Surface chemistry of metastable oxygen. I. Production and loss of the 4–5 eV states

Abstract

In an attempt to isolate the important parameters governing the production of the 4–5 eV states of O2 generated from O‐atom recombination on metallic surfaces, we have investigated the effects of substrate composition and temperature on the intensity of the UV glows that are generated. Of the 22 metals investigated, glow‐producing activity is localized in the VIIIb group on the Periodic Table, led by Ni and including Pt, Co, Pd, Fe, and also Au. One parameter that appears to be directly or indirectly linked to this effect is the electronic heat capacity. The six active metals exhibit unique temperature signatures, with Fe, Ni, and Co being distinctly different from Pd, Pt, and Au, measured over the 220–470 K range. The spectra of the UV glows, consisting of the A–X, A’–a, and c–X transitions of O2, differ only in intensity between the metals, not in state or vibrational level distributions. This suggests that the observed spectra are mediated by gas‐phase interactions. The data indicate that at high temperatures, surfacequenching of the A, A’, and c states is rapid, and that under low‐pressure conditions, this effect can be observed even for excited molecules generated in the gas phase. Studies with nickel alloys indicate that the percentage of Ni in a sample is the overriding determinant of the glow intensity, regardless of the other components. Although existing data on energy accommodation coefficients lead to the conclusion that most of the energy in O2 molecules leaving surfaces should be in vibrational modes of the ground state, we have not been able to substantiate this idea.