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Revisiting the combined photon echo and single-molecule studies of low-temperature dynamics in a dye-doped polymer
Abstract
The photon echo (PE) spectroscopy and single-molecule spectroscopy (SMS) may be combined to give a very powerful tool for comprehensive study of low-temperature dynamics in dye-doped disordered solids (polymers, glasses). At the same time, this type of studies are likely to reveal discrepancies when comparing characteristic times of optical dephasing T2 and single-molecule zero-phonon spectral lines (ZPL) broadening obtained from PE and SMS, correspondingly, for tetra-tert-butylterrylene in polyisobutylene in the temperature range of a few–dozen of Kelvins [see Phys. Status Solidi B 241, 3480 and 3487 (2004)]. Inexplicably, PE-experiments demonstrated T2-times to be much shorter than it is sufficient to cause the corresponding ZPL broadening. Here we experimentally solve this problem and show that at T = 4.5–15 K the incoherent PE gives T2-times which correspond to the narrowest SM ZPL. On the SM-level there is a pronounced additional ZPL-broadening due to spectral diffusion processes which are strongly dependent on the characteristics time of the measurement (tens of nanoseconds for PE and seconds for SMS). There is also a broad distribution of ZPL spectral widths for different SMs due to different local environments, that contribute differently to both the optical dephasing and the spectral diffusion processes, but always in addition to the value of inverse optical dephasing times measured using a PE technique.
... Enhancing the traditional techniques by combination with imaging techniques like confocal microscopy enhances the monitoring of the studied samples. The ultra-super resolution techniques which depend on uorescence, like single-molecule and time-resolved systems [9][10][11][12][13][14]. ...
Selected four gilt-bronze statuettes have been studied after excavation from the burial environment. Soil deposits and corrosion forms were observed on the surface disfiguring the gold layer. Confocal micro-Raman spectroscopy and Portable X-ray fluorescence (pXRF) were used to identify the molecular structure and elements distribution over the statuette’s surfaces. Total internal reflection fluorescence (TIRF) has been utilized to recognize the surface/interface state before the conservation treatment and takes the high-resolution imaging. Chemical analyses indicated that it was made of leaded bronze. A high lead content in the statuettes was reflected by lead corrosion products admixed with green chloride and carbonate salts of copper. Conservation treatment was accomplished using fine and appropriate mechanical tools to recover all the gilding remains and surface details. Finally, the protection process of statuettes was accomplished using two layers of Permalac.
... But the most recent approach in spectroscopy is superresolution spectroscopy like single-molecule spectroscopy, imaging, and mapping spectroscopy which are from state of the art in spectroscopy which is known as microspectroscopy. The backbone of the microspectroscopic techniques is the laser and confocality principle [19][20][21][22][23][24]. 3D laser Raman microspectroscopy is now well established as one of the most important and great spectroscopic tools for a vast range of applications in the research world. ...
Phosphosilicate thin film and monolith co-doped with Er³⁺-Yb³⁺ were prepared by self-modified Sol-Gel. Spin coating was used to deposit the prepared samples. The calculated crystallite sizes were found to be 26 and 28 nm for (S22P0.5E0.5YM) sintered at 500 and 1000 °C, respectively. A thickness of 1.6 μm was obtained for (S22P0.5E0.5YM). Laser-based Raman microspectroscopy presents the homogenous distribution of the Er³⁺&Yb³⁺ions. The Er³⁺&Yb³⁺absorption coefficients were measured. The radiative properties for the metastable level ⁴I13/2 were determined using The Judd–Ofelt model. The electron relaxation time (τr) slightly increased with the Yb³⁺ ions concentration up to 19.41 ms. Photoluminescence increased with increasing the Yb³⁺ ions. Pumping the 4F transition ⁴I13/2→⁴I15/2 of Erbium ions with 650 nm laser emits at around 1500 nm. This study introduces promising results for the prepared samples to support the planar optical waveguide amplifier system that could be used in communication and laser applications. However its cost-effectiveness, on-site measurability is still an interesting matter challenge.
... Nowadays this experimental setup is based on a super-luminescence Rhodamine-dye source (operated in the wavelength range 575-600 nm), which is transversally pumped by secondharmonic of a solid-state Nd:YAG pulse laser LS-2131M-10-FF (LOTIS TII, Belarus). Detailed description of the setup was given in [37,38]. The optical scheme of the spectrometer consisted of two optical delay lines, forming two laser pulses delayed to each other. ...
The dye-doped polymer is commonly used in the field of optoelectronics, given its effectiveness in optimising the device’s performance. This study is devoted to the synthesis and characterisation of Anchusa-Italica-doped Pentacene thin-film. Scanning electronic microscopy structural analysis, Fourier transform spectroscopy, and UV-visible transmittance spectra with a range of 300-900 nm were also carried out. The fundamental optical properties such as the absorption coefficient, optical energy gap, absorption and refractive indices were calculated based on the methods already used in the literature as Tauc’s relationship. The morphology of the samples indicated that dye structure was affected in the doped pentacene. The Fourier transform infrared technique (FT-IR) resulting spectrum of the doped samples also showed a significant absorption peak corresponding to C-H as an index of impurities. The calculated band-gap energy of the impurity sample was reduced and was the lowest compared to both the pure dye and polymer samples. The optical absorption and transmittance spectra revealed that it was positioned in the desirable ranges for optoelectronic applications. An anomaly in the absorption index was also observed through excitation of the resonance mode with transparent indication. This effect was deduced from the calculation of the refractive index. The results presented in this paper significantly contribute to the developments in the field of optoelectronic devices based on dye/polymer organic materials.
