The dipole moment function and vibrational transition intensities of OH

Abstract

The relative intensities of nine pairs of rovibrational transitions of OH in the v=1←0 fundamental have been measured by flash kinetic infrared absorption spectroscopy. Each pair of transitions originates from a common rotational and spin–orbit state, so that relative intensities are independent of the OH number density and quantum state distribution. The relative intensities are strongly J dependent and this dependence provides detailed information about the shape of the OH dipole moment function, μ(r), and hence the absolute infrared transition strengths. In an accompanying paper we present the theoretical basis for extracting μ(r), for an open shell diatomic like OH, from relative infrared intensities and permanent dipole moment measurements (Peterson et al.). In this work we implement those ideas and determine the OH dipole moment function to be: μ(r)=1.6498(6) D+0.561(32) D/Å (r−r_e )−0.75(17) D/Ų (r−r_e )−1.5(11) D/ų(r−r_e )³. The accuracy of μ(r) is excellent near r_e (r_e =0.970 Å), since the data used to derive it are from low vibrational states. The useful range of this function extends from approximately 0.75 to 1.35 Å. The rotationless Einstein A coefficient for the OH fundamental is determined from μ(r) to be 16.7(19) Hz. This is in considerable disagreement with most other experimental and theoretical results, but is in good agreement with theoretical calculations by Mies (18.3 Hz) and by Langhoff et al. (13.8 Hz).