Ultrafast Dynamics of Nonequilibrium Electrons in Metal/Adsorbate Nanosystems Christophe Bauer, Jean-Pierre Abid and Hubert Girault Laboratoire d’Electrochimie Physique et Analytique, ISIC, SB, EPFL, CH-1015 Lausanne, Switzerland Summary Metal/molecule interfaces play a crucial role in topics such as molecular electronics, surface femtochemistry, organic light emitting diodes, catalysis and solar cells. Here, we use adsorbates-covered metal nanoparticles to investigate by femtosecond transient absorption spectroscopy the dynamical behaviour of nonequilibrum electrons (NEs) at metal/molecule interfaces. This approach allows: the separation of internal from external thermalization, the investigation of the size behaviour of internal thermalization and hot electron cooling dynamics, the study of molecular vibrations effect on electron transfer process, the identification of the mechanism of internal thermalization retardation, the observation of complex nonlinear dynamics with feedback loops. Ultrafast Electron Dynamics in Metals Dynamical events: The picture Spectral Map: Nonthermal regime and hot electron gas A: Energy redistribution processes in a metal. B: Metal/molecule/metal interfaces: Heart of molecular electronics Femtosecond Pump-Probe Spectroscopy Metal molecule Metal molecule Metal e - ph e - MV Light Metal molecule Metal molecule Metal e - ph e - MV Light e - ph e - MV Light P I Nascent nonthermal electrons (NNEs) Hot electron gas Hot lattice Surrounding medium electron-electron scattering electron-Molecular Vibration scattering P II P III electron-phonon interaction phonon-phonon interaction P I Nascent nonthermal electrons (NNEs) Hot electron gas Hot lattice Surrounding medium electron-electron scattering electron-Molecular Vibration scattering P II P III electron-phonon interaction phonon-phonon interaction 500 550 600 650 -1.0 -0.5 0.0 0.5 Absorbance Change (a. u.) Wavelength (nm) 200 fs 4 ps 500 5 5 0 6 0 0 6 5 0 -1.0 -0 .5 0 .0 0 .5 Absorbance Change (a. u.) Wavelength (nm) Region I Region II Region III 200 fs 4 ps Gold NP Sulfate Excitation Feedback Energy dissipation toward surrounding medium Gold NP Sulfate Excitation Feedback Energy dissipation toward surrounding medium 0 2 4 6 8 10 12 1.0 1.5 2.0 Lifetime (ps) Nanoparticles Diameter (nm) 0 2 4 6 8 10 12 1.0 1.5 2.0 Nanoparticles Diameter (nm) E = E i -E f LUMO h E F Adsorbates ini fin Density of states Nonthermal electrons LUMO Metal h E F ini fin Density of states Nonthermal electrons E = E i -E f LUMO h E F Adsorbates ini fin Density of states Nonthermal electrons LUMO Metal h E F ini fin Density of states Nonthermal electrons E = E i -E f LUMO h E F Adsorbates ini fin Density of states Nonthermal electrons LUMO Metal h E F ini fin Density of states Nonthermal electrons -1 0 1 2 3 4 5 6 7 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 Absorbance Change (a. u.) Delay Time (ps) 50 100 150 200 250 300 1.5 2.0 2.5 3.0 3.5 4.0 Lifetime (ps) Pump Fluence (nJ/pulse) Inelastic electron tunneling at metal/molecule interface Build-up of a population of hot adsorbates (highly vibrationnaly excited adsorbates) Size effect on hot electron cooling L W k k d-band Conduction band Energy E F E max = E max = L W k k d-band Conduction band Energy E F E max = E max = E max = NNEs (Metal) Adsorbates Hot adsorbates Electron tunneling e-MV interaction Negative feedback NNEs (Metal) Adsorbates Hot adsorbates Electron tunneling e-MV interaction Hot electron emission Negative feedback Retardation Internal thermalization External thermalization UCIS e-e scattering Adsorbates e-ph interaction e-e scattering Adsorbates e-ph interaction Retardation Build-up hot adsorbates Retardation Internal thermalization External thermalization UCIS e-e scattering Adsorbates e-ph interaction e-e scattering Adsorbates e-ph interaction Retardation Build-up hot adsorbates B A Gold band structure around L point of Brillouin zone Nonthermal Thermal Nonthermal regime controls external thermalization 530 nm Probing the d-band to Fermi surface transition: Accordance with Fermi liquid theory Non-adiabatic surface reaction: Break-down of Born-Oppenheimer approximation Low-perturbation regime Nonlinear dynamics Interconnection between the dynamical processes 625 nm 615 nm 605 nm 590 nm 0 2 4 6 8 10 12 -0.04 -0.02 0.00 0.02 0.04 0.06 Absorbance Change (a. u.) Delay Time (ps) 2 4 6 8 10 12 -0.04 -0.02 0.00 0.02 0.04 0.06 Absorbance Change (a. u.) Delay Time (ps) 0 2 -2 0 1 -1 Electron energy f Thermal Nonthermal 0 2 -2 0 1 -1 Electron energy f Thermal Nonthermal Possible when the system exhibits chemical interface damping: Electrons tunnel back and forth between metal and adsorbates Additional damping channel for surface plasmon References Transient absorption data for 2.5 nm Pump fluence dependence for 4.2 nm J. Chem. Phys. 120, 2004, 9302 Chem. Phys. 319, 2005, 409