Showing papers on "Stark effect published in 1996"

Spontaneous emission of localized excitons in InGaN single and multiquantum well structures

Abstract: Emission mechanisms of InGaN single quantum well blue and green light emitting diodes and multiquantum well structures were investigated by means of modulation spectroscopy. Their static electroluminescence (EL) peak was assigned to the recombination of excitons localized at certain potential minima in the quantum well. The blueshift of the EL peak caused by the increase of the driving current was explained by combined effects of the quantum‐confinement Stark effect and band filling of the localized states by excitons.

New plasma diagnosis tables of hydrogen Stark broadening including ion dynamics

Abstract: A set of tables of the Stark width of the first three lines of Lyman and Balmer series of hydrogen, useful for plasma diagnosis purposes, is presented in this work. These tables have been obtained by means of a fully recognized computer simulation technique. Computer experiments have been carried out for density values between and . Simulation temperatures range from 2245 K to 224 856 K. Ion dynamics effects are taken into account in a natural way and the most significative gas mixtures have been covered in the simulations, carried out at the reduced mass of the emitter - perturber pair , 0.6, 0.8, 0.9, 1.0, 1.25, 1.5, 1.75 and 2.0. A detailed discussion of the influence of the relevant plasma parameters on the characteristics of the profiles is also included. The results show an excellent agreement with up-to-date experimental results and confirm these kind of profile calculation techniques as now the most accurate.

Observation of atomic wannier-stark ladders in an accelerating optical potential

Abstract: We report observation of Wannier-Stark ladders with ultracold sodium atoms in an accelerating one-dimensional standing wave of light. Atoms are trapped in a far-detuned standing wave that is accelerated for a controlled duration. A small oscillatory component is added to the acceleration, and the fraction of trapped atoms is measured as a function of the oscillation frequency. Resonances are observed where the number of trapped atoms drops dramatically. The separation between resonant peaks is found to be proportional to the acceleration. We show that this resonant structure can also be understood as a temporal quantum interference effect. [S0031-9007(96)00369-9]

Potential-Dependent Metal−Adsorbate Stretching Frequencies for Carbon Monoxide on Transition-Metal Electrodes: Chemical Bonding versus Electrostatic Field Effects

Abstract: The dependence of the metal−carbon vibrational frequency (νM-C) upon electrode potential (E) for saturated CO adlayers on palladium, platinum, rhodium, and iridium film electrodes is examined in comparison with that for the well-studied intramolecular (C−O) vibration (νCO) by means of surface-enhanced Raman spectroscopy (SERS) in order to evaluate the likely roles of chemical bonding versus the electrostatic field in the electrochemical Stark effect. In each case, the dνM-C/dE values are negative, from ca. −10 to −20 cm-1 V-1, contrasting the positive dνCO/dE values, ca. 30 to 60 cm-1 V-1, observed for adsorbed CO on these Pt group metals. The findings are compared with the predictions of theoretical treatments which account variously for the roles of the interfacial electrostatic field (i.e., the classical vibrational Stark effect) and potential-dependent chemical bonding (i.e., metal−adsorbate orbital overlap). It is necessary to invoke that the latter factor is exerting a major role in the surface−adso...

Laser‐rf double resonance spectroscopy of 174YbF in the X 2Σ+ state: Spin‐rotation, hyperfine interactions, and the electric dipole moment

Abstract: We report the first laser‐radio‐frequency double resonance spectrum of a simple lanthanide compound, the paramagnetic radical 174YbF. Measurements of the rf intervals as a function of the rotational quantum number N allow us to determine precise spin‐rotation and fluorine hyperfine interaction coupling constants for the X 2Σ+(v=0 and v=1) ground states of 174YbF. The results for v=0 are γ0=−13.424 00(16) MHz, γ1=3.982 3(11) kHz, γ2=−25(1) mHz, b0=141.795 6(5) MHz, b1=−0.510(11) kHz, c=85.402 6(14) MHz, C=20.38(13) kHz. For v=1 they are γ0=−33.811 8(7) MHz, γ1=4.323(6) kHz, γ2=−28(9) mHz, b0=139.89(4) MHz, b1=−0.7(4) kHz, c=86.75(5) MHz, C=18.3(1) kHz. A direct microwave measurement of the first rotational interval in X 2Σ+(v=0) gives the rotational constant B0=0.241 292 7(7) cm−1. Finally, the Stark shift of hyperfine transitions in the first two rotational states of X 2Σ+(v=0) are analyzed to determine the electric dipole moment μe=3.91(4) D. We find that although the gross structure of YbF in its ground...

