Size effects in angle-resolved photoelectron spectroscopy of free rare-gas clusters

Abstract

The photoionization of free Xe clusters is investigated by angle-resolved time-of-flight photoelectron spectroscopy. The measurements probe the evolution of the photoelectron angular distribution parameter as a function of photon energy and cluster size. While the overall photon-energy-dependent behavior of the photoelectrons from the clusters is very similar to that of the free atoms, distinct differences in the angular distribution point at cluster-size-dependent effects. Multiple scattering calculations trace their origin to elastic photoelectron scattering. Several decades after its beginnings, cluster research continues to be an exciting area of science as most cluster properties remain much less known than those of their constituent atoms and molecules. Moreover, the scalability of clusters allows interpolation between the individual atom, the surface, and the bulk, bridging the gap between single atoms or molecules and condensed matter systems. This makes clusters unique targets to advance both the fundamental understanding of the many-body problem as well as nanotechnological applications ͓1-3͔.Of particular interest are phenomena exhibiting clustersize dependence that underline the transition from individual atoms and molecules to large cluster systems with typical solid-state behavior, such as changes in cluster geometry and electronic structure. As each atom in the cluster is surrounded by an increasing number of neighbors, its electronic structure is altered by the resulting changes in the cluster potential and the atomic orbitals evolve into the band structure of the solid state. Cluster-size-dependent electronic properties can be probed directly using angle-resolved photoelectron spectroscopy, a well established technique for the study of the electronic structure of atoms, molecules, as well as condensed matter ͓4,5͔. However, while photoelectron spectroscopy of clusters, and in particular rare-gas clusters, has been a growing field since the late 1980s ͓6-16͔, there is a paucity of angle-resolved measurements, mainly due to low target densities in the cluster beam and the resulting low signal intensities in angle-resolved measurements. To date, measurements of the photoelectron angular distribution parameter are only available for small metal clusters ͑e.g., ͓17͔͒. For rare-gas clusters, recent qualitative studies by Öhr-wall et al. ͓10͔ have shown substantial differences in the angular dependence of the photoelectron intensity from Xe clusters compared to free Xe atoms, but their experiment did not provide absolute measurements of the angular distribution parameter.In this paper, we present the first quantitative measurement of the photoelectron angular distribution parameter ␤ ͓18͔ as a function of photon energy and cluster size for any rare-gas cluster system. Our experimental results are supported by multiple scattering calculations, which elucidate the effect of elastic electron scattering on the photoelectron angular distribution.The measurements were carried out with linearly polarized sync...

Extreme ultraviolet ionization of pure He nanodroplets: Mass-correlated photoelectron imaging, Penning ionization, and electron energy-loss spectra

“…In contrast, for He + 2 and He + 3 we find slightly altered PES as compared to He + as well as reduced anisotropy of the PADs, presumably due to elastic scattering of the outgoing photoelectrons with He atoms in the droplets. This effect was previously observed with rare gas clusters (Ar, 23,24 Kr, 23 and Xe 23,25,26 ) even for mean cluster sizes as small as N 100. We find indications for multiple excitations and subsequent decay by Penning-like ionization when irradiating the droplets at hν = 21.6 eV which corresponds to the maximum of the 1s 2 1 S→1s2p 1 P droplet absorption band.…”

The photoelectron angular distribution of water clusters

“…In that respect, our study differs from earlier work on the rare gas clusters, in which the interaction of the electronic charge cloud of different cluster constituents is only via much weaker dispersion forces. [12][13][14][15][16][17] More detailed experimental investigations of this point are certainly suggested.…”

“…40 eV, a significant reduction of β was observed in the cluster case. The reduction did only weakly depend on the orbital looked at (Xe 5p, 13 Kr 4p, 14 Ar 3s, 15 Ar 3p, 17 Xe-Kr-Ar comparison 16 ). The authors concluded that variations of β between atoms and atomic clusters are caused by final state elastic scattering of the outgoing photoelectron, and that orbital-specific effects such as band formation do not play a role for the decrease of β.…”

“…3 PADs have been studied for rare gas clusters though. [12][13][14][15][16][17] These studies compared the PAD of several rare gas core and valence photoelectron lines to the ones for atomic photoemission. For photoelectron lines with a strongly anisotropic atomic emission pattern and a kinetic energy below approx.…”

“…They should be affected much less by intracluster elastic scattering, as the respective cross sections have pronounced maxima at kinetic energies around 10 eV and drop by factors between 3 and 6 for an electron energy of 100 eV. 19 Consequentially, for the Xe 4d line of N = 270 clusters, Rolles et al 13 found identical β values between atoms and clusters, even in an energy range with a pronounced photoelectron anisotropy (photon energies 135-150 eV, kinetic energies 85-100 eV). On the other hand, the same authors found significantly lowered β values for Ar 3s of similar size Ar clusters at kinetic energies up to 70 eV.…”