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

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.

Figure 1

(Color online) 2D photoelectron map for $5p$ ionization of Xe clusters with average cluster size $⟨N⟩=70$ measured with the “magic angle” analyzer $(54.7^{°})$. Individual spectra illustrating the spin-orbit split photolines of the monomer as well as the cluster measured with $h\nu =35.5\mathrm{eV}$ at $0°$ and $54.7°$ are shown above the 2D map. For comparison, the inset shows a $\mathrm{Xe}4d$ spectrum (average cluster size $⟨N⟩=270$) measured at $h\nu =82.0\mathrm{eV}$ with the $54.7°$ analyzer.

Figure 2

(Color online) Photoelectron angular distribution parameter $\beta$ of the spin-orbit components of the $\mathrm{Xe}5p$ and $4d$ photolines in free clusters (circles) compared to single atoms (solid curves). Atomic values were taken from the literature [24] and were used to calibrate the relative spectrometer efficiencies.

Figure 3

(Color online) $\mathrm{Xe}4d$ photoelectron angular distribution parameter at $h\nu =150\mathrm{eV}$ as a function of cluster size.

Figure 4

(Color online) Experimental $\mathrm{Xe}5p$ and $4d$ cluster photoelectron angular distribution parameter (symbols) compared to multiple scattering (MS) calculations for icosahedral clusters of 55 atoms (dashed and dash-dotted lines). The energy axis in the calculation is scaled to match calculated and experimental atomic $\beta$ curves (dotted and solid lines).