Molecules with sufficiently large dipole moments were predicted to form weakly bound negative ions in the dipolar field [1][2][3][4] and such dipole-bound anions have been observed and characterized experimentally. [5][6][7][8][9][10] Negative ions with dipolar molecular cores can have excited dipole-bound states (DBSs) near the detachment threshold, analogous to Rydberg states in neutral molecules. DBSs in excited anions were first observed as resonances in photodetachment crosssections. [11][12][13] Ultrahigh resolution spectroscopy has been reported on excited DBSs of a number of anions near their detachment threshold by autodetachment. [14][15][16] The theory for autoionization from Rydberg states was first developed by Berry, [17] who predicted a Dv = À1 vibrational propensity rule, that is, the Rydberg state undergoes a vibrational relaxation by one vibrational quantum in the molecular core during autoionization, in which the vibrational energy is transferred to the Rydberg electron. The Dv = À1 propensity rule has been observed in autoionization of numerous molecules [18][19][20] and mode-specific autoionization has been observed by photoelectron spectroscopy, [21] in which the kinetic energies of the outgoing electrons are measured. The same Dv = À1 propensity rule should apply in autodetachment from DBSs of excited anions [22] and has been inferred in previous studies.[ 13,16] However, the electron kinetic energies of the outgoing autodetached electrons from the excited DBSs have not been measured by electron spectroscopy.All prior studies of DBSs in excited anions are dominated by rotational effects [23,24] and pure vibrational autodetachment from DBSs has not been reported, even though vibrational autodetachment has been observed in weakly bound anions and has been used effectively as a spectroscopic tool for anions. [25][26][27] Here we report the direct observation of pure vibrational autodetachment from DBSs of cryogenically cooled phenoxide anions using high-resolution photoelectron imaging. Autodetachment from eight vibrational levels of the DBS in optically exited phenoxide anions are observed and the Dv = À1 propensity rule is found to be strictly obeyed.

Molecules with large enough dipole moments can bind an electron by the dipole field, which has little effect on the molecular core. A molecular anion can be excited to a dipole-bound state, which can autodetach by vibronic coupling. Autodetachment spectroscopy of a complex anion cooled in a cryogenic ion trap is reported. Vibrational spectroscopy of the dehydrogenated uracil radical is obtained by a dipole-bound state with partial rotational resolution. Fundamental frequencies for 21 vibrational modes of the uracil radical are reported. The electron affinity of the uracil radical is measured accurately to be 3.4810±0.0006 eV and the binding energy of the dipole-bound state is measured to be 146±5 cm(-1). The rotational temperature of the trapped uracil anion is evaluated to be 35 K.

High-resolution photoelectron imaging and spectroscopy of cold C₆₀⁻ anions are reported using a newly built photoelectron imaging apparatus coupled with an electrospray ionization source and a temperature-controlled cryogenic ion trap. Vibrationally resolved photoelectron spectra are obtained for the detachment transition from the ground state of C₆₀⁻ to that of C60 at various detachment wavelengths from 354.84 nm to 461.35 nm. The electron affinity of C60 is accurately measured to be 2.6835 ± 0.0006 eV. Numerous unexpected vibrational excitations are observed in the photoelectron spectra due to the Jahn-Teller effect in C₆₀⁻ and Hertzberg-Teller vibronic coupling in both C₆₀⁻ and C60. Both the relative intensities of vibrational peaks and their photoelectron angular distributions provide evidence for the vibronic couplings. The observed p-wave-like behavior in the angular distribution of the 0₀⁰ transition suggests that the electron is detached from an s-type orbital.

We report an investigation of the electronic structure and chemical bonding of AuH 2 À using photoelectron spectroscopy and ab initio calculations. We obtained vibrationally resolved photoelectron spectra of AuH 2 À at several photon energies. Six electronic states of AuH 2 were observed and assigned according to the theoretical calculations. The ground state of AuH 2 À is known to be linear, while that of neutral AuH 2 is bent with a :H-Au-H equilibrium bond angle of 129 . This large geometry change results in a very broad bending vibrational progression in the photoelectron spectra for the ground-state transition. The electron affinity of AuH 2 is measured to be 3.030 AE 0.020 eV. A short bending vibrational progression is also observed in the second photodetachment band, suggesting a slightly bent structure for the first excited state of AuH 2 . The linear geometry is a saddle point for the ground and first excited states of AuH 2 , resulting in double-well potentials for these states along the bending coordinate. Spectroscopic evidence is observed for the detachment transitions to the doublewell potentials of the ground and first excited states of AuH 2 . Higher excited states of AuH 2 due to detachment from the nonbonding Au 5d electrons are all linear, similar to the anion ground state. Kohn-Sham molecular orbital analyses reveal surprising participation of H 2p orbitals in the Au-H chemical bonding and an unprecedented weak Au 5dp to H 2pp back donation. The simplicity of the linear AuH 2 À anion and its novel spectroscopic features make it a textbook example for understanding the covalent bonding properties and relativistic effects of Au.

