Switching from molecular to bulklike dynamics in electronic relaxation of a small gold cluster

We have investigated the ultrafast electronic relaxation of Au${}_7^{-}$ using time-resolved photoelectron spectroscopy combined with first-principles simulations of the excited-state dynamics. Unlike previous findings, which have demonstrated molecularlike excited-state relaxation in Au${}_7^{-}$ at low excitation energy (1.56 eV), we show here that excitation with 3.12 eV leads to bulklike electronic relaxation without a considerable change of geometry. The experimental findings are fully supported by theoretical simulations, which reveal a bulklike electron-hole relaxation mechanism in a far band-gap cluster. Our findings demonstrate that small gold clusters in the sub-nm size range can exhibit either molecularlike or bulklike properties, depending on the excitation energy.

Figure 1

(a), (b) Calculated absorption spectra for the two lowest energy isomers of Au${}_7^{-}$. (c)–(e) Kohn-Sham orbitals corresponding to the dominant excitations for characteristic transitions of the isomer 1 in the energy regions (c) I, (d) II and (e) III.

Figure 2

(a) Experimental and (b) simulated TRPES of Au${}_7^{-}$with $h{\nu }_{\mathrm{pump}}$ $=$ 3.12 eV and $h{\nu }_{\mathrm{probe}}$ $=$ 1.56 eV. The full simulated TRPES with a time resolution of 20 fs is shown in (b) as a two-dimensional plot on the top.

Figure 3

Integrated experimental photoelectron yield of the three energy regions X, $X^{\prime }$, and A (energy region <0.5 eV) as a function of time. The photoelectron transients have been fitted by a model consisting of three coupled rate equations [26] numerically solved by the Runge-Kutta method (solid lines). This model provides exponential decay time constants for the intensity of X, ${\tau }_X=0.6\pm 0.1$ ps, and of $X^{\prime }$, ${\tau }_{X^{\prime }}=1.6\pm 0.2$ ps. The intensity of region A increases up to 1.5 ps before decaying with ${\tau }_A=0.7\pm 0.2$ ps. After 6 ps, no significant photoelectron intensity is visible in any of the three energy regions.

Figure 4

(a) Excited state population dynamics of the isomer 1 of Au${}_7^{-}$ for the ground state $X^{-}$ and the states in excitation energy regions I, II, and III simulated using the FISH method with $h{\nu }_{\mathrm{pump}}$ $=$ 3.12 eV. (b) Temporal evolution of the excited and ground electronic state energies averaged over the ensemble of trajectories. The energy regions I, II, and III are indicated in the graph. The averaged nuclear kinetic energies and electronic energies are also shown as thick red (light gray) and black lines.

Figure 5

(a) Excited state population dynamics of the isomer 2 of Au${}_7^{-}$ for the ground state $X^{\prime -}$ and the states in excitation energy regions I, II, and III simulated using the FISH method with $h{\nu }_{\mathrm{pump}}$ $=$ 3.12 eV. (b) Temporal evolution of the excited and ground electronic state energies averaged over the ensemble of trajectories. The energy regions I, II, and III are indicated in the graph. The averaged nuclear kinetic energies and electronic energies are also shown as thick red (light gray) and black lines.

Figure 6

Time evolution of the hole (red, lower line) and electron (blue, upper line) energies. The instantaneous energies of the electron (filled circle) and hole (empty circle) for selected times are shown together with the energies of the Kohn-Sham orbitals (horizontal black lines) at the stated times.