MRCI studies on the electronic structure of AlN and AlN, and the electron affinity of AlN

Using multireference configuration-interaction methods and double to triple-zeta basis sets with semidiffuse and polarization functions, potential energies and spectroscopic constants for low-lying doublet, and quartet states of AlN⁻ were calculated. has R_e=3.280 bohr and . lies 0.17 eV above the ground state. Using an estimated electron affinity of 2.1 eV for AlN, four states of AlN⁻ are found to be stable, namely , , , and . Comparisons with the isovalent anions BN⁻ (three stable states) and AlP⁻ (seven stable states) are made. Photo-detachment of an electron from the state of AlN⁻ can lead to an accurate determination of the energy difference between the two close-lying lowest states of AlN, and , predicted here to be 0.09 eV apart.     

The second hyperpolarizability of CdTe nanocrystals using polarization-resolved degenerate four-wave mixing

Polarization-resolved forward degenerate four-wave mixing (DFWM) in a nonresonant region revealed the effective third-order nonlinear susceptibility of colloidal CdTe nanocrystals (NCs) with the size near the Bohr radius and various concentrations. The second hyperpolarizabilities, and , of the CdTe NCs were ∼1.15 × 10⁻⁴¹ m⁵/V² and ∼3.01 × 10⁻⁴² m⁵/V² from the measurement of the concentration-dependent third-order nonlinear susceptibility of CdTe NCs, respectively. The ratio (/) of the hyperpolarizabilities was ∼0.26, which indicated a large contribution of an electronic polarization process to the third-order nonlinearity of CdTe NCs.     

Profile imaging of reconstructed polar and non-polar surfaces of ZnO

The atomic scale surface structures of ZnO non-polar as well as and ±(0 0 0 1) polar surfaces have been directly imaged by high-resolution transmission electron microscopy (HRTEM). The observations were made on clean surfaces created by irradiating a single ZnO nanobelt using 400 keV electron beam in TEM, under which ZnO dots were grown epitaxially and in situ on the surface of the nanobelt. A technique is demonstrated for directly distinguishing the surface polarity of the ±(0 0 0 1) polar surfaces. For the non-polar surface, HRTEM images and simulation results indicate that the Zn ions in the first and second layer suffer from inward and outward relaxation, respectively; the oxygen ions in the first and second layer prefer shifting to vicinal Zn ions to shorten the bonding distance. For the oxygen-terminated polar surface, the oxygen ions at the outmost top layer were directly imaged. a × 2 reconstruction has also been observed at the surface, and its atomic structure has been proposed based on image simulation. Oxygen-terminated polar surface is flat and shows no detectable reconstruction. For the Zn-terminated (0 0 0 1) polar surface, HRTEM may indicate the existence of Zn vacancies and a possibly c-axis, random outward displacement of the top Zn ions. Our data tend to support the mechanism of removal of surface atoms for maintaining the stability of (0 0 0 1) polar surfaces.     

Formation mechanisms of GaN nanorods grown on Si(1 1 1) substrates

Scanning electron microscopy (SEM) images, transmission electron microscopy (TEM) images, and selected-area electron diffraction (SAED) patterns showed that vertically well aligned GaN nanorods with c-axis-oriented crystalline wurzite structures were grown on Si(1 1 1) substrates by using hydride vapor phase epitaxy. The high-resolution TEM (HRTEM) images showed that the crystallized GaN nanorods contained very few defects and that they were consisted of , {0 0 0 1}, and { } facets. The formation mechanisms for the GaN nanorods grown on Si(1 1 1) substrates are described on the basis of the SEM, TEM, SAED pattern, and HRTEM results.     

L to M shell transitions and model comparisons for radiative opacities of sodium fluoride plasma

The radiative opacities of hot dense sodium fluoride (NaF) plasma have been studied in the case of local thermodynamic equilibrium (LTE). The spectrally resolved opacity in the case of and is taken as an example to illustrate the absorption structures of the NaF plasma. The transitions from L to M shells of sodium and fluoride are the main contributions for the plasma absorptions. The isodense opacities of NaF plasma are studied at a density of and temperatures of 20-60 eV. Detailed comparisons among the results of the detailed term accounting (DTA), detailed configuraion accounting (DCA), unresolved transition array (UTA) and average atom (AA) model codes, which have been developed independently in China in the past few years, are performed for both the spectral resolved and mean opacities. Good agreements are found at a temperature of 40 eV and a density of , but discrepancies appear at a temperature of 50 eV and a density of .     

