Photon-Induced Thermal Desorption of CO from Small Metal-Carbonyl Clusters

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...

Dynamics of O2 photodesorption from metal clusters: A significant difference from bulk behaviour

“…Moreover, the extremely short lifetime also supports our interpretation for direct photodesorption in contrast to thermal photodesorption. The latter has been observed for Au 2 (CO) À , Pt 2 ðCOÞ À 5 and Pt 2 (N 2 ) À [10,11]. Interestingly, the spectra of Ag 4 O À 2 and Ag 8 O À 2 are similar to those of Ag 2 O À 2 .…”

“…So far, direct photodesorption of molecules from metal clusters has not been observed. The few experimental observations [10,11] all proved to be dealing with thermal desorption rather than a direct process: The energy of the photon is thermalized within less than 100 fs, and in a subsequent step, the molecule desorbs from the hot cluster via unimolecular dissociation. In that case, the initial electronic excitation is quenched extremely fast.…”

Electronic structure in real time: mapping valence electron rearrangements during chemical reactions

“…4 shows how Lu¨ttgens et al used TRPES to trace the thermal desorption of CO from Au 2 (CO) À clusters. 54 The rapidly decaying transient spectral features at low binding energies (at below 1 eV) are assigned to the ultrafast relaxation of electrons excited in the cluster by the pump pulse. Their shift to higher binding energies with time indicates ultrafast inelastic scattering of hot electrons within the electronic system of the cluster with a time constant of 65 fs.…”

Time-resolved study of the photodissociation of Au2O−

“…In case of this molecule, TR-PES is able to monitor part of the path of the system propagating on the excited state surface. This is an example of the energy of a visible photon converted into chemical energy by a long-lived excited state, in contrast to fast thermalization observed for many other clusters [17,18]. Further studies of larger clusters are on the way.…”