Evidence for paramagnetism of NO molecules chemisorbed on transition-metal surfaces

Metastable-atom deexcitation spectroscopy using singlet ${(}^1$S ${\mathrm{He}}^{\mathrm{*}}$) as well as triplet ${(}^3$S ${\mathrm{He}}^{\mathrm{*}}$${,}^3$P ${\mathrm{Ne}}^{\mathrm{*}}$${,}^3$P ${\mathrm{Ar}}^{\mathrm{*}}$) species reveals that NO chemisorbed on Pd(111) or Pt(111) surfaces remains paramagnetic (on a time scale ${>10\mathrm{}}^{\mathrm{-}13}$ sec) with its 2π̃ level close to the Fermi level. The 1π̃-5σ̃ and 4σ̃ metastable-atom deexcitation spectroscopy peaks show characteristic multiplet splittings which are, however, considerably smaller than with free NO. No paramagnetic properties are found for NO adsorbed on Cu(110) which is attributed to a stronger coupling between the 2π level and the metal.