Evidence for ultrafast V–E transfer in boron oxide (BO)

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Abstract
Boron atoms react with oxygen (O2) and nitrous oxide (N2O) to yield the A 2Π–X 2Σ+ spectrum of BO. These reactions have been characterized from 10−5 to 10−3 Torr and at Ptotal ∼1 Torr in order to study relaxation and the rapid intramolecular V–E transfer BO(X 2Σ+, v″ =17)+X→BO(A 2Π1/2, v′=4)+X where X=O2, N2O, or a combination of these oxidants with argon. At the lowest pressures, a ground state boron 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 A 2Π vibrational populations follow a markedly non-Boltzmann distribution. At pressures as low as 6×10−5 Torr, the formation of BO A 2Π1/2, v′=4 results from both the direct reaction B+RO→BO*+R and the collisional transfer BO(X 2Σ+)+RO→BO(A 2Π)+RO. The spin orbit interaction in BO connects rovibronic levels of X 2Σ+ and A 2Π1/2 facilitating a route for rapid intramolecular energy transfer. This energy transfer leads to the observation of sharp features in the neighborhood of the BO A 2Π–X 2Σ+ (4,0) band which may be correlated with the J′=18.5–21.5 perturbed rotational levels of BO(A 2Π1/2). We characterize this phenomenon determining population distributions, rotational temperatures, and the temperature dependence (boron source) of the chemiluminescence emission. The effects observed in BO demonstrate that a highly vibrationally excited ground state species interacts in the presence of a collision partner with a cross section substantially in excess of that expected for ’’gas kinetic’’ interaction. 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 lasers.