Evolution of plasmon-like excited states in silver nanowires and nanorods

Silver nanowires and nanorods are useful prototypical systems to study the emergence of plasmons within a quantum mechanical context because their high aspect ratios enable plasmons to emerge in smaller systems than for roughly spherical nanoclusters. Here, we quantify the plasmon-like character of the excited states of silver nanorods and nanowires based on three nearly orthogonal criteria: (1) collectivity, (2) dipole additivity, and (3) superatomic character. Based on these three criteria, we classify the excited states as plasmon-like, collective, single-particle, interband, or as intermediate between these categories. We show that linear nanowires have a longitudinal absorption peak that has single-particle character and a transverse absorption peak that evolves from a single-particle to plasmon-like to a mix of plasmon-like and interband with increasing length. Increasing the width tends to increase the plasmon-like character of the longitudinal excited state. In contrast, increasing the nanorod width tends to decrease the length at which interband transitions start mixing significantly into the transverse plasmon-like excited states.