The photodissociation of ammonia in the absorption system. Part II. Translational excitation of the hydrogen atoms produced, and the mechanism of the predissociation

Abstract

The photolyses of NH3 and ND3 have been studied at 2139, 2062, and 1850 Å in the presence of propane and ethylene. Upper limits (none was actually observed) were established for the quantum yields of molecular dissociation of D2 from ND3 of 0.003 and 0.004 at 2139 and 2062 Å, while at 1850 Å a definite yield of 0.009 was obtained. Similar results were observed with NH3. From the dependence of hydrogen yields on the ratio of ethylene to propane, it was concluded that H and D atoms were produced in the photolysis with excess translational energy. Values of the integrated reaction probability (IRP) of hot H atoms with propane were estimated to be 0.078, 0.070, and 0.045 at 2139, 2062, and 1850 Å respectively, while corresponding values for hot D atoms from ND3 were 0.083, 0.062, and 0.029. Implications of the decrease in IRP with increasing photon energy are discussed, and it is concluded that at the shorter wavelengths a second dissociation channel leading to NH2(2A1) becomes important. A mechanism for the predissociation of the Ã-state of ammonia is presented which accounts for this behaviour and for the deuterium isotope effects observed previously. It is suggested that the dissociation does not follow the state correlation rules for dissociation in the plane of the molecule, at least when the ν2 out-of-plane bending vibration in the Ã-state is excited to levels of υ2 = 2 or higher.