Surface selection, absorption

The areas which were found to be particularly important are absorption surface selection, control of reagent desupersaturation, reagent pH profiles, and closed-loop water balance. 

In the first case, the difference in intensities Ip — Is is computed. Due to the surface selection rule, what results is a spectrum showing absorption bands of species on the surface. 

The three most commonly applied external reflectance techniques can be considered in terms of the means employed to overcome the sensitivity problem. Both electrically modulated infrared spectroscopy (EMIRS) and in situ FTIR use potential modulation while polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) takes advantage of the surface selection rule to enhance surface sensitivity. 

Interestingly, the work of Blum et ai (1986) showed that a surface selection rule operates in X-ray reflection absorption. Thus the synchrotron radiation employed in their experiments was polarised in the plane of reflection and the authors noted that bonds perpendicular to the plane of reflection do not contribute to the SEXAFS (cf. the infrared SSR, discussed above). 

As the isoquinoline molecule reorients in the order listed above, the absorption of infrared radiation by the in-plane vibrational modes would be expected to increase, while that of the out-of-plane modes would be predicted to decrease (in accordance with the surface selection rule as described above). In the flat orientation there is no component of the dipole moment perpendicular to the surface for the in-plane modes, and under the surface selection rule these modes will not be able to absorb any of the incident radiation. However, as mentioned above, infrared active modes (and in some cases infrared forbidden transitions) can still be observed due to field-induced vibronic coupled infrared absorption (16-20). We have determined that this type of interaction is present in this particular system. 

This ethylidyne intermediate has recently been subject to two IRS investigations, by Chesters and McCash and in a more detailed study by Malik et alP Their spectra show three absorption peaks above 800cm which in light of the EELS work could be assigned to the C—C stretch mode at 1120cm , the symmetric CH3 bend mode at 1340cm and the symmetric CH3 stretch mode at 2885 cm In contrast to the surface methoxy discussed in the previous section, there is no peak associated with the asymmetric CH3 stretch mode at 2950cm" This shows, in line with the discussion above, that the CCHj is oriented with the C—C axis normal to the surface (as indicated in Fig. 19) and verifies at the same time the validity of the infrared surface selection rule. 

A variant of IRRAS is polarization modulation IR reflection absorption spectroscopy (PM-IRRAS). In this method, the polarization of the IR beam incident on the sample is modulated between parallel and perpendicular polarization. When the sample is metallic, only the parallel-polarized light yields signals from adsorbed molecules, because the electric field amplitude of perpendicular-polarized light vanishes at the metal surface. This statement is the basis for the metal surface selection rule 100,109). When the medium above the sample (gas or liquid phase) is isotropic, both polarizations are equivalent. The PM-IRRAS method thus enables the measurement of signals from adsorbates on a metal surface in the presence of an absorbing gas or liquid phase. 

Fig. 13. Surface selection rules linking the transition moment orientation and the sign and intensity of the absorption band at the air-water interface. Taken from Ref. [91] with permission from American Chemical Society.

The technique of subtracting spectra taken with linearly s- and p-polarized radiation is based on the surface selection rule for reflection-absorption on a metal surface [20] ... 

---