We have compared and analyzed the parameters of the Franck–Condon (FC) and Herzberg–Teller (HT) interactions, which form the fine-structure spectra of stilbene, 1,4-distyrylbenzene, and tetrafluorodistyrylbenzene—compounds that are similar in their chemical structures but differ in the length of π-conjugation and in the presence of substituents in their benzene rings. The numerical values of the FC intramolecular interaction constants have been obtained, and, simultaneously, the values of the HT parameter have been found as quantitative values of the projections of the electronic transition vector of the dipole moment onto the normal vibrational coordinates. We have solved the question of transferability of these parameters in the homologous series of stilbene molecules containing the same sets of structural elements, which has made it possible to extend the use of the fragmentary approach to describe the fundamental bands of organic molecules of different homological series and to solve the problem of studying the structure of the spectra of large molecules.
We report the results of core-shell (CdSe/CdS) quantum dots study. Quantum dots sizes were evaluated as 2.0 and 2.9 nm from absorbance edge position. We suggest two types of traps, predict properties of these traps based on upconversion luminescence data and previous studies of quantum dots (CdSe cores only) and bulk CdS.
Using the hydrothermal method, we synthesized water soluble YVO 4 : Yb, Er nanoparticles with a size less than 10 nm. Nanoparticles exhibit intense luminescence in the green region due to Er 3+ ions when excited by laser radiation at a wavelength of 980 nm as a result of the up-conversion process. Bright and stable luminescence also persists in an aqueous solution of nanoparticles. Based on experimental data, it can be argued that the objects obtained are promising in biological applications, as well as up-conversion phosphors.
The possibility and conditions of an incoherent exciton echo excitation in a thin layer of the CdSe/CdS/ZnS semiconductor quantum dots spread on a glass substrate are discussed.
The possibility of detecting signals of a two-pulse incoherent photon echo in a thin layer of double-coated semiconductor colloidal quantum dots spread on the surface of a glass substrate at T = 10 K is demonstrated. Possible mechanisms of ultrafast optical dephasing detected at the given temperature are discussed.
Low temperature dynamics (tunneling and vibrational relaxation) in doped polyisobutylene film has been reinvestigated using 2-pulse incoherent photon echo (2IPE) and compared with single-molecule spectroscopy (SMS) data. It has been shown that in a very wide range of low temperatures the 2IPE gives optical dephasing times which correspond to the narrowest zero-phonon lines of single dye molecules.
A simple and effective technique for depositing thin films of semiconductor colloidal quantum dots (QD) on glass substrates is developed. Samples with CdSe/CdS/ZnS quantum dots are fabricated and investigated via luminescence microspectroscopy. Four-wave mixing signals are recorded at room temperature in a solution and in a thin film of quantum dots.
ФЛУОРЕСЦЕНТНАЯ МИКРОСКОПИЯ СВЕРХВЫСОКОГО РАЗРЕШЕНИЯ С ВИЗУАЛИЗАЦИЕЙ ОДИНОЧНЫХ МОЛЕКУЛ (НОБЕЛЕВСКАЯ ПРЕМИЯ ПО ХИМИИ 2014 ГОДА) // Fluorescence super-resolution microscopy (Nobel Prize in Chemistry 2014).
The random first-order transition (RFOT) theory of the structural glass
transition is reviewed in a pedagogical fashion. The rigidity that emerges in
crystals and glassy liquids is of the same fundamental origin. In both cases,
it corresponds with a breaking of the translational symmetry; analogies with
freezing transitions in spin systems can also be made. The common aspect of
these seemingly distinct phenomena is a spontaneous emergence of the molecular
field, a venerable and well-understood concept. In crucial distinction from
periodic crystallisation, the free energy landscape of a glassy liquid is
vastly degenerate, which gives rise to new length and time scales while
rendering the emergence of rigidity gradual. We obviate the standard notion
that to be mechanically stable a structure must be essentially unique; instead,
we show that bulk degeneracy is perfectly allowed but should not exceed a
certain value. The present microscopic description thus explains both
crystallisation and the emergence of the landscape regime followed by
vitrification in a unified, thermodynamics-rooted fashion. The article contains
a self-contained exposition of the basics of the classical density functional
theory and liquid theory, which are subsequently used to quantitatively
estimate, without using adjustable parameters, the key attributes of glassy
liquids, viz., the relaxation barriers, glass transition temperature, and
cooperativity size. These results are then used to quantitatively discuss many
diverse glassy phenomena, including: the intrinsic connection between the
excess liquid entropy and relaxation rates, the non-Arrhenius temperature
dependence of $\alpha$-relaxation, the dynamic heterogeneity, ... (see Comments
for the remainder of Abstract.)
