Al+O3 chemiluminescence: Perturbations and vibrational population anomalies in the B 2Σ+ state of AlO

Abstract

Ozone reacts with aluminum entrained in an argon buffer gas to yield the B ²Σ⁺–X ²Σ⁺ spectrum of AlO. The measured B ²Σ⁺ vibrational populations for this reaction follow a markedly non‐Boltzmann distribution, exhibiting local maxima at vibrational levels v′=6, 8, 12, and 14. This behavior is attributed to an initial chemical reaction Al+O₃→AlO(A ²Π) +O₂ followed by the collision induced rearrangement AlO(A ²Π) +Ar→AlO(B ²Σ⁺)+Ar. The spin–orbit interaction in AlO connects ro‐vibronic levels of the A ²Π and B ²Σ⁺ states. Consequently, collisional energy transfer is particularly efficient for the most strongly perturbed levels of the B ²Σ⁺ state. For Al+N₂O, an approximate Boltzmann distribution in the AlO B ²Σ⁺ state population indicates that this reaction proceeds by a differing mechanism. Consistent with the proposed mechanism for Al+O₃ is the appearance of ’’extra’’ band heads representing normally Franck–Condon forbidden A–X transitions which become allowed because of a small admixture of B ²Σ⁺ character. The relative intensities of the extra and ’’main’’ transitions are strongly dependent upon the argon buffer gas pressure. A quantitative description of this dependence gives an estimate for the amount of mixing between A ²Π and B ²Σ⁺ and for the rate of energy transfer between these two states. The measured rate constant for the most strongly perturbed level is comparable in magnitude to the rate constant expected for hard sphere collisions between argon and AlO. The present results allow some understanding of the continuum emission observed in the upper atmosphere release of aluminized grenades. They also may be pertinent to the construction of a visible chemical laser.