Susan T. Arnold

Negative ion photoelectron spectroscopy of solvated electron cluster anions, (H2O)n− and (NH3)n−

The photodetachment spectra of (H2O)n=2−69/− and (NH3)n=41−1100/− have been recorded, and vertical detachment energies (VDEs) were obtained from the spectra. For both systems, the cluster anion VDEs increase smoothly with increasing sizes and most species plot linearly withn−1/3, extrapolating to a VDE (n=∞) value which is very close to the photoelectric threshold energy for the corresponding condensed phase solvated electron system. The linear extrapolation of this data to the analogous condensed phase property suggests that these cluster anions are gas phase counterparts to solvated electrons, i.e. they are embryonic forms of hydrated and ammoniated electrons which mature with increasing cluster size toward condensed phase solvated electrons.

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...

Reactions of mass-selected cluster ions in a thermal bath gas

Technologicaldevelopments of fast-flow tubes that led to major advances in the study of cluster ion reactions are reviewed, including the coupling of high-pressure cluster ion sources to flowing-afterglow and selected-ion flow tube (SIFT) instruments. Several areas of cluster ion chemistry that have been studied recently in our laboratory, using a SIFT instrument with a supersonic expansion ion source, are reviewed. Firstly the thermal destruction of cluster ions is highlighted by a discussion of the competition between electron detachment and thermal dissociationin hydrated electron clusters (H O) . Rates and activationenergies for the n thermal destruction (dissociation plus detachment) of these clusters are discussed. The reactivity of hydrated electron clusters with several neutral electron scavengers is also reviewed. Secondly cluster ion chemistry related to trace neutral detection of atmospheric species using chemical ionization mass spectrometry is discussed. Recent rate measurements needed for ch...

Photoelectron spectra of hydrated electron clusters: Fitting line shapes and grouping isomers

The photoelectron spectra of (H2O)(n = 2-69) - and (D2O)(n = 2-23) - are presented, and their spectral line shapes are analyzed in detail. This analysis revealed the presence of three different groupings of species, each of which are seen over the range, n = 11-16. These three groups are designated as dipole boundlike states, seen from n = 2-16, intermediate states, found from n = 6-16, and bulk embryonts, starting at n = 11 and continuing up through the largest sizes studied. Almost two decades ago [J. V. Coe et al., J. Chem. Phys. 92, 3980 (1990)], before the present comprehensive analysis, we concluded that the latter category of species were embryonic hydrated electrons with internalizing excess electrons (thus the term embryonts). Recent experiments with colder expansion (high stagnation chamber pressures) conditions by Neumark and coworkers [J. R. R. Verlet et al., Science 307, 93 (2005)] have also found three groups of isomers including the long-sought-after surface states of large water cluster anions. This work confirms that the species here designated as embryonts are in the process of internalizing the excess electron states as the cluster size increases (for n > or = 11).

Ammonia cluster anions and their relationship to ammoniated (solvated) electrons: The photoelectron spectra of (NH3)n=41–1100−

We report the negative ion photoelectron spectra of (NH3)n=41–1100−, recorded using 2.540 eV photons. The largest cluster anion in this series has a diameter of approximately 4.3 nm. The vertical detachment energies (VDEs) of these cluster anions increase smoothly from 0.55 eV for n=41 to 1.05 eV for n=1100. The VDEs throughout this size range are linear with n−1/3 and extrapolate to a VDE (n=∞) value, which is very close to the measured photoelectric threshold energy of condensed phase ammoniated electrons. The linear extrapolation of this data to an analogous condensed phase property implies that these cluster anions are gas-phase counterparts to ammoniated electrons, i.e., they are embryonic forms of ammoniated electrons which will mature with increasing cluster size to become condensed phase-solvated electrons. The VDE data further implies that these embryonic ammoniated electrons were generated in solid ammonia environments, consistent with the source conditions under which they were produced.

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...

Electron attachment to SF5CF3 (296–563 K) and calculations of the neutral and anion thermochemistry

Moller–Plesset (MP) perturbation theory and density functional theory (DFT) were used to examine the structure and bonding of trifluoromethyl sulfurpentafluoride, SF5CF3, and the corresponding anion, SF5CF3−. The structural parameters, charge analysis, and energetics are all consistent with the anion having ion-dipole character (i.e., SF5−–CF3). Results from G2(MP2) theory yield a neutral D2980(SF5–CF3)=301 kJ mol−1 (3.12 eV), anion D2980(SF5−–CF3)=21.5 kJ mol−1 (0.22 eV), EA(SF5CF3)=119 kJ mol−1 (1.24 eV), ΔfH2980(SF5CF3)=−1639 kJ mol−1, and ΔfH298(SF5CF3−)=−1750 kJ mol−1. The calculated value for the standard enthalpy of formation for SF5CF3 differs from the previous estimate by 78 kJ mol−1. DFT was found to perform poorly for quantities related to the neutral SF5–CF3 bond. Calculations were also carried out for SF5, SF5−, CF3, and CF3− fragments, and both DFT and G2(MP2) methods performed well for these open-shell species. Rate constants for electron attachment to SF5CF3 were measured over the temperat...

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...

Rate constants and branching ratios for the reactions of various positive ions with naphthalene from 300 to 1400 K

Abstract Temperature dependent rate constants and branching ratios are reported for the reactions of a variety of ions with recombination energies ranging from 9.26 eV (NO + ) to 21.56 eV (Ne + ) with naphthalene. For most ions, the measurements are made between 300 and 370 K in a variable temperature-selected ion flow tube (VT-SIFT). For the reactions of Ar + and N 2 + , data have also been measured between 300 and 500 K in the selected ion flow tube. In addition, for the reactions of O 2 + and N 2 + , data have been obtained between 500 and 1400 K in a high temperature flowing afterglow (HTFA). These are among the first determinations of branching ratios for ion–molecule reactions measured over 700 K. All reactions are found to proceed at the Langevin collision rate for all temperatures studied. The reactions proceed by nondissociative and dissociative charge transfer except for the reaction involving F + where some of the reactivity is attributed to chemical channels. No dissociative charge transfer is observed for ions with recombination energies equal to or less than that for N 2 + at room temperature. At higher temperatures in the N 2 + reaction and for ions with higher recombination energies (F + , Ne + ), naphthalene cation dissociation is observed, implying a threshold over 16 eV. This value is substantially higher than the known thermodynamic threshold because of kinetic shifts and quenching of the excited state of C 10 H 8 + by collisions with the helium buffer gas. The observed product thresholds and branching ratios are presented within the context of previous work and the implications for combustion chemistry are discussed.