The reaction force constant: an indicator of the synchronicity in double proton transfer reactions

Earlier work, both experimental and computational, has drawn attention to the transition region in a chemical reaction, which includes the traditional transition state but extends along the intrinsic reaction coordinate ξ from perturbed forms of the reactants to perturbed forms of the products. The boundaries of this region are defined by the reaction force F(ξ), which is the negative gradient of the potential energy V(ξ) of the system along ξ. The reaction force constant κ(ξ), the second derivative of V(ξ), is negative throughout the transition region. We have now demonstrated, for a series of twelve double proton transfer processes, that the profile of κ(ξ) in the transition region is an indicator of the synchronicity of the two proton migrations in each case. When they are fully or nearly fully synchronous, κ(ξ) has a single minimum in the transition region. When the migrations are considerably nonsynchronous, κ(ξ) has two minima separated by a local maximum. Such an assessment of the degree of synchronicity cannot readily be made from an examination of the transition state alone, nor it is easily detected in the profiles of V(ξ) and F(ξ).     

Swift heavy-ion irradiated active waveguides in Nd:YAG crystals: fabrication and laser generation

An Nd:YAG planar waveguide laser has been fabricated by ultra-low-fluence (2×10(12) cm(-2)) swift heavy-ion irradiation (60 MeV Ar(4+) ions). The appearance of the buried waveguiding has been associated with an increased refractive index layer as a consequence of the ion-induced electronic damage. Continuous-wave laser oscillations at 1064.2 nm have been observed from the waveguide under 808 nm optical excitation, with the absorbed pump power at threshold and laser slope efficiency close to 26 mW and 5.9%, respectively.     

Microstructuring of Nd:YAG crystals by proton-beam writing

We report on the microstructuring of Nd:YAG crystals by direct proton-beam writing. Buried channel waveguides have been fabricated with full spatial control by the combined variation of crystal position and proton energy. The fluorescence images of the obtained structures have been used to evaluate the potential application of the fabricated structures for laser gain as well as to elucidate the mechanism at the basis of the refractive index increment induced at the end of the proton path. We have concluded that this increment is very likely a local enhancement in the electronic polarizability caused by nuclear collisions.     

Direct laser writing of near-IR step-index buried channel waveguides in rare earth doped YAG

A new (to our knowledge) ultrashort laser pulse irradiation regime that allows us to directly modify and increase the refractive index of rare earth doped YAG polycrystalline ceramics has been identified. Single-mode buried channel waveguides in both Ho:YAG and Er:YAG ceramics at the near-IR wavelengths of 1.55?μm and 1.95?μm are demonstrated by fabricating positive square step-index cores. Minimum propagation losses of 1.5?dB?cm(-1) at a 1.51?μm wavelength have been preliminarily obtained. Confocal microluminescence mapping reveals that the increased refractive index regions retain the near-IR spectral properties of Er3+ ions in the YAG crystalline matrix.     

Whispering-gallery modes in glass microspheres: optimization of pumping in a modified confocal microscope

Whispering-gallery modes (WGMs) on Nd3+-doped glass microspheres with a radius of ～15?μm were measured in a modified confocal microscope, where a dual spatial resolution in both excitation and detection zones was possible. As an alternative to the standard excitation mechanism by an evanescent wave, we used an efficient pumping/detecting scheme, focusing a laser in the microsphere and exciting the Nd3+ ions, whose fluorescent emission produces the WGMs. We have also measured the generated WGMs by changing the detection zone, where higher amplitude resonances were found when exciting in the center and detecting at the edge of the microsphere.     

Development and Investigation of Ultrastable PbS/CdS/ZnS Quantum Dots for Near‐Infrared Tumor Imaging

