Femtosecond Relaxation Dynamics of Au₂₅L₁₈⁻ Monolayer-Protected Clusters

Abstract

I. Technical aspects of transient grating experimentsFemtosecond spectroscopy experiments are based on a Quantronix Q-lite seeded Integra C Titanium Sapphire amplifier generating 130 fs, 800 nm, 2.0 mJ laser pulses at 1 kHz. The laser system pumps two home-built noncollinear optical parametric amplifiers (NOPA). 1,2 The NOPA used for pump pulse generation has a spectral bandwidth corresponding to transform limited 15 fs laser pulses, whereas the NOPA from which probe pulses are derived generates spectra spanning the full 500-750 nm wavelength range. Portions of the probe spectrum are filtered in a fused silica prism compressor for use in experiments. Pump and probe pulses are compressed to 15-20 fs with a time-bandwidth product of 0.5-0.6 where residual third-order dispersion prevents compression to the Fourier transform limit.Transient grating (TG) experiments are performed with the interferometer shown in Figure S1. The interferometer generates a trapezoidal laser beam geometry with diffractive-optics (DO) for passively phase-stabilized interferometric signal detection. [3][4][5][6][7][8][9] The TG interferometer uses a DO (Holoeye) producing an angle of 4.65 degrees between the +/-1 diffraction orders at 590 nm. The pump (pulses 1 and 2) and probe (pulses 3) beams are crossed at angle of approximately 4.6 degrees in the DO. Pulses 1 and 2 arrive at the sample at the same time, and are delayed with respect to pulses 3 and 4 with a motorized translation stage. The signal is phase-matched so that it is automatically collinear with the reference pulse after the sample.

Ultrafast Optical Study of Small Gold Monolayer Protected Clusters: A Closer Look at Emission

“…27,58,73 C. On the Mechanism of Emission. From the emission data, we can identify two luminescence transitions: transition from a short-lived state in the visible range, which is most probably associated with the MPC's core, 21,22,59,64 and that from a longlived surface-related state in near-infrared most probably associated with ligands. 21,22,59,64 Our time-resolved fluorescence data showed that the visible emission lifetime (∼250 fs, Figure 3) is substantially longer than the lifetime of the visible emission of larger nanoparticles.…”

“…From the emission data, we can identify two luminescence transitions: transition from a short-lived state in the visible range, which is most probably associated with the MPC's core, 21,22,59,64 and that from a longlived surface-related state in near-infrared most probably associated with ligands. 21,22,59,64 Our time-resolved fluorescence data showed that the visible emission lifetime (∼250 fs, Figure 3) is substantially longer than the lifetime of the visible emission of larger nanoparticles. 59 The short-lived visible emission mechanism can be explained by the creation of the hole by the excitation in the ground state, which can be subsequently filled with the electron from the higher lying excited state near 2.5 eV (Figure 4, B band).…”

“…This transition is similar to the HOMO-LUMO+1 suggested on the basis of crystal structure for Au 25 (SR) 18 . 21,22 HOMO-LUMO transition corresponds to the transition from ground state to A band on the diagram shown in Figure 4. The energy gap of this transition is lower than that for the observed visible fluorescence.…”

“…62 The proximity of surface plasmon in terms of energy can enhance the fluorescence, thus leading the higher quantum yield as compared with bulk metal. 59 Surface states were used in reported literature 22,57,76 to explain the longer fluorescence lifetime in the NIR for nanoclusters. It was suggested that the A band to ground-state transition is mostly nonradiative and does not produce detectable fluorescence.…”

“…[1][2][3][4][5][6][7][8] Metal clusters had been investigated previously in the gas phase; 2,20 with the discovery of metal clusters in the condensed phase, a great deal of interest is developed in the application, properties, and fundamental physics of these nanosystems. 1,[3][4][5][6][7][8][21][22][23][24][25] The interest in nanoparticles by a large following of scientists and engineers focuses on the properties and applications of small metal topologies. In particular, there is a great interest in the possibility that these small clusters of metal (<2.5 nm) can be used in catalysis, [10][11][12][13][14][15] linear and nonlinear optical effects, [26][27][28] and "quantized" electrochemical applications.…”