Configuration Coordinates Research Articles

Optical properties of chromium (III) in fluoride glasses

The optical properties of Cr 3+ ion in heavy-metal fluoride ZBLA, and BIGaZYT glasses are presented. The thermal quenching of the Cr 3+ broad band luminescence is discussed in terms of the quantum mechanical single configurational coordinate (QMSCC) model using the Struck and Fonger method. A reasonably good agreement with experimental data is achieved. Finally, the analysis of the time evolution of emission spectra provides evidence for the assumption of two distinguishable distributions of Cr 3+ sites in these glasses.

Absorption and emission spectroscopy on chlorofluorocarbons in noble-gas matrices

Although chlorofluorocarbons are chemically very stable, they are easily photolysed by the excitons of a noble-gas matrix. The reaction products Cl and CF 2 can be followed in fluorescence and absorption spectroscopy, respectively. Our results on halogenated methanes (HM) in an Ar matrix show that (i) Cl is photolysed as an atom, (ii) CF 2 is a thermally and photolytically stable radical in the matrix, and (iii) the electronic energies of the CF 2 transition are strongly dependent on the matrix cage or on the split-off molecules, while the vibronic energies and the configuration coordinate remain nearly unchanged.

A Three-Dimensional Dynamic Model for Determining the Equilibrium Configurations of Buoyantly Unstable Icebergs

Abstract This paper describes a dynamic model developed for the purpose of determining the final equilibrium configurations of buoyantly unstable icebergs. The model places no restrictions on the size, shape, or dimensionality of the iceberg, or on the variation range of the configuration coordinates. Furthermore, it includes all six degrees of freedom and is based on a Lagrangian formulation of the dynamic equations of motion. It can be used to advantage in those situations in which the iceberg has a complicated potential function and can acquire enough momentum and kinetic energy in the initial phase of its motion to make its final configuration uncertain on the basis of a static potential analysis. The behavior of the model is examined through several model simulations. The sensitivity of the final equilibrium position to the initial orientation and shape of the iceberg is clearly evident in the model simulations. Model simulations also show that when an iceberg is released from a nonequilibrium initial state, the time taken for it to settle down varies from about 40 s for a growler to nearly 400 s for a large iceberg. While these absolute times may change with better parameterization of the forces, the relative variations with iceberg size are likely to be preserved.

Infrared luminescence properties of Ni 2+ in various chloride lattices: CsCdCl 3, CsMgCl 3, CdCl 2, and MgCl 2

A study of the infrared luminescence of Ni 2+ ( 3A 2g« 3T 2g: O h notation) in four structurally related compounds is presented. Low temperature IR luminescence spectra are presented for the title compounds; energy assignments are given for the fine structure observed in the spectrum of Ni 2+:MgCl 2. Low temperature 3A 2gå 3T 2g absorption spectra for Ni 2+:CsCdCl 3 and Ni 2+:MgCl 2 are shown; fine structure is observed mainly on the high-energy side of these bands. Temperature dependences of luminescence intensities and of luminescence decay rates are given; these dependences are described in terms of radiative and non-radiative models. Non-radiative decay is assumed to be through multi-phonon emission, and we use a single configuration coordinate (SCC) model. For each compound, the ‘effective’ phonon frequency of non-radiative decay is estimated to be approximately the mean frequency of the phonon spectrum. Variation in non-radiative behavior is well described in terms of the variation of the number of phonons involved and the Huang-Rhys parameter.

Spectral line shapes of organic dyes in solutions: the configuration-coordinate model revisited

The concept of the configuration-coordinate (CC) model for calculations of spectral line shapes of organic dyes in solutions is discussed. We propose the reinterpretation and justification of the CC-model in terms of the conditional free energy associated with some significant physical observable (s). We stress the role of possible non-stationarity of the relaxation processes occurring in liquids.

Quantum diagrams of transition-metal-metalloid defect structures.

