Wayne P. Hess

Coprecipitation of Uranium(VI) with Calcite: XAFS, micro-XAS, and luminescence characterization

X-ray absorption and luminescence spectroscopies have been used to characterize the local structure and coordination of uranium(VI) species coprecipitated with calcite (CaCO3) from room-temperature aqueous solutions. Different solution chemistries and pHs are found to result in differences in the equatorial coordination of the uranyl species (UO2 ) incorporated in the calcite, with multiple coordination environ- ments of uranyl evident in one sample. Differences in the equatorial coordination between the aqueous uranyl species and those found in the calcite indicate that coordination changes occur during incorporation of at least some species. This contrasts with previous findings showing no change in equatorial coordination during uranyl incorporation into aragonite, and may explain the greater incorporation in this latter phase. The absence of calcium backscatterers and well defined structure beyond the equatorial shell is consistent with disorder associated with disruption of the local calcite structure. This may indicate an inability of the uranyl unit to assume a stable structural environment in the host calcite, which could decrease the stability of uranyl- containing calcite. Calcite single crystals grown in uranyl-containing solutions exhibit polygonized spiral growth hillocks on (101 ¯4) surfaces composed of four vicinal surfaces, consistent with face symmetry. Micro-X-ray fluorescence reveals that uranium is differentially incorporated between nonequivalent vicinal surfaces, reflecting step- selective incorporation of uranyl species during growth. Micro-X-ray absorption near-edge structure spectra from the nonequivalent vicinal faces fail to reveal any differences in speciation between the vicinals or that might account for the presence of the multiple coordination environments identified by luminescence and X-ray absorption spectroscopies. Copyright

Carrier dynamics in α‐Fe2O3 (0001) thin films and single crystals probed by femtosecond transient absorption and reflectivity

Femtosecond transient reflectivity and absorption are used to measure the carrier lifetimes in α‐Fe2O3 thin films and single crystals. The results from the thin films show that initially excited hot electrons relax to the band edge within 300fs and then recombine with holes or trap within 5ps. The trapped electrons have a lifetime of hundreds of picoseconds. Transient reflectivity measurements from hematite (α‐Fe2O3) single crystals show similar but slightly faster dynamics leading to the conclusion that the short carrier lifetimes in these materials are due primarily to trapping to Fe d‐d states in the band gap. In the hematite single crystal, the transient reflectivity displays oscillations due to the formation of longitudinal acoustic phonons generated following absorption of the ultrashort excitation pulse.

Application of an InGaAsP diode laser to probe photodissociation dynamics: I* quantum yields from n‐ and i‐C3F7I and CH3I by laser gain vs absorption spectroscopy

A room temperature heterostructure InGaAsP laser diode operating at 1315 nm is employed for the first time for detection of I*(2P1/2) and I(2P3/2) atoms. The cw diode probe laser is used to study I* yields in the photodissociation of n‐ and i‐C3F7I and CH3I by the new technique of time‐resolved laser gain vs absorption spectroscopy. Preliminary quantum yields determined at 266 nm for n‐C3F7I, i‐C3F7I, and CH3I are 102±4%, 102±7%, and 73±4%, respectively. With further refinements to the diode laser set‐up, highly accurate quantum yields will be possible.A room temperature heterostructure InGaAsP laser diode operating at 1315 nm is employed for the first time for detection of I*(2P1/2) and I(2P3/2) atoms. The cw diode probe laser is used to study I* yields in the photodissociation of n‐ and i‐C3F7I and CH3I by the new technique of time‐resolved laser gain vs absorption spectroscopy. Preliminary quantum yields determined at 266 nm for n‐C3F7I, i‐C3F7I, and CH3I are 102±4%, 102±7%, and 73±4%, respectively. With further refinements to the diode laser set‐up, highly accurate quantum yields will be possible.

Role of Photoexcitation and Field Ionization in the Measurement of Accurate Oxide Stoichiometry by Laser-Assisted Atom Probe Tomography

The addition of pulsed lasers to atom probe tomography (APT) extends its high spatial and mass resolution capability to nonconducting materials, such as oxides. For a prototypical metal oxide, MgO, the measured stoichiometry depends strongly on the laser pulse energy and applied voltage. Very low laser energies (0.02 pJ) and high electric fields yield optimal stoichiometric accuracy. Correlated APT and aberration-corrected transmission electron microscopy (TEM) are used to establish the high density of corner and terrace sites on MgO sample surfaces before and after APT. For MgO, long-lifetime photoexcited holes localized at oxygen corner sites can assist in the creation of oxygen neutrals that may spontaneously desorb either as atomic O or as molecular O2. The observed trends are best explained by the relative field-dependent ionization of photodesorbed O or O2 neutrals. These results emphasize the importance of considering electronic excitations in APT analysis of oxide materials.

Absolute I∗ quantum yields for the ICN Ã state by diode laser gain-vs-absorption spectroscopy

Absolute I* quantum yields have been measured as a function of wavelength for room temperature photodissociation of the ICN A state continuum. The yields are obtained by the technique of time‐resolved diode laser gain‐vs‐absorption spectroscopy. Quantum yields are evaluated at seven wavelengths from 248 to 284 nm. The yield at 266 nm is 66.0±2% and it falls off to 53.4±2% and 44.0±4% at 284 and 248 nm, respectively. The latter values are significantly higher than those obtained by previous workers using infrared fluorescence. Estimates of I* quantum yields obtained from analysis of CN photofragment rotational distributions, as discussed by other workers, are in good agreement with the I* yields reported here. The results are considered in conjunction with recent theoretical and experimental work on the CN rotational distributions and with previous I* quantum yield results.

