Photolysis of Methane at 1236-Å: Quantum Yield of Hydrogen Formation

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Abstract
Pure CH4 and CD4 have been separately but simultaneously photolyzed at 1236 Å by means of a split‐cell technique in order to obtain accurate relative quantum yields for the various modes of production of H2 and D2. The split‐cell technique has been used to obtain quantum yields by comparison with the CO2 actinometer. Molecular elimination yields for CD4 and CH4 (0.58) are found to be equal, while the H‐atom yield greatly exceeds the D‐atom yield. It is concluded from the results that a significant fraction of the atoms produced disappears by the association reaction M + H + CH3→CH4 + M. The effect of scavengers and inert gas also have been investigated. The total quantum yield for H and H2 formation is greater than unity and fragmentation of CH2 or CH3 formed in the primary process is suggested: CH3→H + CH2, CH2→H + CH. However, the lower limit for the quantum yield for D and D2 formation is 0.76 and indicates that fragmentation of CD2 or CD3 is less important than that of the corresponding protonated species.

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