Resolving State-Specific Energy Flow in Metal Nanoclusters Using 2D Electronic Spectroscopy.

Sub-to-few-nanometer gold nanoclusters exhibit a manifold of electronic states that result in nanocluster- and state-specific mechanisms of energy flow, which present new opportunities for developing photonic materials. Due to the spectral congestion of conventional ultrafast transient methods, mechanistic insights into energy flow are difficult to achieve for these systems. The use of two-dimensional electronic spectroscopy (2DES) to resolve electronic relaxation dynamics with state specificity for metal nanoclusters is described, along with prospects for future research. Excitation-detection frequency correlations inherent to 2D measurements resolve the electronic relaxation within specific gold superatom states. The state specificity of 2DES is extended to distinguish the influences of the electronic state symmetry on carrier relaxation using polarization-dependent measurements. Crosspeak-specific 2DES has also been used to distinguish sequential relaxation through nondegenerate electronic state manifolds of nanoclusters from the collective dynamics of metallic nanoparticles. These results demonstrate the power of 2DES for aiding the understanding of metal nanocluster photophysical properties.