UNIMOLECULAR DECOMPOSITION OF NO3 - THE NO-2 THRESHOLD REGIME(O)

The unimolecular decomposition of expansion-cooled NO3 has been invest igated in the threshold regime of the NO+O-2 channel. Photoexcitation in the region 16780-17090 cm(-1) (596-585 nm) prepares ensembles of mo lecular eigenstates, each of which is a mixture of the B(2)E' bright s tate and lower electronic states. The X(2)A(2)' ground state is believ ed to be the probable terminus of (2)E' radiationless decay, though pa rticipation of A(2)E '' is also possible. For these photon energies, u nimolecular decomposition occurs exclusively via the NO+O-2 channel, a nd NO yield spectra and state distributions have been obtained. The yi eld spectra are independent of the rotational state monitored, as expe cted for a large reverse barrier. The state distributions are insensit ive to the photolysis photon energy and can be rationalized in terms o f dynamical bias. The NO yield goes to zero rapidly above the O+NO2 th reshold (17090+/-20 cm(-1)). Because of tunneling, the NO+O-2 channel does not have a precise threshold; the value 16780 cm(-1) is the small est photon energy that yielded signals under the present conditions. V ery small decomposition rates were obtained via time-domain measuremen ts in which reactive quenching of long-lived NO3 fluorescence was obse rved. The rates varied from 1x10(4) at 16780 cm(-1) to 6x10(7) s(-1) a t 16880 cm(-1), and their collision free nature was confirmed experime ntally. These data were fitted by using a one-dimensional tunneling mo del for motion along the reaction coordinate combined with the thresho ld Rice-Ramsperger-Kassel-Marcus (RRKM) rate. The top of the NO+O-2 ba rrier is estimated to lie at 16900+/-15 cm(-1). Translational energy m easurements of specific NO cm (X(2) Pi(Omega),upsilon,J) levels showed that O-2 is highly excited, with a population inversion extending to energies above the a(1) Delta(g) threshold, in agreement with previous It is possible that the main O-2 product is X(3) Sigma(-) though some participation of a(1) Delta(g) cannot be rulted out. Within the exper imental uncertainty, b(1) Sigma(g)(+) is not produced. (C) 1996 Americ an Institute of Physics.