R. N. Compton

Gas-Phase Multiply Charged Anions

Singly charged negative ions in the gas phase have attracted considerable experimental and theoretical attention over the past decades. However, the existence of free doubly or multiply charged negative ions, in particular those of small systems, has remained a curiosity and a matter of some controversy. Recent experimental and quantum mechanical studies show that multiply charged negative ions of small molecules and clusters can exist as isolated entities.

Attributing Effects of Aqueous C60 Nano-Aggregates to Tetrahydrofuran Decomposition Products in Larval Zebrafish by Assessment of Gene Expression

Background C60 is a highly insoluble nanoparticle that can form colloidal suspended aggregates in water, which may lead to environmental exposure in aquatic organisms. Previous research has indicated toxicity from C60 aggregate; however, effects could be because of tetrahydrofuran (THF) vehicle used to prepare aggregates. Objective Our goal was to investigate changes in survival and gene expression in larval zebrafish Danio rerio after exposure to aggregates of C60 prepared by two methods: a) stirring and sonication of C60 in water (C60–water); and b) suspension of C60 in THF followed by rotovaping, resuspension in water, and sparging with nitrogen gas (THF–C60). Results Survival of larval zebrafish was reduced in THF–C60 and THF–water but not in C60–water. The greatest differences in gene expression were observed in fish exposed to THF–C60 and most (182) of these genes were similarly expressed in fish exposed to THF–water. Significant up-regulation (3- to 7-fold) of genes involved in controlling oxidative damage was observed after exposure to THF–C60 and THF–water. Analyses of THF–C60 and THF–water by gas chromatography–mass spectrometry did not detect THF but found THF oxidation products γ-butyrolactone and tetrahydro-2-furanol. Toxicity of γ-butyrolactone (72-hr lethal concentration predicted to kill 50% was 47 ppm) indicated effects in THF treatments can result from γ-butyrolactone toxicity. Conclusion This research is the first to link toxic effects directly to a THF degradation product (γ-butyrolactone) rather than to C60 and may explain toxicity attributed to C60 in other investigations. The present work was first presented at the meeting “Overcoming Obstacles to Effective Research Design in Nanotoxicology” held 24–26 April 2006 in Cambridge, Massachusetts, USA.

Collisional ionization of Na, K, and Cs by CO2, COS, and CS2: Molecular electron affinities

The negative ion products resulting from collisions between orthogonal beams of alkali metal atoms (Na, K, Cs) and the linear triatomic molecules CO2, COS, and CS2 have been studied from threshold to ∼400 eV (lab). Ions with masses corresponding to the parent molecules CO2, COS, and CS2 are detected for all collision permutations except for Na colliding with CO2. The following electron affinities are deduced from measurements of the threshold for the ion pair production reactions: CO2(−0.60±0.2 eV), COS(+0.46±0.2 eV), and CS2(1.0±0.2 eV). The CO−2 ion was found to be metastable with respect to autodetachment. This result is compatible with the negative electron affinity for CO2 and in agreement with our earlier observations of CO−2* and with recent theoretical calculations. The lifetime of CO−2* (9±2×10−5 sec) was measured to be independent of collision energy over the region of energy studied (threshold to ∼20 eV c.m.). The fragment ions O−/CO2, O−/COS, S−/COS, and S−/CS2 were detected at a threshold ene...

Nondissociative Electron Capture in Complex Molecules and Negative‐Ion Lifetimes

The formation and decay of parent negative ions have been studied with electron‐swarm and electron‐beam techniques. The rate of attachment at thermal energies under electron‐swarm conditions for SF6 and C6H5NO2 was determined to be 1.24×109 and 2.1×107 sec−1·torr−1, respectively. Autoionization lifetimes for SF6−, C6H5NO2−, and (CH3CO)2− are reported to be 25, 40, and 12 μsec, respectively. The absolute rate of electron attachment in SF6 was found to be independent of the temperature (298°≤T≤418°K), and is thus consistent with an electron capture cross section for SF6 which varies inversely with the speed of the electron. The attachment rates and lifetimes were combined through the principle of detailed balance to calculate the ratio of density of states of the negative ion to that of the neutral plus electron. A simple theoretical treatment of long‐lived negative ions is presented, and the possibility of estimating electron affinities from measurements of attachment cross sections and negative‐ion lifeti...

