Rydberg states in triatomic molecules

For molecules, just as for many-electron atoms, one uses equation (2.8) to describe the series. As before, the quantum defect p characterises the whole Rydberg series but now absorbs the influence of the molecular core. In practice, its value still depends largely upon the atomic symmetry of the Rydberg orbital (more detail on its properties for polyatomic molecules will be given in section 3.11). 

Two further features are often observed in vacuum ultraviolet absorption spectra which reflect the molecular core structure associated with the Rydberg transitions. 

The second feature that may become apparent in vacuum ultraviolet spectra is rotational and/or vibrational structure. This can be more or less obtrusive, depending on the molecule concerned and, in some cases, may mask the Rydberg series unless steps (e.g. cooling of the molecule) are taken to simplify the observed spectra. 

The most important molecule of this sequence is of course H2O [88]. In terms of analysis, this is actually the most complex case. This is because (i) H2O is a strongly asymmetric rotor, which complicates the rotational spectra the properties of a symmetric rotor are shown in fig. 2.14 (2) a large number of vibronic transitions are observed and (3) the elec- 

This provides a simple example of how a spectrum becomes complex in structure and exhibits irregularity of energy spacings as a result of symmetry being broken. The dramatic effects of broken symmetry on the complexity of spectra and the emergence of erratic level spacings will be discussed in more detail in chapter 10. 

---