We suggest a novel approach for spatially resolved probing of local fluctuations of the refractive index n in solids by means of single-molecule (SM) spectroscopy. It is based on the dependence T1(n) of the effective radiative lifetime T1 of dye centres in solids on n due to the local-field effects. Detection of SM zero-phonon lines at low temperatures gives the values of SM natural spectral linewidth (which is inverse proportional to T1) and makes it possible to reveal the distribution of the local n values in solids. Here we demonstrate this possibility on the example of amorphous polyethylene and polycrystalline naphthalene doped with terrylene. Particularly, we show that the obtained distributions of lifetime limited spectral linewidths of terrylene molecules embedded into these matrices are due to the spatial fluctuations of the refractive index local values.
We develop a first principles theoretical description of femtosecond double-pump single-molecule
signals of molecular aggregates. We incorporate all singly excited electronic states and vibrational
modes with significant exciton-phonon coupling into a system Hamiltonian and treat the ensuing system dynamics within the Davydov D1 Ansatz. The remaining intra- and inter-molecular
vibrational modes are treated as a heat bath and their effect is accounted for through lineshape
functions. We apply our theory to simulate single-molecule signals of the light harvesting complex
II. The calculated signals exhibit pronounced oscillations of mixed electron-vibrational (vibronic)
origin. Their periods decrease with decreasing exciton-phonon coupling
Parasitic two-level tunnelling systems originating from structural material defects affect the functionality of various microfabricated devices by acting as a source of noise. In particular, superconducting quantum bits may be sensitive to even single defects when these reside in the tunnel barrier of the qubit's Josephson junctions, and this can be exploited to observe and manipulate the quantum states of individual tunnelling systems. Here, we detect and fully characterize a system of two strongly interacting defects using a novel technique for high-resolution spectroscopy. Mutual defect coupling has been conjectured to explain various anomalies of glasses, and was recently suggested as the origin of low-frequency noise in superconducting devices. Our study provides conclusive evidence of defect interactions with full access to the individual constituents, demonstrating the potential of superconducting qubits for studying material defects. All our observations are consistent with the assumption that defects are generated by atomic tunnelling.
The detection of individual molecules has found widespread application in molecular biology, photochemistry, polymer chemistry, quantum optics and super-resolution microscopy. Tracking of an individual molecule in time has allowed identifying discrete molecular photodynamic steps, action of molecular motors, protein folding, diffusion, etc. down to the picosecond level. However, methods to study the ultrafast electronic and vibrational molecular dynamics at the level of individual molecules have emerged only recently. In this review we present several examples of femtosecond single molecule spectroscopy. Starting with basic pump-probe spectroscopy in a confocal detection scheme, we move towards deterministic coherent control approaches using pulse shapers and ultra-broad band laser systems. We present the detection of both electronic and vibrational femtosecond dynamics of individual fluorophores at room temperature, showing electronic (de)coherence, vibrational wavepacket interference and quantum control. Finally, two colour phase shaping applied to photosynthetic light-harvesting complexes is presented, which allows investigation of the persistent coherence in photosynthetic complexes under physiological conditions at the level of individual complexes.
The initial steps of photosynthesis comprise the absorption of sunlight by pigment-protein antenna complexes followed by rapid
and highly efficient funneling of excitation energy to a reaction center. In these transport processes, signatures of unexpectedly
long-lived coherences have emerged in two-dimensional ensemble spectra of various light-harvesting complexes. Here, we demonstrate
ultrafast quantum coherent energy transfer within individual antenna complexes of a purple bacterium under physiological conditions.
We find that quantum coherences between electronically coupled energy eigenstates persist at least 400 femtoseconds and that
distinct energy-transfer pathways that change with time can be identified in each complex. Our data suggest that long-lived
quantum coherence renders energy transfer in photosynthetic systems robust in the presence of disorder, which is a prerequisite
for efficient light harvesting.
Experimentally observed narrowing of spectral holes in a glass under hydrostatic pressure confirms our theoretical finding that the external pressure, in addition to increasing the frequencies of soft localized modes, also reduces their number. This occurs because the majority of soft localized modes in glasses is shown to have a negative cubic anharmonicity. For that reason the applied pressure not only enhances the stiffness of these modes, but also transforms a fraction of them into tunneling two-level systems, whereas the simultaneous reverse transformations of some other two-level systems into soft localized modes are less numerous.
The field of viscous liquid and glassy solid dynamics is reviewed by a process of posing the key questions that need to be answered, and then providing the best answers available to the authors and their advisors at this time. The subject is divided into four parts, three of them dealing with behavior in different domains of temperature with respect to the glass transition temperature, Tg, and a fourth dealing with “short time processes.” The first part tackles the high temperature regime T>Tg, in which the system is ergodic and the evolution of the viscous liquid toward the condition at Tg is in focus. The second part deals with the regime T∼Tg, where the system is nonergodic except for very long annealing times, hence has time-dependent properties (aging and annealing). The third part discusses behavior when the system is completely frozen with respect to the primary relaxation process but in which secondary processes, particularly those responsible for “superionic” conductivity, and dopart mobility in amorphous silicon, remain active. In the fourth part we focus on the behavior of the system at the crossover between the low frequency vibrational components of the molecular motion and its high frequency relaxational components, paying particular attention to very recent developments in the short time dielectric response and the high Q mechanical response. © 2000 American Institute of Physics.
