James V. Beitz

Exothermic rate restrictions on electron transfer in a rigid medium

Many highly exothermic (2–3 eV) electron transfer reactions are shown to be slower than moderately exothermic reactions by factors as large as 105. The decrease occurs in a regular way with increasing exothermicity, tending to confirm theoretical predictions of Franck–Condon restrictions on strongly exothermic electron transfer reactions. Deviations from the above trends occur if the reaction product, a molecular anion, has a low‐lying electronic excited state into which the reaction may occur with more moderate vibrational exothermicity. Then greatly enhanced rates are found. The rates are enhanced to a lesser extent for acceptors likely to undergo configurational changes upon accepting an electron. These effects are found in measurements of rates of electron tunneling reactions between trapped electrons and 48 organic electron acceptors in rigid 2‐methyltetrahydrofuran glass at 77 K. Electron tunneling rates were observed from 10−6 to 102 s. Measured tunneling distances were 15–40 A. In most cases the o...

Long range transfer of positive charge between dopant molecules in a rigid glassy matrix

Pulse radiolysis has been used to observe and measure the kinetics for intermolecular positive charge (hole) transfer from biphenyl+ or pyrene+ ions to TMPD molecules in rigid 2‐chlorobutane glass at 77 K. These hole transfers occur over distances of about 17 A at 10−6 s, increasing to about 34 A at 102 s. The kinetic data are interpreted in terms of current theories which treat electron transfer processes as radiationless transitions. Estimates of the required electron exchange interactions based on the usual electron tunneling models can not explain the fast reactions observed, even when Coulombic effects on the ’’barrier’’ are considered. A superexchange model is proposed which involves interactions propagated by both negative and positive ion states of the solvent. This model adequately interprets the data in terms of a dominant role of the solvent positive ion states, and is also applicable to negative charge transfer in the condensed phase. In samples containing only one solute (biphenyl or pyrene),...

Aqueous complexation of trivalent lanthanide and actinide cations by N,N,N′,N′-tetrakis(2-pyridylmethyl)ethylenediamine

Abstract The aqueous complexation reactions of trivalent lanthanide and actinide cations with the hexadentate ligand N,N,N′,N′-tetrakis(2-pyridylmethyl)ethylenediamine (TPEN) have been characterized using potentiometric and spectroscopic techniques in 0.1 M NaClO4. At 25°C, the stability constant of Am(TPEN)3+ is two orders of magnitude larger than that of Sm(TPEN)3+, reflecting the stronger interactions of the trivalent actinide cations with softer ligands as compared to lanthanide cations.

f-State luminescence of trivalent lanthanide and actinide ions in solution

Detailed studies of the luminescence of aquated Am 3+ are presented in the context of prior work on lanthanide and actinide ions. The luminescing state of aquated Am 3+ is confirmed to be 5 D 1 based on observed emission and excitation spectra. The luminescence lifetime of Am 3+ in H 2 O solution is (22±3) ns and (155±4) ns in D 2 O solution at 295 K. Judd-Ofelt transition intensity theory qualitatively describes the observed Am 3+ relative integrated fluorescence intensities. Recent luminescence studies on complexed trivalent f-element ions in solution are reviewed as to the similarities and differences between the lanthanide ion 4f state and the actinide ion 5f state properties

Anion Effects in the Extraction of Lanthanide 2-Thenoyltrifluoroacetone Complexes into an Ionic Liquid

The extraction of trivalent lanthanides from an aqueous phase containing 1 M NaClO4 into the room temperature ionic liquid 1-butyl-3-methylimidazolium nonafluoro-1-butanesulfonate by the β-diketone extractant 2-thenoyltrifluoroacetone (Htta) was studied. Radiotracer distribution, absorption spectroscopy, time-resolved laser-induced fluorescence spectroscopy, and X-ray absorption fine structure measurements point to the extraction of multiple lanthanide species. At low extractant concentrations, fully hydrated aqua cations of the lanthanides are present in the ionic liquid phase. As the extractant concentration is increased 1:2 and 1:3 lanthanide:tta species are observed. In contrast, 1:4 Ln:tta complexes were observed in the extraction of lanthanides by Htta into 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide.

