The pure rotational spectrum of ZnO in the X¹Σ⁺ and a³Π_i states

The pure rotational spectrum of ZnO has been measured in its ground X¹Σ⁺ and excited a³Π_i states using direct-absorption methods in the frequency range 239-514 GHz. This molecule was synthesized by reacting zinc vapor, generated in a Broida-type oven, with N₂O under DC discharge conditions. In the X¹Σ⁺ state, five to eight rotational transitions were recorded for each of the five isotopologues of this species (⁶⁴ZnO, ⁶⁶ZnO, ⁶⁷ZnO, ⁶⁸ZnO, and ⁷⁰ZnO) in the ground and several vibrational states (v = 1-4). Transitions for three isotopologues (⁶⁴ZnO, ⁶⁶ZnO, and ⁶⁸ZnO) were measured in the a³Π_i state for the v = 0 level, as well as from the v = 1 state of the main isotopologue. All three spin-orbit components were observed in the a³Π_i state, each exhibiting splittings due to lambda-doubling. Rotational constants were determined for the X¹Σ⁺ state of zinc oxide. The a³Π_i state data were fit with a Hund's case (a) Hamiltonian, and rotational, spin-orbit, spin-spin, and lambda-doubling constants were established. Equilibrium parameters were also determined for both states. The equilibrium bond length determined for ZnO in the X¹Σ⁺ state is 1.7047 Å, and it increases to 1.8436 Å for the a excited state, consistent with a change from a π⁴ to a π³σ¹ configuration. The estimated vibrational constants of ω_e ∼ 738 and 562 cm^-1 for the ground and a state agreed well with prior theoretical and experimental investigations; however, the estimated dissociation energy of 2.02 eV for the a³Π_i state is significantly higher than previous predictions. The lambda-doubling constants suggest a low-lying ³Σ state.