Diagnostics of Spectrally Resolved Transient Absorption: Surface Plasmon Resonance of Metal Nanoparticles

Time and frequency resolved transient absorption measurements yield two-dimensional images that map the dynamical correlation between the center and width of the scattering function. Global analysis of such data allows unique diagnostics of the mechanics underlying the time evolution. We specialize in the case of surface plasmon resonances of optically driven nanoparticles. We present a catalog of 2D maps that can be used to fingerprint physically meaningful cases, and we provide two experimental examples to illustrate the diagnostic value of the maps and their utility in extracting the various time constants at play. In silver nanorods, the experiment shows a π/2 phase shift between the oscillations of the center and the width of the plasmon resonance. Inspection of the maps allows the assignment that the center of the plasmon resonance tracks the strain in shape-oscillations, while the width tracks the strain rate. This finding is the basis of the novel mechanism of plasmon damping due to electron scattering from the electrophoretic potential generated by the motion of the interfacial double layer in colloidal nanoparticles. Measurements in gold nanoparticles show over-damped oscillations, which obscure the phase correlation between the center and width of the plasmon. The damping is dominated by inhomogeneous dephasing, and the time dependence of the width, which follows the temperature of the nanoparticles, and is diagnostic of the interband transition contribution to the plasmon resonance.