The optical and optical/stark spectrum of iridium monocarbide and mononitride

Abstract: Supersonic molecular beam samples of iridium monocarbide IrC and iridium mononitride IrN were generated using a laser ablation/reaction source and characterized using high resolution (Δν<30 MHz FWHM) laser induced fluorescence spectroscopy. This is the first identification of gaseous IrN. Numerous strong band systems in the 18 800 to 14 360 cm−1 spectral range were assigned as the (v′,0) progression of the A 1Π–X 1Σ+ band system of IrN. The (1,0) and (0,0) bands were analyzed to produce a set of fine and hyperfine parameters. The electric field induced effects on the R(0) line of these bands were analyzed to produce permanent electric dipole moments: A 1Π(v=0) 2.78(2) D, A 1Π(v=1) 2.64(2) D, X 1Σ+(v=0)=1.66(1) D. The (0,0) band of the D2 Φ7/2−X 2Δ5/2 system of IrC was recorded and analyzed to produce a set of fine and hyperfine parameters. The electric field induced effects on the R(2.5) branch feature were analyzed to produce permanent electric dipole moments: D 2Φ7/2(v=0) 2.61(6) D and X 2Δ5/2(v=0) 1.60(7) D. Plausible electronic configurations consistent with the experimental observations are given.

External Electric Field Effects on Fluorescence in an Electron Donor and Acceptor System: Ethylcarbazole and Dimethyl Terephthalate in PMMA Polymer Films

Abstract: Fluorescence of ethylcarbazole doped in PMMA polymer films is quenched by an external electric field in the presence of dimethyl terephthalate, indicating that the rate of the intermolecular electron transfer from a photoexcited molecule of ethyl carbazole to dimethyl terephthalate is enhanced by an applied electric field. The field effect on the electron transfer as well as the transfer rate at zero field increases with a decrease of the electron donor−acceptor distance. The free energy change of the electron transfer reaction from photoexcited ethylcarbazole to dimethyl terephthalate is estimated to be about −0.7 eV in PMMA polymer films, based on the external electric field effect on fluorescence intensity of ethylcarbazole combined with the fluorescence lifetime. The molecular polarizability of fluorescent exciplex formed in a mixture of ethylcarbazole and dimethyl terephthalate is also estimated, based on the Stark shift of the exciplex fluorescence.

Wannier excitons and Franz-Keldysh effect of polydiacetylene chains diluted in their single crystal monomer matrix

Abstract: Electric field induced changes of the absorption spectra are used to study the \ensuremath{\pi} electron states of isolated 4BCMU and 3BCMU polydiacetylene (PDA) chains. The strong excitonic transition, common to all PDA chains, responds to moderate electric fields according to a quadratic Stark effect. Weak transitions below this exciton respond identically. The polarizability of all these excitons is the same and equal to that in fully polymerized PDA's. About 580 meV higher in energy an additional response broadening with increasing field is observed whose amplitude increases sublinearly with field. This behavior is in perfect agreement with what is expected for the Franz-Keldysh effect of continuum states, yielding a reduced mass of only 0.05${\mathit{m}}_{0}$. This small mass is consistent with an exciton size of 5 repeat units, estimated from the Stark shift of the excitons. The spectral range of the well resolved Franz-Keldysh oscillations points to a coherence length of \ensuremath{\pi} states of 200-400 \AA{}. \textcopyright{} 1996 The American Physical Society.