The structural transition between transition-metal--metalloid compounds with NiAs, MnP, and related structures is reexamined with the aid of modern quantum configuration coordinates. Older coordinate (Mooser-Pearson) maps of these distorted NaCl structures often produced contradictory coincidences. I obtain a successful separation of 40 pnictide compounds in four structures using three coordinates, but these coordinates are different from those used recently by Villars to separate 3000 ``normal'' compounds in 76 structures. This difference may be related to the defect nature of these structures.

Nonequilibrium Statistical Operator of a Vibronic System; Decay of the Squeezed State

A nonequilibrium statistical operator of a vibronic system in the representation of the configurational coordinate and the corresponding characteristic Wigner function are considered. In the formulas obtained, quadratic quantum fluctuations in the nonstationary state are expressed via integrals from the dynamical Green's functions; the expressions at small and large times pass over into the Callen-Welton formula. The formalism proposed can be used to describe squeezed pulses.

Hartree–Fock calculation of the electronic structure of a Cu+ impurity in NaCl

Hartree–Fock cluster calculations have been carried out for the ground 3d10 and excited 3d94s configurations of the Cu+ ion in a NaCl host. Special emphasis has been given to providing an accurate representation of the Coulomb potential due to the remainder of the lattice. Configuration coordinate curves were determined for the symmetric displacement of the nearest-neighbor Cl− ions and are compared to recent Xα calculations. The Hartree–Fock equilibrium Cu–Cl distance was found to be 5.327 bohr, slightly shorter than the calculated nearest-neighbor distance of 5.353 bohr for the pure NaCl host. Comparison of the Hartree–Fock and Xα ground and excited state energies, shows that the Xα approximation reverses the ordering of the 3T2g and 1Eg states, overestimates the equilibrium nearest-neighbor distance, and predicts the a1g vibrational frequency to be about twice the Hartree–Fock value. Using the Franck–Condon factors found with the ab initio potential energy curves, the calculated bandwidths for the 1,3T2g states are found to be in excellent agreement with the low-temperature absorption spectra.

Triplet states of CO trapped in rare gas crystals

Vibrational progressions due to the a 3Π, a′ 3Σ+, d 3Δ, and e 3Σ− states of CO in Ar, Kr, and Xe crystals are observed in absorption. Two groups of phonon sidebands separated from the zero phonon lines by about 4 and 8 meV for the a′ 3Σ+ and d 3Δ states are attributed to librations and to localized phonons. The broad absorption bands, the even broader emission bands, and the large Stoke shift of the a 3Π transition are treated in terms of a configuration coordinate model by a momentum analysis. A quadratic coupling has to be invoked with relaxation energies in the excited states increasing from 14 meV for Ar to 95 meV for Xe and with relaxation energies in the ground state increasing from 29 meV for Ar up to 1278 meV for Xe. The changes of the configuration coordinate of 0.2 to 1.5 Å are also discussed with the help of two pair potential models.

Description of radiation effects by means of potential curve graphs

Abstract The formation of radiation-induced centres on irregular nodes is considered by means of potential curves in configuration coordinates. The results of the study of radiation effects on paramagnetic centres in silicate and quartz glasses at helium temperatures are described on the basis of a relationship between the dispersion of potential states and tunnelling.

Inside Submicron MOSFETS

ABSTRACTNanofabrication techniques allow us to carve out narrow (40 nm) conducting channels and multiple voltage probes inside a submicron portion of a “conventional” silicon MOSFET. The repeated capture and emission of single electrons at a particular interface trap can be observed, because this changes the number of scatterers, producing sudden switching of the conductance. The dependence of capture and emission rates on lattice temperature, electron temperature, and gate voltage are consistent with a simple “configuration coordinate” model of the coupling between the trapped electron and the surrounding lattice. At low temperatures, the electrons being scattered diffuse considerable distances Lϕ before losing quantum phase information. When measured at scale Lϕ, random quantum interference causes a typical (rms) conductance change of e2/h = (25.8 kΩ)−1 (“universal conductance fluctuations”). From this, the resistance fluctions at longer and shorter voltage probe spacings can be predicted. Each scatterer substantially affects the quantum interference throughout a region of size Lϕ.