The uranyl tetrachloride dianion (UO(2)Cl(4)(2-)) is observed in the gas phase using electrospray ionization and investigated by photoelectron spectroscopy and relativistic quantum chemical calculations. Photoelectron spectra of UO(2)Cl(4)(2-) are obtained at various photon energies and congested spectral features are observed. The free UO(2)Cl(4)(2-) dianion is found to be highly stable with an adiabatic electron binding energy of 2.40 eV. Ab initio calculations are carried out and used to interpret the photoelectron spectra and elucidate the electronic structure of UO(2)Cl(4)(2-). The calculations show that the frontier molecular orbitals in UO(2)Cl(4)(2-) are dominated by the ligand Cl 3p orbitals, while the U-O bonding orbitals are much more stable. The electronic structure of UO(2)Cl(4)(2-) is compared with that of the recently reported UO(2)F(4)(2-) [P. D. Dau, J. Su, H. T. Liu, J. B. Liu, D. L. Huang, J. Li, and L. S. Wang, Chem. Sci. 3 1137 (2012)]. The electron binding energy of UO(2)Cl(4)(2-) is found to be 1.3 eV greater than that of UO(2)F(4)(2-). The differences in the electronic stability and electronic structure between UO(2)Cl(4)(2-) and UO(2)F(4)(2-) are discussed.

Bare uranyl tetrafluoride (UO 2 F 4 2À) and its solvation complexes by one and two water or acetonitrile molecules have been observed in the gas phase using electrospray ionization and investigated by photoelectron spectroscopy and ab initio calculations. The isolated UO 2 F 4 2À dianion is found to be electronically stable with an adiabatic electron binding energy of 1.10 AE 0.05 eV and a repulsive Coulomb barrier of $2 eV. Photoelectron spectra of UO 2 F 4 2À display congested features due to detachment from U-O bonding orbitals and F 2p lone pairs. Solvated complexes by H 2 O and CH 3 CN, UO 2 F 4 (H 2 O) n 2À and UO 2 F 4 (CH 3 CN) n 2À (n ¼ 1, 2), are also observed and their photoelectron spectra are similar to those of the bare UO 2 F 4 2À dianion, suggesting that the solvent molecules are coordinated to the outer sphere of UO 2 F 4 2À with relatively weak interactions between the solvent molecules and the dianion core. Both DFT and CCSD(T) calculations are performed on UO 2 F 4 2À and its solvated species to understand the electronic structure of the dianion core and solute-solvent interactions. The strong U-F interactions with partial (d-p)p bonding are shown to weaken the U]O bonds in the [O]U]O] 2+ unit. Each H atom in the water molecules forms a H-bond to a F atom in the equatorial plane of UO 2 F 4 2À , while each CH 3 CN molecule forms three H-bonds to two F ligands and one axial oxygen.

While uranyl halide complexes [UO2(halogen)n](2-n) (n = 1, 2, 4) are ubiquitous, the tricoordinate species have been relatively unknown until very recently. Here photoelectron spectroscopy and relativistic quantum chemistry are used to investigate the bonding and stability of a series of gaseous tricoordinate uranyl complexes, UO2X3(-) (X = F, Cl, Br, I). Isolated UO2X3(-) ions are produced by electrospray ionization and observed to be highly stable with very large adiabatic electron detachment energies: 6.25, 6.64, 6.27, and 5.60 eV for X = F, Cl, Br, and I, respectively. Theoretical calculations reveal that the frontier molecular orbitals are mainly of uranyl U-O bonding character in UO2F3(-), but they are from the ligand valence np lone pairs in the heavier halogen complexes. Extensive bonding analyses are carried out for UO2X3(-) as well as for the doubly charged tetracoordinate complexes (UO2X4(2-)), showing that the U-X bonds are dominated by ionic interactions with weak covalency. The U-X bond strength decreases down the periodic table from F to I. Coulomb barriers and dissociation energies of UO2X4(2-) → UO2X3(-) + X(-) are calculated, revealing that all gaseous dianions are in fact metastable. The dielectric constant of the environment is shown to be the key in controlling the thermodynamic and kinetic stabilities of the tetracoordinate uranyl complexes via modulation of the ligand-ligand Coulomb repulsions.

High-resolution state-selective autodetachment photoelectron spectroscopy via dipole-bound states and photodetachment spectroscopy of cryogenically cooled deprotonated thymine anions are reported.