Photoelectron spectra of $(\mathrm{HF}{)}_{3}^{\ensuremath{-}}$ reveal coexistence of two anionic isomers with vertical electron detachment energies (VDE) of 0.24 and 0.43 eV. The results of electronic-structure calculations, performed at the coupled cluster level of theory with single, double, and noniterative triple excitations, suggest that the two isomers observed experimentally are an open, zigzag, dipole-bound anion and an asymmetric solvated electron, in which the dipole-bound anion of $(\mathrm{HF}{)}_{2}$ is solvated by one HF monomer at the side of the excess electron. The theoretical VDE of 0.21 and 0.44 eV, respectively, are in excellent agreement with the experimental data.
The geometric and electronic structures of both neutral and negatively charged lead sulfide clusters, (PbS)n/(PbS)n− (n = 2–10) were investigated in a combined anion photoelectron spectroscopy and computational study. Photoelectron spectra provided vertical detachment energies (VDEs) for the cluster anions and estimates of electron affinities (EA) for their neutral cluster counterparts, revealing a pattern of alternating EA and VDE values in which even n clusters exhibited lower EA and VDE values than odd n clusters up until n = 8. Computations found neutral lead sulfide clusters with even n to be thermodynamically more stable than their immediate (odd n) neighbors, with a consistent pattern also being found in their HOMO–LUMO gaps. Analysis of neutral cluster dissociation energies found the Pb4S4 cube to be the preferred product of the queried fragmentation processes, consistent with our finding that the lead sulfide tetramer exhibits enhanced stability; it is a magic number species. Beyond n = 10, computational studies showed that neutral (PbS)n clusters in the size range, n = 11–15, prefer two-dimensional stacking of face-sharing lead sulfide cubical units, where lead and sulfur atoms possess a maximum of five-fold coordination. The preference for six-fold coordination, which is observed in the bulk, was not observed at these cluster sizes. Taken together, the results show a preference for the formation of slightly distorted, fused cuboids among small lead sulfide clusters.
Nucleic acid base anions play an important role in radiation-induced mutagenesis. Recently, it has been shown that isolated (gas-phase) nucleobases form an exotic form of negative ions, namely, dipole bound anions. These are species in which the excess electrons are bound by the dipole fields of the neutral molecules. In the condensed phase, on the other hand, nucleobase anions are known to be conventional (covalent) anions, implying the transformation from one form into the other due to environmental (solvation) effects. Here, in a series of negative ion photoelectron spectroscopic experiments on gas-phase, solvated uracil cluster anions, we report the observation of this transformation.
Vibrationally resolved photoelectron spectra of MgO- and ZnO- have been recorded at several photon energies under varied experimental conditions. Peaks in these highly structured spectra have been assigned to photodetachment transitions from the MgO- and ZnO- ground state (X2Σ+) to vibrational progressions in the ground and several low lying neutral excited states. In addition, a high-temperature MgO- spectrum shows spectral features due to photodetachment from an excited electronic state of the MgO- anion, which has been assigned to an A2Π anionic state. From the MgO- spectra, the electron affinity of the MgO ground state (X1Σ+) is determined to be 1.630 (0.025) eV. Four electronic excited states of MgO, a3Π, A1Π, b3Σ+, and B1Σ+, were found to lie 2510, 3390, 8390, and 20 000 cm-1 above the X1Σ+ neutral ground state, respectively. The excited MgO- A2Π anion state was found to lie 4791 cm-1 above the MgO- X2Σ+ anion ground state. The photoelectron spectra of ZnO-, presented here at higher photon energies, extend a previous photoelectron study by Fancher et al. to the first two excited neutral states, a3Π and A1Π, which have been found to lie 2460 and 4960 cm-1 above the X1Σ+ ground state, respectively. From Franck−Condon analyses of the well-resolved vibrational progressions for each electronic transition, equilibrium internuclear distances and fundamental vibrational frequencies of the MgO and ZnO neutral electronic states were determined. Moreover, because the sources employed produced vibrationally hot anions, the bond length and vibrational frequencies of both the MgO- ground and excited states were found from the vibrational hot band transitions.
The anions of the nucleic acid bases, uracil and thymine, were studied by negative ion photoelectron spectroscopy. Both monomer anions exhibit spectroscopic signatures that are indicative of dipole bound excess electrons. The adiabatic electron affinities of these molecules were found to be 93±7 meV for uracil and 69±7 meV for thymine. No conventional (valence) anions of these molecules were observed.
We present the mass spectral and photoelectron spectroscopic results of our study of (HF)2−. Our main findings are as follows. The (HF)2− anion was observed experimentally for the first time, confirming the 20 year old prediction of Jordan and Wendoloski. The photoelectron spectrum of (HF)2− exhibits a distinctive spectral signature, which we have come to recognize as being characteristic of dipole bound anions. The vertical detachment energy (VDE) of (HF)2− has been determined to be 63±3 meV, and the adiabatic electron affinity (EAa) of (HF)2 was judged to be close to this value as well. Relatively weak spectral features, characteristic of intramolecular vibrations in the final (neutral dimer) state, were also observed. We have interpreted these results in terms of slight distortions of the dimer anion’s geometric structure which lead to an enhanced dipole moment. This interpretation is supported to a considerable extent by theoretical calculations reported in the companion paper by Gutowski and Skurski.
