OH− and OD− threshold photodetachment

Abstract

Threshold photodetachment cross sections for OH⁻ and OD⁻ at photon energies close to their electron binding energies have been obtained using a crossed tunable laser‐negative ion beam apparatus at a resolution of 25 μeV. The data reveal many clear features associated with photodetachment of single rotational levels of the negative ion, producing single rotational levels of the neutral and a near zero energy electron. From the frequencies of the observed thresholds, the spectroscopic constants of OH⁻ and OD⁻ are found. The rotational constants of the vibrational ground state are B₀ = 18.7409(45) cm⁻¹ and D₀ = 2.052(45)×10⁻³ cm⁻¹ for OH⁻ and B₀ = 9.9852(48) cm⁻¹ and D₀ = 0.553(33)×10⁻³ cm⁻¹ for OD⁻. The electron affinities of OH and OD are measured to be 14 741.03(17) cm⁻¹ and 14 723.92(30) cm⁻¹, respectively. The observed bound–free transitions satisfy selection rules only slightly different from the selection rules observed in bound–bound spectroscopy. The relative intensities of the transitions show the transition from Hund’s case (a) to case (b) as the rotation in OH increases. The intensities are in agreement with a theory based on an intermediate complex which dissociates into jj‐coupled constituents. The shape of the cross section near threshold, a probe of the long range electron–OH interaction, is found to depend on the final rotational state of the OH neutral. This dependence is studied in detail. Analogous studies on photodetachment of OD⁻ confirm our deductions.