Electronic spectroscopy and excited state dynamics of the Al–N2 complex Original Research Article

The weakly bound Al⋯N2 complex was prepared in a pulsed supersonic beam and studied with laser fluorescence excitation spectroscopy. Transitions to bound vibrational levels in electronic states correlating with the Al(5s, 4d)+N2 asymptotes have been observed. Resonance fluorescence from the excited levels could not be detected. These excited levels decay nonradiatively, and the excitation spectrum was obtained by monitoring emission from the lower Al atomic levels. The band systems were dominated by progressions in the excited state Al–N stretch vibrational mode. Vibrational assignments were made through analysis of nitrogen isotope shifts. The rotational contours were obscured by Lorentzian line broadening from the nonradiative decay. However, for excitation to the Al(5s)⋯N2 state the rotational structure of bands with small linewidths was consistent with a 2Σ+–2Π electronic transition of a linear molecule, in accord with the previously calculated linear structure of the ground state [G. Chaban, M.S. Gordon, J. Chem. Phys. 107 (1997) 2160]. The excited state binding energies were estimated to be 1218±10 cm−1 for the Al(5s)⋯N2 state [2Σ+ symmetry] and 2705±165 cm−1 for the Al(4d)⋯N2 state of 2Δ symmetry. From the threshold for formation of Al(5s) from the nonradiative decay of Al(4d)⋯N2, an upper bound to the ground state dissociation energy D0″≤354±2 cm−1 was obtained. For comparison with high-resolution scans over Al⋯N2 bands, we have also recorded and analyzed similar scans over several bands of the AlAr H2Σ+–X2Π1/2 transition, which correlates with the Al 5s←3p atomic transition. Excited-state rotational constants were derived and employed to obtain Be′=0.1128±0.0020 cm−1, and hence Re′=3.05±0.03 Å. A weak predissociation, leading to formation of Al(3d), was observed for AlAr(H2Σ+).