A voltage‐tunable multicolor triple‐coupled InGaAs/GaAs/AlGaAs quantum‐well infrared photodetector for 8–12 μm detection

Abstract: A voltage‐tunable multicolor triple‐coupled quantum‐well infrared photodetector (TC‐QWIP) has been developed for 8–12 μm detection. The TC‐QWIP consists of three coupled quantum wells formed by an enlarged Si‐doped InxGa1−xAs quantum well and two undoped GaAs quantum wells separated by two thin AlyGa1−yAs barriers. Two TC‐QWIP structures with varying indium and aluminum compositions were designed and demonstrated. Due to the strong coupling effect of the asymmetrical quantum wells, three bound states (E1, E2, E3) are formed inside the quantum wells of the TC‐QWIP. The main detection peak wavelength is due to E1→E3 bound states transition for both devices, while two secondary detection peaks due to E1→E2 and E1→Ec continuum states transitions under different biases were also observed. In addition, a strong quantum‐confined Stark shift effect for the E1→E3 transition was observed in the wavelength range of 8.2–9.1 and 10.8–11.5 μm for QWIP‐A and QWIP‐B, respectively; both devices exhibit a linear dependence of detection peak wavelength on the applied bias voltage. A spectral responsivity of Ri=0.05 A/W and background limited performance (BLIP) detectivity DBLIP*=6.1×109 cm√Hz/W were obtained at Vb=5 V, λp=8.6 μm, and TBLIP=66 K for QWIP‐A, while Ri=0.33 A/W and DBLIP* =1.63×1010 cm√Hz/W at Vb=4 V, λp=11.2 μm, and TBLIP=50 K were obtained for QWIP‐B.

Imaging of oriented molecules.

Abstract: Hexapole state selection and orientation of parent molecules is combined with two-dimensional ion imaging of photofragments to study the direct photolysis of deuterated methyl iodide molecules (CD3I) at 266 nm. The combination of these two techniques allows us to create an essentially single quantum state-selected beam of oriented molecules, which are subsequently photodissociated, and to measure the final state-, velocity-, and angle-resolved recoil distribution of the photofragments. Extensive measurements have been done on the variation of the recoil distribution with the orientation field strength applied. These measurements allow a quantitative comparison of the orientation distribution of state-selected methyl iodide molecules with theoretical Stark effect calculations. For the prompt dissociation of CD3I at 266 nm the angular recoil distribution of the I(2P1/2) photofragment reflects predominantly the initial spatial orientation of the state-selected parent. A comparison is presented between the fr...

Lifetimes of Rydberg states in ZEKE experiments. III. Calculations of the dc electric field dependence of predissociation lifetimes of NO

Abstract: Results are presented of calculations on the experimentally observed lifetime enhancement of high principal quantum number nf(N+=2) and np(N+=0) Rydberg states of the NO molecule in small dc electric fields. A Hund’s case (d) molecular Hamiltonian is introduced, which describes intramolecular interactions between low angular momentum Rydberg states as a result of core penetration and l mixing caused by the electric field. Eigenstates of the molecule in the electric field are obtained by diagonalization of the Hamiltonian and an effective decay time is determined which describes the multiexponential decay of the superposition of (Stark) states which is excited experimentally. The calculations reproduce the striking onset of the lifetime enhancement of the np(N+=0) series, which was observed experimentally. Both for the nf(N+=2) and np(N+=0) Rydberg series, core penetration by low‐l states leads to extensive interseries coupling for several values of the principal quantum number.

Spectroscopic measurements of streamer filaments in electric breakdown in a dielectric liquid

Abstract: Emission spectroscopy has been utilized to provide information about the electron density and temperature in streamers and breakdown arcs in transformer oil. Recorded spectra include strongly broadened hydrogen Balmer- lines and vibration/rotation band profiles of the molecule. The origin of the observed broadening of hydrogen lines is discussed and it is concluded that it arises mainly from collisions with charged particles, so-called dynamic Stark broadening. By assuming that the broadening is due solely to dynamic Stark broadening, electron densities between and were obtained for the rear of positive streamer filaments during the later stages of propagation. For negative streamers we obtained an upper limit of and for breakdown arcs electron densities up to . The temperature information in the emission profiles and the intensity ratio of the hydrogen Balmer lines are discussed. Rough estimations of the temperature are presented both for positive and for negative streamers.

