Spectroscopic interrogation of heterocluster isomerization. II. Spectroscopy of (9,10 dichloroanthracene)⋅(rare gas)n heteroclusters

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Abstract
In this paper we report on the electronic spectroscopy of mass‐resolved heteroclusters of 9,10‐dichloroanthracene (DCA) with rare‐gas atoms, i.e., DCA⋅Arn (n=1–54), DCA⋅Krn (n=1–20), and DCA⋅Xen (n=1–14), which were interrogated by resonant two‐photon two‐color ionization in conjunction with mass spectrometric detection. We measured the spectral shifts δν and the linewidths (FWHM) Δ of the S0→S1 transition, the ionization potential shifts δI from S0, and the change Δτr in the pure radiative lifetime of S1, relative to the corresponding values for the bare DCA molecule. We were able to provide a global assignment of the structure of isomers of DCA⋅An (A=Ar,Kr) heteroclusters, which rests on six independent sources of information, i.e., the application of experimental combination rules and on the comparison between the results of the molecular dynamics (MD) simulations and the experimental data for δν, δI, and Δτr. The size dependence and the isomer specificity of δν, δI, and Δτr are well accounted for by microscopic theories for these spectroscopic observables in conjunction with MD simulations. The size dependence of the experimental linewidths was analyzed in terms of finite temperature MD simulations, which incorporate the effects of homogeneous and inhomogeneous line broadening. Our analysis provides an adequate description of the qualitative difference between the gradual line narrowing of DCA⋅Arn (n=20–34) and the abrupt onset of line narrowing of DCA⋅Krn (n≥16) heteroclusters, which are attributed to the reduction of inhomogeneous line broadening originating from the combined effects of homogeneous line broadening and of the isomer specificity of the spectral shifts. The temperature dependence of the homogeneous linewidths provides means for internal cluster thermometry. The isomerization phenomena inferred from MD simulations of nuclear dynamics could not be identified by the cluster size dependence of δν, δI, Δτr, and Δ, in accord with the conclusions drawn from the MD simulations of these spectroscopic observables. Finally, we explored the evolution of condensed matter properties with increasing the heterocluster size, advancing cluster size equations, which quantitatively accounts for the smooth ‘‘transition’’ of the spectral shift and of the homogeneous line broadening of DCA⋅Arn heteroclusters to those of DCA in bulk solid Ar.