Spectral Diffusion Processes Research Articles

Electron spin–lattice relaxation in radicals containing two methyl groups, generated by γ-irradiation of polycrystalline solids

The effects of methyl rotation on electron spin–lattice relaxation times were examined by pulsed electron paramagnetic resonance for the major radicals in γ-irradiated polycrystalline α-amino isobutyric acid, dimethyl-malonic acid, and l-valine. The dominant radical is the same in irradiated dimethyl-malonic acid and α-amino isobutyric acid. Continuous wave saturation recovery was measured between 10 and 295 K at S-band and X-band. Inversion recovery, echo-detected saturation recovery, and pulsed electron–electron double resonance (ELDOR) data were obtained between 77 and 295 K. For the radicals in the three solids, recovery time constants measured by the various techniques were not the same, because spectral diffusion processes contribute differently for each measurement. Hyperfine splitting due to the protons of two methyl groups is resolved in the EPR spectra for each of the samples. Pulsed ELDOR data were obtained to characterize the spectral diffusion processes that transfer magnetization between hyperfine lines. Time constants were obtained for electron spin–lattice relaxation ( T 1e), nuclear spin relaxation ( T 1n), cross-relaxation ( T x1), and spin diffusion ( T s). Between 77 and 295 K rapid cross-relaxation (Δ M s=±1, Δ M I=∓1) was observed for each sample, which is attributed to methyl rotation at a rate that is approximately equal to the electron Larmor frequency. The large temperature range over which cross-relaxation was observed suggests that methyl groups in the radical and in the lattice, with different activation energies for rotation, contribute to the rapid cross-relaxation. Activation energies for methyl and amino group rotation between 160 and 1900 K (1.3–16 kJ/mol) were obtained by analysis of the temperature dependence of 1/ T 1e at S-band and X-band in the temperature intervals where the dynamic process dominates T 1e.

Two-color three pulse photon echo peak shift spectroscopy

Two-color three pulse photon echo peak shift spectroscopy (2C3PEPS) has been used to probe correlation in electronic transitions in two different regions of the electronic spectrum of a chromophore in the condensed phase. The 2C3PEPS can be done in an “uphill” or a “downhill” sense, where the first two interactions are of lower (higher) frequency than the final interaction with the radiation field. In both cases no correlation between the two spectral regions is observed at very short times. Different parts of the spectrum gain correlation owing to ultrafast solvent motion and the accompanying Stokes shift. We propose a model for the spectral shift that incorporates a conditional probability distribution for transition frequencies in the two spectral regions. The model qualitatively reproduces all the features of complete numerical simulations. Our results show that memory is partially conserved during the spectral diffusion process from the pump to the probe region. The downhill difference peak shift is very sensitive to the time scale of the inertial component of solvation and seems very promising for an accurate determination of this time scale.

Time-dependent fluctuations in single molecule spectroscopy: a generalized Wiener-Khintchine approach.

We consider the theory of time-dependent fluctuations in single molecule spectroscopy. In particular, the relation between stationary spectral diffusion processes and photon counting statistics is investigated. Within linear response theory, a general relation between Mandel's Q parameter and a three time dipole correlation function is obtained. For a prototype spectral diffusion process, an exact solution of Q is found which exhibits rich types of physical behaviors.

Thermally induced spectral diffusion of Rhodamine 3B in viscous polyols

Energy shifts of the ν 00 vibronic near the fluorescence maxima of Rhodamine 3B, in 1,5-pentanediol and glycerol, are studied in the range 275< T<341 K. The frequencies red shift sharply with the temperature increase. The spectral diffusion can be related with temperature induced changes from the relaxation pattern of the polyols on the basis of simple continuum calculations. The correlation of the spectral diffusion process with the time scale of dipole memory loss and with the α-relaxation process is presented. Average solvation times in glycerol are estimated using a mode coupling theory description of the solvation autocorrelation function.

Fluorescence correlation spectroscopy as a tool to investigate single molecule probe dynamics in thin polymer films.

