Chemiluminescence and rotational alignment in Mn + O2: Direct observation of the MnO*(A′6Π) state

Abstract

‘Hot’ and ‘ cold’ beam chemiluminescence excitation functions have been determined for the Mn + O₂ reaction in the 560–700 nm (‘red ’) and 770–900 nm (‘IR ’) regions. While the former is the well-known MnO A⁶Σ⁺ → X⁶Σ⁺ band system, the latter is identified, by ab initio calculations and the observed alignment, as a new lower-lying A′⁶Π → X ⁶Σ⁺ system. Analysis of the excitation functions by the multiple line-of-centres approach indicates that Mn*(a⁴D_J) atoms react ia two distinct potential surfaces to yield both MnO* emitting states in common processes. A more excited metastable state also generates products, but the lower z⁸P_J state yields A ⁶Σ⁺ chemiluminescence only. The product rotational alignments 〈P₂( ĵ′·)〉 in the higher-threshold processes are initially high, reflecting a line-of-normals energy dependence in which the relative velocity k lies at first in the Mn–O–O plane. For a⁴D_J atoms, the initial A′⁶Π alignment is lower than that of A⁶Σ⁺, suggesting that the former requires either near-linear or C_2v geometry, and the latter less restricted configurations, on the same potential surface.