Metastable-defect behavior in silicon: Charge-state-controlled reorientation of iron-aluminum pairs.

We report the observation of a novel example of defect metastability in silicon. The phenomenon, monitored by deep-level transient spectroscopy, takes place at a well-identified point defect, i.e., the interstitial-iron--substitutional-aluminum pair (${\mathrm{Fe}}_{(\mathrm{i})}$${\mathrm{Al}}_{(\mathrm{s})}$). The charge state of the defect during sample cooldown to low temperature is found to control a reversible transmutation behavior between two defect energy levels, at ${E}_{V}$+0.20 eV (H1) and ${E}_{V}$+0.13 eV (H2). A kinetic study of the transformation has led to a detailed microscopic description of the phenomenon. It is shown to arise from a charge-state-controlled, electrostatically driven, reorientation of ${\mathrm{Fe}}_{(\mathrm{i})}$${\mathrm{Al}}_{(\mathrm{s})}$ pairs between 〈111〉 and 〈100〉 configurations. Levels H1 and H2 are thus ascribed to (${\mathrm{Fe}}_{(\mathrm{i})}$${)}^{2+}$-(${\mathrm{Fe}}_{(\mathrm{i})}$${)}^{+}$ transitions at the nearest and next-nearest tetrahedral sites adjacent to aluminum, respectively. A configuration-coordinate (CC) description of the center, based on the simple ionic model of iron-acceptor pairs, is shown to account for all features of the reaction. No very large lattice relaxation is needed to understand the phenomenon. The CC model of the ${\mathrm{Fe}}_{(\mathrm{i})}$${\mathrm{Al}}_{(\mathrm{s})}$ pair is then extended to non-purely-ionic defect complexes. A complete new class of metastable centers is thus proposed. Metastable phenomena involving other semiconductor defects (A center in silicon, EL2 center in GaAs, M center in InP) are discussed in the light of these new CC models.

Effect of melt temperature on the dc conductivity of GeO2 glasses

The dc electrical properties of a series of high purity GeO2 glasses fused and equilibrated at various temperatures (Tφ) in air were measured. Tφ ranged from 1200 to 1690 °C. The observed changes are not correlated changes in concentration of any of the impurities as determined by neutron activation analyses or IR measurements of OH concentrations. The resistivity was found to obey an Arrhenius function with enthalpy of activation of approximately 1.0 eV for all Tφ’s except for Tφ =1200 °C. The charge carrier was concluded to be the Na+ ion. The mobility of the Na+ ion was calculated and was found to be a function of Tφ. We suggest that the change in Tφ results in a change in the configuration coordinates of the average interstitial sites through which the Na+ ion moves. This change results in differences in the vibrational energy structure of the interstitial sites. These differences are manifested in the entropy of activation. With changes in the entropy of activation, the preexponential term changes, producing the variation in mobilities which is observed.

Luminescence and Tem-Investigation of Laser Induced Defects in (AI,Ga)As Heterostructures

AbstractLaser induced defects in (AI,Ga)As heterostructures have been investigated. Luminescencetopography reveals three different defects, a luminescent B, a nonradiative D as well asdark line defects (DLD). Luminescence and excitation spectra together with TEM measurements indicate a point defect or point defect complex for B and D. Defect B is described by a configuration coordinate (CC-) model with low vibrational energies. Defect DLD consists of extended dislocations.

A complete generalized solution to the inverse kinematics of robots

The kinematic transformation between task space and joint configuration coordinates is nonlinear and configuration dependent. A solution to the inverse kinematics is a vector of joint configuration coordinates that corresponds to a set of task space coordinates. For a class of robots closed form solutions always exist, but constraints on joint displacements cannot be systematically incorporated in the process of obtaining a solution. An iterative solution is presented that is suitable for any class of robots having rotary or prismatic joints, with any arbitrary number of degrees of freedom, including both standard and kinematically redundant robots. The solution can be obtained subject to specified constraints and based on certain performance criteria. The solution is based on a new rapidly convergent constrained nonlinear optimization algorithm which uses a modified Newton-Raphson technique for solving a system nonlinear equations. The algorithm is illustrated using as an example a kinematically redundant robot.

