Electronic adsorption spectral maxima

Of these four properties, spectral shifts are the most sensitive to environmental changes and also the most readily measured. As a result the majority of investigations into electronic absorption spectral changes resulting from surface adsorption have been confined to measurements of spectral shifts. While the shift of the 0-0 bands is the most meaningful measurement to make, these 0-0 bands are not always discernible, especially when the molecules are adsorbed on polar surfaces, so it has become common practice simply to measure the shift of the absorption maximum. In most cases this measurement would correspond to the shift of the 0-0 band, in others, however, adsorption processes can produce unequal displacement of the ground and excited state potential curves, resulting in a different vibronic band shape. 

In the foregoing examples the spectral data indicated a Lewis acid-base reaction on the surface where the alkali and alkaline earth cations acted as the electron acceptors while the adsorbates were the electron donors. It is quite natural that the reverse situation might be possible that is, the adsorbent be basic while the adsorbate show acidic properties so that in the chemisorption electron transfer will occur in the reverse direction. Several examples of such adsorption have already been discussed in this chapter. Kortiim (22) found another example in the adsorption of symmetrical trinitrobenzene on magnesia and on alumina. Whereas trinitrobenzene adsorbed on calcium fluoride or silica was colorless, on magnesia it was red with an absorption maximum at 4650 A (Fig. 26) and the spectrum of the adsorbed species was very... 

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