Luminescence imaging has been adapted to facilitate alignment and focusing of multiple non-collinear laser beams in pump–probe experiments. The technique has been validated in an experiment on four-wave mixing in a doped polymer placed into a high-pressure sapphire-anvil cell, itself inside an optical cryostat.
We investigated the spectra of a large number of single tetra-tert-butylterrylene molecules embedded in an amorphous polyisobutylene matrix and analyzed the distributions of their linewidths (widths of single spectral peaks). The measurements were performed at 2, 4.5, and 7 K. This is a temperature region, where the standard two-level system (TLS) model of low-temperature glasses begins to fail. At T=2 K the temporal behavior (history of frequency jumps) of most of the measured spectra and their linewidth distributions were found to be consistent with the TLS model. At higher temperatures the main features of individual spectra (number of spectral peaks, temperature variation of peak widths, ratio of intensities of different peaks, etc.) still appear consistent with the predictions of this model. An increase of temperature leads mainly to the broadening of spectral peaks. A detailed analysis of the linewidth distributions reveals deviations from a standard TLS model at T=4.5 and 7 K. This difference is attributed to the influence of quasi-local low-frequency modes (LFM) of the amorphous matrix. By comparing the measured linewidth distributions with computer simulations, we quantitatively determined the LFM contribution to the single-molecule spectra in our dye-matrix system at different temperatures. (C) 2003 American Institute of Physics.
Numerous experiments have shown that the low-temperature dynamics of a wide variety of disordered solids is qualitatively universal. However, most of these results were obtained with ensemble-averaging techniques which hide the local parameters of the dynamic processes. We used single-molecule (SM) spectroscopy for direct observation of the dynamic processes in disordered solids with different internal structure and chemical composition. The surprising result is that the dynamics of low-molecular-weight glasses and short-chain polymers does not follow, on a microscopic level, the current concept of low-temperature glass dynamics. An extra contribution to the dynamics was detected causing irreproducible jumps and drifts of the SM spectra on timescales between milliseconds and minutes. In most matrices consisting of small molecules and oligomers, the spectral dynamics was so fast that SM spectra could hardly or not at all be recorded and only irregular fluorescence flares were observed. These results provide new mechanistic insight into the behavior of glasses in general: At low temperatures, the local dynamics of disordered solids is not universal but depends on the structure and chemical composition of the material.
The normal modes and the density of states (DOS) of any material provide a basis for understanding its thermal and mechanical
transport properties. In perfect crystals, normal modes are plane waves, but they can be complex in disordered systems. We
have experimentally measured normal modes and the DOS in a disordered colloidal crystal. The DOS shows Debye-like behavior
at low energies and an excess of modes, or Boson peak, at higher energies. The normal modes take the form of plane waves hybridized
with localized short wavelength features in the Debye regime but lose both longitudinal and transverse plane-wave character
at a common energy near the Boson peak.
The line width dependence of zero-phonon lines and phonon sidebands on temperature, bath dissipation, and electron-phonon coupling is studied for an underdamped Brownian oscillator model with an Ohmic dissipative bath. Factors determining the line widths vary from the zero-phonon lines to the phonon sidebands. The control-parameter space of line broadening has been mapped out, revealing that the line widths of the zero-phonon lines and phonon sidebands are linearly dependent on both the temperature and the Huang-Rhys factor. It is also found that the dependence of the line widths on the bath damping factor is linear for the zero-phonon lines and quadratic for the phonon sidebands.
The interaction of sound waves with tunneling, relaxational, and resonant vibrational states in glasses is investigated within the soft-potential model. The same bilinear coupling constant is assumed for all three different kinds of soft modes. The model reproduces the results of the tunneling model at low temperatures and frequencies. In addition, it explains the fast rise of the relaxational absorption above 1 K and the plateau in the thermal conductivity around 5 K. The universal features of the sound absorption in glasses are described with good accuracy up to 20 K.
Single Molecule Spectroscopy is one of the hottest topics in today's chemistry. It brings us close to the the most exciting vision generations of chemists have been dreaming of: To observe and examine single molecules! While most of chemistry deals with myriads of molecules, this books presents the latest developments for the detection and investigation of single entities. Written by internationally renowned authors, it is a thorough and comprehensive survey of current methods and their applications. © VCH Verlagsgesellschaft mbH, D-69451 Weinheim (Federal Republic of Germany), 1997. All rights reserved.
New experimental data on the times of phase relaxation in the amorphous polyisobutylene doped with chromophore molecules of tetra-tert-butylterylene are obtained by the incoherent photon echo method at temperatures of 5, 7, 10, and 15 K. A comparative analysis of the results is performed and data on the widths of zero-phonon spectral lines of single molecules in this impurity system are presented.