Line-by-line method of calculating emission coefficients for thermal plasmas consisting of monatomic species

Abstract: A theoretically rigorous method for obtaining radiation property data of thermal plasmas composed of monatomic, nonhydrogenic species is presented. An exact line-by-line technique is utilized in order to avoid assumptions that are commonly made in handling line radiation. Doppler, natural, Van der Waals, resonance, and quadratic Stark broadening mechanisms are included in determining the line half-widths. In addition to line radiation, free-bound continuum and free-free continuum for both ions and neutral particles are considered. The fundamental radiation property determined by the program is the spectral emission coefficient. From this property the spectral absorption coefficient can easily be determined. Properties that are more suitable for engineering analysis, such as the wavelength integrated emission coefficient and the net emission coefficient, are also calculated. Comparisons to experimentally determined wavelength integrated emission coefficients for argon plasmas and analytically determined net emission coefficients for argon/copper plasmas are presented.

Wavelength dependent refractive effects and Stark broadening in laser‐induced YBa2Cu3Ox plasma plumes

Abstract: Spectral lines observed at short delay times and close to the target during laser ablation of YBa2Cu3Ox show strongly distorted line shapes arising partly from optical refraction by the large density gradients within the expanding plume. Spatially and temporally resolved linewidths, corrected for refraction effects, are reported for the Ba(I) 553.5 nm absorption transition and Stark widths, deduced from these corrected linewidths, indicate that the maximum electron number density at 1.0 mm from target is ∼1×1019 cm−3.

A diode laser absorption study on a 100 MHz argon ICP

Abstract: Diode laser absorption on the - argon transition is used to measure heavy particle temperatures in a 100 MHz argon inductively coupled plasma. Radial profiles of this temperature are obtained from the Gaussian part of the absorption profile with an accuracy of about 500 K, for four different input powers and at two different heights. The integrated profile is used to calculate the 4s level density and to trace the ionizing and recombining plasma parts. The measurements also show that the method of attributing the Lorentzian width only to Stark broadening for calculating electron densities is not correct for this argon transition in atmospheric plasmas. A second broadening process with Lorentzian shape, Van der Waals broadening, has to be taken into account. Under the measured conditions at the hottest positions in the plasma about 50% of the Lorentz component is due to Van der Waals broadening and this increases to almost 100% at the edges of the plasma.

Optical absorption intensities and fluorescence dynamics of ? ions in ?

Abstract: Optical absorption and fluorescence spectra have been systematically investigated in -doped crystals. The energy position and symmetry character of the Stark levels in the near-infrared region are deduced. The room-temperature absorption line strengths have been experimentally determined and the theory of Judd and Ofelt is used to obtain the radiative lifetimes and branching ratios of the emitting and states. Non-radiative relaxation processes are found to contribute to the transition probabilities of both states. In the case of the multiplets, a multiphonon relaxation process with an effective phonon energy of accounts for the temperature dependence of the lifetime. The -state luminescence shows a non-exponential decay time which indicates the existence of an energy transfer process. This energy transfer process exhibits a Boltzmann-type temperature dependence and is attributed to two-phonon-assisted energy transfer.

Absence of Stark shift in strained Si1−xGex/Si type‐I quantum wells

Abstract: Excitonic photoluminescence in a transverse electric field has been studied in strained Si1−xGex/Si type‐I quantum wells (QWs) in the weak field regime. With increasing electric field, a steady blue‐shift of the free exciton emission energies has been observed, thereby suppressing the anticipated downward shift due to a quantum confined Stark effect. It is revealed that a field‐driven decrease of the exciton binding energies gives a good account of the experimental blue‐shift for a wide range of well widths, which is also in good agreement with variational calculations. We also find that the extremely small conduction band offset characteristic of stained Si1−xGex/Si type‐I QWs is responsible for the absence of the Stark red‐shift and the mixed dimensionality.

Direct calculation of Stark resonances in hydrogen.

