Respective Ground States Research Articles

Carrier statistics in quantum-dot lasers

The description of quantum dot ensembles using solely average carrier populations is insucient. Bookkeeping of the probability of all micro-states and soluiton of the master equations for the transitions between them allows proper modeling of the phonon bottleneck and laser properties. We predict that single exciton shows gain and calculate the dependence of threshold current density of the type of capture process. The gain-current relation in quantum dot lasers is linear. 1. Quantum dots represent a unique electronic system which was recently successfully implemented for novel semiconductor lasers [1n4]. The charge carriers populate discrete electronic levels and (at least at suciently low temperature) all dots in an ensemble are laterally decoupled from each other. Therefore an event in a quantum dot, e. g. a recombination process, does not depend on the average carrier density (involving all other dots) but only on the particular population of the present dot with electrons and holes [5]. The ensemble has to be averaged over the probability distribution of the dierent micro-states. Dierent probability distributions of micro-states can have the same average carrier population but show dierent properties like recombination current. In order to illustrate this point with a simple example, we compare two QD ensembles with the same average carrier population: in ensemble I all electrons are in dierent dots than the holes: no radiative recombination takes place. In ensemble II electrons and holes populate the dots by pairs, leading to radiative recombination. We further discuss modeling of the phonon bottleneck eect. 2. Since the epitaxially created self-ordered QDs [6,7] are in the strong confi nement regime, it is adequate to model electronic levels in the single particle picture, i. e. electrons and holes populate single particle levels. Ground state luminescence is said to originate from the radiative recombination of an electron and a hole in their respective single particle ground states (exciton). The lifetime of the exciton shall be denoted be X. The Coulomb correlation shall not fundamentally alter that picture. One eect is the shift of recombination energy of the biexciton (two electrons and two holes in their spin-degenerate ground states) with respect to that of the exction. For InAs/GaAs quantum dots this shift is expected to be small (< 2m eV )[ 8 ] ;f or IInVI compounds this shift will be larger. The radiative lifetime XX of the biexciton in the strong confi nement limit is XX = X= 2 (as if two independent exciton decay);

Time-Resolved Fluorescence of Glucagon Studied by Global Compartmental Analysis

The photophysics of the polypeptide hormone glucagon in aqueous phosphate buffer (λex = 295 nm, pH 6.7, 17.2 °C) in the presence of the added quencher sodium iodide is investigated using standard simultaneous biexponential and global compartmental analyses of its fluorescence decay surface. The use of the scanning procedure in global compartmental analysis allows distinction between reversible and irreversible transformation processes in the excited state (J. Phys. Chem. 1995, 99, 8959−8971). Furthermore, by comparison of the species-associated with the decay-associated emission spectra, a unidirectional process in the excited state can be distinguished from the case where no interconversion occurs between the two excited-state species. Scanning different rate constants and comparing the species-associated with the decay-associated emission spectra indicate that the dual exponential fluorescence decays of the single tryptophan-containing polypeptide glucagon are due to two non-interconverting excited-state species resulting from excitation of their respective ground states. On the time scale of fluorescence there is no interconversion between the two excited-state species. The composite deactivation rate constant values for monomeric glucagon in phosphate buffer (k01 = 1.01 × 109 s-1 and k02 = 2.7 × 108 s-1) are separated into contributions via fluorescence (kF1 = 0.05 × 109 s-1 and kF2 = 1.6 × 108 s-1) and nonradiative processes (kNR1 = 0.96 × 109 s-1 and kNR2 = 1.1 × 108 s-1). The fluorescence quantum yields in the absence of added quencher are = 0.05 and = 0.58. The relatively high values (>1 × 109 M-1 s-1) of the rate constants of quenching are indicative of a tryptophyl residue exposed to the surrounding aqueous environment. The obtained values of the rate constants may indicate that the decay component of 1 ns is associated with a tryptophyl residue in a flexible extended random coil conformation of the polypeptide chain while its 3.7 ns counterpart corresponds to a tryptophan in a compact, quite rigid backbone conformation. The method used to elucidate the origin of the biexponential fluorescence decays of glucagon is generally applicable to any intramolecular two-state excited-state process and will be useful in the study of peptides and proteins with dual exponential fluorescence decay kinetics.

