## Abstract

The flash photolysis of cyanogen, cyanogen bromide and cyanogen iodide has been studied under isothermal conditions. Vibrationally excited ($v'' \leqslant$ 6) cyanogen radicals were produced and observed spectroscopically in absorption, in the $\Delta v$ = 0, $\pm$ 1 and -2 sequences of the violet $(B^2\Sigma \leftarrow X^2\Sigma)$ system. The CN radical produced in the reaction $\mathrm{CN} R + hv \rightarrow \mathrm{CN}(X^2\Sigma,v = 0) + R$ is excited electronically, $\mathrm{CN}(X^2\Sigma, v = 0) + hv \rightarrow \mathrm{CN} (B^2\Sigma, v = 0, 1, 2, \ldots),$ and then returns to various vibrational levels of the ground state by fluorescence or collision $\mathrm{CN}(B^2\Sigma, v = 0, 1, 2, \ldots) \rightarrow \mathrm{CN}(X^2\Sigma, v = 0, 1, 2, \ldots) + hv,\\ \mathrm{CN}(B^2\Sigma, v = 0, 1, 2, \ldots) + M \rightarrow \mathrm{CN}(X^2\Sigma, v = 0, 1, 2, \ldots) + M.$ Frequent repetition of this type of excitation in the absence of relaxation would lead to a vibrational `temperature' which may be described as virtually infinite, and in any case is extremely high when relaxation is relatively slow. The alternative reactions by which vibrationally excited radicals could be produced, namely $\mathrm{CN}R + hv \rightarrow \mathrm{CN}(X^2\Sigma, v \leqslant 6) + R$ and $\mathrm{CN} R + hv \rightarrow \mathrm{CN}(A^2\Pi, v > 0)+R,$ followed by $\mathrm{CN}(A^2\Pi, v > 0) \rightarrow \mathrm{CN}(X^2\Sigma, v > 0),$ were shown to account for < 6% of the vibrationally excited radicals observed and may be entirely inoperative. The probability of energy transfer to CNBr from the fourth vibrational level of CN, P$_{4-3}$, was found to be approximately 1 x 10$^{-2}$. The rate constant for the recombination of cyanogen radicals at room temperature was found to be $\sim$ 6 x 10$^{11}$ ml. mole$^{-1}$ s$^{-1}$ or $\sim$ 1.7 x 10$^{16}$ ml.$^2$ mole$^{-2}$ s$^{-1}$ with nitrogen as third body.