J. H. Hendricks

Dipole bound, nucleic acid base anions studied via negative ion photoelectron spectroscopy

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.

The dipole bound-to-covalent anion transformation in uracil

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.

On the binding of electrons to nitromethane: Dipole and valence bound anions

Conventional (valence) and dipole‐bound anions of the nitromethane molecule are studied using negative ion photoelectron spectroscopy, Rydberg charge exchange and field detachment techniques. Reaction rates for charge exchange between Cs(ns,nd) and Xe(nf ) Rydberg atoms with CH3NO2 exhibit a pronounced maximum at an effective quantum number of n*≊13±1 which is characteristic of the formation of dipole‐bound anions [μ(CH3NO2)=3.46 D]. However, the breadth (Δn≊5, FWHM) of the n‐dependence of the reaction rate is also interpreted to be indicative of direct attachment into a valence anion state via a ‘‘doorway’’ dipole anion state. Studies of the electric field detachment of CH3NO−2 formed through the Xe(nf ) reactions at various n values provide further evidence for the formation of both a dipole‐bound anion as well as a contribution from the valence bound anion. Analysis of the field ionization data yields a dipole electron affinity of 12±3 meV. Photodetachment of CH3NO−2 and CD3NO−2 formed via a supersonic...

Photoelectron spectroscopy of the solvated anion clusters O−(Ar)n=1–26,34: Energetics and structure

Negative ion photoelectron spectra of the solvated anion clusters O−(Ar)n=1–26,34 have been recorded. Vertical detachment energies obtained from the cluster anion spectra were used to determine total as well as stepwise stabilization energies. An examination of these energetic values as a function of cluster size demonstrates that the first solvation shell closes at n=12. Furthermore, magic numbers in the energetic data and in the mass spectrum suggest O−(Ar)n clusters of sizes n=12–34 are structurally very similar to homogeneous rare gas clusters and follow a polyicosahedral packing pattern, implying O−(Ar)12 has an icosahedral structure and O−(Ar)18 has a double icosahedral structure. The solvated cluster anion photoelectron data were also analyzed using a generalized cluster size equation, which relates the cluster anion data to bulk parameters. The data for O−(Ar)n≥12 is well represented by the theoretical prediction and was therefore used to estimate several bulk parameters, including the photoemissi...

Negative ion photoelectron spectroscopy of the ground state, dipole-bound dimeric anion, (HF)2−

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.

Lithium cluster anions: Photoelectron spectroscopy and ab initio calculations

Structural and energetic properties of small, deceptively simple anionic clusters of lithium, Li(n)(-), n = 3-7, were determined using a combination of anion photoelectron spectroscopy and ab initio calculations. The most stable isomers of each of these anions, the ones most likely to contribute to the photoelectron spectra, were found using the gradient embedded genetic algorithm program. Subsequently, state-of-the-art ab initio techniques, including time-dependent density functional theory, coupled cluster, and multireference configurational interactions methods, were employed to interpret the experimental spectra.

Anion solvation at the microscopic level: Photoelectron spectroscopy of the solvated anion clusters, NO−(Y)n, where Y=Ar, Kr, Xe, N2O, H2S, NH3, H2O, and C2H4(OH)2

The negative ion photoelectron spectra of the gas-phase, ion-neutral complexes; NO−(Ar)n=1–14, NO−(Kr)1, NO−(Xe)n=1–4, NO−(N2O)n=3–5, NO−(H2S)1, NO−(NH3)1, and NO−(EG)1 [EG=ethylene glycol] are reported herein, building on our previous photoelectron studies of NO−(N2O)1,2 and NO−(H2O)1,2. Anion solvation energetic and structural implications are explored as a function of cluster size in several of these and as a result of varying the nature of the solvent in others. Analysis of these spectra yields adiabatic electron affinities, total stabilization (solvation) energies, and stepwise stabilization (solvation) energies for each of the species studied. An examination of NO−(Ar)n=1–14 energetics as a function of cluster size reveals that its first solvation shell closes at n=12, with an icosahedral structure there strongly implied. This result is analogous to that previously found in our study of O−(Ar)n. Inspection of stepwise stabilization energy size dependencies, however, suggests drastically different st...

Photoelectron spectroscopy of lithium hydride anion

We present negative ion photoelectron spectra of the smallest stable molecular negative ion, the lithium hydride anion. Photoelectron spectra, recorded using 2.540 eV photons, are reported for the LiH(D) [X 1Σ+]+e−←LiH(D)−[X 2Σ+] transitions of 7LiH− and 7LiD−. Adiabatic electron affinities of 0.342±0.012 eV and 0.337±0.012 eV were determined for 7LiH and 7LiD, respectively. The experimentally determined electron affinities led to anion dissociation energy (D0) values of 2.017±0.021 eV for 7LiH− and 2.034±0.021 eV for 7LiD− relative to their Li[2S1/2]+H−(D−)[1S0] asymptotes. Franck–Condon analyses yielded the following molecular parameters for the ground state of 7LiH−: Be=6.43±0.18 cm−1, re=1.724±0.025 A, and ωe=920±80 cm−1; and the following parameters for the ground state of 7LiD−: Be=3.62±0.06 cm−1, re=1.724±0.015 A, and ωe=650±45 cm−1. In addition, we have observed the alkali hydride anions: 7LiH−2, 7LiD−2, Li2D−, NaD−, NaD−2, NaD−3, and NaD−4. No photodetachment signal was observed for the lithium d...

Photoelectron spectroscopy of alkali metal tetramer anions: The anomalous spectrum of Li−4

We present the photoelectron spectrum of Li−4. This spectrum displays a spectral pattern that is strikingly different from that of the other alkali tetramer anions. Using the photoelectron spectrum of Li−4 along with our previously measured photoelectron spectra of Na−4, K−4, and Rb−4 plus other existing evidence, we find that Li−4 does not have a linear geometry, as do the tetramer anions of sodium, potassium, and rubidium. This observation indicates that for both anions and neutrals, lithium clusters appear to take on higher dimensional structures at smaller sizes than do sodium and probably other alkali clusters. By examining the clues found in its photoelectron spectrum, we then speculate as to what the structure of Li−4 may be and also summarize the present state of theoretical progress on this problem.

Characterization of theX2 ∑ u + state of7Li 2 − via negative ion photoelectron spectroscopy

The negative ion photoelectron spectrum of7Li2− is reported at 488 nm (2.540 eV). Three electronic bands are observed in this spectrum and are assigned to the following photodetachment transitions:7Li2,X1 ∑g++e− ←7Li2−,X2 ∑u+;7Li2,a3 ∑u++e− ←7Li2−,X2 ∑u+; and7Li2,A1 ∑u++e− ←7Li2−,X2 ∑u+. The electron affinity of7Li2 is determined to be 0.437±0.009 eV, leading to an anion dissociation energy,D0, of 0.865±0.022 eV for the ground state of7Li2−. A Franck-Condon analysis of the7Li2,X1 ∑g++e− ←7Li2−,X2 ∑u+ band yields the following spectroscopic constants for the ground state of7Li2−:Be=0.502±0.005 cm−1,re=3.094±0.015 Å, and ωe=232±35 cm−1.