Motional Narrowing Effect Research Articles

Unconventional motional narrowing in the optical spectrum of a semiconductor quantum dot

Motional narrowing refers to the striking phenomenon where the resonance line of a system coupled to a reservoir becomes narrower when increasing the reservoir fluctuation. A textbook example is found in nuclear magnetic resonance, where the fluctuating local magnetic fields created by randomly oriented nuclear spins are averaged when the motion of the nuclei is thermally activated. The existence of a motional narrowing effect in the optical response of semiconductor quantum dots remains so far unexplored. This effect may be important in this instance since the decoherence dynamics is a central issue for the implementation of quantum information processing based on quantum dots. Here we report on the experimental evidence of motional narrowing in the optical spectrum of a semiconductor quantum dot broadened by the spectral diffusion phenomenon. Surprisingly, motional narrowing is achieved when decreasing incident power or temperature, in contrast with the standard phenomenology observed for nuclear magnetic resonance.

A transferable electrostatic map for solvation effects on amide I vibrations and its application to linear and two-dimensional spectroscopy

A method for modeling infrared solvent shifts using the electrostatic field generated by the solvent is presented. The method is applied to the amide I vibration of N-methyl acetamide. Using ab initio calculations the fundamental frequency, anharmonicity, and the transition dipoles between the three lowest vibrational states are parametrized in terms of the electrostatic field. The generated map, which takes into account the electric field and its gradients at four molecular positions, is tested in a number of common solvents. Agreement of solvent shift and linewidths with experimental Fourier transform infrared (FTIR) data is found to within seven and four wave numbers, respectively, for polar solvents. This shows that in these solvents electrostatic contributions dominate solvation effects and the map is transferable between these types of solvents. The effect of motional narrowing arising from the fast solvent fluctuations is found to be substantial for the FTIR spectra. Also the two-dimensional infrared (2DIR) spectra, simulated using the constructed map, reproduce experimental results very well. The effect of anharmonicity fluctuations on the 2DIR spectra was found to be negligible.

Time-resolved CO_2 thermometry for pressures as great as 5 MPa by use of pure rotational coherent anti-Stokes Raman scattering

Pure rotational coherent anti-Stokes Raman scattering measurements of pure CO2 have been performed in a temperature range from 300 to 773 K and for pressure from 0.1 to 5 MPa for the purpose of time-resolved CO2 thermometry. Particular emphasis was put on the comparison of several line-width approximations to model the experimental spectra. Generally good agreement of the temperature mean values with the thermocouple reference has been found for all models over almost the whole pressure and temperature range investigated. The standard deviations, which increased with temperature, were comparable with or better than the results gained for single-shot measurements of pure N2 or O2-N2 mixtures. Yet for high particle densities close to the critical point of CO2 the limitation of the models became obvious, owing to the strongly increased influence of motional narrowing effects. The characteristics of these effects have been demonstrated by measurements even closer to the critical conditions.

Amide I IR, VCD, and 2d IR spectra of isotope‐labeled α‐helix in liquid water: Numerical simulation studies

AbstractAmide I infrared (IR), vibrational circular dichroism (VCD), and two‐dimensional IR (2D IR) spectra of various isotope‐labeled α‐helical polyalanines in liquid water were theoretically calculated by combining semiempirical quantum chemistry calculation, Hessian matrix reconstruction, fragmentation approximation, and molecular dynamics simulation methods. The solvation‐induced amide I frequency shift was found to be about −20 cm−1. Properly taking into account the motional narrowing effect in the ensemble averaging, we show that the simulated IR, VCD, and 2D IR spectra are quantitatively in good agreement with experimental results. Depending on the relative positions of the 13C and/or 13C18O labeled peptides in a given α‐helix, the IR absorption line shape, IR intensity distribution, positive‐negative VCD pattern, and diagonal/off‐diagonal 2D IR echo spectral features were found to change dramatically. It is shown that these different spectroscopic observations can be described in a consistent manner by using the present simulation method. We will show that the properly designed isotopomers and their IR, VCD, and 2D IR spectrum analyses can provide incisive information on the three‐dimensional polypeptide structure and dynamics. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005

