Relativistic effects Resonance frequency

Over recent years there has been a steady growth of interest in vibrational effects in the context of ab initio calculations of linear and non-linear molecular response functions. It has been realized that in some cases vibrational may rival electronic contributions to the parameters controlling non-linear optical responses. This is particularly likely where the molecule is of higher symmetry (quadrupolar or octupolar rather than dipolar), and for lower frequency effects where there is little pre-resonant enhancement of the electronic contribution. The main features of the theoretical methodology for the calculation of vibrational response functions were established several years ago and the fundamental papers were reviewed in the previous volume. Recent developments have been the introduction of field induced co-ordinates, improved integration techniques and the first relativistic studies. ... 

The advantages of the collinear geometry and a purely relativistic Doppler effect can be combined in a resonant two-photon absorption experiment. The two photons are absorbed from the direct and the retroreflected laser beam of frequency pl, and a few atomic systems have intermediate states that can be tuned into resonance at a beam velocity selected by... 

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