Distribution of the Excess Energy in CN B 2Σ Produced in Photodissociation of Cyanogen Halides and Hydrogen Cyanide

Abstract

The vibrational and rotational excitations of CN $B^2\Sigma$ produced in photodissociation of cyanogen halides and hydrogen cyanide in the vacuum ultraviolet have been measured as a function of incident wavelengths. More than 70% of the total population is in levels up to $\text{υ′\hspace{0.17em}=\hspace{0.17em}3}$ . No population inversion was observed, indicating the photodissociation takes place without much change in the equilibrium C–N bond distance. The percent conversion of the excess energy to vibration is approximately 20% for cyanogen halides and about 12% for hydrogen cyanide, irrespective of the amount of excess energy, which is much less than that expected from the equipartition of energy in all vibrational degrees of freedom of the molecule. The rotational levels of CN $B^2\Sigma$ are highly excited for all molecules. The extent of conversion of the excess energy to rotation is of the order of 10%–20% for ICN and ClCN, and only several percent for BrCN and HCN. The remaining energy must be distributed as kinetic energy between CN $B^2\Sigma$ and atoms. The vibrational and rotational distributions are discussed in terms of the equilibrium geometries of the ground and excited molecules and of the final products. The CN $A^2\Pi$ state is also formed in photodissociation and apparently not directly but through the $\mathrm{CN}{B}^2\Sigma$ . The intensity of the CN red emission is much less than that of the violet. Various other photodissociation processes are discussed. In general, the extent of conversion of the excess energy to vibration is much less than that expected from the equipartition theory for most triatomic molecules. No vibrational population inversion has been observed.