Stark quantum-beat spectroscopy: The electric dipole moment of NO (A2Σ+)

The γ-system (A2Σ+ X2Π) of nitric oxide (NO) is observed in many environments including combustion and atmospheric applications. Molecular properties of the A2Σ+ state are thus important in collision theories used to predict A-X radiative efficiencies. These theories often involve electric-multipole expansions,1,2 in which the permanent electric dipole moment of A2Σ+ (µA) is the leading term for long-range interactions. Moreover, µA is a basic property of the molecular charge distribution and has often been used to gauge the accuracy of ab initio quantum chemistry calculations.3-6 Here we describe measurements of Stark quantum beats in the fluorescence of NO from which we derive a precise value for µA in v'=0. This is the vibrational level most often excited in laser-based methods for detecting NO. Bergeman and Zare,7 using a radio frequency-optical double-resonance technique, provide the only other measurement of µA in a different vibrational level (v'=3). We thus also determine the change in µA with v', which provides a sensitive test of computed dipole moment functions.