Theoretical study of local lattice structure of Mn in perovskite fluorides A2MF4 (A=K, Rb; M=Mg, Zn, Cd) series

A theoretical method for investigating the inter-relation between the molecular structure and electronic structure has been established on the basis of the 252×252 complete energy matrices for a 3d⁵ configuration ion in a tetragonal ligand field. By means of this method, which is independent of the X-ray diffraction, the local structure of the paramagnetic Mn²⁺ ion in perovskite fluorides A₂MF₄ (A=K, Rb; M=Zn, Mg, Cd) are determined directly by analyzing the EPR spectrum of octahedral Mn²⁺ center in A₂MF₄ crystals and the optical absorption spectrum of the (MnF₆)⁴⁻ cluster. It is shown that, comparing with the octahedral cubic structure, the local micro-structure in the vicinity of Mn²⁺ displays an elongated distortion when and a compressed distortion when , and ΔR vs. as well as ΔR vs. in the distortion region is, respectively, approximately linear. Simultaneously, the theoretical zero-field-splitting parameters , and are in good agreement with the experimental values.     

Near-infrared diode laser spectroscopy of the HCSi radical

The , , and band spectra of HCSi radical were investigated by means of near-infrared diode laser spectroscopy to determine precise molecular constants for the and states. The detailed analysis of the rotationally resolved band spectra, studied for the first time in the present investigation, leads to the precise determination of molecular constants for the state associated with the Renner-Teller interaction. We obtained −0.15126663(53) and 495.00698(30) cm⁻¹ as the Renner-Teller parameter ε and the bending vibrational frequency ω₂, respectively. Based on the molecular constants for the and states, the rotational levels of the state were analyzed to obtain molecular constants and information on upper state perturbations. Using the available spectroscopic data, valence force fields for both the and states were estimated to aid in understanding the vibrational energy levels of the HCSi radical.     

First-principles study on the electronic structures and absorption spectra for the PbWO4 crystal with lead vacancy

The electronic structures and absorption spectra for the perfect PbWO₄ (PWO) crystal and the crystal containing lead vacancy have been calculated using density functional theory code CASTEP with the lattice structure optimized. The calculated absorption spectra of the PWO crystal containing exhibit seven absorption bands peaking at 1.72 eV (720 nm), 2.16 eV (570 nm), 2.81 eV (440 nm), 3.01 eV (410 nm), 3.36 eV (365 nm), 3.70 eV (335 nm) and 4.0 eV (310 nm), which are very close to the experimental values. It predicts that the 330, 360, 420, 500-750 nm absorption bands are related to the existence of in the PWO crystal.     

Measurement of the 4d-photoionization cross section via two-photon and two-step excitation in sodium

We present a comparison of the photoionization cross sections of the 4d excited levels of sodium by using the two-step resonant laser excitation and by two-photon non-resonant excitation from the ground state. Dye lasers, pumped with a Nd: YAG laser, have been used in conjunction with a thermionic diode ion detector to measure cross sections and the atomic densities as a function of laser energy. By applying the saturation technique, the measured values of the cross sections and atomic densities for the 4d level by two-photon excitation are 12.2(2.4) Mb and , respectively. Where as in the case of two-step excitation, the cross section and number density for the 4d level via 3p level and for the 4d levels via 3p level are determined as 9.6(1.9) Mb, and 12.8(2.5) Mb, , respectively.     

The luminescence of Sm in alkaline earth borophosphates

The temperature-dependent luminescence of Sm²⁺ ions in MBPO₅ was studied. At low temperature, Sm²⁺ in this series shows 4f⁶→4f⁶ luminescence with only a single emission line observed for the transition, revealing that only one crystallographic cationic site is available for Sm²⁺ in all the hosts. With increasing temperature, the emission intensity of the transition increases whereas that of the transitions decreases. The transitions of Sm²⁺ were observed in BaBPO₅ and its intensity increases with increasing temperature. At , a broad band of the 4f⁵5d→4f⁶ luminescent transition of Sm²⁺ in SrBPO₅ and BaBPO₅ with maximum at appears due to the thermal population. The lifetime of the transition was recorded at different temperatures, showing a single exponential decay for Sm²⁺ in SrBPO₅ and BaBPO₅ but a non-single-exponential decay in CaBPO₅.     

