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

Dialuminum monoxide, AlO, 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, NO. 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 and to five of its hot bands, originating from excited levels of the ν symmetric stretching mode and the ν bending mode. The measurements encompass 11 vibrational energy states ( ). The ro-vibrational transitions show spin statistical line intensity alternation of 7:5, which is caused by two identical aluminum nuclei of spin = / 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 and higher, while rotational levels within vibrational modes exhibited thermal population, with rotational temperatures around = 115 K. Molecular parameters for 11 vibrational states were derived, including rotation and centrifugal distortion constants and -type doubling constants for the states ( ) = (0 1 0) and (0 1 1) and an -type resonance between the states (0 2 0) - (0 2 0) and (0 2 1) - (0 2 1). From the experimental results, rotational correction terms and the equilibrium bond length were derived. The measurements were supported and guided by high-level quantum-chemical calculations that agree well with the derived experimental results.