Mechanisms of a change in the refractive index of an intensely pumped Yb:YAG crystal

Mechanisms of a change in the refractive index appearing in an intensely diode-pumped Yb:YAG-laser disk element are studied with the help of polarisation interferometry and dynamic grating testing. It is found that changes in the electronic component of the refractive index arising upon changing the populations of electronic levels of Yb ions (the ground F state and the upper F level of the laser transition) and caused by the difference in the polarisability of these levels are an order of magnitude greater than thermal changes in the refractive index. It is shown that the difference Δp in the polarisability at the probe wavelength of 633 nm is 1.9 × 10 cm and at the laser transition wavelength of 1029 nm is 1.6 × 10 cm. ©2006 Kvantovaya Elektronika and Turpion Ltd.

Microstructural characterization of electron beam evaporated tungsten oxide films for gas sensing applications

Tungsten trioxide is one of the potential semiconducting materials used for sensing NH3, CO, CH4 and acetaldehyde gases. The current research aims at development, microstructural characterization and gas sensing properties of thin films of Tungsten trioxide (WO3). In this paper, we intend to present the microstructural characterization of these films as a function of post annealing heat treatment. Microstructural and elemental analysis of electron beam evaporated WO3 thin films and iron doped WO3 films (WO3:Fe) have been carried out using analytical techniques such as Transmission electron microscopy, Rutherford Backscattered Spectroscopy and XPS analysis. TEM analysis revealed that annealing at 300oC for 1 hour improves cyrstallinity of WO3 film. Both WO3 and WO3:Fe films had uniform thickness and the values corresponded to those measured during deposition. RBS results show a fairly high concentration of oxygen at the film surface as well as in the bulk for both films, which might be due to adsorption of oxygen from atmosphere or lattice oxygen vacancy inherent in WO3 structure. XPS results indicate that tungsten exists in 4d electronic state on the surface but at a depth of 10 nm, both 4d and 4f electronic states were observed. Atomic force microscopy reveals nanosize particles and porous structure of the film. This study shows e-beam evaporation technique produces nanoaparticles and porous WO3 films suitable for gas sensing applications and doping with iron decreases the porosity and particle size which can help improve the gas selectivity.

Synthesis and physico-chemical properties of diruthenium(III, II) tetra-aryl carboxylato compounds

Monochloro-tetra-μ-aryl-carboxylatodiruthenium(III, II) compounds Ru2Cl (O2CAr)4 (Ar = -C6H5; -C6H4-p-OCH3), are prepared and characterized. The compounds have magnetic moments that correspond to three unpaired spins per dimer. The Rusingle bondRu bond order is 2.5 and the ground electronic configuration is σ2π4δ2(δ*π*)3. The visible spectral band is observed at ca 450 nm along with a shoulder near 580 nm in DMF solution. The compounds undergo a one-electron Ru(III)Ru(II) → Ru(II)Ru(II) quasi-reversible reduction in DMF near 0.0 V vs sce.

Optical and magnetic properties of the exact PPP states of biphenyl

The low-lying singlets and triplets of biphenyl are obtained exactly within the PPP model using the diagrammatic valence bond method. The energy gaps within the singlet manifold as well as the lowest singlet-triplet gap are found to be in good agreement with experimental results. The two weak absorptions between 4·1 and 4·2 eV reported experimentally are attributed to the two states lying below the optical gap that become weakly allowed on breaking electron-hole and inversion symmetries. The observed blue shift of the spectral lines, attributed to a change in dihedral angle, on going from crystalline to solution to vapour phase is also well reproduced within the PPP model. The bond orders show that the ground singlet state is benzenoidal while the dipole excited state as well as the lowest triplet state are quinonoidal and planar. Comparison with the experimental spin densities and the fine structure constants D and E in the triplet state point to slightly weaker correlations than assumed by the PPP model. The introduction of a 1-8 bond to mimic poly(paraphenylene)s gives an optical gap that is in good agreement with experiment.

