Unimolecular decomposition of NO3: The NO+O2 threshold regime

Mikhaylichenko, K.; Riehn, C.; Valachovic, L.; Sanov, A.; Wittig, C.
October 1996
Journal of Chemical Physics;10/22/1996, Vol. 105 Issue 16, p6807
Academic Journal
The unimolecular decomposition of expansion-cooled NO3 has been investigated in the threshold regime of the NO+O2 channel. Photoexcitation in the region 16 780–17 090 cm-1 (596–585 nm) prepares ensembles of molecular eigenstates, each of which is a mixture of the B 2E′ bright state and lower electronic states. The X 2A2′ ground state is believed to be the probable terminus of 2E′ radiationless decay, though participation of A 2E″ is also possible. For these photon energies, unimolecular decomposition occurs exclusively via the NO+O2 channel, and NO yield spectra and state distributions have been obtained. The yield spectra are independent of the rotational state monitored, as expected for a large reverse barrier. The state distributions are insensitive to the photolysis photon energy and can be rationalized in terms of dynamical bias. The NO yield goes to zero rapidly above the O+NO2 threshold (17 090±20 cm-1). Because of tunneling, the NO+O2 channel does not have a precise threshold; the value 16 780 cm-1 is the smallest photon energy that yielded signals under the present conditions. Very small decomposition rates were obtained via time-domain measurements in which reactive quenching of long-lived NO3 fluorescence was observed. The rates varied from 1×104 at 16 780 cm-1 to 6×107 s-1 at 16 880 cm-1, and their collision free nature was confirmed experimentally. These data were fitted by using a one-dimensional tunneling model for motion along the reaction coordinate combined with the threshold Rice–Ramsperger–Kassel–Marcus (RRKM) rate. The top of the NO+O2 barrier is estimated to lie at 16 900±15 cm-1. Translational energy measurements of specific NO (X 2ΠΩ,v,J) levels showed that O2 is highly excited, with a population inversion extending to energies above the a 1Δg threshold, in agreement with previous work. It is possible that the main O2 product is X 3∑g-...


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