Electronically excited nitric oxide by vacuum ultraviolet photodissociation of nitroso-compounds and nitrites

Abstract

The gas-phase photolysis of CF,NO and EtONO a t 123 and 147 nm and of ButNO a t 147 nm gives rise to electronically excited nitric oxide which fluoresces. The strongest emission is that of the A2Zt-XZTT transition ( ybands), although other transitions are also observed. The vibrational level populations in the NO(A2X+) state show some correlation with the energy of the incident radiation and the strength of the R-NO bond. Addition of helium to the CF,NO or EtONO produces an enhancement of emission from the v' = 2 and 3 levels of the A2C+ state, while emission from v' = 0 and 1 is almost unchanged. Addition of low pressures of N, and H, also gives some enhancement of v' = 2 and 3. These observations are discussed in terms of the possible excited states of NO produced during the photolysis, and the deactivating effects of the added gas. Photolysis of ONCN a t 147 nm yields electronically excited cyanide radicals and the (B2C-X2C) violet bands are observed. There is evidence for considerable rotational excitation of the CN radical, an effect well known in the vacuum U.V. photolysis of other cyanides, e.g. ICN. THERE have been a considerable number of studies of the electronically excited nitric oxide molecule. Most of these studies involve either low pressure discharge tubes, or direct excitation of pure nitric oxide with U.V. light. The review of Heicklen and Cohen summarises much of the data. Combination of N and 0 atoms in low pressure flow systems 2 9 3 and electron impact have also been used to excite nitric oxide fluorescence. There are very few reports on work in which the electronically excited nitric oxide is produced by the photodissociation of an NO containing molecule. Photodissociation of N,O leads to NO fluoresence, but it has been established that secondary processes involving N and 0 atoms are the source of the f l ~o r e s e n c e . ~. ~ Photodissociation of NO, at wavelengths (130 nm and of NOCl at wavelengths <170 nm has been shown to produce electronically excited nitric oxide.' The only report of NO fluorescence in the photodissociation of polyatomic molecules is that of Style and Ward,& who observed NO emissions when methyl and ethyl nitrite were excited through a fluorite window with a hydrogen arc. No details of the NO emission were given.The work reported here is concerned with the observation of electronically excited NO produced in the vacuum U.V. photodissociation of some nitroso-compounds and nitrites, and supplements the data given in our previous communication.s EXPERIMENTAL A conventional greaseless mercury-free vacuum system was used for all gas handling. The microwave-powered resonance lamps were attached to the photolysis cell with 64/40 0 ring joints. The krypton (123 nrn) and xenon (147 nm) lamps were fitted with 2 mni thick IiI; windows. I n some cases a second window was attached to the lanip. Use of a CaF, window eliminates the 116 nm krypton line, while a sapphire window eliminates the 129 nm xenon line. There was no significant change in the relative intensities of

Energy Partitioning in Molecular Photodissociation: Methyl Nitrite Photolysis

“…However addition of helium (0. [1][2][3][4][5][6][7][8][9][10] to the nitrite makes the vibrational structure sharper and simpler to assign. From the helium pressure effect it can be deduced that collisions relax predominantly the rotational excitation without affecting the relative vibrational population of CHaO A 2A 1.…”