Using a stochastic model, we examine disorder-induced changes in the
absorption line shape of a chromophore embedded in a matrix of
noninteracting two-level systems (TLSs) with randomly oriented dipole
moments. By systematically controlling the degree of TLS positional
disorder, a perfectly crystalline, glassy or a combination of the two
environments is obtained. The chromophore is assumed to interact with
TLSs via long-range dipole-dipole coupling. At long times and in
the absence of disorder, Gaussian line shapes are found, which morph
into Lorentzian for a completely disordered environment owing to strong
coupling between the chromophore and a TLS in close vicinity.
We performed simulations of the prototypical femtosecond "double-slit" experiment with strong pulsed laser fields for a chromophore in solution. The chromophore is modeled as a system with two electronic levels and a single Franck-Condon active underdamped vibrational mode. All other (intra- and inter-molecular) vibrational modes are accounted for as a thermal bath. The system-bath coupling is treated in a computationally accurate manner using the hierarchy equations of motion approach. The double-slit signal is evaluated numerically exactly without invoking perturbation theory in the matter-field interaction. We show that the strong-pulse double-slit signal consists of a superposition of N-wave-mixing (N = 2, 4, 6[ellipsis (horizontal)]) responses and can be split into population and coherence contributions. The former reveals the dynamics of vibrational wave packets in the ground state and the excited electronic state of the chromophore, while the latter contains information on the dephasing of electronic coherences of the chromophore density matrix. We studied the influence of heat baths with different coupling strengths and memories on the double-slit signal. Our results show that the double-slit experiment performed with strong (nonperturbative) pulses yields substantially more information on the photoinduced dynamics of the chromophore than the weak-pulse experiment, in particular, if the bath-induced dephasings are fast.
The effect of macroscopic parameters of a substance on the optical characteristics of impurity particles is investigated. A generalized control equation is derived for two-level emitters forming an ensemble of optical centers in a transparent dielectric medium. In this equation, the effective values of the acting pump field and the radiative relaxation rate of an optical center are taken into account. The formalism developed here is a completely microscopic approach based on the chain of the Bogoliubov-Born-Green-Kirkwood-Yvon equations for reduced density matrices and correlation operators for material particles and modes of a quantized radiation field. The method used here makes it possible to take into account the effects of individual and collective behavior of emitters, which are associated with the presence of an intermediate medium, consistently without using phenomenological procedures. It is shown that the resultant analytic expression for the effective lifetime of the excited state of an optical center conforms with experimental data.
Optical photon echo measurements on seven doped organic amorphous systems (resorufin (Res) in D- and D6-ethanol (EtOD and EtOD6), tetra-tert-butylterrylene (TBT) in polyisobutylene and in polymethylmetacrylate (PMMA), zinc-tetraphenylporphine (ZnTPP) in EtOD and EtOD6 and in PMMA) have been performed in a wide temperature range (0.35–50 K). This wide temperature interval (more than two decades) permits to clearly separate for the first time the two different contributions to the line width (optical dephasing): low-temperature broadening which follows a power law, and high-temperature broadening which demonstrates an exponential-like behavior. The values of the exponent α obtained at low temperatures in the cases of TBT and ZnTPP in PMMA matrix show some marked discrepancy with theoretical predictions, which can be attributed to a failure of the standard TLS model for a treatment of these systems or to an inaccuracy of the PMMA parameters calculated in literature. A comparison of the high-temperature part of the line broadening of different dye molecules (ZnTPP and Res), embedded in the same matrices (EtOD and EtOD6) shows that the optical dephasing is determined by two contributions – the dynamics of matrix itself and the dynamics which is related to the doped molecules.
The Brownian oscillator model has been successfully employed for modeling solvation dynamics in numerous femtosecond measurements. To a very large extent, this work has been interpreted on the basis of high-temperature limits of the theory. We present an analysis of the low temperature limit, which is particularly important for hole burning, photon echo, and single molecule spectroscopic experiments. Several forms for the bath spectral density are employed to compute zero phonon absorption line shapes. We show that in all cases the zero phonon linewidth vanishes at low temperatures, and that the line becomes asymmetric with a sharp rise at the red edge, as expected qualitatively.
Using a superluminescent diode, we observed femtosecond accumulated photon echoes in 1,1′-diethyl-4,4′-quinodicarbocyanine iodide embedded in polyvinyl acetate. The output of the superluminescent diode directly excited the sample to produce photon echoes. By using a simple experimental system we obtained a temporal resolution as high as 100 fsec. This resolution is determined by the spectral width of the superluminescent diode and the noncollinear excitation beam configuration of the system.
Photon echoes are generated by excitation pulses of chaotic (thermal) light. Echo signals are large and display a modulation with pulse separation due to quantum beating of hyperfine levels.