Ultraviolet photoelectron spectroscopy of the hydrogen bonded heterodimer H2Scdots, three dots, centeredHCl

The HeI photoelectron spectrum of the hydrogen bonded hetero-dimer H2Scdots, three dots, centeredHCl shows two vertical ionization energies at 10.91 and 12.16 eV. Ab initio MO calculations reveal that these features are due to the sulphur and chlorine lone pair ionizations respectively. Results show that while the ground ionic state is repulsive the first excited ionic state is strongly bound. The photoelectron spectrum of the diethyl sulphidecdots, three dots, centeredHCl complex is similar to that of H2Scdots, three dots, centeredHCl.

Role of dicarboxylate linkers in Mn-III-salicylaldoximate based extended structures: synthesis, structures, and magnetic behavior

Four new oxo-centered Mn-III-salicylaldoximate triangle-based extended complexes (Mn6O2)-O-III(salox)(6)(EtOH)(4)(phda)](n)(saloxH(2))(n)(2H(2)O)(n) (1), (Mn6O2)-O-III(salox)(6)(MeOH)(5)(5-I-isoph)](n)(3MeOH)(n) (2), (Mn6O2)-O-III(salox)(6)(MeOH)(4)(H2O) (5-N-3-isoph)](n)(4MeOH)(n) (3) and (Mn3NaO)-Na-III(salox)(3)(MeOH)(4)(5-NO2-isoph)](n)(MeOH)(n) (H2O)(n) (4) salox=salicylaldoximate, phda=1,3-phenylenediacetate, isoph=isophthalate] have been synthesized under similar reaction conditions. Single crystal X-ray structures show that in 1, only one type of Mn-6 cluster is arranged in 1D, whereas in 2 and 3 there are two types of clusters, differing in the way the triangle units are joined and assembled. In complex4, however, the basic building structure is heteronuclear and based on Mn-3 units extended in 2D. Susceptibility measurements (dc and ac) over a wide range of temperatures and fields show that the complexes1, 2, and 3 behave as single molecule magnets (SMMs) with S=4ground state, while 4 is dominantly antiferromagnetic with a ground spin state S=2. Density functional theory calculations have been performed on model complexes to provide a qualitative theoretical interpretation for their overall magnetic behavior.

Strain-Induced Hierarchy of Energy Levels in CdS/ZnS Nanocrystals

Aside of size and shape, the strain induced by the mismatch of lattice parameters between core and shell in the nanocrystalline regime is an additional degree of freedom to engineer the electron energy levels. Herein, CdS/ZnS core/shell nanocrystals (NCs) with shell thickness up to four monolayers are studied. As a manifestation of strain, the low temperature radiative lifetime measurements indicate a reduction in Stokes shift from 36 meV for CdS to 5 meV for CdS/ZnS with four monolayers of overcoating. Concomitant crossover of S- and P-symmetric hole levels is observed which can be understood in the framework of theoretical calculations predicting flipping the hierarchy of ground hole state by the strain in CdS NCs. Furthermore, a nonmonotonic variation of higher energy levels in strained CdS NCs is discussed.

Two-colour laser-induced electron dynamics in two-level quantum system

To attempt to control the quantum state of a physical system with a femtosecond two-colour laser field, a model for the two-level system is analysed as a first step. We investigate the coherent control of the two-colour laser pulses propagating in a two-level medium. Based on calculating the influence of the laser field with various laser parameters on the electron dynamics, it is found the electronic state can be changed up and down by choosing the appropriate laser pulses and the coherent control of the two-colour laser pulses can substantially modify the behaviour of the electronic dynamics: a quicker change of two states can be produced even for small pulse duration. Moreover, the oscillatory structures around the resonant frequency and the propagation features of the laser pulses depend sensitively on the relative phase of the two-colour laser pulses. Finally, the influence of a finite lifetime of the upper level is discussed in brief.