Multiphonon transitions between adiabatic potential curves

Multiphonon transitions are discussed semiclassically using an extension of the configuration-coordinate diagram to a complex plane of the configuration coordinate. The author considers the tunnelling regime, complementary to the region of applicability of the Landau-Zener formula, where the crossing point of the potential curves is classically inaccessible. An expression is obtained for the vibrational matrix element in terms of a contour integral, which can be evaluated analytically for a wide class of adiabatic potential curves. He obtains a general expression for the multiphonon transition rate at T=0, and this is used to calculate the transition rate between non-intersecting Morse curves.

An elastic hinge model for dynamics of stiff chains. I. Viscoelastic properties

The behavior of linear macromolecules with some flexibility is considered. A ‘‘trumbell’’ model is used which consists of three beads connected by two rigid rods and a hinge with a restoring force proportional to the angle of bending at the middle bead. This model, which has four curvilinear nonorthogonal configurational coordinates, seems to be the most complex that can be solved completely at present. The relaxation spectrum λi is found for different values of the relative bending force constant Z=ε/kT by numerical integration of Kirkwood’s diffusion equation using Hassager’s collocation method. The viscoelastic behavior is calculated by extending Hassager’s procedure. The results show two main contributions: (i) a slow relaxation λ1 which is nearly pure rotation and which tends to the rigid-rod limit as Z increases, (ii) a fast mixed bend-rotation relaxation λ2 proportional to Z. A pure-bending mode of relaxation also exists, but it does not contribute to the viscoelastic behavior of the model.

Vibrational relaxation and hot luminescence in impurity centres with a strong vibronic coupling

The results of theoretical and experimental study of vibrational relaxation processes in impurity centres with a strong electron-phonon interaction are presented. General expressions are obtained which describe the temporal evolution of the configurational-coordinate distribution function and the envelope of the emission spectrum consisting of ordinary and hot luminescence. The problem is solved exactly in the harmonic approximation with allowance made for both linear and quadratic vibronic coupling providing a small number of actual configurational coordinates. The hot luminescence bands in KI-Tl positioned at 3.1 and 2.25 eV are detected. The computations show that in Tl+ centres the main portion of the excess vibrational energy relaxes in a time shorter than the mean period of vibrations ( approximately=0.5 ps) and, therefore, the observed spectral features are caused by the first classical turning point in the configurational-coordinate relaxation law. Thus, the centres under study serve as an example of ultrafast relaxing systems. Other weak spectral features are related with the complicated structure of adiabatic potential surfaces in the excited state and are interpreted as a barrier and post-tunnel hot luminescence.

Hot luminescence and ultrafast configurational relaxation of F centers

Hot luminescence of F centers in RbCl, KBr and KCl single crystals at liquid helium temperatures is discovered and interpreted as the emission from the classical turning points of a configurational coordinate. The energy relaxation time is estimated by two different methods providing interesting information about the excited state relaxation dynamics.

Coordinate rotation studies of H−, He−, Be−, Mg− resonances: Basis set and configuration list dependence

AbstractWe have performed coordinate rotated configuration interaction calculations on well‐studied Feshbach resonances of H− and He− and on 2P shape resonances of Be− and Mg−. The focus of our efforts was the dependence of computed resonance energies on both the quality of the atomicorbital basis and the level of treatment of electron correlation. Our results indicate that great care must be taken to guarantee that a basis is adequate; commonly used quantum‐chemistry bases are probably far from satisfactory. Our findings also indicate that a proper treatment of inner‐shell orbitals within coordinate rotation calculations is a formidable task. We are therefore encouraged to look carefully for modified coordinate rotation techniques that focus on the active valence‐level orbitals and may avoid spurious complex energies arising from improper treatment of inner shells.