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

'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 A 6Σ+ → X 6Σ+ 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*(a 4DJ) atoms react via two distinct potential surfaces to yield both MnO* emitting states in common processes. A more excited metastable state also generates products, but the lower z 8PJ state yields A 6Σ+ chemiluminescence only. The product rotational alignments 〈P2(ĵ′ · k̂)〉 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 a 4DJ atoms, the initial A′ 6Π alignment is lower than that of A 6Σ+, suggesting that the former requires either near-linear or C2v geometry, and the latter less restricted configurations, on the same potential surface.