Rovibrational fine structure

To perform refiable quantum chemical simulations of electronic spectra, a detailed understanding of the different state and possible transition types is necessary. Figure 1 provides a schematic overview of the processes usually observed for molecules where, for simplicity, the rovibrational fine structure of the electronic levels has been neglected. 

UV-VIS spectra are complex as they contain the fine structure from rovibrational transitions. Upon adsorption this structure can only rarely be observed. Due to the blurring of the rovibrational levels mainly broad unresolved bands remain with the complete loss of structural information. The assignment of bands to certain electronic transitions is carried out in correspondence with the direction of the frequency shift or with the aid of semiempirical concepts. At present there is no strict theory relating the spectral changes to the nature of molecular interaction. New surface complexes and intermediate surface compounds have to be determined by comparison with the spectra of the same compounds in solution. 

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