The electronic emission spectrum of benzaldehyde vapour

The electronic emission spectrum of benzaldehyde vapour excited in a discharge consists mainly of (a) over tilde (3)A '' - (X) over tilde (1)A' phosphorescence but also contains some weak (A) over tilde (1) A ''-(X) over bar (1)A' fluorescence. The spectrum has been photographed under high resolution with a view to establishing which a" vibrations are active in the (a) over tilde-(X) over tilde system. We have shown, with the help of rotational band contours, that the vibration in which the substituent as a whole moves out-or-plane is weakly active and that the substituent torsional vibration is more weakly active. These are the only a" viibrations which are active with sufficient intensity to be observed in the spectrum. In the (A) over tilde-(X) over tilde fluorescence an a" wagging vibration of the hydrogen atoms of the phenyl ring is moderately strongly active, as is the corresponding excited state vibration in the (A) over tilde-(X) over tilde absorption system.