Spectroscopic techniques exhibit different sensitivities for line broadening processes in amorphous solids. Photon echo and hole-burning spectroscopy yield averages over the chromophore ensemble. At low temperatures, the results can usually be fitted with a combination of a power-law term — corresponding to the relaxations of two-level systems- and of an exponentially activated contribution of pseudo-local phonon modes. Single-molecule spectroscopy (SMS). in contrast, can resolve the behavior of single dye molecules and yields a distribution of power laws as well as of activation energies. We compare photon echo results for tetra-tert-butylterrylene (TBT) in polyisobutylene (PIB) with SMS data for the same system. The latter were used to simulate numerically the data which would be obtained in an ensemble-averaging experiment. The results of the numerical calculation can be well fitted without assuming a distribution of parameters.
The line width distributions for single terrylene molecules in a naphthalene crystal have been measured at temperatures down to 30 mK. The line width distribution becomes narrower with decreasing temperature, and has a full-width at half-maximum of approximately 4.3(13) MHz at 30 mK and an average line width of 42.7(3) MHz.
The theory of optical photon echo and hole burning spectroscopies in low temperature glasses is discussed within the framework of the tunneling two-level system and stochastic sudden jump models. Exact results for the relevant theoretical quantities involve certain averages over the distributions of the two-level system energies and relaxation rates. The standard approximations for these averages are critically examined, for experimentally realistic parameters, via comparison to numerically exact calculations. The general conclusion is that the standard approximations are often used under conditions where they are not expected to be quantitatively accurate.
Fossil amber offers the opportunity to investigate the dynamics of glass-forming materials far below the nominal glass transition temperature. This is important in the context of classical theory, as well as some new theories that challenge the idea of an 'ideal' glass transition. Here we report results from calorimetric and stress relaxation experiments using a 20-million-year-old Dominican amber. By performing the stress relaxation experiments in a step-wise fashion, we measured the relaxation time at each temperature and, above the fictive temperature of this 20-million-year-old glass, this is an upper bound to the equilibrium relaxation time. The results deviate dramatically from the expectation of classical theory and are consistent with some modern ideas, in which the diverging timescale signature of complex fluids disappears below the glass transition temperature.
We investigated the spectra of single tetra-tert-butylterrylene (TBT) molecules in the amorphous matrix poly(isobutylene) (PIB). The distribution of line widths of TBT in PIB was measured and compared to that of TBT in poly(ethylene). The fluorescence intensity autocorrelation function as well as the two-point frequency autocorrelation function were determined for different single TBT molecules. Logarithmic-like decays of the fluorescence autocorrelation function could be reproduced by assuming a 1R fluctuation rate distribution for the two-level-tunnelling systems.
Pressure- and temperature-dependent photon echo results are obtained for pentacene doped polymethyl methacrylate (PMMA). A unique pressure effect is observed in which the optical dephasing rate increases as the pressure is increased from ambient pressure to 4 kbar, above which the optical dephasing rate is pressure independent up to 43 kbar. The present results are also compared with pressure- and temperature-dependent photon echo results for rhodamine 101 in PMMA, in which the optical dephasing rate was completely insensitive to pressure over the range 0 to 30 kbar. A negative correlation is also observed between the optical dephasing rate and the spectral hole burning efficiency. Line broadening due to pressure induced spectral diffusion may be responsible for both the increased dephasing rate and the reduced spectral hole-burning at high pressure. © 1997 American Institute of Physics.
A joint analysis of spectroscopic data obtained at liquid–helium temperatures by three line-narrowing techniques, photon echo (PE), persistent hole burning (HB), and single molecule spectroscopy (SMS), is presented. Two polymer systems, polyisobutylene (PIB) and polymethylmethacrylate (PMMA), doped with tetra-tert-butylterrylene (TBT) were studied via PE and HB techniques and the results are compared with literature data [R. Kettner et al., J. Phys. Chem. 98, 6671 (1994); B. Kozankiewicz et al., J. Chem. Phys. 101, 9377 (1994)] obtained by SMS. Both systems behave quite differently. In TBT/PIB a rather strong influence of a dispersion of the dephasing time T2 was found which plays only a minor role in TBT/PMMA. We have also measured the temperature dependence of T2 for both systems in a broad temperature range (0.4–22 K). Using these data we separated the two different contributions to the optical dephasing — due to an interaction with two-level systems and due to coupling with local low-frequency modes. The data are compared with calculations using a numerical and a semianalytical model in the presence of a large dispersion of the single molecule parameters. Furthermore, we discuss the differences of the linewidths as measured by different experimental methods. © 1998 American Institute of Physics.
We show that a linear specific heat at low temperatures for glass follows naturally from general considerations on the glassy state. From the same considerations we obtain the experimentally observed anomalous low-temperature thermal conductivity, and we predict an ultrasonic attenuation which increases at low temperatures. Possible relationships with the linear specific heat in magnetic impurity systems are pointed out. We suggest experimental study of the relaxation of thermal and other properties.