Achieving bright, reliable, robust, and stable probes for in vivo imaging is becoming extremely urgent for the cancer imaging research community. To date very few works have reported on elucidating in the varied and chemically complex biological milieu. The authors report detailed investigations of the synthesis of near‐infrared, water dispersive, strongly luminescent, and highly stable PbS/CdS/ZnS core/shell/shell quantum dots (QDs). These QDs are extremely stable, they could keep their initial morphology, dispersion status, and photoluminescence (PL) in phosphate buffered saline buffer for as long as 14 months. The QDs also show excellent photostability and could keep ≈80% of their initial PL intensity after 1 h continuous, strong UV illumination. More interestingly, they show negligible toxicity to cultured cells even at high QDs concentration. Given these outstanding properties, the QDs are explored for in vivo, tumor imaging in mice. With one order of magnitude lower QD concentration (0.04 mg mL^–1), significantly weaker laser intensity (0.04 W cm^–2 vs ≈1 W cm^–2), and considerably shorter signal integration time (≤1 ms vs hundreds of ms) as compared to the best reported rare earth doped nanoparticles, the QDs show high emission intensity even at injection depth of ≈2.5 mm.     

Cr<Superscript>3+</Superscript> spectroscopy study in ZnO-codoped and Zn-in-diffused congruent LiNbO<Subscript>3</Subscript>:Cr crystals

This paper reports on the spectroscopy properties, absorption and luminescence, of Cr³⁺ ions in singly doped, ZnO-codoped, and Zn in-diffused LiNbO₃:Cr crystals. In addition to the broad absorption, inter-ionic transitions ascribed to Cr³⁺ ions located in Li⁺ and Nb⁵⁺ sites; [Cr]_Li and [Cr]_Nb centres two absorption bands at higher energy are reported and ascribed to the charge transfer transitions of the Cr³⁺ ions of the two defect centres. The charge transfer transitions are used as optical probe to study the role of the Zn ions in the Zn in-diffused LiNbO₃:Cr samples. It has been observed that the Zn-in-diffused processes created [Cr]_Nb centres in the diffusion zone. The location of the diffused Zn²⁺ ions is considered to be in Li⁺ site, displacing the Cr³⁺ ions from the Li⁺ sites, [Cr]_Li, to the Nb⁵⁺ positions, [Cr]_Nb.     

Thermoluminescence and electron spin resonance after room-temperature X-ray irradiation of NaCl:Mn2+

A parallel investigation of thermoluminescence (TL) and electron spin resonance (ESR) spectra on room-temperature (RT) X-irradiated NaCl:Mn²⁺ has been performed. The TL spectra in the range 20–300°C consist of five glow peaks, numbered from I to V. Temperatures at maximum height are 41°, 68°, 118°, 152° and 216°C, respectively. Peaks I, II and IV obey first-order kinetics, whereas peaks III and V fit second-order behavior. The wavelength spectrum for all glow peaks consists of two bands centered at 595 and 400 nm. The 595 nm emission is attributed to hole capture by Mn⁺ and subsequent deexcitation of Mn²⁺. The 400 nm emission is produced as a consequence of hole-F center recombination.The correlation of TL glow peaks to various defects has been investigated. Peak II is clearly related to manganese-vacancy dipoles and peak I can be roughly associated to free cation vacancies. Peak IV appears to relate to large Mn-aggregates, whereas peak V is intrinsic and not related to impurities.On the other hand, ESR data indicate that each glow peak in the 595 nm emission is associated to the annihilation of a given Mn-center; Peak I to Mn⁰C, peak II to Mn⁰C and Mn⁺, peak III to Mn⁺ and peaks IV and V to Mn⁰-D.     

Thermoluminescence and recovery processes in pure and doped NaCl after 20 K irradiation

The thermoluminescence (TL) spectra after X-ray irradiation at 20 K have been investigated for pure as well as divalent cation doped NaCl. The F-centre decay has also been determined in pure and Ca and Mg doped NaCl for comparison purposes. A clear decrease in F-centre concentration appears to correlate with glow peaks at 44 and 50 K for pure and Ca-doped samples. Main glow peak appearing at 69 K is not associated to any appreciable F-centre decay step. Below liquid nitrogen temperature (LNT) all peaks show both σ and π exciton emission bands. Above LNT, the glow peaks for doped samples show the σ emission together with another band at 410 nm, whereas pure samples still present the intrinsic emission bands.     

Comparison between microscopic and phenomenological analysis of ionic thermocurrents (ITC)

The classical ITC model and a numerical calculation using a microscopic theory are compared for the NaCl:Mn²⁺ system. The comparison is extended from LNT to 600 K to cover both ITC and dielectric losses (tg δ) ranges.