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Flash photolysis of NO2 and SO2. Morse, Robert Donald

Abstract

Vibrationally excited oxygen with up to 15 quanta of energy has been produced in the flash photolysis of NO₂ . When the NO₂ was selectively photolysed such that only radiation below 4000 A° was admitted to the reaction vessel the highest level observed was reduced to v" = 11 which in terms of vibrational energy corresponds to the exothermicity of the reaction O + NO₂ —> NO + O₂ Relative population ratios for the successive vibrational levels v" = 6 to 11 were calculated for both the filtered and unfiltered photolysis of NO₂ using two independently calculated arrays of Franck-Condon Factors. In both cases the ratios showed that the population distribution was definitely non Boltzmann. in that the levels were almost equally populated. The distribution for the unfiltered experiments gave a pronounced dip at v" =11 indicating that the levels 12 ≤ v" ≤ 15 are formed by a second reaction. This reaction has been shown to be Q+ NO*₂—> O*₂ v" ≥12 +NO where the NO₂ is probably electronically excited. The adiabatic flash photolysis of both NO₂ and SO₂ has been investigated. These spectra show weakening of their banded structure up to 100 microseconds after the flash and then gradual strengthening until back to normal by 10 milliseconds. This effect is well known for SO₂ and has been postulated to be the result of the formation of a high temperature isomer of SO₂. Subsequently the postulated isomer had a transient spectrum in the vacuum ultraviolet assigned to it. We have found that the weakening of both the NO₂ and SO₂ banded structure is the result of temperature broadening. The spectrum assigned to the SO₂ isomer has been extended. It has also been produced from the flash photolysis of SO₂ under isothermal conditions showing that its assignment to a high temperature isomer is incorrect. From kinetic data this spectrum in the 1800 A° region and another the 1400 A° region has been tentatively identified as two triplet systems of SO.

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