional-to T1/2), which is borne out by molecular dynamics (MD) simulati ons. We have derived cluster size equations (CSE) relating the spectro scopic observables of the finite cluster with those of the infinite bu lk system. The excluded volume corrections for deltanu(n) is-proportio nal-to n-1 and for DELTA(n) is-proportional-to n-3 are different, in a ccordance with the nonuniversability principle for CSEs. The predictio ns of the CSEs concur with experimental and MD simulation data for del tanu(n) and with MD simulation data for DELTA(n). Finally, we have add ressed dimensionality scaling of spectroscopic cluster properties, app licable for the situation of a common dimensionality (D not-equal 3) f or the cluster and for the infinite bulk. This analysis may be useful for the spectroscopic interrogation of doped clusters of low dimension ality, e.g., wire clusters (D= 1) and planar clusters (D = 2), as well as fractal clusters, e.g., clusters of porous materials.