Ultraviolet spectroscopy forbidden transitions

The ultraviolet spectroscopy of formaldehyde has been studied almost exhaustively, and there is an excellent review on this subject (171). A majority of the bands in the electric-dipole-forbidden vibronically allowed A 2 +X Aj transition have been assigned mostly due to the work of Brand (37), Robinson and DiGiorgio (196), Callomon and Innes (44), and Job, et al. (124). As briefly mentioned earlier, the ground electronic state (X) is planar and the first excited singlet state (A) is pyramidal. It is valid to use the C2V point group symmetry for both electronic states, rather than the C2 point group symmetry (see ref. 171), although the emission could certainly be treated as a -A" - 1A transition. 

An additional advantage of ESA and ESE is the possibility of localizing high energy levels, which could not be accessed using conventional spectrophotometers. This is the case for some bands in the ultraviolet spectral region. Moreover, ESA could allow the observation of optical transitions, which are forbidden by one-photon spectroscopy (Malinowski et al, 1994)... 

The selection rules help to predict the probability of a transition but are not always strictly followed. If the transition obeys the rules it is allowed, otherwise it is forbidden. A molecule can become excited in a variety of ways, corresponding to absorption in different regions of the spectrum. Thus certain properties of the radiation that emerges from the sample are measured. The fraction of the incident radiation absorbed or dissipated by the sample is measured in optical (ultraviolet and visible) absorption spectroscopy and some modes of nuclear magnetic resonance spectrometry (NMR). Because the relative positions of the energy levels depend characteristically on the molecular structure, absorption spectra provide subtle tools for structural investigation. 

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