Knowledge Bank

The rotational analysis of the high resolution emission spectrum of formaldehyde in the near ultraviolet region gives evidence that the upper state of this transition has an equilibrium configuration that is slightly non-planar. The inertial defect, $A-IC-IB-IA$, calculated for the upper level of the $\alpha$-band is small but definitely negative. For a rigid planar structure it should be zero, while in the lowest level of the planar ground $state1$ it is of the same order of magnitude as here but positive due to zero-point vibrations. A re-examination of part of the absorption $spectrum2$ similarly shows negative defects. If the equilibrium configuration of the upper state is non-planar as suggested by the rotational analysis, but with the potential energy of the planar configuration only slightly higher, one would expect inversion doubling of the lowest upper state vibrational levels. The puzzling $125cm-1$ energy difference between the lowest observed vibrational states of the emission system ($\alpha$-band) and absorption system (A-band) may thus be interpreted. As a consequence of this difference in geometry between the ground and excited electronic states, progressions of the out-of-plane bending vibration are observed as predicted by the Franck-Condon Principle.