Fluorescence correlation experiments were performed on rhodamine 6G in PDMS spin-coated films on glass surfaces. With polarised excitation, ensemble bleaching of the dye and single molecule intensity fluctuations were observed. From the statistics of single molecule intensity data taken at different positions in the film, correlation functions were calculated. Two modes of motion with exponential decay shapes and correlation times of tau(c) = 0.15 s and tau(c) = 0.7 s could be detected. Potential origins of intensity fluctuations are lateral diffusion, rotational diffusion or intramolecular fluctuations of dyes involving spectral diffusion or photoinduced processes. From the experimental results, lateral diffusion can be ruled out as a motional mode. Single molecule fluctuations are assigned to changes of the molecular configuration of the dyes, which are rigidly bound to the glass. To assess the environmental influence on such molecular motions, the bulk viscosity of the PDMS was varied over two orders of magnitude, leading to changes of tau(c) of the slow mode by a factor of four. This result proves the sensitivity of the single molecule fluctuations to the molecular scale dynamics of the surrounding polymer matrix and makes the correlation time a measure of the local environment of dye probes.

Multipolar interaction mediated spectral diffusion in disordered mixed molecular solid: A Monte Carlo study

A Monte Carlo simulation of a model of multipolar interaction mediated spectral diffusion in binary system of host and guest has been studied. The dynamics and the mechanisms of the spectral diffusion process have been investigated by monitoring the flow of energy within a Gaussian distribution of energy profile. The importance of acceptor concentration, excitation transfer topology and time dependence for the onset of spectral diffusion has been pointed out. The dependence of spectral diffusion upon the interaction strength has also been studied. The existing analytical models of isotropic energy transfer in mixed solid is found to be applicable in explaining the spectral diffusion process. Switching of spectral diffusion is interpreted as a result of the formation of a maxi-cluster of acceptors, which contains the donor.

Coherent echoes in glasses and crystals with point defects

Dielectric echo experiments have proven to be a very suitable tool for the investigation of the dynamics of tunneling states in solids. In particular, rotary echoes have been used to determine the distribution of the tunnel splitting of amorphous solids and crystalline defect systems like (KBr)1−x (KCN)x, and to investigate the tunneling motion of pairs of strongly coupled Li+ ions in KCl. Recently progress has been made in understanding the decay of spontaneous echoes in glasses which is caused by spectral diffusion processes. The expected non-exponential decay of the echo amplitude has been observed for several glasses. While the temperature dependence of the phase memory time is in good agreement with theoretical predictions, the cross-over from the non-exponential to the exponential decay regime occurs at much shorter times than expected.

Small Molecules in Hole-Burning Systems: A Local Antenna for Infrared-Induced Spectral Diffusion Processes

Abstract Light-induced spectral diffusion in photochemical hole-burning systems is investigated with respect to its dependence on the frequency of the irradiated infrared light in the wavelength range between 2 μm and 11 μm. The investigated system is polymethylmethacrylate (PMMA) containing up to one volume percent water molecules and doped with free-base phthalocyanine (H2Pc). Different resonant contributions to the infrared-induced spectral diffusion show that the microscopic origin of the observed diffusion process is a reorientation of H2O molecules. A simple kinetic model, based on the idea of two possible sites for each H2O molecule, allows to describe the dependence of the hole-broadening on infrared intensity and illumination time in a very good quantitative fashion.

Manifestation of irreversible relaxation in multiplication of binary numbers by the photon echo method

An investigation is reported of the manifestation of relaxation of optical excitation in multiplication of binary numbers by the photon echo method. Calculations are carried out on the basis of the Bloch model and two model spectral diffusion processes are taken into account. The relationships obtained are used to correct the amplitudes of sets of echo signals representing the product of two numbers and thus exclude the relaxation factor from the multiplication procedure.

Decay of spontaneous echoes in glasses

At low temperatures the phase memory time τ2 of tunneling states in glasses is limited by spectral diffusion processes. The origin of this dephasing mechanism are interactions between two-level systems which are in resonance with the applied field with other tunneling systems in the near neighborhood. Using two-pulse polarization echoes we have investigated the temperature dependence of dephasing in Suprasil I. We discuss our results in the framework of the theory of spectral diffusion.

A Monte Carlo simulation of thermally activated spectral diffusion for an energetic and substitutionally disordered system

The dynamics and the mechanism of exchange-mediated spectral diffusion processes with energetic as well as substitutional disorder have been studied employing the Monte Carlo simulation technique. The thermally activated energy transfer has been incorporated in the simulation to investigate the temperature dependent characteristics of spectral diffusion. The importance of acceptor concentration, temperature and the excitation transfer topology for the onset of spectral diffusion has been pointed out. We observed that the onset of spectral diffusion depends sharply on temperature and at a fixed temperature on excitation transfer topology, acceptor concentration, and site-selective excitation.

