Plasmon Dynamics in Nanoclusters: Dephasing Revealed by Excited States Evaluation

The photocatalytic efficiency of materials such as graphene and noble metal nanoclusters depends on their plasmon lifetimes. Plasmon dephasing and decay in these materials is thought to occur on ultrafast time scales, ranging from a few femtoseconds to hundreds of femtoseconds and longer. Here we focus on understanding the dephasing and decay pathways of excited states in small lithium and silver clusters and in plasmonic states of the π-conjugated molecule anthracene, providing insights that are crucial for interpreting optical properties and photophysics. To do this, we study the time dependence of the electronic density matrix of these molecules using a new approach that expresses the density matrix in terms of TDDFT eigenstates (ESs) of the TDDFT Hamiltonian. This approach, which involves combining linear response time-dependent density functional theory (LR-TDDFT) and real-time time-dependent density functional theory (RT-TDDFT), leads to an analysis of the electron dynamics in terms of ESs, rather than individual molecular orbital (MO) transitions as has typically been done. This circumvents the complexities and subjective biases that traditional MO-based analysis provides. We find in an analysis of the induced dipole moment in these molecules that what had previously been considered to be energy relaxation is actually dephasing associated with the eigenstates that are stationary after the excitation pulse is turned off. We conclude that the ES-basis analysis has significant potential to advance understanding of the electron dynamics of plasmonic nanomaterials, aiding their optimization for photocatalytic and technological applications.