$3p$ valence photoelectron spectrum of Ar clusters

The shape of the outer valence $\left(3p\right)$ photoelectron spectrum of Ar clusters is investigated by vacuum ultraviolet photoionization with synchrotron radiation. We show the dependence of the spectrum on cluster size and the change in its shape with photon energy. Inelastic losses due to intracluster photoelectron scattering are most important for changes in the photoelectron main line and explain the appearance of additional peaks. A comparison of our results to earlier work on bulk condensed Ar and Ar thin films is given. Evidence for a deviation of the photoionization cross sections for clusters from the atomic ones has not been found.

Figure 1

(Color online) Photon energy dependence of the $3p$ photoelectron spectrum of medium-sized Ar clusters $\left(⟨N⟩=66\right)$. The photoelectron lines from uncondensed monomers are designated. The different widths of the latter reflect the photon energy dependence of the beamline resolution at fixed exit slit width. The spectrum at 80.1 eV was repeatedly taken for reference purposes, and an average over all acquisitions is shown. Spectra have been normalized to equal total area. The photon energy behavior of the three regions designated in the topmost panel is further discussed below (Fig. 5). Letters $\text{A}–{\text{C}}^{\prime }$ in the middle panel follow the designations by Carnovale et al. (Ref. 6), while arrows marked with a and v designate the adiabatic and the vertical ionization potentials of the Ar dimer in its ground state, respectively (Refs. 1, 3). In the bottom panel additional photoelectron spectra of an Ar monolayer for emission along the surface normal (thin solid line) and under an angle of $\theta =40°$ (dotted line) with respect to the normal are shown (Ref. 13).

Figure 2

Photoelectron spectrum of Ar clusters taken at 35 eV photon energy and showing the energy region of satellites pertaining to inelastic scattering of $3p$ photoelectrons. Energies designated by letters relate the binding energy axis to the panels in Figs. 1, 3. The $\text{Ar}3s$ main lines, coincidentally overlapping in energy, are designated (two broader features from cluster bulk and surface states; one sharp line from monomers). The energies of surface and bulk excitons known from optical absorption measurements of bulk condensed Ar, and shifted by 15.0 eV, are marked as well (Ref. 30). We attribute the remaining structure to interband transitions excited by $3p$ photoelectron scattering (see text for details).

Figure 3

(Color online) $3p$ photoelectron spectra of large Ar clusters $\left(⟨N⟩=1950\right)$ at photon energies around the onset of inelastic losses. The spectra of $\left(⟨N⟩=66\right)$ clusters given in Fig. 1 are repeated for comparison (dotted lines). Each set of three spectra was normalized to equal area of the atomic $3p_{1/2}$ photoline. A scaling constant between the two sets of data was chosen to allow for an easy comparison.

Figure 4

Ratio of the $3p$ cluster feature divided by the $3p$ monomer feature, displayed vs kinetic energy of the $3p_{3/2}$ atomic photoelectron line. (This can be considered a lower limit of the kinetic energy of the respective cluster photoelectrons.) The span of the data points at a kinetic energy of approximately 61.3 eV, which were repeatedly taken during acquisition of the data set, is indicative of the error due to variations in the cluster beam parameters with time. Error bars at low kinetic energies reflect the uncertainty of the transmission function correction. Compared to that, the statistical error of the data is unimportant. See text for details.

Figure 5

(Color online) Ratio of three different regions within the $3p$ valence spectrum of $\left(⟨N⟩=66\right)$ Ar clusters to the $\text{Ar}3p$ monomer lines, with regions as indicated in the top panel of Fig. 1. An enlarged view is displayed for low kinetic energies (left panel). As part 3 represents a wider energy interval than the other two regions, its values have been divided by two to allow for a better comparison. Some points have been connected to guide the eyes. The arrow marks the onset of inelastic losses as seen in Fig. 2. Black symbols (+) pertain to feature ${\text{C}}^{\prime }$ of Carnovale et al. (Ref. 6), red symbols (circle) to A (with some overlap also with B), and blue symbols (x) to B and C. The kinetic energy refers to the $\text{Ar}3p_{3/2}$ monomer line.

Figure 6

(Color online) $\text{Ar}3s$ and $3p$ photoelectron lines recorded at equal kinetic energies. For the $3s$ line (bottom panel), the results of a least-squares fit by Gaussian functions are shown (dotted: total, solid: bulk, and surface component, not convoluted with apparatus broadening).