Temperature dependence of dephasing relaxation in dense media

Temperature dependence of dephasing relaxation is studied within the framework of impact theory. The temperature dependence of a linewidth calculated by perturbation theory is non-monotonic; as T rises, the spectral line first narrows, then broadens. Motion narrowing of the line is attributed to the effect of attraction forces.     

Size dependent optical spectroscopy of II–VI semiconductor nanocrystallites (quantum dots)

We use low temperature (10K) optical hole-burning and fluorescence line narrowing spectroscopy to investigate the electronic properties of CdSe nanocrystallites (quantum dots) as a function of crystallite diameter (20–80Å). We discuss how the homogeneous linewidth of the HOMO-LUMO transition, the energy shift between the absorbing and emitting state, and the LO phonon frequency vary with nanocrystallite size.     

Non-markovian theory of molecular relaxation. I. Vibrational relaxation and dephasing in condensed phases

The cumulant expansion is used to derive two formally different master equations for a two-level molecular system interacting with a bath, starting wit The two master equations reduce to the same form in the markovian limit for the bath (where its correlation time is much shorter than the relaxation pr A detailed comparison is made between the predictions of the two approaches which enables us to understand their range of validity and limitations. We apply the formalism to the vibrational relaxation and dephasing of a molecular impurity in a solid matrix and obtain a closed expression for the vib In contrast to the simple stochastic approaches we predict that the line shape in the non-markovian limit contains information regarding the interactio However, the fluctuations in the mean interaction energy of the two-level system with the bath, if correlated with the frequency modulation, result in     

Four-level optical line shape of a single molecule coupled to a single tunneling two-level system

We propose a density-matrix theory for the four-level system consisting of a single optical two-level system (OTLS) coupled to a single two-level system tunneling along a vibrational coordinate (VTLS). Phonons induce jumping rates of the VTLS, but coherent tunneling has to be considered explicitly as well, because the Born-Oppenheimer potential of the tunnel variable may change upon optical excitation. The OTLS is subject to spontaneous emission and driven by a laser wave with arbitrary strength. Numerical simulations for various coupling cases reproduce limiting behaviors previously discussed separately in the literature, such as motional narrowing, cross transitions, optical saturation and pumping, and nonlinear effects. Our model also perfectly fits recent measurements of the spectra of a single molecule coupled to a single tunneling system in a disordered crystal.     

Exciton-phonon coupling and disorder in the excited states of CdSe colloidal quantum dots

We study the origin of the spectral line shape in colloidal CdSe nanocrystal quantum dots. The three-pulse photon echo peak shift (3PEPS) data reveal a temperature-independent fast decay, obscuring the quantification of the homogeneous linewidth. The optical gap and Stokes shift are found to have an anomalous behavior with temperature, which is size, capping group, and surrounding polymer matrix independent. Using these results and combining them with simulations, we discuss the role of exciton-phonon coupling, static inhomogeneity, exciton fine structure, and exciton state disorder in the linewidth of the nanocrystal. In particular, our analysis shows that the disorder due to surface imperfections and finite temperature effects, as well as the relaxation within the fine structure, can have significant impact on the steady-state absorption spectrum, 3PEPS data, and dephasing processes.     

Low-pressure collisional narrowing in CO2

We report the observation of collisional narrowing of the Q branch of the Raman spectrum for the (000) → (100) transition in CO₂ at very low pressures. The minimum linewidth is reached at ≈250 Torr. An estimate for the difference in rotational constants of the (100) state and the ground state is obtained. The narrowing of the linewidth and the changes in line-shape are interpreted in terms of velocity changing collisions and rotationally inelastic collisions between the CO₂ molecules.     

23Na NMR in urethane cross-linked polyether solid polymer electrolytes

Sodium triflate/polyether urethane polymer electrolytes ranging in concentration from 0.05 molal to 1.75 molal have been investigated via ²³Na static solid-state NMR. Room temperature spectra and spin lattice relaxation times were consistent with a single narrow resonance indicating the presence of only mobile ionic species. The concentration and temperature dependence of relaxation times, chemical shifts, and linewidth have been investigated. The results suggest either a single species or rapid exchange between a number of species (even at temperatures below the glass transition temperature, T_g). The linewidth decreases with increasing concentration of ions and remains temperature independent below T_g. Below T_g a maximum quadrupolar interaction constant of 2 MHz is calculated. The addition of plasticizer to the polymer electrolyte causes significant chemical shift changes that depend on the solvent donicity of the plasticizer. The linewidth and T₁ relaxation times also depend on the T_g of the plasticized systems. Previous ²³Na NMR literature results are reviewed and qualitative models developed to account for the variation in results. © 1994 John Wiley & Sons, Inc.     

