Mobility of CO on the (110) plane of Tungsten

Abstract

The measurement of time autocorrelation functions of field emission current fluctuations has been applied to the adsorption of CO on the (110) plane of tungsten. For molecularly adsorbed virgin-CO, no evidence for diffusion was found at any coverage, although a weak, exponentially decaying correlation function, suggesting flip-flop between differing ad-sites, was seen. A hysteresis in the mean square fluctuation could be clearly identified with conversion to β-CO. For the latter state a flip-flop signal was seen for 280 ⩽ T ⩽ 650 K, and for T $ ̆ 650 K a correlation function corresponding to diffusion was found. The activation energy of diffusion was 23 kcal, independent of coverage. This value agrees closely with that found for oxygen diffusion at O W = 0.5. For CO readsorbed on a β-layer (α-CO) neither diffusion nor flip-flop was seen.