We present a theoretical framework for analyzing the distribution of optical line shapes in low-temperature glasses, as measured by single-molecule spectroscopy. The theory is based on the standard tunneling two-level system model of low-temperature glasses and on the stochastic sudden jump model for the two-level system dynamics. Within this framework we present an explicit formula for the line shape of a single molecule and employ Monte Carlo simulation techniques to calculate the distribution of single-molecule line shapes. We compare our calculated line-width distributions to those measured experimentally. We find that the two-level system model captures the features of the experimental line-width distributions very well, although there are discrepancies for small line widths. We also discuss the relation of single-molecule line-shape distributions to more traditional “line shapes”, as measured by hole-burning and photon echo spectroscopies. Using the results from our analysis of the single-molecule line-width distributions, with no adjustable parameters we can compare theoretical predictions with experiment for photon echo decay times and hole widths. In general, the agreement is good, providing further evidence that the standard tunneling model in glasses is basically correct. For two systems, however, theory and experiment do not agree quantitatively.
We give a short overview of the selective spectroscopy of organic molecules in solid solutions, starting from Shpol'skii matrices up to single molecule spectroscopy. We discuss the general principles of selectives and different applications of this technique to molecular and solid-state studies. We examine in more detail two new fields to which we have contributed: persistent spectral hole burning in Langmuir-Blodgett (LB) films and the study of individual molecules. We show how persistent spectral hole burning provides information about structure and dynamics of LB films and how energy transfer can be studied in concentrated films. We probed the dynamics of the LB matrix as a function of the depth of the dye in a multilayer. We show that the surface monolayer presents specific dynamics, which we attribute to the long hydrophobic chains. The shift and broadening of a spectral hole under an applied electric field allows us to determine the orientation and direction of the chromophore axes. We then present the new field of single molecule spectroscopy, including the latest results. So far, the observations were made in a molecular crystal and in a polymer. We first consider the general appearance of fluorescence excitation lines and the sudden jumps of their resonance frequencies. The external electric field effects are then discussed. The correlation properties of the light emitted by single molecules give new insight about intramolecular dynamics and spectral diffusion, which would be impossible to obtain in experiments with ensembles of molecules. We demonstrate how single molecule spectroscopy gives truly local information, eliminates averages and populations, and gives access to distributions of molecular parameters in solids.
It is shown that many center excitations are responsible for the universal low energy spectral properties in an arbitrary ensemble of defect centers with an internal degree of freedom. Universality means a quasiuniform distribution of the energy and the logarithm of the tunneling amplitude together with a disappearance of the dependence on the primary defect parameters.
Linewidth distributions for single terrylene molecules in polyethylene have been measured in the temperature range from 30 mK to 1.83 K. The temperature dependence of the average linewidth is best described by a linear relationship over the full temperature range. At 30 mK, the linewidth distribution has a full-width at half-maximum of &18.6 MHz and an average linewidth of 42.8(6) MHz. 2000 Elsevier Science B.V. All rights reserved.
By means of single molecule (SM) spectroscopy we investigated elementary matrix excitations in a disordered solid, i.e., quasi-localized low-frequency vibrational modes (LFMs). To this end we recorded the spectra of single tetra-tert-butylterrylene molecules embedded in an amorphous polyisobutylene matrix in a temperature region, where the LFM contribution to line broadening dominates. The individual parameters of LFM in a polymer glass can be determined from the temperature-dependent linewidths of single molecules. The magnitude of the LFM contribution to SM spectra was obtained by the statistical analysis,of the distribution of linewidths of SMs. Pronounced distributions of LFM frequencies and SM-LFM coupling constants were found. This result can be regarded as the first direct experimental proof of the localized nature of LFMs. (C) 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weimheim.
Spectra of single tetra-tert-butylterrylene molecules incorporated in purely amorphous polyisobutylene matrix have been measured at 2, 4.5 and 7 K (244, 381 and 187 molecules, correspondingly). This is a temperature region, where the main assumptions of the standard two-level system (TLS) model of low-temperature glasses begin to be not valid. At T = 2 K the main parameters of most of the registered spectra were found to be consistent with the standard TLS model. At T = 4.5 and 7 K some deviations from predictions of this model were observed. The detailed analysis reveals that increasing of temperature leads to additional, in comparison with the predictions of the standard TLS model, line broadening of spectral peaks. This additional line broadening was attributed to the influence of quasi-local low-frequency modes (LFMs) of the amorphous matrix in system under study at T = 4.5 and 7 K. Distributions of single spectral peak widths of the detected spectra (the line width distributions) have been calculated and compared with the line width distributions simulated for the same system. Comparative analysis of experimental and simulated distributions allows to evaluate the value of LFM contribution at 4.5 and 7 K. The single molecule spectroscopy data were compared with the literature values of inverse optical dephasing times, 1/piT(2), as measured for the same system by photon echo (S.J. Zilker et al., J. Chem. Phys. 109 (1998) 6780). (C) 2003 Elsevier B.V. All rights reserved.
