An X-ray photoelectron spectroscopic study of the interaction of oxygen and nitric oxide with aluminium

The interaction of oxygen and nitric oxide with clean aluminium surfaces has been investigated by X-ray photoelectron spectroscopy. Studies have been in the main confined to the temperature range 80-290 K and pressure range $10^{-6}-10^{-2}$ Pa. The Al(2p) level is shown to exhibit a shifted component at a binding energy of $\sim $ 74.5 eV after oxygen interaction at 80 K. A curve-fitting and deconvolution analysis of the shifted peak indicates that it is made up of two components, one at a binding energy of 74.0 eV and the other at 75.3 eV. The lower binding energy component (designated $\alpha $) develops preferentially at 80 K while the higher binding energy one $(\beta)$, assigned to Al$_{2}$O$_{3}$, is dominant at 290 K. We suggest that $\alpha $ is an incorporated oxygen structure which is a precursor to the formation of Al$_{2}$O$_{3}$. The initial sticking probability of oxygen at 80 K is 0.07 while at 290 K it is 0.02. The plasmon-loss features associated with the Al(2s) peak are shown to be sensitive to oxygen adsorption and therefore useful in confirming the surface monolayer. At 290 K and an oxygen pressure of $10^{4}$ Pa the oxide thickness is estimated to be about 0.9 nm. When NO was adsorbed at 80 K three distinct N(1s) peaks were observed at binding energies of about 397, 403 and 407 eV. We assign the 397 eV peak to N$_{\text{ads}}^{\delta -}$ arising from dissociation of the molecule and the two higher binding energy peaks to NO$_{\text{ads}}^{\delta -}$ and N$_{2}$O$_{\text{ads}}$. The N(1s) peak characteristic of $\text{NO}_{\text{ads}}^{\delta -}$ is close to the lower binding energy peak of the two N(1s) peaks associated with $\text{N}_{2}\text{O}_{\text{ads}}$ while the O(1s) of $\text{NO}_{\text{ads}}^{\delta -}$ overlaps with the O(1s) peak of the surface oxide at a binding energy of about 532 eV. Mass spectroscopic analysis of the gas phase indicated that on warming to 85 K, the adlayer formed at 80 K, $\text{N}_{2}\text{O}$ was desorbed, confirming our assignment of the core-level spectra. The $\text{NO}^{\delta -}$ $\text{and}$ $\text{N}_{2}\text{O}$ species are not observed at 290 K while at 80 K exposure of the adlayer to water vapour results in the complete removal of weakly adsorbed $\text{N}_{2}\text{O}$. By monitoring the intensities of the Al(2p), N(1s) and O(1s) peaks, estimates were made of the absolute and relative concentrations of the various species and various molecular processes delineated.