Theoretical spectrum of AlN

The electronic states of AlN with excitation energies below about 35 000 cm^-1 have been studied at the CASSCF and MRCI levels of theory. Less elaborate calculations have been performed for the low-lying states of AlN⁺. Although the results are not definitive, they strongly suggest a ³Π ground state for AlN with a very low-lying ³Σ^- state, in analogy with the Al₂ molecule. All of the strong transitions however involve the lowest ³Π state. The ³Π-³Π AlN band system observed in emission is assigned as the C³Π-X ³Π transition. The C ³Π radiative lifetime is estimated as 151, 163, and 175 ns for v'=0,1,2, respectively. However, these lifetimes may be significantly shortened due to predissociation by the (1) ⁵Π state. The calculated D_e of 2.35 eV for the ground state is about half that of the isovalent BN molecule. Thus the bonding in AlN is significantly weaker than for BN, owing to the smaller overlaps of the valence orbitals. Finally, transition moment functions are presented for many of the dipole-allowed transitions to aid further spectroscopic studies of AlN.