SHAPE PHASE-TRANSITIONS IN THE ABSORPTION-SPECTRA OF ATOMIC CLUSTERS

A quantum-mechanical framework for calculating the photoabsorption cro ss section of atomic clusters, which explicitly includes the thermal m otion of the ions, is proposed. We compute the photoabsorption cross s ection of Na-8 for ionic temperatures below and above melting. At equi librium 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 incr easing temperatures, the cross section undergoes a shape phase transit ion such that, at T = 300 K, the double-peak structure has changed int o a broad and asymmetric line shape dominated by a single peak, provid ing an excellent fit to the available experimental data.