Ground‐state splitting of S‐state ion Cm3+ in LaCl3

We report optical‐spectroscopic studies of the ground‐state splitting of the nominal S‐state ion Cm3+ in LaCl3. A total splitting of 1.97±0.05 cm−1 for the 8S7/2 ground state of this system has been measured for the first time. Both electric and magnetic dipole transitions were observed. Energy levels of all four Kramers doublets of the 8S7/2 manifold were resolved by means of the site‐selective excitation and emission spectra of 8S7/2↔6D7/2 and 8S7/2↔6P5/2 transitions. Our results show that the lowest component of the ground manifold is a μ=5/2 state dominated by M=‖±7/2〉 in contrast to a previously reported μ=1/2 state. The crystal‐field model, established through analyzing optical spectra of the excited states, predicts a ground‐state splitting with the right ordering but greater magnitude. In comparison with the lanthanide S‐state ion Gd3+, much stronger spin–orbit coupling and reduced electrostatic interaction in concert with greater crystal field interaction significantly influence the characteristi...

Fluorescence studies of neptunium and plutonium hexafluoride vapors

The first observation of fluorescence from gas phase, electronically excited, transuranic hexafluorides is reported. Fluorescence peaking at 1360 nm was observed from 237NpF6 gas excited at 1064 nm. The measured fluroescence lifetime was 3.53±0.01 ms, independent of NpF6 pressure. Fluourescence peaking at 2300 nm has been observed from 242PuF6 gas excited at 1064 nm. The measured fluorescence lifetime was 204±12 μs, independent of PuF6 pressure. In both cases the emitting state is assigned as a vibronic component of the first excited electronic state of the hexafluoride based on previously reported absorption spectra and energy level calculations. Fluorescence in the 1900 and 4800 nm region was observed from PuF6 gas excited at 532 nm. The lifetime of this fluorescence was 86±4. The emitting state giving rise to this shorter‐lived fluorescence was not identified. Estimated fluorescence quantum yields are reported.

High sensitivity photoacoustic spectrometer for variable temperature solution studies

A single beam, dual cell, photoacoustic spectrometer has been developed for use with solutions from ambient temperature to the boiling point of the liquid. The optical absorbance sensitivity of the apparatus is 3×10−7 absorbance units per cm (base 10 logarithm, 1000 pulse average, 13 mJ/pulse) for solutes in aqueous solution at 90 °C, using an excimer‐pumped dye laser as the light source. The cell assembly is designed to thermally isolate the acoustic detector and prevent evaporative loss of the solvent. High accuracy subtraction of solvent optical absorption is achieved using preamplifiers with continuously variable gain, matched electrical bandpass filters, and a high common mode rejection ratio differential amplifier. Design constraints resulting from factors such as the temperature dependence of solute photoacoustic signal amplitude are discussed and results of experimental tests probing some limits of the identified constraints are presented. A data analysis method is described which substantially re...

Laser-induced fluorescence studies of europium extractant complexes in organic phases☆

Abstract Studies of Eu3+-extractant complexes in organic phases of interest in solvent extraction of lanthanides and actinides have been carried out using time- and wavelength-resolved laser-induced fluorescence (LIF). Even at low metal loading, the Eu3+ is shown to be present in the organic phase as a single metal ion-extractant complex in which complete dehydration of the metal ion has occurred when the aqueous phase was nitric acid and di(2-ethylhexyl)phosphoric acid or octyl(phenyl)-N,N-diisobutylcarbamoyl-methylphosphine oxide were used as extractants.

Nonlinear optical dynamics and Eu3+ spectral holeburning in strontium barium niobate thin film grown by pulsed laser deposition

Optical quality SrxBa1−xNb2O6 (SBN) thin films, both undoped and Eu3+-doped, of thickness less than 0.5 μm have been successfully grown on fused quartz substrates using a pulsed laser deposition technique. Optical properties of these films were characterized in high-resolution spectroscopic experiments in time and frequency domains. For undoped SBN thin films, broadband emission in the UV region extending to the visible was observed following excitation at 355 nm. This emission is attributed to exciton luminescence of the SBN film. Nonlinear optical response in the picosecond regime and the third-order nonlinear susceptibility, χ(3), were studied using degenerate four-wave-mixing methods. In transverse alignment, χ(3) is enhanced by two orders of magnitude in comparison with its bulk counterpart. A thermal annealing process, monitored via changes in spectral properties of Eu3+, was employed to convert the as-grown amorphous film into a polycrystalline film. High-resolution spectroscopic measurements in th...