Two photon photoemission of deposited silver clusters

Abstract

We use time resolved two photon photoemission to study the stability of size selected silver clusters deposited onto highly oriented pyrolytic graphite (HOPG) substrates. Size-selected Ag + n clusters (n = 2 − 9) are deposited at low coverage onto HOPG surfaces at liquid nitrogen temperatures. After deposition, the samples are irradiated by a series of ultrashort laser pulse pairs. Photoelectrons created by two photon photoemission are collected in a magnetic bottle type time-of-flight photoelectron spectrometer. Their kinetic energy distribution is recorded as a function of the delay time between subsequent light pulses. With the exception of Ag 3 the size dependence of the photoelectron spectra reveals a pronounced odd/even effect, which is well known for gas phase silver clusters. This indicates that the deposited clusters retain their size and identity on the sample. The lifetime of the photoexcitation rises with cluster size. This is attributed to an increasing electronic density of states for larger clusters.PACS. 36.40.Sx Diffusion and dynamics of clusters -61.46.+w Clusters, nanoparticles, and nanocrystaline materials -79.60.-i Photoemission and photoelectron spectra -78.47.+p Time-resolved optical spectroscopies and other ultrafast optical measurement in condensed matter

Diffusion and aggregation of Ag n-clusters ( n=2-9) on HOPG probed by fs-two-photon-photoemission

“…A steep increase of the photoelectron yield is visible at 155 K. This increase shows, that the plasmon resonance of the grown nanoparticles begins to shift into the photon energy. The excitation path changes compared to the low temperature regime [6], where the first photon is absorbed by the substrate. With the formation of silver nanoparticles the predominant part of the signal results from absorption of both photons inside the adsorbate.…”

“…To our knowledge no experiments, which investigate the temperature dependent coalescence of the silver clusters, exist so far. In earlier 2PPE-experiments [6] we observed the cluster size dependence of the 2PPE-spectra at a sample temperature of 100 K. The work function of the sample showed a pronounced odd/even effect, which is caused by the different charge transfer of odd-and even-numbered silver clusters to the substrate. From these observations we concluded that the clusters do not coalesce but retain their identity on the substrate at 100 K. The surface of the HOPG substrate is characterized by large flat terraces, which are separated by nearly parallel steps [8].…”

“…It opens up the possibility to analyze the kinetic energy of the emitted photoelectrons as well as to conduct timeresolved measurements with femtosecond laser-pulses in order to get informations about the carrier dynamics [1][2][3][4][5][6]. The method is specially suited for cluster covered surfaces since it allows to analyze the influence of the morphology of the adsorbate on the excitation path.…”