Quasielastic and low vibrational lineshapes in atom–surface diffusion

The diffusion and low frequency vibrational motions of atoms and molecules onsurfaces can be measured by means of quasielastic helium atom scattering. In thispaper, we discuss and investigate different analytical approximations, based on thetheory of stochastic processes, to the dynamic structure factor, considering thetwo motions on an equal footing. Special emphasis is put on the nature of thecorresponding lineshapes, explained in terms of the motional narrowing effect. We alsodiscuss the influence of the diffusional and vibrational coupling and several ways ofexperimentally separating the two types of contributions to the dynamic structure factor.In particular, we propose that the so-called inelastic focusing singularity fromatom–surface scattering controls the lineshapes of quasielastic and vibrational peaks.

A combined time correlation function and instantaneous normal mode study of the sum frequency generation spectroscopy of the water/vapor interface

Theoretical approximations to the interface specific sum frequency generation (SFG) spectrum of O–H stretching at the water/vapor interface are constructed using time correlation function (TCF) and instantaneous normal mode (INM) methods. Both approaches lead to a (SSP polarization geometry) signal in excellent agreement with experimental measurements; the SFG spectrum of the entire water spectrum, both intermolecular and intramolecular, is reported. The observation that the INM spectrum is in agreement with the TCF result implies that motional narrowing effects play no role in the interfacial line shapes, in contrast to the O–H stretching dynamics in the bulk that leads to a narrowed line shape. This implies that (SSP) SFG spectroscopy is a probe of structure with dynamics not represented in the signal. The INM approach permits the elucidation of the molecular basis for the observed signal, and the motions responsible for the SFG line shape are well approximated as local O–H stretching modes. The complexity of the broad structured SFG signal is due to O–H stretching motions facing toward the bulk or vacuum environments that are characteristic of the interface. The success of both approaches suggests that theory can play a crucial role in interpreting SFG spectroscopy at more complex interfaces. It is also found that many-body polarization effects account for most of the observed signal intensity.

Electronic structure, physical properties and ionic mobility of LiAg2Sn

The stannide LiAg2Sn was synthesized from the elements by reaction in a sealed tantalum tube in a resistance furnace. LiAg2Sn crystallizes with a ternary ordered version of the cubic BiF3 structure, space group Fmm: a = 659.2(2) pm, wR2 = 0.0450, 69 F2 values, 5 variables. The silver and tin atoms form an antifluorite structure of composition Ag2Sn (285 pm Ag–Sn) in which the lithium atoms fill octahedral voids. Electronic structure calculations reveal weak Ag–Ag and strong Ag–Sn bonding within the Ag2Sn substructure. LiAg2Sn is weakly Pauli paramagnetic and a good metallic conductor. Nevertheless, the modestly small 7Li Knight shift is consistent with a nearly complete state of lithium ionization. The high local symmetry at the tin site is reflected by the absence of a nuclear electric quadrupolar splitting in the 119Sn Mössbauer spectra and a small chemical shift anisotropy evident from 119Sn solid state NMR. Static 7Li solid state NMR spectra reveals motional narrowing effects above 300 K, consistent with lithium atomic mobility on the kHz timescale.

Spin coherence in a two-dimensional electron gas with Rashba spin-orbit interaction

We study spin-relaxation phenomena in a two-dimensional electron gas in the presence of Rashba spin-orbit coupling. A tight-binding model including Rashba spin-orbit coupling is used to study spin relaxation and spin diffusion in a two-dimensional electron gas within Landauer-B\"uttiker formalism. It is shown that the spin-diffusion length is not independent of the mean free path as predicted by the motional narrowing effect. Further it is demonstrated that spin relaxation is anisotropic and can show a nonmonotonic dependence on Fermi energy due to nonparabolicity of the band.

Isotropic Raman scattering of HF in dense gas and liquid SF6: Study of the motional narrowing effect

The isotropic Raman spectra of HF diluted in dense gas and liquid SF6 have been calculated and compared with existing experimental data. The motional narrowing that arises in the transition from dense gas to liquid phase has been studied globally and line-by-line. The influence of mixing and memory effects and the role played by the different terms of the solute–solvent interaction are especially considered.

