H+O₃ Fourier-transform infrared emission and laser absorption studies of OH (X ²Π) radical: An experimental dipole moment function and state-to-state Einstein a coefficients

The relative intensities of 88 pairs of rovibrational transitions of OH (X ²Π) distributed over 16 vibrational bands (υ′≤9, Δυ = -1, -2) have been measured using Fourier transform infrared (FTIR) emission/absorption spectroscopy. Each pair of transitions originates from a common vibrational, rotational, and spin-orbit state, so that the measured relative intensities are independent of the OH number density and quantum state distribution. These data are combined with previous υ = 1←0 relative intensity absorption measurements and υ = 0, 1, and 2 permanent dipole moments to determine the OH dipole moment function as a cubic polynomial expanded about r_e, the equilibrium bond length. The relative intensities provide detailed information about the shape of the OH dipole moment function μ(r) and hence the absolute Einstein A coefficients. The intensity information is inverted through a procedure which takes full account of the strong rotation-vibration interaction and spin uncoupling effects in OH to obtain the dipole moment function (with 95% confidence limits): μ(r) = 1.6502(2) D+0.538(29) D/Å (r-r_e) -0.796(51) D/Ų (r - r_e)² -0.739(50) D/ų (r - r_e),³ with a range of quantitative validity up to the classical turning points of the υ = 9 vibrational level (i.e., from 0.70 to 1.76 Å). The μ(r) determined in this study differs significantly from previous empirical analyses which neglect the strong effects of rotation-vibration interaction and spin uncoupling. The present work also permits distinguishing between the various ab initio efforts. Best agreement is with the dipole moment function of Langhoff, Werner, and Rosmus [J. Mol. Spectrosc. 118, 507 (1986)], but their theoretical predictions for higher overtone transitions are still outside of the 2σ experimental error bars. Absolute Einstein A coefficients from the present μ(r) are therefore presented for P, Q, R branch transitions for Δυ = 1, 2, 3, υ′≤9, J′≤14.5, in order to provide the most reliable experimental numbers for modeling of near IR atmosphere OH emission phenomena.