Velocity Modulation Laser Spectroscopy of Negative Ions: The v₃ Band of Azide (N₃^-)

Thirty-four transitions in the v₃(asymmetric stretch) fundamental of the azide anion have been measured in an NH₃/N₂O discharge by using diode laser velocity modulation spectroscopy. The data, ranging from P(42) to R(62), were fit to a linear molecule effective Hamiltonian through quartic terms. The vibrational band origin was determined to be v₃= 1986.4672 (19) cm^-1and the rotational constants are Bʺ = 0.426 203 (57) and Bʹ = 0.422 572 (55) cm^-1. Comparisons with ab initio calculations and condensed phase measurements are presented. The observed 2:1 intensity alternation in successive rotational lines establishes the centrosymmetric structure of the anion (r₀= 1.188402 (82) Å).