Spectroscopic Measurements of Gaseous CN I. Dissociation in the Electric Discharge

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Abstract
Free CN radicals in the gaseous state are formed in the electric discharge through cyanogen. By the use of a very intense low pressure spark as a background the 0,0 and 1,1 bands of the 2Σ→2Σ transition of CN have been observed in absorption at short intervals of time after the end of this discharge. The intensities of the lines of the bands were measured by means of microphotometer traces of the spectra and of the characteristic curve of the photographic plate. They were corrected for the incomplete resolution of the spectrograph. Relative concentrations of CN were calculated from the sums of the intensities of the lines. The rate of disappearance of CN after the end of the discharge was studied and found to be the same over a large range of pressure and dilution with argon. A mechanism is proposed to account for this. In all cases the disappearance was exponential with a mean half-life of 11 milliseconds. The half-life of the first excited vibration state was 3 milliseconds, during which time about 10,000 collisions occurred. By dilution of the cyanogen in the discharge tube with argon until the CN bands were no longer visible the partial pressure of CN could be calculated on the assumption that near this point all the cyanogen present was observed in the form of CN. The minimum detectable pressure of CN was of the order of 2×10—6 mm. The absolute f value of the 2Σ→2Σ transition is 0.026 (±0.006). By using the corresponding absolute absorption coefficient and the observed temperature distribution in a carbon arc the partial pressure of CN present in an arc is calculated to be of the order of 10 mm.

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