Solvent response and dielectric relaxation in supercooled butyronitrile

We have measured the dynamics of solvation of a triplet state probe, quinoxaline, in the glass-forming dipolar liquid butyronitrile near its glass transition temperature T(g)=95 K. The Stokes shift correlation function displays a relaxation time dispersion of considerable magnitude and the optical linewidth changes along the solvation coordinate in a nonmonotonic fashion. These features are characteristic of solvation in viscous solvents and clearly indicate heterogeneous dynamics, i.e., spatially distinct solvent response times. Using the dielectric relaxation data of viscous butyronitrile as input, a microscopic model of dipolar solvation captures the relaxation time, the relaxation dispersion, and the amplitude of the dynamical Stokes shift remarkably well.     

Electron paramagnetic resonance investigation of the electron-doped manganite La(1-x)Te(x)MnO3 (0.1 ≤ x ≤ 0.2)

The electron paramagnetic resonance (EPR) properties of the electron-doped manganite La(1-x)Te(x)MnO(3) (0.1 ≤ x ≤ 0.2) are investigated based on the data of EPR spectra, resistivity, and magnetic susceptibility. With decreasing temperature from 400 K, the EPR linewidth ΔH(PP) decreases and passes through a minimum at T(min), then substantially increases with further decreasing temperature. The broadening of the EPR linewidth above T(min) can be understood in terms of the increase in the relaxation rate of spin of e(g) polarons to the lattice with increasing temperature due to the similarity between the temperature dependence of the linewidth ΔH(pp)(T) and the conductivity σ(T). For the samples with x = 0.1 and 0.15, the conductivity activation energy E(σ) is comparable with the activation energy E(a) deduced from the linewidth. Whereas for the x = 0.2 sample, there is a large difference between E(σ) (0.2206 eV) and E(a) (0.0874 eV).     

Long-Lived Coherence Originating from Electronic-Vibrational Couplings in Light-Harvesting Complexes

We theoretically investigate the evolutions of two-dimensional, third-order, nonlinear pho-ton echo rephasing spectra with population time by using an exact numerical path integral method. It is shown that for the same system, the coherence time and relaxation time of excitonic states are short, however, if the couplings of electronic and intra-pigment vibra-tional modes are considered, the coherence time and relaxation time of this vibronic states are greatly extended. It means that the couplings between electronic and vibrational modes play important roles in keeping long-lived coherence in light-harvesting complexes. Particularly, by using the method we can fix the transition path of the energy transfer in bio-molecular systems.     

Vibrational coherence transfer characterized with Fourier-transform 2D IR spectroscopy

Two-dimensional infrared (2D IR) spectroscopy of the symmetric and asymmetric C[Triple Bond]O stretching vibrations of Rh(CO)(2)acac in hexane has been used to investigate vibrational coherence transfer, dephasing, and population relaxation in a multilevel vibrational system. The transfer of coherence between close-lying vibrational frequencies results in extra relaxation-induced peaks in the 2D IR spectrum, whose amplitude depends on the coherence transfer rate. Coherence transfer arises from the mutual interaction of the bright CO stretches with dark states, which in this case reflects the mutual d-pi(*) back bonding of the Rh center to both the terminal carbonyls and the acetylacenonate ligand. For 2D IR relaxation experiments with variable waiting times, coherent dynamics lead to the modulation of peak amplitudes, while incoherent population relaxation and exchange results in the growth of the relaxation-induced peaks. We have modeled the data by propagating the density matrix with the Redfield equation, incorporating all vibrational relaxation processes during all three experimental time periods and including excitation reorientation effects arising from relaxation. Coherence and population transfer time scales from the symmetric to the asymmetric stretch were found to be 350 fs and 3 ps, respectively. We also discuss a diagrammatic approach to incorporating all vibrational relaxation processes into the nonlinear response function, and show how coherence transfer influences the analysis of structural variables from 2D IR spectroscopy.     

Relaxation in a double potential well

Kinetic equations are deduced for the density matrix describing the relaxation in a two-level system interacting with a heat reservoir. It is assumed that the frequency of transition between the levels is small relative to the characteristic frequency of fluctuation in the reservoir; the interaction may be of any strength. The equations are used to discuss the relaxation in such a system.     

Photochemical and nonphotochemical hole burning in dimethyl-s-tetrazine in a polyvinyl carbazole film

Hole burning as well as fluorescence line narrowing experiments have been performed on the system dimethyl-s-tetrazine in polyvinyl carbazole films at low temperatures. The first singlet electronic absorption bands are typical (300 cm^?3 wide) of inhomogeneously broadened bands of guest molecules in amorphous organic hosts. Evidence is presented for both photochemical and nonphotochemical hole burning. The narrowest holes observed were lorentzian, had a width of 0.44 cm^?1 at 1.8 K, and are believed to be of nonphotochemical origin. A model which envisions the guest molecules to occupy different sites in the polymer host with a distribution of energy barriers between sites is used to describe these observations. The fast (20 ps) relaxation time implied by the 0.44 cm^?1 lorentzian linewidth is interpreted as indicative of the rate of site interconversion in the excited state.     