According to the modern conception, the dynamics of amorphous solids in the intermediate interval of low temperatures (from a few up to dozens of Kelvins) is determined mainly by quasi-localized low-frequency vibrational modes (LFMs). Up to now, it is known very little about a nature and properties of these excitations in disordered solids. High-selective laser spectroscopy of impurity centres, embedded to transparent disordered matrix as a probe, is a very powerful method for deriving information about LFMs. In the two presented papers the results of our photon echo (PE) and single molecule spectroscopy (SMS) studies of LFMs in organic amorphous solids are discussed. In the first part we review the recent results of our PE-studies. Two cases are analyzed: (a) a coupling of chromophores with a continuous broad spectrum of LFMs, which shape was taken from light scattering experiments, and (b) a coupling of chromophores with continuous LFMs spectra, calculated on the base of the soft potentials model. In the second part we consider the results of our studies of LFMs in a glassy polymer on microscopic level using SMS. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
Laser spectroscopic techniques at low temperature, such as fluorescence line-narrowing and hole burning, enable an increase
of spectral resolution by a factor of 103–105 compared to conventional spectroscopy at room temperature. With these methods, it is possible to retrieve a fingerprint of
the species involved and to measure the rates of dynamic processes that normally remain hidden in the broad absorption bands.
A few applications carried out in our laboratory will be discussed: (1) the determination of energy transfer rates in the
peripheral LH2 complex of purple bacteria; (2) the study of spectral diffusion and its implications in three types of systems:
(a) the B820 and B777 subunits of the LH1 complex of purple bacteria, (b) the photosystem II reaction center (PSII RC) and
CP47 antenna complex of green plants, and (c) an organic glass doped with bacteriochlorophyll a; (3) the unraveling of 0-0 transitions and the pathways of photoconversion between a number of conformations of the green
fluorescent protein mutant S65T; (4) the measuring of electron-phonon coupling strengths in PSII RC and the red fluorescent
protein DsRed; and (5) the determination and comparison of the homogeneous linewidths and optical dephasing in photosynthetic
chromoprotein complexes and autofluorescent proteins.
A linear temperature dependence of the specific heat in amorphous solids at very low temperatures is shown to follow from an ionic tunneling model. Moreover, this model predicts both the observed temperature dependence and the magnitude of the thermal conductivity and also explains the anomalous results obtained for the phonon free path by means of stimulated Brillouin scattering.
The concept of “homogeneous” spectral linewidths in doped amorphous solids and some possibilities to separate the linewidth parts related to the optical dephasing and spectral diffusion (SD) are discussed. The results of the model calculations of the photon echo (PE) decay under conditions of a large dispersion of homogeneous linewidths are presented. The deviations of the PE decay from an exponential due to this dispersion are discussed. The experimental data on incoherent PE in the terrylene/polyethylene system are presented and compared with literature single molecule spectroscopy (SMS) data. It is shown that in this case the dephasing time dispersion plays an important role for the linewidth distribution in SMS. A simple method for separation of the dephasing and SD linewidths in the SMS, based on intensity saturation effects, is considered. The facilities of this method using some SMS data are demonstrated.
The study of a new dye-matrix system-quickly frozen ortho-dichlorobenzene weakly doped with terrylene--via single-molecule (SM) spectroscopy is presented. The spectral and photo-physical properties, dynamics, and temperature broadening of SM spectra at low temperatures are discussed. The data reveal a broad inhomogeneous distribution, which indicates a high degree of matrix inhomogeneities, but at the same time, huge fluorescence emission rates and extraordinary SM spectral and photochemical stability with almost complete absence of blinking and bleaching. These unusual properties render the new system a promising candidate for applications in photonics, for example, for delivering single photons on demand.
Measurements of the elastic and inelastic neutron scattering from vitreous silica in the frequency range 0.3 to 4 THz and with scattering vectors in the range 0.2 to 5.3 Å-1 are analyzed in conjunction with heat-capacity measurements on the same samples to provide a microscopic description of low-frequency vibrational modes. The results show that additional harmonic excitations coexist with sound waves below 1 THz, and that these excitations correspond to relative rotation of SiO4 tetrahedra.
The Pb(Zr,Ti)O3 (PZT) disordered solid solution is widely used in piezoelectric applications owing to its excellent electromechanical properties. Six different structural phases have been observed for PZT at ambient pressure, each with different lattice parameters and average electric polarization. It is of significant interest to understand the microscopic origin of the complicated phase diagram and local structure of PZT. Here, using density functional theory calculations, we show that the distortions of the material away from the parent perovskite structure can be predicted from the local arrangement of the Zr and Ti cations. We use the chemical rules obtained from density functional theory to create a phenomenological model to simulate PZT structures. We demonstrate how changes in the Zr/Ti composition give rise to phase transitions in PZT through changes in the populations of various local Pb atom environments.
We present a single molecule fluorescence study that allows one to probe the nanoscale segmental dynamics in amorphous polymer matrices. By recording single molecular lifetime trajectories of embedded fluorophores, peculiar excursions towards longer lifetimes are observed. The asymmetric response is shown to reflect variations in the photonic mode density as a result of the local density fluctuations of the surrounding polymer. We determine the number of polymer segments involved in a local segmental rearrangement volume around the probe. A common decrease of the number of segments with temperature is found for both investigated polymers, poly(styrene) and poly(isobutylmethacrylate). Our novel approach will prove powerful for the understanding of the nanoscale rearrangements in functional polymers.