Fig. 3.

Measurements of hot electron cooling parameters. (A) The electronic temperature of the thiolated (Right) and aminated (Left) Au NP-ligand systems, determined by fitting the TA spectra as a function of time (Fig. 2). The kinetic traces show an initial thermalization period (within the first 300 fs), followed by a period in which the hot electron gas equilibrates with available vibrations (1–5 ps), and a final period in which the lattice dissipates thermal energy to the solvent. The red lines are linear fits to the regime of interest, the cooling of the hot electron gas via electron–phonon coupling, assuming a first-order decay of the electronic temperature. (B) The square of the initial electronic temperature of the system (after thermalization of the plasmon) minus the square of the electronic temperature before photoexcitation, plotted against the energy of the laser excitation normalized by the unit volume of the Au NP. The slope of this plot is inversely proportional to the electronic heat capacity constant for the aminated (γ = 62 ± 2 J⋅m⁻³⋅K⁻²) and thiolated (γ = 87 ± 1 J⋅m⁻³⋅K⁻²) Au NPs. Note that the x-axis is U from Eq. 1 normalized by the volume of the Au NP. (C) The observed lifetime of hot electrons versus the average number of photons absorbed per NP, for the aminated (black) and thiolated (red) Au NPs. Expectation values were calculated with extinction coefficients of 4.71 × 10⁶ M⁻¹⋅cm⁻¹ for the aminated NPs and 3.07 × 10⁶ M⁻¹⋅cm⁻¹ for the thiolated NPs at 520 nm.