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-8 for ionic temperatures below and above melting. At equilibrium the line shape of Na-8 is dominated by two peaks separated by 200 meV. 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.