Photodissociation of ClN3 at 193 and 249 nm

Abstract

Experiments directed toward elucidating the nature of fragments produced by the photodissociation of ClN₃ have been performed. The utlraviolet absorption spectrum of this molecule exhibits three features between 300 and 170 nm, with λ_max = 250, 205, and 170 = 5000 l mol⁻¹ cm⁻¹. Photolysis of ClN₃ at 193 nm produces N₂ and NCl fragments in both the singlet and triplet manifolds. In the singlet manifold of fragments, emission from electronically excited NCl(b ¹Σ⁺) was observed. The NCl(b ¹Σ⁺) molecules are also vibrationally excited, and emission from levels as high as v′ = 10 was recorded. Time resolution of the NCl(b ¹Σ⁺) emission indicated that the higher vibrational levels were quenched by the gas mixture (∼3% ClN₃ in helium) much more rapidly than the lower vibrational levels. The rate constant for quenching of the low v′ levels (v′ = 0–2) of NCl(b ¹Σ⁺) was determined to be k_Q = (1.72±0.05)×10³ s⁻¹ Torr⁻¹; the rate constant for quenching of the higher levels (v′?8) was found to be k_Q = (3.8±0.2)×10⁴ s⁻¹ Torr⁻¹. The spontaneous emission rate for NCl(b ¹Σ⁺), v′ = 0–2, was measured to be A = 1590±160 s⁻¹. In the triplet manifold of fragments, photolysis at 193 nm produces excited N₂(A ³Σ⁺_u) metastable molecules. The presence of this species was determined from the observation of N₂(C ³Π_u)→(B ³Π_g) emission produced by energy pooling processes, and from the excitation of NO by energy transfer from the N₂ metastables. Photolysis of ClN₃ produces at 193 nm excited singlet and triplet fragments with comparable probability. Photolysis at 249 nm also generates both excited singlet and triplet NCl and N₂ fragments. Emission from vibrationally excited NCl(b ¹Σ⁺) was again observed, and the initial vibrational distribution appeared to be similar to that found for photolysis at 193 nm. The yields for production of NCl(b ¹Σ⁺) at 193 and 249 nm are approximately equal. Photolysis at 249 nm also produces N₂(A ³Σ⁺_u) fragments, although in substantially smaller yield than found at 193 nm.