Spectroscopic manifestations of phase changes in CaAr n clusters: finite size effects

The finite temperature optical spectroscopy of CaArn clusters in the range 6 ≤ n ≤ 146 is investigated using a Diatomics-In-Molecule (DIM) Hamiltonian and classical parallel tempering Monte Carlo simulations. The absorption spectrum is calculated in the vertical approximation at various temperatures between 2 K and 50 K. Several typical situations are reported. CaAr6 shows a strong thermal broadening and shift of the spectral lines, possibly associated with isomerization. CaAr13 only shows some broadening. CaAr37 exhibits features corresponding to coexisting isomers at low temperature. Finally, the abrupt changes in the absorption spectrum in CaAr146 at about 20 K are indicative of surface diffusion.