Direct vibrational energy transfer in zeolites

With two-color picosecond infrared laser spectroscopy the dynamics of O–H and O–D stretch vibrations in zeolites are investigated. Zeolites appear to be good model systems to study transfer of vibrational energy in a solid. For the O–D vibrations, transient spectral holes are burnt in the inhomogeneously broadened absorption bands by saturating the absorption with a strong pump pulse. From the spectral hole widths the homogeneous absorption linewidths are obtained. The excited population lifetimes are determined using a time-resolved pump–probe technique, and in combination with the homogeneous linewidth the pure dephasing time is revealed as well. For high concentrations of O–H oscillators the vibrational stretch excitations are found to diffuse spectrally through the inhomogeneous absorption band. This spectral diffusion process is explained by direct site-to-site transfer of the excitations due to dipole–dipole coupling (Förster transfer). The dependences of the transient spectral signals on oscillator concentration and the results of one-color polarization resolved experiments confirm this explanation. The spectral transients are satisfactorily described by simulations in which the site-to-site transfer by dipole–dipole coupling is taken into account.

Spectral diffusion induced by energy transfer and high-pressure effects in doped organic glasses: A hole-burning study

Time-resolved hole-burning (HB) experiments have been performed on the S 1 ← S 0 0−0 transitions of free-base chlorin in polystyrene (H 2Ch in PS) and bacteriochlorophyll-a in triethylamine (BChl-a in TEA) as a function of temperature (1.2–4.2 K), excitation wavelength, concentration, and delay time between burning and probing. At concentrations higher than 5 × 10 −4M a new type of spectral diffusion process takes place, even at T → 0, which is induced by energy transfer. Moreover, HB-experiments on BChl-a in TEA with delay times between microseconds and hours at high ( Δp ≈ 30 kbar) pressure demonstrate that a transition from a glassy- to a locally ordered structure occurs in this sample, suggesting that the number of “two-level-systems” in the glass responsible for spectral diffusion is reduced by compression.

Hahn echo experiments on a single triplet electron spin

We present results on Hahn echo experiments with a single molecular triplet electron spin. The surprising possibility of observing an echo phenomenon with an individual electron spin arises as a result of a spectral diffusion process of the magnetic resonance transition frequency in the course of the optical excitation cycle and illustrates the equivalence of time and ensemble average. The spin dephasing times as measured by the decay times of the Hahn echo for different molecules varies between 2 and 7 μs. By using standard theories of nuclear spin diffusion these differences may be attributed to variations in the local environment close to the chromophore which can be estimated quantitatively.

The effect of pressure, time and concentration in hole-burning studies

The effective homogeneous line width Γ′ hom of the S 1 ← S 0 0-0 transitions of bacteriochlorophyll-a (BChl-a) in the glass triethylamine (TEA) and of free-base chlorin (H 2Ch) in polystyrene (PS) have been studied, respectively, under the influence of high pressure (~30 kbar) and for a range of concentrations (~ 10 −4–10 −2 M). The aim was to get a better understanding of spectral diffusion and energy transfer processes in amorphous systems. The dependence of Γ′ hom on temperature (1.2–4.2 K), excitation wavelength (within the 0-0 band) and delay time (10 −6–10 5 s) was measured for both systems by means of spectral hole burning.

Spectral spin diffusion undern= 0 rotational resonance

The spectral spin diffusion process between the 31P nuclei present in the dipotassium salt of α-D-glycopyranose-1-phosphate is investigated under magic-angle spinning conditions. The 31P nuclei in such a system are characterized by rather weak direct coupling interactions in addition to exhibiting identical isotropic chemical shifts with differing orientations of the shielding tensor. First-order average Hamiltonian theory is used to provide a particularly simple physical insight into the spin diffusion mechanism, with the assumption that the 31P-31P interaction may be introduced as a small perturbation of the interaction between the 31P and 1H nuclei. The spin diffusion rate is investigated as a function of the 1H radiofrequency decoupling field. The time constants describing the spin diffusion process were obtained using a hybrid of TOSS and NOESY experiments with the simple modification of decoupling the proton influence to various extents in successive experiments by varying the effective 1H RF field ...

