A Spectroscopic Study of the Decomposition and Synthesis of Organic Compounds by Electrical Discharges: Electrodeless and Glow Discharges

Abstract

In the chemical reactions which occur in electrical discharges through organic vapors and in the maintenance of the discharge, the simple ionization of the molecules, and the resultant formation of clusters, is found to be less important, and the breaking down into molecular, atomic and ionic fragments, more important than has heretofore been supposed. The fragments revealed by the spectroscope are CH, OH, NH, C2, CN, N2, CO, CO+, CS, S2, H2, C, C+, H, and S, and in addition H2O and NH3 are known to be formed. When the electrodeless discharge is in benzene vapor the pressure is rapidly lowered until the discharge is extinguished, but this effect is not found with the glow discharge. As compared with the electrodeless discharge, the spectrum from the glow discharge differs in the following respects: the C2 bands are much less intense and only the fourth group of Swan bands appears appreciably. The line spectrum of C is less intense, and the prominent λ4267 line of C+ disappears. The CH bands are very clear and that at λ3900 is closer in intensity to that at λ4300 than in the electrodeless discharge. The Balmer series of hydrogen is slightly more intense, and the many-line spectrum of H2, not present with the electrodeless discharge, is prominent. In general the greatest intensity of the spectrum is given by the cathode glow and the edge of the negative glow near the cathode. The Crookes dark space is a region of very low intensity. The Balmer lines are of nearly uniform intensity throughout except for a specially high intensity just at the cathode. The many line spectrum of hydrogen differs from the others in that it is scarcely visible in the cathode glow. With phenol many bands due to CO and CO+ dominate the spectrum of the glow discharge, though they are not found in the electrodeless discharge. In the electrodeless discharge the rate and nature of the reaction and the spectrum emitted are affected by the hydrogen to carbon ratio in such a way that the rate of reaction decreases as this ratio increases, and the fraction of gaseous products and the intensity of the spectra emitted by molecules which contain hydrogen increase with this ratio. Thiophene is decomposed into sulfur atoms (S), and molecules (S2), and carbon monosulfide molecules (CS) in addition to molecules of carbon (C2) and of monohydrocarbon (CH), atoms of hydrogen (H) and of carbon (C), and ions of carbon (C+) as had been shown before for the hydrocarbons. In the glow discharge both saturated and unsaturated hydrocarbons are decomposed at about the same rate to form the molecules and atoms listed above. The decomposition products from the glow unite to form brown or black solids somewhat similar to those formed in the electrodeless discharge. From the way in which the product is deposited and the intensity of the spectrum, the reaction is seen to be most rapid at the cathodeward edge of the negative glow.