Shape Phase Transitions in the Absorption Spectra of Atomic Clusters

A quantum-mechanical framework for calculating the photoabsorption cross section of atomic clusters, which explicitly includes the thermal motion of the ions, is proposed. We compute the photoabsorption cross section of Na{sub 8} for ionic temperatures below and above melting. At equilibrium, the line shape of Na{sub 8} is dominated by two peaks separated by 200meV. This structure is found to survive up to T=100 K . With increasing temperatures, the cross section undergoes a shape phase transition such that, at T=300 K , the double-peak structure has changed into a broad and asymmetric line shape dominated by a single peak, providing an excellent fit to the available experimental data. {copyright} {ital 1997} {ital The American Physical Society}