Ro-vibrational Spectrum of Linear Dialuminum Monoxide (Al 2 O) at 10 μm.

Dialuminum monoxide, Al 2 O, 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, N 2 O. 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 ν 3 and to five of its hot bands, originating from excited levels of the ν 1 symmetric stretching mode and the ν 2 bending mode. The measurements encompass 11 vibrational energy states ( v 1 v 2 l v 3 ). 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 ( v 1 v 2 l v 3 ) = (0 1 1 0) and (0 1 1 1) and an l -type resonance between the states (0 2 0 0) - (0 2 2 0) and (0 2 0 1) - (0 2 2 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.