High-resolution threshold photodetachment spectroscopy of OH-

Relative photodetachment cross sections for rotational thresholds of the hydroxide anion were measured using a coaxial laser-ion beam spectrometer. The thresholds correspond to transitions from the J=1 and J=2 states of X (1) Sigma(+) OH- to the J=3/2 state of X (2) Pi(3/2) OH. Best fits of the data give cross sections that scale with energy above threshold as (E-E(th))(0.18+/-0.06) and (E-E(th))(0.21+/-0.05) for the two thresholds, respectively. This non-Wigner law energy dependence of the cross section results from the long-range r(-2) interaction between the dipole and the departing electron. Experimental results were compared with the predictions of a strong-coupling model of charge-dipole interaction devised by Engelking and good qualitative agreement was observed. When the resolution and range of the data for transitions that terminate in the lower Lambda-doublet component is such that the Lambda-doublet splitting is resolved, one observes the (E-E(th))(1/2) behavior predicted by the Wigner law. This result is explained in terms of the effect of the Lambda doubling upon the r(-2) charge-dipole interaction. From the threshold frequencies. an OH electron affinity of 14 741.02(3) cm(-1) was determined.