IR photon energies

Elsewhere, in the mid-IR, photon energy is sufficient to modify the quantized terms vib and iJjo in expression 10.2. This is therefore a vibration-rotation spectrum, that is, several tens of rotational transitions accompany each vibrational transition. For the simplest molecules it is possible to interpret particular aspects of the absorption bands. Experience and theory have enabled rules of the permitted transitions to be drawn up. Small molecules as carbon monoxide and hydrogen chloride (Figure 10.5) have been intensely studied from this point of view. 

To date, only IR photon energies in the H-stretching range have been used in IR-induced population transfer experiments, which for most systems results in excitation far above the isomerization barrier, impeding an experimental determination of the isomerization barriers directly. Zwier and coworkers presented an elegant alternative, where stimulated emission pumping (SEP) is combined with HF or PTS methods, see Fig. 2d. This combination allows one to probe the barrier to conformational isomerization [56]. The method consists of two steps a pump-dump SEP followed by a probe laser interrogation to determine the new cmifomiational distribution. In the early part of the molecular beam expansimi, SEP prepares the... 

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