Second-order reaction Selection rules

A relatively new method for studying chemisorbed species is sum-frequency generation (SFG) (see Table 4.1 for references). This is a second-order non-linear process, requiring both a fixed visible and a tuneable laser the selection rules determine that a vibrational mode must result in changes both to dipole moment and to polarisability for the effect to occur, and this limits it to a medium which lacks inversion symmetry, i.e. to the surface and not the gas phase. This, coupled with the fact that excitation is by photons, not electrons, leads to the inestimable benefit of being usable in the presence of a high gas pressure, and therefore enables in situ examination of the surface under reaction conditions. 

Second, if for some reactions multiple experimental data are available, kinetic parameters obtained by different authors using different methods as a rule are not in a close agreement, and in some cases are even contradictory. In this case we have to employ some reference argumentation in order to choose the one to be used. Some criteria for such selection are already discussed in Sections I.D and III.B. In any case, the final decision about each value is going to be taken by a particular investigator, no matter if he/she gives credence to the authors of preceding studies or makes his/her own evaluations. 

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