Observation of conformers with laser electronic spectroscopy in supersonic molecular beams

We discuss the ability and effectiveness of electronic spectroscopy to reveal rotational isomerism by presenting some examples of the S1 implied by S0 electronic spectra of non-rigid molecules. One or two photon electronic spectra have multiple features when the molecule has more than one conformational preference. Torsional bands showing up in the spectrum complicate the assignment of conformers. Hole burning experiments give definite conclusions on the existence of rotational isomerism and an example from the literature is given.