Electronic spectra of polyatomic molecules
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Abstract
Chapter 2 dealt with the analysis of high resolution electronic spectra of diatomic molecules where the electronic transition also involved simultaneous changes in vibrational and rotational quantum numbers. For an N-atomic molecule there are up to three degrees of rotational freedom and (3N – 6) vibrational modes (3N – 5 if the molecule is linear). Although some of these may be degenerate if the molecular symmetry is high enough, the overall appearance of the spectrum is as a result generally very complex indeed. For example, in the analysis of the 455 nm absorption band of the asymmetric molecule glyoxal many hundreds of individual rotational transitions are observed in each of the vibrational progressions. The analysis of such spectra is a complex task and the adventurous reader is directed to the book by Herzberg [4.1] and for a modem review to Innes [4.2] . Molecular geometry parameters associated with the molecule in ground and excited electronic states constitute one of the end products and these results have been used to test qualitative and quantitative theories of bonding and molecular structure. In particular the simple molecular orbital scheme developed by Walsh [4.3] some years ago, and often called Walsh’s rules, have proven very useful in this respect for small molecules.
When the electrons in the atom are excited, for example by being heated, the additional energy pushes the electrons to higher energy orbitals. When the electrons fall back down and leave the excited state, energy is re-emitted in the form of a photon. ... These emitted photons form the element's spectrum.✨