Direct observation of the k 3Πstate of 12C18O

A weak rotationally resolved absorption band of 12C18O has been identified from photographic VUV spectra and assigned to the k 3Π (v = 3) ← X1Σ+ (v = 0) forbidden transition. The experimentally determined band origin and upper state rotational constant are in close agreement to that derived from isotopic scaling. A consideration of the intensity structure of the band suggests that the band gains its intensity mainly through a k 3Π (v = 3)-E1Π (v = 0) interaction. This is the first direct observation of the k 3Π state for 12C18O.