Molecular rotation and the observation of dipolebound states of anions J. Chem. Phys. 99, 7586 (1993); 10.1063/1.465688 Spectroscopy and dynamics of the dipolesupported state of acetyl fluoride enolate anion J. Chem. Phys. 88, 6785 (1988); 10.1063/1.454424Threshold resonances in the electron photodetachment spectrum of acetaldehyde enolate anion. Evidence for a lowlying, dipolesupported state Ultrahigh-resolution photodetachment spectroscopy of acetaldehyde enolate negative ion has revealed -50 narrow resonances near threshold, corresponding to excitation to a diffuse state in which the electron is weakly bound by the field of the molecular dipole. A complete analysis of rotational transitions between the ground valence state and the excited dipole-bound state has been carried out, yielding spectroscopic constants and geometries for both states. In analogy to Rydberg states, the structure of the "neutral core" of the dipole-bound state is like that of the neutral radical. The dependence of autodetachment lifetimes upon the rotational quantum numbers of the dipole-bound state has been measured. Bound levels of the dipole-bound anion state are readily electric-field detached. The selection rules and dynamics of autodetachment from the dipole-bound state are discussed.

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.

High-resolution threshold photodetachment of H 2 C-CHO~ has resulted in the observation of -50 narrow resonances (some < 20 MHz full width at half maximum). These resonances are due to autodetachment from a diffuse negative-ion complex consisting of an electron weakly bound ( ~ 5 cm -1 ) by the dipolar electric field of the neutral. Electric fields of < 70 V/cm are adequate to detach the electron from the lower (bound) levels where autodetachment is not possible. Such states should exist for all neutrals with dipole moments greater than about 2 D.