Predissociation in the Spectrum of OH; the Vibrational and Rotational Intensity Distribution in Flames

In the main 3064 Å system of OH higher rotational levels of bands with v' = 1 and all levels with v' ≥ 2 are affected by a weak predissociation. One spin component of the 2Ʃ+ levels is more strongly affected than the other. Of the three molecular states, 4II, 4Ʃ¯ and 2Ʃ¯ which could conceivably cause the predissociation, only the 2Ʃ¯ could produce this effect. The predissociation is observed in the OH bands from a discharge and from the reaction zone of an oxy-acetylene flame at low pressure, and also to some extent in a flame at atmospheric pressure. The predissociation, and its reverse process, a ‘pre-association’, may also affect the vibrational intensity distribution if there is departure from equilibrium. Striking anomalies of this type in hydrogen flames are interpreted in this way. For flames at 1 atm. pressure there appears to be an excess of free atoms in the flame, and at low-pressure emission of radiation from the flame disturbs the equilibrium.