A Monte Carlo simulation of spectral diffusion in the presence of energetic and substitutional disorder

The isotropic exchange mediated spectral diffusion process in one- and two-dimensional systems with energetic as well as substitutional disorder has been studied by employing a Monte Carlo simulation technique. The dynamics and the mechanism of the spectral diffusion process have been investigated by monitoring the flow of energy of an excitation started at specified sites with a Gaussian distribution of the energy profile. The importance of acceptor concentration and the excitation transfer topology for the switching of spectral diffusion has been observed. The concentration-dependent excitation transport efficiency in two dimensions is found to be consistent with the percolation model.

H2-Octaethylporphin in Amorphous Polystyrene Matrix: Spectral Hole Burning and Spectral Diffusion

Abstract All experimental data available on a low‐temperature optical dephasing for H,‐octaethylporphin guest molecules in an amorphous polystyrene host are discussed in relation to spectral diffusion processes. Some data published earlier have once more been thoroughly studied and some corrections introduced, also some new data are presented. A notable dependence of the holewidth on a burning wavelength within the inhomogeneously broadened 0–0 absorption band at T = 0.05 K has been observed. The spectral diffusion is found to be actual at T = 1.45 – 7 K and is not at T< 0.05 K and at T>8 K.

Spin-fluctuation-induced optical spectral diffusion in Pr3+:YAlO3.

We report a quantitative study of spectral diffusion in a solid-state system by measuring the stimulated echo relaxation. The spectral diffusion process in ${\mathrm{Pr}}^{3+}$:${\mathrm{YAlO}}_{3}$ at low temperatures (2.0 K) is found to be dominated by the relaxation of the ``frozen core'' of nearest neighboring Al spins. The decay of the echo signal is found to be fully described by a simple spin-flip diffusion model and by a first-principle calculation. The complete understanding of the spectral diffusion process in this simple model system establishes a quantitative basis for using coherent optical techniques to study slow dynamics in complex systems.

Electron spin-lattice relaxation and spectral diffusion measurements on tyrosine radicals in proteins

The electron spin-lattice relaxation and spectral diffusion properties at 22 K have been examined for three tyrosine radicals: the stable tyrosine radical Y D + in photosystem II (PSII), the tyrosine radical in the B2 subunit of ribonucleotide reductase from Escherichia coli, and a model L-tyrosine radical prepared in a frozen glass by UV irradiation at 77 K. The relaxation transients obtained by the method of saturation recovery, where recovery from saturation following a microwave pulse is detected by a low continuous observation microwave level, were compared to those obtained by the inversion-recovery and spin-echo-detected saturation-recovery techniques. The same values for the electron spin-lattice relaxation rate 1 T 1 were obtained by saturation recovery and spin-echo-detected saturation recovery provided that spectral diffusion channels were saturated by a sufficiently long microwave pulse. The inversion-recovery spin-echo method produced relaxation transients that were more rapidly decaying than those of either of the saturation-recovery methods, owing to the presence of spectral diffusion. Two contributions to spectral diffusion were observed. For the UV-generated model L-tyrosine radical, the dominant contribution to spectral diffusion occurred on the time scale of the electron spin-spin relaxation time T 2 and was saturated with a microwave pulse lasting only 50 μs. The spectral diffusion processes for the tyrosine radicals in the proteins occurred on the time scale of T 1; a saturating microwave pulse of duration greater than 3–5 ms was required to completely saturate these spectral diffusion processes. These slow spectral diffusion processes are likely to involve electron-nuclear spin-spin interactions. It is probable that the large separation between radicals imposed by the dimensions of the protein prevents spectral diffusion paths due to interactions between tyrosine radicals on the time scale of T 2. The UV-generated l-tyrosine radical and the tyrosine radical in ribonucleotide reductase exhibited predominantly single-exponential relaxation kinetics when very long saturating pulses were employed. The tyrosine radical in PSII, however, exhibited nonexponential relaxation kinetics even at the extrapolated limit of infinitely long saturating pulses, where contributions from spectral diffusion have been eliminated. We suggest that spin-spin interactions of the tyrosine radical with neighboring paramagnetic site (s) are the source of the deviation from exponential relaxation kinetics in PSII.