Wannier-Mott exciton formed by electron and hole separated in parallel quantum wires

We analyze a Wannier-Mott exciton in which the electron and hole are constrained to move in two separated and parallel quantum wires. We expand the electron-hole interaction potential in terms of multipoles by assuming that both the electron and hole experience transverse harmonic confinements in the $x$ and $y$ directions, both being in their respective transverse ground states. For the resulting $z$-dependent Schr\"odinger equation, we calculate in detail eigenenergies and eigenfunctions for the exciton ground and first excited states as functions of the transverse dimension of the wires and their separation distance. Also, we calculate the higher eigenenergies and eigenfunctions approximately by using a WKB formalism.

Nonclassical features in the dynamics of a new quadratic quantum model of the radiation - matter interaction in a confined space

We introduce a new solvable effective Hamiltonian model describing a two-level atom coupled to a bimodal degenerate cavity field by two-photon processes. We study the evolution of the system assuming the atom and one mode in their respective ground states and the other mode in an excited Fock state. We find that the dynamics of the system displays nonclassical behaviour exhibiting, in particular, a peculiar sensitivity to the parity of the initial number of photons. We show that this quantum effect may be exploited to generate the amplification of an initially given small difference in the mode populations also enabling the preparation of quantum superpositions of macroscopically distinct field states.

Structure of 294,302120 Nuclei Using the Relativistic Mean-Field Method

We have studied the structure of superheavy nuclei 294,302120 in the framework of a relativistic mean-field formalism, using three different parameter sets (NL1, NL–SH and TM1) in an axially deformed harmonic oscillator basis. The calculated shapes are found to be parameter-dependent, e.g. NL1 parameter set predicts 302120 as a spherical and 294120 a very weakly oblate deformed nucleus whereas NL–SH and TM1 parameter sets predict both the nuclei with strongly prolate/oblate deformed configurations, in their respective ground states. This result, coupled with the calculated single-particle energy spectrum for NL1 parameter set, supports for Z=120 nuclei the spherical magic shell more at N=184 than at N=172. Even for the spherical 302120 nucleus, many new closed shells are predicted and some of the known magic numbers are found absent. Also, the binding energies of the various isotopes of Z=104–111 nuclei are calculated whose comparisons with experimental data favor the NL1 parameter set.

Magnesium as a representative analyte metal in argon inductively coupled plasmas. II. Population mechanisms in analytical zones of different spectrochemical systems

The extensive collisional-radiative model is applied to the actual argon inductively coupled plasmas (ICP) in order to clarify consistently the mechanisms by which the excited levels of magnesium atoms and ions are populated in analytical zones of these ICP systems. Computations are carried out for various sets of input parameters, such as the electron temperature the T e , atom temperature T a , the electron number density n e and the optical escape factors A mn and A m for Mg atoms, and A mn + and A m + for Mg + ions. The predicted values of the populations in the excited levels and the quantities derived from them are fairly close to the corresponding experimental results. The effect of the changes in the discharge parameters T e , T a , n e , A 1 n , and A 1 n + on the population mechanisms, together with the role played by the upward excitation flows of electrons from the respective ground states and by the charge transfer between argon and magnesium, are also shown. © 1997 Elsevier Science B.V.

Electron impact excitation of magnesium and zinc atoms in the relativistic distorted-wave approximation

A calculation based on a relativistic distorted-wave approximation has been carried out for the electron impact excitation of the , , and -states of magnesium and the and -states of zinc from their respective ground states. Differential cross sections, coherence and correlation parameters as well as the spin-polarization parameters were obtained at incident energies of 10, 20 and 40 eV, respectively. The results for magnesium are analysed and compared with other available theoretical and experimental data while those for the zinc atom are reported for the first time.

Transient effects can change identifiability criteria for complex excited-state processes

The identifiability of two-state excited-state processes is studied when transient effects are present. When the emission bands of the excited species are well separated, the deactivation rate constants of the two excited species to their respective ground states, k 01 and k 02, can be determined from two decay traces of the individual excited species at one concentration. Thus, two decay traces are sufficient for the recovery of k 01 and k 02 for inter- and intra-molecular two-state excited-state processes. When transient effects are absent more kinetic information is needed for the recovery of k 01 and k 02.

Electron momentum spectroscopy experiments and calculations for the production of excited states of He+ and

The (e,2e) cross-section for transitions to the n = 2 final state of He+ and the 2sσg, final state of [Formula: see text] have been measured, relative to the cross-section for the transitions to the respective ground state ions, using a highly sensitive momentum dispersive multichannel electron momentum spectrometer. The experimental results for He are compared with plane wave impulse approximation (PWIA) cross-section calculations carried out using two previously published GI wavefunctions and also with two cross-section calculations based on explicitly correlated wavefunctions with energy errors of less than 10 nHartree. The H2 results are compared with calculations by J.W. Liu and V.H. Smith Jr. (Phys. Rev. A, 31, 3003 (1985); erratum: Phys. Rev. A, 39, 3703 (1989)). For both He and H2, significant differences are observed between the measured relative cross-sections and those calculated using the PWIA. While the measurements for He differ from previous work, the results for H2 are consistent with some earlier measurements.

