High-resolution threshold photodetachment spectroscopy of ${\mathrm{OH}}^{\mathrm{-}}$

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${\mathrm{}}^1$${\mathrm{\Sigma }}^{+}$ ${\mathrm{OH}}^{\mathrm{-}}$ to the J=3/2 state of X${\mathrm{}}^2$${\mathrm{\Pi }}_{3\mathrm{/}2}$ OH. Best fits of the data give cross sections that scale with energy above threshold as (E-${\mathrm{E}}_{\mathrm{th}}$${)}^{0.18\pm 0.06}$ and (E-${\mathrm{E}}_{\mathrm{th}}$${)}^{0.21\pm 0.05}$ for the two thresholds, respectively. This non-Wigner law energy dependence of the cross section results from the long-range ${\mathrm{r}}^{\mathrm{-}2}$ 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 Λ-doublet component is such that the Λ-doublet splitting is resolved, one observes the (E-${\mathrm{E}}_{\mathrm{th}}$${)}^{1\mathrm{/}2}$ behavior predicted by the Wigner law. This result is explained in terms of the effect of the Λ doubling upon the ${\mathrm{r}}^{\mathrm{-}2}$ charge-dipole interaction. From the threshold frequencies, an OH electron affinity of 14 741.02(3) ${\mathrm{cm}}^{\mathrm{-}1}$ was determined.