Phase shift caused by microwave field based on light storage

We have theoretically investigated the phase shift of a probe field for a four-level atomic system interacting successively with two fields tuned near an EIT resonance of an atom, a microwave field, and a coupling field. It has been found that the phase of retrieved signal has been shifted due to the cross-phase modulation when the stored spin wave was disturbed by a microwave. Because of the low relaxation rates of the ground hyperfine state, our proposed technique can impart a large phase rotation onto the probe field with low absorption of retrieved field and very low intensity of the microwave field.

Electric-field switching of exciton spin splitting in coupled quantum dots

We investigate theoretically the spin splitting of the exciton states in semiconductor coupled quantum dots (CQDs) containing a single magnetic ion. We find that the spin splitting can be switched on/off in the CQDs via the sp-d exchange interaction using the electric field. An interesting bright-to-dark exciton transition can be found and it significantly affects the photoluminescence spectrum. This phenomenon is induced by the transition of the ground exciton state, arising from the hole mixing effect, between the bonding and antibonding states. (C) 2008 American Institute of Physics.

Hole levels and exciton states in CdS nanocrystals

We have studied the hole levels and exciton states in CdS nanocrystals by using the hole effective-mass Hamiltonian for wurtzite structure. It is found that the optically passive P-x state will become the ground hole state for small CdS quantum dots of radius less than 69 Angstrom. It suggests that the "dark exciton" would be more easily observed in the CdS quantum dots than that in CdSe quantum dots. The size dependence of the resonant Stokes shift is predicted for CdS quantum dots. Including the Coulomb interaction, exciton energies as functions of the dot radius are calculated and compared with experimental data.

Applicability of the reaction window theory

Based on the molecular Coulombic over barrier model for description of slow ion-atom collisions, the reaction window theory related to projectile velocity is presented briefly. According to the theory, the state-selective differential cross sections of single electron capture in O8+ -H, A(8+) -H, Ar8+-He, Ne10+-He and Ar18+-He collisions at different collision velocities are calculated and compared with experimental results. Calculations are also done for single, double, and triple electron capture in N-15(7+)-Ne collisions at fixed velocity of 0.53 a.u., and are compared with experimental data. It is found that the predictions of the final electronic state distribution of captured electron(s) are in agreement with experimental data, and both theory and experiments show that the widths of the reaction window increase with the projectile velocity. The differential cross sections predicted by the theory are larger for smaller Q-values, vice versa, when compared with experimental data.

Photodissociation dynamics of the methyl radical at 212.5 nm: Effect of parent internal excitation

Photodissociation dynamics of the CH3 radical at 212.5 nm has been investigated using the H atom Rydberg tagging time-of-flight method with a pure CH3 radical source generated by the photolysis of CH3I at 266 nm. Time-of-flight spectra of the H atom products from the photolysis of both cold and hot methyl radicals have been measured at different photolysis polarizations. Experimental results indicate that the photodissociation of the methyl radical in its ground vibrational state at 212.5 nm excitation occurs on a very fast time scale in comparison with its rotational period, indicating the CH3 dissociation at 212.5 nm occurs on the excited 3s Rydberg state surface. Experimental evidence also shows that the photodissociation of the methyl radical in the nu(2)=1 state of the umbrella mode at 212.5 nm excitation is characteristically different from that in the ground vibrational state. (C) 2004 American Institute of Physics.

Above Threshold Dissociation of Vibrationally Cold HD+ Molecules

The experimental study of molecular dissociation of H2+ by intense laser pulses is complicated by the fact that the ions are initially produced in a wide range of vibrational states, each of which responds differently to the laser field. An electrostatic storage device has been used to radiatively cool HD+ ions enabling the observation of above threshold dissociation from the ground vibrational state by 40 fs laser pulses at 800 nm. At the highest intensities used, dissociation through the absorption of at least four photons is found to be the dominant process.