Using time-resolved photoelectron spectroscopy we show that electron relaxation processes via inelastic electron-electron scattering are efficient energy dissipation channels not only in bulk metals but also in extremely small transition metal clusters. The photoelectron spectra of optically excited Pd 3 Ϫ , Pd 4 Ϫ , and Pd 7 Ϫ reveal effective electron relaxation times of less than 100 fs. Moreover the relaxation times vary with cluster size. In comparison to simple metal clusters the bulklike inelastic scattering rates in open d-shell transition metal clusters are attributed to the larger valence electron level density. An energy transfer to the vibrational degrees of freedom occurs within 10 ps.

Photon-induced ultrafast energy dissipation in small isolated Ni 3 Ϫ has been studied by two-color pumpprobe photoelectron spectroscopy. The time-resolved photoelectron spectra clearly trace the path from a singleelectron excitation to a thermalized cluster via both inelastic electron-electron scattering and electronvibrational coupling. The relatively short electron-electron-scattering time of 215 fs results from the narrow energy spread of the partially filled d levels in this transition-metal cluster. The relaxation dynamics is discussed in view of the cluster size and in comparison to the totally different relaxation behavior of s/p-metal clusters.

No abstract

Thermal CO desorption from photoexcited free metal-carbonyl clusters has been resolved in real time using two-color pump-probe photoelectron spectroscopy. Sequential energy dissipation steps between the initial photoexcitation and the final desorption event, e.g., electron relaxation and thermalization, have been resolved for Au 2 ͑CO͒ 2 and Pt 2 ͑CO͒ 5 2 . The desorption rates for the two clusters differ considerably due to the different numbers of vibrational degrees of freedom. The unimolecular CO-desorption thresholds of Au 2 ͑CO͒ 2 and Pt 2 ͑CO͒ 5 2 have been approximated by means of a statistical Rice-Ramsperger-Kassel calculation using the experimentally derived desorption rate constants. DOI: 10.1103/PhysRevLett.88.076102 PACS numbers: 68.43.Vx, 33.80.Eh, 36.40. -c, 42.65.Re Thermal desorption of a molecule from an equilibrated surface takes place due to statistical energy fluctuations among all degrees of freedom. There is a certain probability that energy is accumulated in a specific chemisorptive bond that exceeds the desorption threshold of an adsorbed molecule. As thermal desorption is a statistical process, which is usually described in terms of stochastic time evolution of the energy content of the dissociative mode [1], the desorption rate depends first of all on the desorption threshold and the total number of degrees of freedom. In isolated particles, the energy cannot simply be released to the surrounding by diffusion or heat transport as in solids. Therefore in small metal-adsorbate clusters a finite probability exists to release the energy by thermal electron emission (thermionic emission) or thermal desorption of a ligand molecule after reaching thermal equilibrium. The energy redistributes either into a particular degree of freedom, i.e., a particular electronic state having one electron less, or a dissociative final state.For photon-induced desorption processes, many dissipation steps are involved between the initial energy absorption and final ligand evaporation. Energy dissipation in photoexcited systems proceeds via electron relaxation and vibrational relaxation where the former process is usually faster than electron-vibrational coupling. Inelastic electron-electron scattering with time constants much less than 100 fs have been observed in optically excited bulk metals [2]. Similar relaxation times have also been observed for Pd and Au nanoparticles [3]. Even the smallest transition metal clusters of Pt, Pd, and Ni show electron relaxation times of about 100 fs [4,5]. Thermalization between the electronic and vibrational system is usually slower ranging to the ps regime.Transition metal-carbonyl clusters such as Pt n ͑CO͒ m 2 are excellent candidates to study photon-induced thermal desorption processes. In these clusters decarbonylation thresholds are smaller than both the electron affinity (thermionic emission threshold) and the metal-metal dissociation energy [6,7]. Besides fluorescence, evaporation of a ligand molecule should thus be the only process by which energy can be rele...

Photoelectron detachment spectra of M2(C6H6)− (M=Pt, Pd, Pb) have been measured in the gas phase using photon energies of a Nd:YAG laser. The vibrationally resolved ground state transition from the anion to the neutral reveals an adiabatic electron affinity of (2.01±0.05) eV and (0.88±0.05) eV for Pt2(C6H6) and Pd2(C6H6), respectively. A ground state vibrational energy of (24.2±1) meV has been resolved for Pt2(C6H6). The corresponding vibrational energy of Pt2(C6H6)− amounts to (19.0±1.0) meV. The ground state vibrational energies of Pd2(C6H6) and Pd2(C6H6)− are (20.3±1.0) meV and (18.0±2.0) meV, respectively. The small vibrational frequencies suggest a perpendicular coordination (C6v-symmetry) of the benzene-adsorbed transition metal dimers. Pb2, on the other hand, is bound parallel to the benzene plane (C2v-symmetry). A closed shell ground state electron configuration is postulated for Pb2(C6H6) in contrast to the triplet ground state of unreacted Pb2. The vertical electron affinity of Pb2(C6H6) is (1.95±0.05) eV.