Abstract: We propose an alternative way of calculating the resonances of the Stark effect in hydrogen. The method is based on a rational approximation of the logarithmic derivative of the eigenfunction, and leads to a quantization condition for the complex energies of the metastable states. We present accurate results for the ground and some excited states for several field intensities. {copyright} {ital 1996 The American Physical Society.}

High‐efficiency Stark‐geometry photorefractive quantum wells with intrinsic cladding layers

Abstract: Output diffraction efficiencies approaching 40% and input diffraction efficiencies approaching 3% have been achieved in photorefractive p‐i‐n quantum well diodes operating in the longitudinal Stark geometry. The device structure consists of a low‐temperature‐grown multiple quantum well isolated from the doped contacts by an intrinsic standoff layer of Al0.5Ga0.5As. All charge trapping and screening occurs within the quantum wells without the need for trapping in specialized buffer layers used in previous designs. This new design operates at lower voltages (18 V) and lower fields (6 V/μm) than previously demonstrated.

Potential chirp-free characteristics (negative chirp parameter) in electroabsorption modulation using a wide tensile-strained quantum well structure

Abstract: The possibility of nearly chirp‐free electroabsorption (EA) characteristics in intensity modulation at an operating wavelength of 1.55 μm is studied theoretically. The TE‐polarized EA spectra, their Kramers–Kronig (K–K) transformed refractive index changes, and the chirp parameters derived under both small‐signal and large‐signal conditions are calculated including excitonic effect within the framework of the effective mass theory. We investigate on the basis of the simulated EA results how to keep chirp parameters always negative. This can be accomplished by following two successive steps: (1) Widen the well width to increase the Stark shift. This makes it easier for the chirp parameter to turn negative at smaller applied electric field. (2) Apply tensile strain to the wide QW to induce a negative refractive index changes even in the vicinity of zero electric fields, indicating that there exists a desirable condition where the chirp parameter is negative independently of applied fields.

Effect of a Protein Electric Field on the CO Stretch Frequency. Finite Difference Poisson−Boltzmann Calculations on Carbonmonoxycytochromes c

Abstract: We report electrostatic potential and electric field calculations at the CO ligand of the heme in both horse heart and yeast cytochrome c, obtained from a finite difference solution to the Poisson−Boltzmann equation. This method takes into account the protein shape and charge distribution, as well as the solvent and generalized ionic strength effects. The calculations support recent experimental and theoretical evidence suggesting that polar interactions can significantly affect the vibrational frequency of the ligand. Our work shows that the ionizable amino acid residues and polar contributions of the protein matrix can induce a Stark effect on the CO stretch frequency of the carbonmonoxycytochromes. The observed CO stretches, at 1965.9 and 1960.1 cm-1 for horse and yeast cytochromes c, respectively, show that the experimental shift is of the order of 6 cm-1. This is in good agreement with a calculated value of 8 cm-1 (±1 cm-1) for a vibrational Stark shift due to the different charge distributions in bo...

Electric field effects on excitons in gallium nitride.

Abstract: Electric-field effects on Wannier exciton are observed in GaN thin films Using both absorption and photocurrent measurements, we have studied the excitonic Franz-Keldysh effect in thin epitaxial GaN films at temperatures between 80 and 300 K We have measured the Stark shift, quenching, and broadening of the exciton peak with applied field These results are compared with theoretical calculations from the literature The physics of exciton ionization at varying temperatures is discussed, which explains the interplay between absorption and photocurrent \textcopyright{} 1996 The American Physical Society

The formation and evolution of negative central magnetic shear current profiles on DIII-D

Abstract: Using the combination of early neutral beam injection, ramping of the plasma current, low electron density and controlled L - H transitions, robust discharges with negative central magnetic shear are reproducibly obtained on the DIII-D tokamak. The effects of these factors on the formation and evolution of the q profile during the initial phase of these discharges are documented. Details of the evolution of the inverted q profile are obtained from measurements of the internal field pitch using a 16-channel motional Stark effect (MSE) system. Time-dependent MSE data are used to directly construct the profile of the toroidal electric field and allow a straightforward calculation of the non-inductive current density profile.

Asymptotic completeness for long-range many-particle systems with Stark effect. II

Abstract: We prove the existence and the asymptotic completeness of the Dollard-type modified wave operators for many-particle Stark Hamiltonians with long-range potentials