Vibrationally excited nitric oxide produced in the flash photolysis of nitrosyl halides

Abstract

The flash photolysis of nitrosyl chloride and nitrosyl bromide has been studied under isothermal conditions. Vibrationally excited nitric oxide molecules were produced and all levels from v'' = 0 to v'' = 11 were observed in absorption from the ground electronic states in the $\beta$, $\gamma$, $\delta$ and $\epsilon$ systems. Some of these bands have not previously been reported. The mechanism of the production is either directly $NOR + hv \rightarrow NO(X^2\Pi, v \leqslant 11) + R(^2P),$ or by the sequence which includes the reactions $NOR + hv \rightarrow NO(^4\Pi) + R,$ $NO\bullet^4\Pi + M \rightarrow NO(X^2\Pi, v>0)+M.$ In the latter case, the $^4\Pi$ state of NO lies not more than 3.5 eV above the ground state. Other possible mechanisms and models accounting for the direct production of vibrationally excited NO in its ground electronic states are discussed. By flashing chlorine in the presence of NOCl it was shown that the reaction $Cl + NOCl \rightarrow Cl_2 + NO (v > 0)$ does not occur, thus providing direct evidence that in reactions of the type $A + BCD \rightarrow AB + CD$ only the AB molecule containing the newly formed bond can be vibrationally excited. Vibrational relaxation is very rapid and probably occurs by step-wise degradation involving resonance vibrational energy transfer. NOCl and NOBr are very efficient and with NO itself the reaction $NO(v = n) + NO(v = 0) \rightarrow NO(v = n-1) + NO(v = 1)$ can be followed.