Photoabsorption Spectra of Na_n+ clusters: Thermal Line-Broadening Mechanisms

Photoabsorption cross sections of small sodium cluster cations (Na$_n^+$, n=3,5,7 and 9) were calculated at various temperatures with the time-dependent local-density-approximation (TDLDA) in conjunction with ab initio molecular dynamics simulations, yielding spectra that agree with measured ones without ad-hoc line broadening or renormalization. Three thermal line-broadening mechanisms are revealed: (I) lifting of level degeneracies caused by symmetry-breaking ionic motions, (II) oscillatory shifts of the entire spectrum caused by breathing vibrations, and (III) cluster structural isomerizations.