Vertical motional narrowing of exciton-polaritons in GaN based quantum wells

Numerical simulation of time-resolved light reflection from GaN/AlGaN single and multi-quantum wells (MQWs) revealed a pronounced vertical motional narrowing effect. This effect consists in reduction of the inhomogeneous broadening of the exciton peaks and increase of the decay-time of the time-resolved reflection due to the averaging of the vertical disorder in the structure by extended exciton–polariton modes. The theory predicts pronounced oscillations in the time-resolved reflection of MQWs resulting from the interference of exciton–polariton modes in the structure.

Propagation of Exciton-Polaritons in Nitride-Based Multiple Quantum Wells

We present a semi-classical theory of exciton–polariton effects in disordered multiple quantum well (MQW) structures in application to GaN/AlGaN MQWs. An original micro-model is proposed that allows to calculate all types of coherent optical spectra of MQWs. We show that GaN/AlGaN MQWs should exhibit pronounced propagation effects in the resonant Rayleigh scattering and the vertical motional narrowing effect in the time-resolved transmission. We predict polariton-induced oscillations in the time-resolved coherent optical spectra of GaN/AlGaN MQWs having a period of the order of hundreds of femtoseconds.

Propagation of Exciton–Polaritons in Nitride-Based Multiple Quantum Wells

We present a semi-classical theory of exciton–polariton effects in disordered multiple quantum well (MQW) structures in application to GaN/AlGaN MQWs. An original micro-model is proposed that allows to calculate all types of coherent optical spectra of MQWs. We show that GaN/AlGaN MQWs should exhibit pronounced propagation effects in the resonant Rayleigh scattering and the vertical motional narrowing effect in the time-resolved transmission. We predict polariton-induced oscillations in the time-resolved coherent optical spectra of GaN/AlGaN MQWs having a period of the order of hundreds of femtoseconds.

Motional narrowing effect in certain random binary lattices

We present a model for a class of random binary lattices by introducing a one-dimensional system where impurities are placed in one sublattice while host atoms lie on the other sublattice. The source of disorder is the stochastic fluctuation of the impurity energy from site to site. We study the optical absorption spectra and the peculiarities of the motional narrowing effect at the band edges for perturbative and nonperturbative degrees of disorder. Analytical results agree well with numerical simulations.

Doping-induced variation of electron spin relaxation behavior in polypyrroles

Electron spin relaxation behaviors in polypyrrole (PPy) are examined using a pulsed electron spin resonance spectroscopy (ESR) technique. We have observed remarkable changes in T 1 −1 as well as T 2 −1 induced from the doping level variation for three types of PPy. T 2 −1 particularly exhibits a transition in the temperature dependence with the enhancement in doping levels from a motional narrowing effect into the Elliott mechanism by way of its intermediate state. Thus, the spin relaxation behaviors are interpreted in terms of the superstition of the above two mechanisms. Such a consideration enables us to estimate two parameters, the correlation rates τ c −1 and the spin relaxation rate due to the Elliott mechanism T e −1. The former gives a measure of the hopping rate for a quasi-three-dimensional motion over several metallic regions, and the latter accounts for the spin relaxation process of the spins residing in the metallic crystalline region. Our treatment demonstrates that the spin relaxation behavior in PPy changes with the dopant levels in relation to several physical properties such as the ratio of the Pauli susceptibility to the total one, the metallic behavior in the conductivity, and the polaron band observed in the optical spectrum.

Vertical motional narrowing of exciton polaritons in GaN based multiple quantum wells

Numerical simulation of time-resolved light reflection from GaN/AlGaN single and multiquantum wells (MQWs) revealed a pronounced vertical motional narrowing effect. This effect consists in reduction of the inhomogeneous broadening of the exciton peaks and increase of the decay time of the time-resolved reflection due to the averaging of the vertical disorder in the structure by extended exciton-polariton modes. The theory predicts pronounced oscillations in the time-resolved reflection of MQWs resulting from the interference of exciton-polariton modes in the structure.

