Laser induced fluorescence spectra of expansion-cooled NOz/Ne samples (1 and 2 K) are reported for transitions that originate from the lowest rovibronic levels and terminate on levels near Do. At I K, nearly all transitions originate from N"=O. With the present resolution of 0.02 cm -1, the I K spectra are resolved rather well. The high density of transitions is due to couplings between rovibronic levels with different Nand K quantum numbers and with electronic characters that borrow oscillator strength from bright B 2 vibronic species of the mixed 2 A 1;2 B 2 electronic system. Just above reaction threshold, such rovibronic species comprise the manifold of levels sampled by optically prepared wave packets. However, at higher energies we argue that the density of B z vibronic species is a more relevant parameter to describe the nature of unimolecular reactions. Nuances of the optical excitation process are discussed.

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−, though some participation of a 1Δg cannot be ruled out. Within the experimental uncertainty, b 1∑+g is not produced.

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Reactions of photolytically prepared hot deuterium atoms with OCS have been investigated: (i) under gas phase, single collision, arrested relaxation (i.e., bulk) conditions; and (ii) by photoinitiating reactions within weakly bound OCS–DI complexes. Nascent SD(X 2Π, v=0) rotational, spin–orbit, and Λ-doublet populations were obtained for the photolysis wavelengths 250, 225, and 223 nm by using A 2Σ←X 2Π laser induced fluorescence (LIF). The reason for using deuterium is strictly experimental: A 2Σ predissociation rates are considerably smaller for SD than for SH. The SD (v=0) rotational distribution was found to be very cold and essentially the same for both bulk and complexed conditions; the most probable rotational energy is ∼180 cm−1. No bias in Λ-doublet populations was detected. Spin–orbit excitation for bulk conditions was estimated to be [2Π1/2]/[2Π3/2]∼0.25, where 2Π1/2 is the upper spin–orbit component. This ratio could not be obtained with complexes because of limited S/N. The complete set of present and past experimental findings, combined with recent theoretical results of Rice, Cartland, and Chabalowski suggest a mechanism in which SD derives from a very short lived HSCO precursor. This can result from direct hydrogen attack at the sulfur and/or the transfer of hydrogen from carbon to sulfur via the HCOS intermediate.

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