Vibrational spectroscopy maximum interaction

The TPD spectra of three different adsorbate systems, corresponding to zeroth-, first- and second-order kinetics, are shown in Figure 2.9. Each trace corresponds to a different initial adsorbate coverage, as indicated in the figure. The simplest case in TPD corresponds to first-order desorption kinetics, represented by the CO/Rh(lll) series in Figure 2.9 [17, 18]. For CO coverages up to 0.5 monolayer (ML), the CO molecules do not interact on Rh(lll) and the desorption traces all fall in the same temperature range, all with the same peak maximum temperature. Hence, the rate of desorption is proportional to the surface concentration of CO. Above 0.5 ML, CO starts to populate additional sites (from vibrational spectroscopy studies we know that in addition to on-top sites also threefold hollow sites are occupied see Fig. 8.15), and a faster reaction channel for desorption opens up, as seen by the development of a shoulder at lower temperatures [18]. 

I, modes which are strongly affected by intermolecular interactions and which show significant changes in the vibrational intensity, and also in the band-maximum position between gaseous and liquid-like states and type II, modes without any change in the vibrational intensity when pressure and temperature are varied. Materials showing type-II behaviour are perfectly suited for quantitative analysis via online absorption spectroscopy. 

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