Estimating molecular collision diameters using computational methods

The collision diameters of a series of 34 atoms and both nonpolar and polar molecules were obtained from ab initio calculations. Molecular volumes, V M , were determined from a Monte Carlo integration of the electron density distribution of the optimized HF/6-31G ∗ structures. The isotropic collision diameter, d vol , defined as ( 6V M π ) 1 3 was compared in all cases with the respective Lennard-Jones potential minima positions, σ m . An excellent linear correlation between d vol and σ m for the 34 species was found, with slope and intercept of 1.025 and −0.07 Å, respectively. It is thus suggested that d vol may be used directly as an estimate of the isotropic collision diameter of a species. d vol values for several atoms and radicals, as well as for the electronically excited states of NH 3 and acetone, are included. The results show that d vol values for the Rydberg excited states are larger than those of the respective ground states.

Photodissociation dynamics of à state ammonia molecules. II. The isotopic dependence for partially and fully deuterated isotopomers

The technique of H(D) Rydberg atom photofragment translational spectroscopy has been used to investigate the photodissociation dynamics of the mixed isotopomers NH2D and NHD2 following the excitation to the v2′=0 and 1 levels of their lowest lying à 1B1 (C2v) excited electronic states. Peaks in the resulting total kinetic energy release (TKER) spectra are assigned to levels of the NH2, NHD, or ND2 fragments with a wide range of quantum numbers Ka for rotation about their a inertial axes, and with N=Ka, N=Ka+1, or N=Ka+2 as appropriate. These data provide the first measurements of high rotational levels for the ground electronic state of the NHD radical. The least squares fitting of all these spectra, and those resulting from NH3 and ND3, to the best calculated NH2, NHD, and/or ND2 rotational term values provides accurate estimations of the respective N–H and N–D bond dissociation energies D00 across the whole series. These values are D00(H–NH2)=37 115±20 cm−1 (4.602±0.002 eV); D00(H–NHD)=37 240±50 cm−1; D00(H–ND2)=37 300±30 cm−1; D00(D–NHD)=37 880±60 cm−1; and D00(D–ND2)=38 010±20 cm−1. The differences between these values are fully consistent with differences in zero-point energies and lead to a mean value of De=40 510±25 cm−1. Dissociation of NH2D or NHD2 through their (Ã−X̃) 210 bands to give an NHD product leads to TKER spectra with a much higher statistical character than those leading to an NH2 or ND2 product, and to those obtained following excitation through the 000 bands. This is rationalized in a semiquantitative manner in terms of a varying contribution to the dissociation rate of the parent molecules from internal conversion (IC) to high levels of their respective ground states. Nuclear permutation symmetry appears to play an important role both for the IC rates and for the subsequent branching between product channels.

How Large Are Low-Lying Molecular Rydberg States? Comparisons Between Experiment and Theory

The "sizes" of low-lying molecular Rydberg states, estimated experimentally from the results of pressure perturbation spectroscopy, are compared with the results of ab initio molecular orbital calculations. Lennard-Jones 6-12 parameters associated with absorber-perturber pairs were assigned to the ground and excited states on the basis of pressure perturbation spectroscopy. These data are reported for the lowest Rydberg transitions of NH3, acetone, and CH3I, as well as for the first two transitions of 1-azabicyclo[2.2.2]octane (ABCO). Increases in the sizes of these excited states relative to the respective ground states were compared with the results of ab initio calculations of the root-mean-square values of the electronic displacement, RMSR. Excited state calculations using configuration interaction (singles) with the 6-31+G* basis set (augmented in the case of CH3I) were performed. Calculations are also reported for CH2O, SO2, CS2, and 1,4-diazabicyclo[2.2.2]octane. Results show that increases in molecular size for intravalence transitions are nearly zero, while for the lower Rydberg transitions, Δ(RMSR) values range between 0.23 Å (for ABCO) and 0.95 Å (for NH3). The calculations on ABCO and acetone indicate the involvement of C atom 3s orbitals in the lowest excited (Rydberg) states. These findings are consistent with the experimental results obtained from pressure perturbation spectroscopy.

