Fully Quantum-State Resolved Study of NO2 Photodissociation: Correlated NO(2.PI..OMEGA., .nu. = 0,J,.LAMBDA.) + O(3Pj) Distributions

Abstract

Relative O(3Pj=~,~,~) spin-orbit populations correlated with specific N0[2nQ=l/2,3/2; Y = 0; J; A = II(A’), n(A”)] product states were obtained following photolysis of NO2 at excess energies Zi? = 390, 425, and 1054 cm-I. These fully quantum state-resolved measurements were carried out by recording spatial profiles of recoiling N0(213~,J,A) products using polarized radiation for photolysis and state-selective laser ionization detection. The relative O(3P,) populations correlated with each N0(213~,J,A) state show marked fluctuations at each excess energy as a function of rotational state and A-doublet component. The relative populations also fluctuate as a function of excess energy. The O(3Pj) spin-orbit population ratios, when averaged over all measurements, exhibit distributions that are colder than statistical, in agreement with previous results. In particular, we find that, on average, O(3P~):O(3P2) population ratios correlated with the ground N0(2171/2) state are colder than the corresponding ratios correlated with the excited NO(TI3/2) spin-orbit state. These results are in agreement with the state-specific calculations of Katigiri and Kat0 [J. Chem. Phys. 1993, 99, 88051 and are discussed in terms of long-range nonadiabatic transitions among electronic states correlating asymptotically with different spin-orbit states of the ground NO(TI) + O(3P) dissociation channel. I. Introduction The photodissociation of NO:! has attracted considerable experimental and theoretical attention. Due to its unique properties, NO:! has become a model system for testing statistical theories of unimolecular reactions. Usually, in a bond fission reaction proceeding without a barrier, high density of states and intramolecular vibrational redistribution in the excited molecule lead to product state distributions that can be well described by statistical theories, i.e., the product levels are populated as per their degeneracies and quantum state-specific effects are averaged out. In contrast, the unimolecular decomposition of NO:! represents a case in which both statistical and state-specific dynamical effects are manifest.’-19 Due to vibronic chaos at excitation energies > 16 000 cm-I, which results from strong couplings between the bright 12B2 state and the ground 12A1 state via a conical inter~ection,~.~~ the eigenstates in the vicinity of the dissociation threshold (DO = 25 130.6 cm-1)lc-14 are strongly mixed and have predominantly 12A1 chara~ter.~~~~~. ~~ Dissociation following 12B2