Free nanoscale sodium clusters studied by core-level photoelectron spectroscopy

Free sodium metal clusters have been studied by probing the $\mathrm{Na}2p$ core level using x-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES). The development of electronic structure with size has been studied and discussed in comparison with the atom, dimer, and solid. Information on cluster metallic properties, size, and temperature has been deduced from the XPS measurements. For the large $⟨N⟩>{10}^3$ Na clusters, the surface and bulk sites have been separated in the photoelectron signal. Auger spectra allowed extracting the information on the valence band. The present study introduces core-level spectroscopies XPS and AES into the field of free neutral metal cluster research.

Figure 1

(Color online) High-resolution sodium $2p$ XPS spectrum of large $⟨N⟩\approx {10}^4–{10}^6$ clusters measured at $h\nu =74\mathrm{eV}$ (crosses). The spectrum is fitted assuming bulk (solid line) and surface (dotted line) features for each of the $2p_{3∕2}$ and $2p_{1∕2}$ spin-orbit components. The total resolution is estimated to be around $40\mathrm{meV}$.

Figure 2

$2p$ XPS spectra of large $⟨N⟩\approx {10}^4–{10}^6$ sodium clusters measured at three different photon energies: 74, 120, and $198\mathrm{eV}$. Argon lines have been subtracted from the cluster signal. All spectra are normalized to the height of the $2p_{3∕2}$ bulk signal. The instrumental resolution is $85\mathrm{meV}$.

Figure 3

Photoelectron spectra for two different sizes of sodium clusters measured at photon energies of (a) $74\mathrm{eV}$ and (b) $120\mathrm{eV}$. The smaller cluster spectra (gray line) are shifted relative to the larger cluster spectra (black line) toward higher binding energies by $\approx 40\mathrm{meV}$. All spectra are normalized to the height of the $2p_{3∕2}$ bulk signal.

Figure 4

Normal Auger spectrum of sodium clusters recorded using $90\mathrm{eV}$ photon energy. Dimer Auger feature width and position are also shown.

Figure 5

Comparison of atomic (Ref. 14), dimer (Ref. 16), and sodium cluster $2p$ XPS spectra.

Figure 6

$2p$ binding-energy difference for Na clusters as a function of $1∕R$ and $⟨N⟩$. The straight line is the graphical presentation of Eq. (1) using the experimental points for $⟨N⟩=6\times {10}^5$ atoms and $E_{\mathit{\text{cluster}}}-E_{\mathit{\text{atom}}}\approx -4.4\mathrm{eV}$.