Magnetic properties of O${}_2$ adsorbed on Cu(100): A spin-polarized metastable He beam study

Magnetic properties of O${}_2$ adsorbed on Cu(100) were investigated by monitoring the spin dependence in Penning ionization of metastable He(2${}^3$$S$) under external magnetic fields of 0–5 T. A clear spin polarization was found for the 3${\sigma }_g$ and 1${\pi }_u$ orbitals of physisorbed O${}_2$ under external fields, while the spin polarization disappeared when O${}_2$ was changed into the chemisorbed state at $>$50 K. The magnetic susceptibility at the surface of multilayer and monolayer of physisorbed O${}_2$ on Cu(100) was similar to that for the bulk liquid O${}_2$. Observed exchange splittings and spin polarization suggest that a physisorbed O${}_2$ molecule has a magnetic moment close to that for an isolated O${}_2$ molecule even at submonolayer coverages, while a density functional theory calculation predicts a much reduced magnetic moment for O${}_2$ directly adsorbed on Cu(100).

Figure 1

Metastable deexcitation spectra of Cu(100) at O${}_2$ exposures are shown. The dashed line corresponds to the background assumed for the secondary electrons and ionic decay channel of (2).

Figure 2

The yield of electrons ejected by He${}^{*}$ at various O${}_2$ exposures. The measurements were conducted at ${\mu }_{0}H$ $=$ 5 T.

Figure 3

The computed local density of states of oxygen in (a) an isolated O${}_2$ molecule and (b) at the surface of $\alpha$-O${}_2$. The surface structure of $\alpha$-O${}_2$ is shown in (c). The Fermi energy was set to the position of the highest occupied molecular orbital. The experimental interatomic distance and the bulk lattice parameters have been used for the calculations.

Figure 4

The sample voltage dependence in the spin asymmetry measured at ${\mu }_{0}H=5$ T and at the O${}_2$ exposures shown.

Figure 5

The change in (a) the spin asymmetry and (b) the sample current at ${\mu }_{0}H=5$ T when warming the Cu(100) surface exposed to 20 L of O${}_2$ at 18 K. The sample current was unable to measure correctly at 40–50 K because of the O${}_2$ desorption.

Figure 6

The magnetic field dependence of the spin asymmetry measured at the sample voltage of 3 V and at an O${}_2$ exposure of 15 L.

Figure 7

(a) The local density of states computed for the oxygen atom (O1) in an O${}_2$ molecule adsorbed on Cu(100). The nearest-neighbor Cu-O distance was set to be 0.24 nm. (b) The variation of the magnetic moment of O${}_2$ as a function of the Cu-O distance. The O-O distance was set to be 0.123 nm for all of the Cu-O distances shown.