Ro-vibrational Spectrum of Linear Dialuminum Monoxide (Al₂O) at 10 μm

Dialuminum monoxide, Al₂O, has been investigated in the laboratory at mid-IR wavelengths around 10 μm at high spectral resolution. The molecule was produced by laser ablation of an aluminum target with the addition of gaseous nitrous oxide, N₂O. Subsequent adiabatic cooling of the gas in a supersonic beam expansion led to rotationally cold spectra. In total, 848 ro-vibrational transitions have been assigned to the fundamental asymmetric stretching mode ν₃ and to five of its hot bands, originating from excited levels of the ν₁ symmetric stretching mode and the ν₂ bending mode. The measurements encompass 11 vibrational energy states (v₁ v₂ v₃). The ro-vibrational transitions show spin statistical line intensity alternation of 7:5, which is caused by two identical aluminum nuclei of spin I = ⁵/₂ at both ends of the centrosymmetric molecule of structure Al-O-Al. The less effective cooling of vibrational states in the supersonic beam expansion allowed measurement of transitions in excited vibrational states at energies of 1000 cm^-1 and higher, while rotational levels within vibrational modes exhibited thermal population, with rotational temperatures around T_rot = 115 K. Molecular parameters for 11 vibrational states were derived, including rotation and centrifugal distortion constants and l-type doubling constants for the states (v₁ v₂ v₃) = (0 1¹ 0) and (0 1¹ 1) and an l-type resonance between the states (0 2⁰ 0) - (0 2² 0) and (0 2⁰ 1) - (0 2² 1). From the experimental results, rotational correction terms and the equilibrium bond length r_e were derived. The measurements were supported and guided by high-level quantum-chemical calculations that agree well with the derived experimental results.