Nonequilibrium product distributions observed in the multiple collision chemiluminescent reaction of Sc with NO2. Perturbations, rapid energy transfer routes and evidence for a low-lying reservoir state

Abstract

Nitrogen dioxide reacts with scandium to yield the B 2Σ+–X 2Σ+ spectrum of ScO. This reaction has been characterized from 10−5 to 1 Torr in order to study relaxation and rapid intramolecular E–E transfer among ScO excited states. At the lowest pressures, a ground state metal atom interacts with a tenuous atmosphere of oxidant gas (beam-gas configuration). These ’’single collision’’ studies are extended in a controlled manner to higher pressure by entraining the metal atoms in argon and subsequently carrying out the oxidation of this mixture. At all pressures, the measured B 2Σ+ vibrational populations follow a markedly non-Boltzmann distribution. At the lowest pressures, the formation of ScO B 2Σ+ results directly from the reaction Sc+NO2→ScO*+NO. At higher pressures, the B 2Σ+ state is also populated via rapid intramolecular energy transfer from long-lived, weakly emitting ’’reservoir’’ states via the sequence Sc+NO2+Ar→ScO(res)+NO+Ar and ScO(res)+Ar→ScO(B 2Σ+)+Ar. Spin orbit and Coriolis interactions in ScO connect rovibronic levels of B 2Σ+ and low-lying 4Πr or 2Πi reservoir states resulting in the observation of substantial perturbations in B 2Σ+. Collisional energy transfer is particularly efficient for the most strongly perturbed levels of the B2Σ+ state. This energy transfer is manifest by the appearance of ’’extra’’ band heads representing normally forbidden (small electronic transition moment or Franck–Condon factor) ’’reservoir state’’– ground state transitions which become allowed because of a small admixture of B 2Σ+ character. The relative intensities of the extra and ’’main’’ B 2Σ+–X 2Σ+ transitions are strongly dependent on argon buffer gas pressure. A quantitative description of this dependence gives an estimate for the amount of mixing between the reservoir state and B 2Σ+ and for the rate of energy transfer between these two states. Collisional transfer to ScO B 2Σ+ v′=3–9 s found to proceed at rates which for certain levels approach 100 times gas kinetic. The effects observed in ScO demonstrate that the excited states of this molecule interact in the presence of a collision partner as if they were large diffuse entities. These effects are not pathological. This behavior may have important implications for the modeling of energy systems as well as the ability to create population inversions requisite for the construction of visible chemical laser systems.