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

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Abstract
The relative intensities of 88 pairs of rovibrational transitions of OH (X 2Π) distributed over 16 vibrational bands (v’≤9, Δv=−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 v=1←0 relative intensity absorption measurements and v=0, 1, and 2 permanent dipole moments to determine the OH dipole moment function as a cubic polynomial expanded about re, 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−re)−0.796(51) D/Å2 (r−re)2−0.739(50) D/Å3 (r−re), 3 with a range of quantitative validity up to the classical turning points of the v=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. 1 1 8, 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 Δv=1, 2, 3, v’≤9, J’≤14.5, in order to provide the most reliable experimental numbers for modeling of near IR atmosphere OH emission phenomena.