Dynamic effects in small-angle electron differential cross sections for excitation of H 2 2P, He 2 1P and Li 2 2P levels

Experimental and theoretical small-angle electron differential cross sections (DCSS) for excitation of H 2 2P, He 2 1P and Li 2 2P states from their respective ground states are investigated through the generalized oscillator strength (GOS) and compared with the GOS from the recent universal function in an attempt to delineate the behaviour of dynamic effects as a function of impact energy, E. The results cover the impact energy range from near threshold to the strong Born approximation regime. We find the surprising result that for the Li 2 2S-2P transition the dynamic effects first decrease as E increases from 10 to about 20 eV where they reach a minimum, manifested through reasonable agreement between measurement and the universal curve over a wide range of the momentum transfer squared, K2 values, 0.01<or=K2<or=0.1 au. Thereafter, for a given K2, away from the asymptotic region, their importance increases until the Born approximation regime is reached. In contrast, dynamic effects in H 2 2P and He 2 1P persist over a wide range of E values all the way to the Born approximation limit where it agrees with the universal function GOS, indicative of their diminished significance to zero.

Crystal Structure and Polarized Electronic Absorption Spectra of NaCo 2(SeO 3) 2(OH)

The crystal structure of hydrothermally synthesized NaCo 2 (SeO 3) 2(OH) has been determined by direct and Fourier methods using single crystal X-ray diffraction data up to sin θ/λ = 0.8 Å -1 [orthorhombic, space group Pbnm, a = 8.338(1) Å, b = 13.268(3) Å c = 6.042(1) Å Z = 4; R w = 0.023 for 1407 unique reflections with F o > 2σ(F o)|. The structure of NaCo 2(SeO 3) 2(OH) consists of [4 + 2]-coordinated Na atoms, distorted CoO 6 octahedra, pyramidal SeO 3 groups, and hydroxyl groups. The CoO 6 polyhedra form 1 ∞ [Co 4O 14] 20- zigzag chains, analogous to the respective Me 2+ chains in the olivine structure type. These chains are connected via the selenite groups and sodium atoms. The O atom of the hydroxyl group belongs to three CoO 6 polyhedra and forms a bifurcated hydrogen bond (3.312 Å, 2×). Furthermore, polarized electronic absorption spectra of NaCo 2(SeO 3) 2(OH) have been measured in the range from 4000 to 40,000 cm -1 using microscope spectrometric techniques. The spectra are characterized by electric dipole transitions at the acentric Co 2+ site [Co(2)]. An interpretation of the band splitting and polarization behavior is attempted in terms of a pseudotetragonal field component and the imposed actual monoclinic site pertubation, yielding 4 A 2 (F) and 4 A″( F) as respective ground states. Tetragonal crystal field calculations give Dq = 630, Dt = -135, Ds = -111, B = 843, and C = 3520 cm -1, which corresponds to a "cubic" Dq of 709 cm -1. The sign and magnitude of the tetragonal parameter Dt is in good agreement with the pseudotetragonal compression of the Co(2)O 6 polyhedron.

Dual phosphorescence from 2-(2′-hydroxyphenyl) benzoxazole in amorphous solid solution. Temperature dependence of dispersive kinetics fo nonexponential triplet decay

2-(2′-hydroxyphenyl) benzoxazole (HBO) can exist in two tautomeric forms, in an enol form (E) and a keto form (K). In the metastable triplet state of HBO in liquid alkanes, both tautomers have roughly the same energy, and a dual phosphorescence, 3K *→ 1K and 3E *→ 1E, is observed; both components of the dual phosphorescence exhibit the same monoexponential decay. In solid solution, the keto phosphorescence decays faster than the enol phosphorescence, and both decays are nonexponential. The dispersive kinetics of the dual phosphorescence from a solid solution can be described by a simple model with a minimum of parameters: The rate constants k K and k E for the decay of 3K * and 3E * to the respective electronic ground state and a distribution function for the energy difference between 3K * and 3E * in solid solution. The true distribution function seems to be better approximated by a Lorentzian than by a Gaussian. k K and k E are virtually independent of temperature.

The polygon quantum-billiard problem

The present work addresses the quantum polygon billiard problem with attention given to analytic and degeneracy properties of energy eigenstates. The ‘‘polygon ground-state theorem’’ is proven which states that the only polygons that contain respective ground states that are analytic in the closed domain of the entire polygon are the ‘‘elemental polygons’’ (defined in the text). The ‘‘polygon first excited-state theorem’’ is established which states that for every N-sided regular polygon, N equivalent first excited states exist, each of which contains a nodal curve that is a line of mirror symmetry of the related polygon. A vector description of nodal diagonal eigenstates is introduced to establish the second component of this theorem which indicates that the space of first excited states for the N-sided regular polygon is spanned by any two of these N nodal-diagonal eigenstates (i.e., the first excited state is twofold degenerate). At various levels of the discussion attention is drawn to the correspondence between the quantum and classical solutions for these configurations. Thus, for example, correspondence is demonstrated in the common source of integrability in the classical and quantum domains for three inclusive, nonoverlapping classes of polygons. Stemming from the preceding conclusions, a discussion is included on the possibility of employing transverse magnetic (TM) modes in a metal waveguide to determine if an equivalent quantum billiard configuration is chaotic.

