Observation of electronic energy bands in argon clusters

The $3p$ valence region of argon clusters has been investigated with photoemission near the photoionization threshold. A strong feature between 14.6 and 15.3 eV binding energy shows a photon-energy dependence indicative of electronic-energy band dispersion. A similar feature at approximately the same binding energy and with a similar dispersion occurs in photoemission spectra of both the ordered and disordered solids. The effect is already fully developed for scaling-law mean cluster sizes $⟨N⟩$ of approximately 200 atoms, thus showing an early onset of bulklike electronic properties.

Figure 1

(Color online) High-resolution $3p$ (valence band) photoelectron spectra of Ar clusters at photon energies from 15.5 to 18.3 eV. The mean cluster size (from the scaling laws) is $⟨N⟩=1670$. Data recorded with the Scienta analyzer. The sharp, spin-orbit split atomic lines at 15.76 and 15.94 eV binding energy are below the usable energy range of the analyzer at $h\upsilon =16.1\text{eV}$. A horizontal arrow marks the extent of the observed dispersion.

Figure 2

(Color online) Contour plot showing high-resolution $3p$ photoelectron spectra of Ar clusters with mean size $⟨N⟩=1670$ as a function of both photon energy and binding energy. The peak maxima of the $\approx 15\text{eV}$ feature showing dispersion are indicated by black dots. High-resolution data recorded with the Scienta analyzer. The lines designated 1 and 2 derive from the photoemission data for Ar(111) measured by Kassühlke et al. (Ref. 18). See text.

Figure 3

(Color online) Low-resolution $3p$ (valence-band) photoelectron spectra of Ar clusters at different photon energies recorded with the magnetic bottle analyzer. The mean cluster size is $⟨N⟩=41$.