Spectroscopic interrogation of heterocluster isomerization. I. Simulations of nuclear dynamics and electronic spectroscopy

Abstract

In this paper we explore the structure, energetics, nuclear dynamics, isomerization, and electronic-vibrational spectroscopy of large finite systems by constant energy molecular dynamics (MD) simulations of heteroclusters of 9,10-dichloroanthracene (DCA) with rare-gas atoms, i.e., DCA⋅Arn (n=1–34) and DCA⋅Krn (n=1–18). We consider the spectroscopic implications of heterocluster size effects, isomer-specificity, nuclear dynamics, and isomerization as manifested by the spectral shifts (δν) and homogeneous linewidths (Δ) for the S0→S1 transition, the ionization potential shifts (δI) from S0, and the shifts (Δτr) of the pure radiative lifetimes of S1 for these heteroclusters, with δν, δI, and Δτr being given relative to the corresponding observable for the bare DCA molecule. MD simulations of the energetics, rigid and nonrigid structures, fluctuations in structural parameters, and transport properties over a broad temperature region (8–70 K) allowed for the identification of several isomerization phenomena, i.e., correlated surface motion, surface melting, side crossing, wetting–nonwetting, and nearly rigid–nonrigid transitions. The size dependence of the temperature onsets for various isomerization processes was established. The hierarchical occurrence of several isomerization processes for a single heterocluster composition, together with the sequential occurrence of distinct isomerization processes with increasing the cluster size, was documented. In the size domain explored herein the temperature onsets of surface melting, side crossing, and wetting–nonwetting transitions tend to decrease with increasing the heterocluster size, in contradiction to the rule that the cluster melting temperature increases with increasing its size, reflecting finite microsurface effects. The spectroscopic observables were simulated using quantum mechanical expressions for δν, δI, and Δτr at a fixed nuclear configuration, and were summed over the entire MD trajectory. The heterocluster isomerization phenomena could not be identified by the temperature and/or size dependence of δν, Δ, and δI, while Δτr data exhibit a marked structural sensitivity, providing a promising approach for the characterization of heterocluster isomerization. The MD simulations elucidate the size dependence and isomer specificity of spectral shifts, homogeneous linewidths, ionization potential shifts, and pure radiative lifetimes of heteroclusters. The intriguing phenomenon of spectral line narrowing with increasing the cluster size for these heteroclusters, is attributed to the size dependent reduction of inhomogeneous broadening which originates from the cumulative contribution of the isomer-specific spectral shifts and of homogeneous broadening effects.