Quadrupolar Transition Probabilities for Open-Shell Atomic Systems

view Abstract Citations (6) References (32) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Quadrupolar Transition Probabilities for Open-Shell Atomic Systems Ghosh, T. K. ; Das, A. K. ; Mukherjee, P. K. Abstract Excitation energies, oscillator strengths, and transition probabilities for quadrupolar excitations up to principal quantum number n = 6 have been calculated for the astrophysically important open-shell ions C, N+, O2+, N, O+, and O from their respective ground states using the time-dependent linear response theory. Dynamic quadrupolar polarizabilities have been evaluated over a range of frequencies and analytic representations of the quadruply excited Rydberg states are obtained, and their quantum defects are estimated. Most of the data are new, and the results compare favorably with the existing data, wherever available. Publication: The Astrophysical Journal Pub Date: December 1993 DOI: 10.1086/173536 Bibcode: 1993ApJ...419..855G Keywords: ATOMIC DATA; ATOMIC PROCESSES full text sources ADS |

Electronic structure of U4+, Np4+, and Pu4+ doped into ThSiO4 single crystal

The optical spectra of tetravalent ions of early actinides, U4+, Np4+, and Pu4+, were interpreted via a semiempirical Hamiltonian in D2d site symmetry, in terms of electronic transitions within their respective 5fn ground state configurations. Systematic trends in the semiempirical parameters describing the intraconfiguration transitions have suggested that those for U4+ ion are not representative of the light actinide series.

Ab initio calculation of the vertical excitation energies of small helium cluster ions

Quantum chemical ab initio calculations have been performed for the vertical excitation energies and oscillator strengths of all low-lying electronically excited states of small helium cluster ions, Hen+,n=2, ..., 7. The geometrical structures of the ions were fixed at the equilibrium geometries of the respective ground states, for He4+ and He5+ also one alternative structure was considered. The low-lying excited states can be classified into two categories: the electronic transition can occur either within the central He2+ or He3+ unit or from the peripheral weakly bound He atoms to this unit. The latter transitions are very weak (f≈0.001), closely spaced, with vertical excitation energies of about 5.7 eV. The He2+ and He3+ units have strong transitions at 9.93 and 5.55 eV, respectively; these transitions are only slightly blue-shifted if He2+ or He3+ are placed as “chromophores” into the centre of a larger Hen+ cluster. The large difference in the vertical excitation energy of the strong transition should enable an experimental decision of the question whether the cluster ions have He2+ or He3+ cores.

Ab initio study of geometrically metastable multiprotonated species: MHk+n

The geometries and stabilities relative to fragmentation of H4O2+, H4S2+, H3F2+, H3Cl2+, H2Ne2+, H2Ar2+; H4F3+, H4Cl3+, H3Ne3+, H3Ar3+; H4Ne4+, and H4Ar4+ have been studied using the quadratic configuration interaction (with single and double excitations plus approximate triple excitations included) QCISD(T)/6–311G(2df,2p) method at second-order Mo/ller–Plesset optimized geometries MP2(full)/TZP+ZPE/6–31G**. All of the triply charged H4F3+, H4Cl3+, H3Ne3+, and H3Ar3+ and more highly charged H4Ne4+ and H4Ar4+ ions, as well as doubly charged H2Ne2+ were not found to possess local minima on their zero-point corrected ground state surfaces, although H4Cl3+ is only slightly unstable. Tetrahedral (Td) structures for H4O2+ and H4S2+, planar triangular (D3h) H3F2+, triangular pyramidal (C3v) H3Cl2+ and bent (C2v) H2Ar2+ were found to be local minima on the respective ground state surfaces. The latter five species lie above their respective ion-plus-ion dissociation products by 61, 91, 111, 67, and 116 kcal/mol, and have barriers to dissociation of 38, 20, 12, 34, and 5 kcal/mol (all energies being zero-point corrected). Such multiply charged cations store a great deal of energy, which may be released by the addition of a single extra electron to form the corresponding cation of one less charge.