Absorption spectra of dipolar Frenkel excitons in two-dimensional lattices with configurational disorder: Long-range interaction and motional narrowing effects

We present results of numerical simulations of optical absorption line shape of Frenkel excitons in two-dimensional disordered lattices. Disorder is generated by Gaussian randomness in the molecular positions. The intersite interaction is considered to be of dipole origin, including coupling to far neighbors. Results of simulations are compared with those obtained in the frame of the nearest-neighbor approximation, showing remarkable differences in the absorption line shape. The motional narrowing effect is found to be essentially different from that previously reported for the case of diagonal disorder as well as for that produced by randomness in nearest-neighbor hopping integrals.

Motional narrowing effect in one-dimensional Frenkel chains with configurational disorder

We analyze the peculiarities of the motional narrowing effect in disordered one-dimensional Frenkel chains with off-diagonal disorder, induced by uncorrelated Gaussian fluctuations in the positions of the host units. A clear difference in the scaling laws with respect to the magnitude of the positional disorder and the chain size is found, as compared to those for uncorrelated diagonal disorder as well as for off-diagonal disorder modeled by uncorrelated randomness in the nearest-neighbor couplings. The origin of such a difference is discussed in detail.

Frenkel excitons in one-dimensional systems with correlated disorder

We consider two different models to study the effects of correlations in disorder on optical properties of one-dimensional Frenkel excitons. The first model is described by means of pairwise correlated Gaussian diagonal disorder, where two-level molecules form dimers of equal transition energy. In the second model, disorder is produced by uncorrelated fluctuations in molecular positions, yielding nevertheless correlations in the hopping integrals. The peculiarities of the motional narrowing effect for both models of disorder are discussed in detail. In particular, it is shown that for the configurational disorder, this effect sufficiently differs from that with uncorrelated randomness in the nearest-neighbor coupling.

Adiabatic quantum approach to optical line shapes in the condensed phase

It is shown that absorption line shapes in the condensed phase can be well described with an adiabatic quantum model. This model reproduces the results of other line-shape models and includes spectral-diffusion and motional-narrowing effects. The motional narrowing is described in a purely quantum-mechanical manner and is found to result from the delocalized character of the wave functions of the low-frequency modes to which the optically active mode is coupled. In many line-shape models it is assumed that an increase in the rate of spectral diffusion automatically leads to motional narrowing of the absorption line. However, this assumption is no longer correct if the low-frequency motion is affected by a change in the quantum state of the high-frequency mode. It is demonstrated that this quantum effect can easily be accounted for in the adiabatic quantum model and that this effect leads to different dependencies of motional narrowing and spectral diffusion on the low-frequency motion.

Nuclear magnetic relaxation of 19F in fluoride glasses

Temperature and frequency dependence of the 19F nuclear spin relaxation of the fluoroindate glass, 40InF 3–20ZnF 2–20SrF 2–2GaF 3–2NaF–16BaF 2 and the fluorozirconate glass, 50ZrF 4–20BaF 2–21LiF–5LaF 3–4AlF 3, are reported. Measurements were undertaken on pure and Gd 3+ doped samples, in the temperature range of 185–1000 K, covering the region below and above the glass transition temperature, T g. The temperature and frequency dependence of the spin-lattice relaxation rate, T 1 −1, measured in the glassy state at temperature <300 K, is less than the observed dependence at higher temperatures. At temperatures > T g, the fluorine mobility increases, leading to a more efficient spin-lattice relaxation process. Activation energies, for F − motion, are 0.8 eV for the fluoroindate glass and 1 eV for the fluorozirconate glass. The addition of Gd 3+ paramagnetic impurities, at 0.1 wt%, does not alter the temperature and frequency dependence of T 1 −1, but increases its magnitude more than one order of magnitude. At temperatures <400 K, the spin–spin relaxation time, T 2 −1, measured for all samples, is determined by the rigid-lattice nuclear dipole–dipole coupling, and it is temperature independent within the accuracy of the measurements. Results obtained for the pure glass, at temperatures >400 K, show that T 2 −1 decreases monotonically as the temperature increases. This decrease is explained as a consequence of the motional narrowing effect caused by the onset of the diffusive motion of the F − ions, with an activation energy around 0.8 eV. For the doped samples, the hyperfine interaction with the paramagnetic impurities is most effective in the relaxation of the nuclear spin, causing an increase in the T 2 −1s observed at temperatures >600 K.