Direct nanosecond Nd→Ce nonradiative energy transfer in cerium trifluoride laser crystals

Using third harmonics of LiF:F₂⁺ tunable color center laser excitation and selective fluorescence detection the temperature and concentration dependencies of fluorescence decay curves of the high-lying manifold of the Nd³⁺ ion were measured in CeF₃ crystals. As a result the temperature dependence of energy transfer kinetics from the manifold of the Nd³⁺ donor ions to the manifold of the acceptor Ce³⁺ ions in the ordered practically 100% filled crystal lattice was determined for 13-. Based on the temperature dependence the mechanisms and the channels of the Nd→Ce nonradiative energy transfer have been recognized. The net growth of the resonance Nd→Ce energy transfer rate in the temperature range from 25 to is found to be almost 3 orders of magnitude from 9.0×10⁴ to .In a crystal a significant contribution of the Nd→Nd resonance energy transfer to the manifold quenching is found for 20- and its channel and mechanism are suggested.Discussion of the possibility of subpicosecond and picosecond nonradiative energy transfer in rare-earth doped laser crystals is provided.     

Anomalous thermostat and intraband discrete breathers

We investigate the dynamics of a macroscopic system which consists of an anharmonic subsystem embedded in an arbitrary harmonic lattice, including quenched disorder. The coupling between both parts is bilinear. Elimination of the harmonic degrees of freedom leads to a nonlinear Langevin equation with memory kernels and noise term for the anharmonic coordinates . For zero temperature, i.e. for , we prove that the support of the Fourier transform of and of the time averaged velocity-velocity correlation functions of the anharmonic system cannot overlap. As a consequence, the asymptotic solutions can be constant, periodic, quasiperiodic or almost periodic, and possibly weakly chaotic. For a sinusoidal trajectory with frequency we find that the energy E_T transferred to the harmonic system up to time T is proportional to T^α. If equals one of the phonon frequencies ω_ν, it is α=2. We prove that there is a zero measure set L such that for in its full measure complement R?L, it is α=0, i.e. there is no energy dissipation. Under certain conditions L contains a subset L^′ such that for the dissipation rate is nonzero and may be subdissipative (0≤α<1) or superdissipative (1<α≤2), compared to ordinary dissipation (α=1). Consequently, the harmonic bath does act as an anomalous thermostat, in variance with the common belief that elimination of a macroscopically large number of degrees of freedom always generates dissipation, forcing convergence to equilibrium. Intraband discrete breathers are such solutions which do not relax. We prove for arbitrary anharmonicity and small but finite coupling that intraband discrete breathers with frequency exist for all in a Cantor set C(k) of finite Lebesgue measure. This is achieved by estimating the contribution of small denominators appearing for , related to . For the small denominators do not lead to divergencies such that is a smooth and bounded function in t.     

First-principles study of arsenic impurity clusters in molecular beam epitaxy (MBE) grown HgCdTe

The understanding of the microstructures of the arsenic tetramer , dimer , and singlet of HgCdTe is important to explain the high electrical compensation of molecular beam epitaxy (MBE) samples and the conversion to p-type behavior. The stable configurations were obtained from the first-principles calculations for the arsenic cluster defects [ (n=1, 2, and 4)] in as-grown HgCdTe. According to the defect formation energies calculated under Te-rich conditions, the most probable configurations of , , and have been established. For the optimized and the energy is favorable to combine in a nearest neighboring mercury vacancy , and the corresponding configurations can be used to explain the self-compensated n-type characteristics in as-grown materials. is likely to be more abundant than in as-grown materials, but arsenic atoms are more strongly bounded in than in , thus more substantial activation energy is needed for than that for . The atomic relaxations as well as the structural stability of the arsenic defects have also been investigated.