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ABSTRACT: High resolution Stimulated Emission Pumping (SEP) spectra of acetaldehyde in a pulsed molecular beam have been recorded over a wide range of final ground state (SO) energies ($0.4000cm-1$). One of a group of molecules that undergo similar $\pi +\leftarrow n$ C-type transitions, acetaldehyde is distinguished by extremely complex spontaneous emission and SEP spectra even under cold conditions. We suggest that the increased complexity arises from a structurally mediated coupling of vibrational modes via the methyl rotor. This coupling leads to a greatly reduced intensity in the strong progression anticipated in the $C=O$ stretching vibration from a Franck-Condon factor argument, and also results in an increase in the intensity of a manifold of other, normally less active, modes.