Internal population distribution of the B state of AlO formed by fast ion beam bombardment or laser ablation of an Al 2O 3 (Al) surface

Abstract

A complex emission spectrum in the 440–550 nm region has been recorded by bombarding targets of Al 2O 3 (and Al under low O 2 partial pressure) polycrystalline surface with fast (5 keV) Kr + ion beams under high vacuum (10 −8 Torr). This spectrum has been compared with the chemiluminescence spectrum of AlO formed by laser ablation of an Al target in the presence of oxygen. Both spectra exhibit strong Al lines, weak Al + lines and very weak molecular features, among which bandheads of the Δv=0,±1,±2 sequences of the AlO B 2 Σ +–X 2 Σ + system have been identified. The internal energy distribution of the AlO B state, determined by simulation of the B–X system, shows that both techniques yield highly vibrationally and rotationally excited AlO molecules in the B state. Local equilibrium is achieved ( T rot =T vib ≃4000 K ) for the B 2 Σ + state of AlO formed via laser ablation, in agreement with previous studies and with a reaction scheme proceeding through a long-lived intermediate. In contrast, the AlO molecules formed in the B 2 Σ + state through fast ion beam bombardment are much more rotationally than vibrationally excited. The rovibronic population distributions, most probably highly non-thermal, can be roughly represented by thermal populations corresponding to rotational and vibrational temperatures of 13 000 and 5000 K, respectively.