EXAFS study of rare-earth element coordination in calcite

Extended X-ray absorption fine-structure (EXAFS) spectroscopy is used to characterize the local coordination of selected rare-earth elements (Nd3+, Sm3+, Dy3+, Yb3+) coprecipitated with calcite in minor concentrations from room-temperature aqueous solutions. Fitting results confirm substitution in the Ca site, but first-shell Nd-O and Sm-O distances are longer than the Ca-O distance in calcite and longer than what is consistent with ionic radii sums for sixfold coordination in the octahedral Ca site. In contrast, first-shell Dy-O and Yb-O distances are shorter than the Ca-O distance and are consistent with ionic radii sums for sixfold coordination. Comparison of Nd-O and Sm-O bond lengths with those in lanthanide sesquioxides and with ionic radii trends across the lanthanide series suggests that Nd3+ and Sm3+ have sevenfold coordination in a modified Ca site in calcite. This would require some disruption of the local structure, with an expected decrease in stability, and possibly a different charge compensation mechanism between Nd and Sm vs. Yb and Dy. A possible explanation for the increased coordination for the larger rare-earth elements involves bidentate ligation from a CO3 group. Because trivalent actinides such as Am3+ and Cm3+ have ionic radii similar to Nd3+, their incorporation in calcite may result in a similar defect structure.

Mass spectrometry of low molecular mass solids by matrix-assisted laser desorption/ionization

Matrix-assisted laser desorption/ionization combined with time-of-flight mass spectrometry (MALDI/TOF-MS) was used for the analysis of low molecular mass compounds. Three classes of molecules were studied: organic acids, salts of oxyanions and amine-based chelating compounds. Mass spectra from samples of citric, propionic, butyric, oxalic and stearic acid; ethylenediaminetetraacetic acid (EDTA),N-(2-hydroxyethyl) ethylenediaminetriacetic acid (HEDTA), ethylenediamineN,N′-diacetic acid (EDDA) and nitrilotriacetic acid (NTA); and sulfate, nitrate, nitrite and phosphate salts were obtained. These species were analyzed alone and as mixtures in both the positive and negative ion modes. The organic acids and oxyanion salts displayed much stronger signals in the negative ion detection mode whereas chelating compounds, which contain basic amine functional groups, yielded stronger signals in the positive ion mode. This implies that detection sensitivity is often better for a particular ion mode in the analysis of small molecules containing limited classes of functional groups. In all analyses, the presence of high concentrations of sodium was found to quench the MALDI signals. To increase the detection sensitivity, some samples were processed through an ion-exchange column to remove sodium ions. This step was found to enhance the signal by two orders of magnitude over untreated samples.

Plasmonic field enhancement of individual nanoparticles by correlated scanning and photoemission electron microscopy

We present results of a combined two-photon photoemission and scanning electron microscopy investigation to determine the electromagnetic enhancement factors of silver-coated spherical nanoparticles deposited on an atomically flat mica substrate. Femtosecond laser excitation of the nanoparticles produces intense photoemission, attributed to near-resonant excitation of localized surface plasmons. Enhancement factors are determined by comparing the respective two-photon photoemission yields measured for single nanoparticles and the surrounding flat surface. For p-polarized, 400 nm (∼3.1 eV) femtosecond radiation, a distribution of enhancement factors is found with a large percentage (67%) of the nanoparticles falling within a median range. A correlated scanning electron microscopy analysis demonstrated that the nanoparticles typifying the median of the distribution are characterized by spherical shapes and relatively smooth silver film morphologies. In contrast, the largest enhancement factors were produced by a small percentage (7%) of particles that displayed silver coating defects that altered the overall particle structure. Comparisons are made between the experimentally measured enhancement factors and previously reported calculations of the localized near-field enhancement for isolated silver nanoparticles.

PHOTODISSOCIATION OF ACETYL CHLORIDE : CL AND CH3 QUANTUM YIELDS AND ENERGY DISTRIBUTIONS

Jet cooled acetyl chloride is dissociated at 236 nm via excitation of the 1[n, π* (C=O)] transition. Chlorine atom photofragments Cl(2P3/2) and Cl*(2P1/2) are detected using (2+1) multiphoton ionization. The relative Cl* yield φ*=[Cl*]/([Cl]+[Cl*]) is measured to be 0.4±0.02, and the mean translational energy of the Cl atoms is 0.28±0.02 eV. Methyl fragments are also detected following primary dissociation and the internal state distributions are probed using (2+1) multiphoton ionization through the 4pz Rydberg state at 286 nm and the 3pz Rydberg state at 334 nm. The quantum yield for CH3 formation is estimated to be 28%, relative to the CH3 yield from methyl iodide photolysis. A small fraction of the available energy is channeled into methyl fragment angular momentum as the rotational state distribution extends only to N‘=5. Our results indicate that CH3CO, generated as a primary photoproduct in the dissociation of acetyl chloride, subsequently decomposes to produce CH3 and CO. Energetic constraints in t...

Selective laser desorption of ionic surfaces: Resonant surface excitation of KBr

We demonstrate evidence of selective laser-induced desorption of ground state Br(2P3/2) and spin–orbit excited state Br(2P1/2) atoms from KBr single crystals following 6.4 eV irradiation. Laser excitation tuned selectively to a surface resonance below the first bulk absorption band excites surface states preferentially leading to surface specific reactions while inducing relatively insignificant bulk reaction. The experimental results are supported by embedded cluster ab initio calculations that indicate a reduced surface exciton energy compared to that of the bulk exciton with a slight further reduction for steps and kink sites. Low fluence irradiation of cleaved KBr crystals, near the calculated surface exciton energy of 6.2 eV, produces hyperthermal Br(2P3/2) emission without a significant thermal or Br(2P1/2) component. The hyperthermal emission is shown theoretically to be characteristic of surface induced reaction of exciton decomposition while thermal emission is attributed to bulk photoreaction.