Temperature dependence of the homogeneous linewdth of the 5D0-7F0 transition of Eu3+ in amorphous hosts at high temperatures

The temperature dependence of the homogeneous linewidth of the ⁵D₀-⁷F₀ transition of Eu³⁺ in different amorphous solids is determined in the 295–800 K temperature range. The nearly quadratic temperature dependence observed is discussed in terms of Raman broadening and mechanisms based on two-level systems known to exist in disordered systems.     

On the role of coupling in mode selective excitation using ultrafast pulse shaping in stimulated Raman spectroscopy

The coherence of two coupled two-level systems, representing vibrational modes in a semiclassical model, is calculated in weak and strong fields for various coupling schemes and for different relative phases between initial state amplitudes. A relative phase equal to pi projects the system into a dark state. The selective excitation of one of the two, two-level systems is studied as a function of coupling strength and initial phases.     

CARBON-13 NMR STUDY ON MOLECULAR MOTION OF 1, 2-POLYBUTADIENE——TEMPERATURE DEPENDENCE OF MOLECULAR MOTION

Proton-decoupled, ~(13)C FT-NMR (operating at 50.3 MHz) is used to determine spin-latticerelaxation time (T_1), nuclear Overhauser enhancement (NOE), linewidth and chemical shift of 1,2-polybutadiene as a function of temperature in CDCl_3 solution and the temperature dependence ofmolecular motion of 1,2-polybutadiene has been investigated with these NMR relaxation parameters.It is found that jumps of NOE and linewidth vs. temperature appear between-1℃and -30℃. Theminimum of nT_1 vs. temperature for all carbons occur at about -45℃.     

Nonmonotonic temperature dependence of heat capacity through the glass transition within a kinetic model

The heat capacity of a supercooled liquid subjected to a temperature cycle through its glass transition is studied within a kinetic model. In this model, the beta process is assumed to be thermally activated and described by a two-level system. The alpha process is described as a beta relaxation mediated cooperative transition in a double well. The overshoot of the heat capacity during the heating scan is well reproduced and is shown to be directly related to delayed energy relaxation in the double well. In addition, the calculated scan rate dependencies of the glass transition temperature T(g) and the limiting fictive temperature T(f) (L) show qualitative agreement with the known results. Heterogeneity is found to significantly reduce the overshoot of heat capacity. Furthermore, the frequency dependent heat capacity has been calculated within the present framework and found to be rather similar to the experimentally observed behavior of supercooled liquids.     

π-Electron-Assisted Relaxation of Spin Excited States in Cobalt Phthalocyanine Molecules on Au(111) Surface

We present our investigation on the spin relaxation of cobalt phthalocyanine (CoPc) films on Au(111) (CoPc/Au(111)) surface using scanning tunneling microscopy and spectroscopy. The spin relaxation time derived from the linewidth of spin-flip inelastic electron tunneling spectroscopy is quantitatively analyzed according to the Korringa-like formula. We find that although this regime of the spin relaxation time calculation by just considering the exchange interaction between itinerant conduction electrons and localized d-shells (s-d exchange interaction) can successfully reproduce the experimental value of the adsorbed magnetic atom, it fails in our case of CoPc/Au(111). Instead, we can obtain the relaxation time that is in good agreement with the experimental result by considering the fact that the π electrons in CoPc molecules are spin polarized, where the spin polarized π electrons extended at the Pc macrocycle may also scatter the conduction electrons in addition to the localized d spins. Our analyses indicate that the scattering by the π electrons provides an efficient spin relaxation channel in addition to the s-d interaction and thus leads to much short relaxation time in such a kind of molecular system on a metal substrate.     

Electron spin relaxation of N@C60 in CS2 in CS2

We examine the temperature dependence of the electron spin relaxation times of the molecules N@C60 and N@C70 (which comprise atomic nitrogen trapped within a carbon cage) in liquid CS2 solution. The results are inconsistent with the fluctuating zero-field splitting (ZFS) mechanism, which is commonly invoked to explain electron spin relaxation for S> or =1 spins in liquid solution, and is the mechanism postulated in the literature for these systems. Instead, we find an Arrhenius temperature dependence for N@C60 , indicating the spin relaxation is driven primarily by an Orbach process. For the asymmetric N@C70 molecule, which has a permanent ZFS, we resolve an additional relaxation mechanism caused by the rapid reorientation of its ZFS. We also report the longest coherence time (T2) ever observed for a molecular electron spin, being 0.25 ms at 170 K.