Absorption spectra of low-pressure flames

Absorption spectra of low-pressure flames, which have a relatively thick reaction zone, have been studied by means of a multiple-reflexion system to increase the effective path length in the flame gases. To obtain a background source of sufficient brightness temperature to reverse the absorption bands, a flash tube and rotating sector device has been employed. For a rather rich oxyacetylene flame, the (0, 0) Swan band of C2 and the 3143 Å band of CH have been observed in absorption for the first time in steady flames. A value for the reversal temperature of C2 has been obtained. Rich-limit low-pressure flames of ethers and aldehydes show formaldehyde absorption. In rich flames of dimethyl ether, acetaldehyde, acetone and methane, a band of the CH3 radical at 2157 Å has been observed, and in flames of diethyl ether, and of ethane, a new band with maxima at 2220 and 2242 Å is provisionally assigned to the C2H5 radical. The absorption by OH and OD radicals in flames of mixed C2D2 + H2 have been examined. Results are discussed.