Threshold Electron Impact Excitation of Atoms and Molecules: Detection of Triplet and Temporary Negative Ion States

A technique which utilizes SF6 as a scavenger of low‐energy electrons is employed to study the “threshold” excitation spectra of He, N2, HCl, H2O, D2O and a number of aromatic molecules. For electrons within ∼0.03 eV of threshold, it is found that the probability of exciting the 23S state of helium is approximately 1.5 times larger than that for the 21S state. For HCl all of the optically allowed transitions are observed including a Rydberg series leading to the ionization potential. Temporary negative ion resonances are observed below the first electronic state for all of the aromatic molecules studied. For benzene, in addition to the optically allowed transitions, the first (3.9 eV) and second (4.7 eV) triplet states are detected, while for naphthalene a new intense level at 5.4 eV as well as the lower triplet states are observed.

Resonantly enhanced multiphoton ionization of pyrrole, N‐methyl pyrrole, and furan

The resonantly enhanced multiphoton ionization (REMPI) spectra of pyrrole (C4H5N), N‐methyl pyrrole (C5H7N), and furan (C4H4O) have been measured in the wavelength region from 365 to 680 nm. New and previously observed Rydberg states are reported for pyrrole and furan. Vibrational constants are presented for most of the Rydberg series. Accurate ionization potentials are derived for pyrrole (8.207±0.003 eV) and N‐methyl pyrrole (7.94±0.02 eV). A strong two photon allowed transition is observed in N‐methyl pyrrole at 41 193 cm−1 (0,0) and is attributed to an 1A2 state. The corresponding state is not seen in pyrrole; however, it may be obscured by overlapping Rydberg series. Mass spectra following REMPI for benzene, pyrrole, and furan are reported. The degree of ionic fragmentation depends upon laser power density and wavelength.

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...

Electron attachment to van der Waals polymers of carbon dioxide and nitrous oxide

Negative ions formed by electron attachment to carbon dioxide and nitrous oxide, expanded through a sonic nozzle, are reported. In each case the dominant ions have the formula O−(M)n, where M is the parent monomer. The appearance potential of O− in each case indicates the absence of ’’hot bands.’’ Ions with the formula (CO2)−n, with n?2, evidently formed by evaporative electron attachment, seem to be stable with respect to autodetachment. The ’’three‐body’’ mechanism for attachment of thermal electrons to nitrous oxide and oxygen is discussed.

Production of negative ions from CH3X molecules (CH3NO2, CH3CN, CH3I, CH3Br) by electron impact and by collisions with atoms in excited Rydberg states

Thermal electron attachment to nitromethane, methylcyanide, methyliodide, and methylbromide is compared with capture of electrons by these molecules from highly excited Rydberg states of atoms. Data on thermal electron attachment to CH3NO2 are consistent with a three‐body attachment process, with the nature of the third body being important. The thermal energy electron attachment rate constant for CH3CN is ≤1.24×10−14 cm3 sec−1. Some results on dissociative electron attachment and ion pairing processes in CH3NO2, CH3CN, CH3I, and CH3Br are also presented.

Collisional ionization between fast alkali atoms and selected hexafluoride molecules

Negative ion products resulting from collisions between orthogonal, crossed beams of alkali metal atoms (Na, K, Cs), and the octahedral hexafluorides MF6 (M=S, Se, Te, Mo, W, Re, Ir, and Pt) have been examined in the energy range from ∼0 to 40 eV (lab). Studies of the dependence of the reaction thresholds upon the temperature of the target molecules SF6, SeF6, and TeF6 have provided electron affinities for these molecules; E.A.(SF6) =0.46±0.2, E.A.(SeF6) =2.9±0.2, and E.A.(TeF6) =3.3±0.2 eV. Energy loss measurements of the alkali, A, in the reaction A+MF6→A++MF−6 at small scattering angles are consistent with these values. Measurements for SF4 together with temperature dependent thresholds for the formation of SF−5 from SF6 and SF−3 from SF4 combined with known bond dissociation energies for D (SF5–F) and D (SF3–F) yield electron affinity values for the SFn series; E.A.(SF6) =0.46±0.2, E.A.(SF5) =2.71±0.2, E.A.(SF4) =0.78±0.2, and E.A.(SF3) =3.07±0.2 eV. Lower limits of ∼5 eV for the electron affinities o...