Pseudolocal Mode Research Articles

Optical dephasing of organic dye molecules doped in cross-linked polyvinyl alcohol derivatives: Incoherent photon echo and hole-burning studies

Optical dephasing of the zero-phonon line (ZPL) of organic dye molecules doped in polyvinyl alcohol (PVA) and several cross-linked PVA derivatives was studied by using the incoherent photon echoes and the photophysical persistent hole-burning. It was found in the incoherent photon echo measurements that optical dephasing time of the ZPL increases with increasing the length of a cross-linker introduced to the PVA backbone. The difference in the temperature dependence of the dephasing time was also observed between a dye doped in PVA with and without the cross-link. When a longer cross-linker was introduced, the phonon sideband component in the incoherent photon echo signal became dominant in contrast to that in PVA with a relatively short cross-linker, while the dephasing time of the ZPL was still longer than that in PVA without the cross-link. Hole-burning studies showed that the phonon sideband spectrum of a dye doped in the cross-linked PVAs is essentially similar to that in PVA. However, the depth of the zero-phonon hole burnt with the same burning power was found to depend on the PVA hosts with the cross-linkers. These our findings suggest that the introduced cross-link does not alter the electron–phonon coupling modes or pseudo-local mode, but it drastically reduces the effect of the two-level tunneling systems in PVA on the optical dephasing of a doped dye. As a result, the dephasing time of the dye in the cross-linked PVAs becomes longer than that in PVA without the cross-link. The decrease of the optical dephasing by introducing the cross-link can be interpreted by assuming a void space in PVA, which is created near to the cross-linker. In addition, absorption and fluorescence spectra in these systems were measured, which imply the occurrence of weak dye–dye interaction when the dye is doped in PVA with a much longer cross-linker.

Ultrafast dephasing of Resorufin in d-ethanol glass from 17 to 40 K studied by incoherent photon echo

The temperature dependence of the electronic dephasing of Resorufin in d-ethanol glass was measured from 1.7 to 40 K by incoherent photon echo (IPE) techniques. At T = 8–11 K good agreement between IPE and two-pulse picosecond photon echo (PPE) results was observed. For T 11 K confirm the hypothesis that high-temperature dephasing in this system is due mainly to interaction with a pseudolocal mode.

Homogeneous lineshapes and shifts of the b 1∑ + ← X 3∑ − transition in matrix-isolated NH: comparison with quadratic coupling theory

The lack of linear electron-phonon coupling in the b 1∑ + ← X 3∑ − transition of matrix-isolated NH in rare gases gives rise to narrow rotational lines without phonon sidebands. Asymmetric homogeneous broadening and a systematic shift to higher frequencies of the QP (0) ZPL in solid Kr has been studied systematically in the range 5–40 K. An improved version of a recently proposed non-perturbative theory of quadratic vibronic coupling is presented, and applied to the experimental result. Assuming coupling to a single low-frequency pseudolocal mode of 10 cm −1, and a state-to-state frequency change of 1.5 cm −1, excellent agreement between observed and calculated T-dependent lineshapes has been obtained. In particular, the increasing asymmetry of the line, which is purely Lorentzian at low temperature, is reproduced quantitatively.

Solvation shell effects and spectral diffusion: Photon echo and optical hole burning experiments on ionic dyes in ethanol glass

Results of picosecond photon echo and optical hole burning experiments are reported for four ionic dyes in ethanol glass. At low temperatures, the dephasing times deduced from the hole widths are as much as nine times shorter than those measured by the two-pulse echo because of the effect of spectral diffusion. The temperature dependences found are of the form aTα+b exp (−ΔE/kT) due to glass two level system dynamics (T<4 K) and a process that activates exponentially at higher temperatures, possibly from a pseudolocal mode or glass optical phonon. Comparing the ratios of echo to hole burning measured dephasing times for the four dyes suggests that the dephasing is influenced by the existence of distinct local ethanol solvation shells in addition to the dynamics of the bulk solvent. A theoretical description of solvent shell effects is achieved through the use of a two spatial domain model of the glass dynamics. Calculations of dynamic perturbations from distinct solvation shell and bulk solvent regions show that the observed differences between the dyes’ dephasing ratios can be explained if the ionic chromophores alter glass dynamics locally.

Pseudolocal modes of guest molecules in mixed molecular crystals: Photon echo experiments and computer simulations

The temperature-dependent optical dephasing of anthracene, 9-methylanthracene, and 2-methylanthracene monomers in phenanthrene host crystals has been measured using photon echo experiments. Despite large linear electron–acoustic phonon coupling, all three systems dephase because of coupling to pseudolocal modes (local motions of the guest molecule). Computer simulations of the three systems calculate the pseudolocal mode eigenvalues and eigenvectors. In contrast to previous discussions in the literature which describe pseudolocal modes as librations, the predicted eigenvalues are in reasonable agreement with the measured pseudolocal mode energies. The predicted eigenvectors are combinations of translational motion along the long molecular axis and rotational motion about the out-of-plane axis of the guest. Differences in site energies for various locations and orientations of the methyl group are calculated.

Solute-solvent dynamics and interactions in glassy media: Photon echo and optical hole burning studies of cresyl violet in ethanol glass

Photon echo and non-photochemical hole burning experiments on cresyl violet in ethanol glass are reported. At 1.5 K the optical dephasing time measured by the picosecond echo is eight times longer than the time measured by hole burning because of the difference in the time scales associated with the measurements. The observed temperature dependence arises from the glass's two-level systems and from a pseudolocal phonon mode at higher temperatures. Comparing the ratios of the echo to hole burning dephasing times for cresyl violet and resornfin, suggests that their dephasing is influenced by the nature of their ethanol solvation shells in addition to the dynamics of the bulk ethanol.

Dynamics in low temperature glasses: Theory and experiments on optical dephasing, spectral diffusion, and hydrogen tunneling

Temperature dependent photon echo (PE) and nonphotochemical hole burning (NPHB) measurements are reported on resorufin in three organic glasses: ethanol (1.5–11 K), glycerol (1.1–25 K), and d-ethanol (1.5–11 K). In all cases, the NPHB results are broadened considerably from the PE results at low temperatures, but the two measurements coalesce at high temperatures. The temperature dependences are found to deviate from the power law dependence expected for two-level system dephasing, and the deviation is attributed to dephasing by a pseudolocal mode. The appropriate correlation functions for PE and hole burning experiments are shown to be different from each other. They also differ from the correlation function for the optical absorption (OA) experiment, which has been the basis for most calculations of optical dephasing in glasses. The broadening of hole widths beyond the PE result is shown to be a measure of the slow spectral diffusion processes in the glass. Other types of dephasing measurement are also analyzed and each measurement is shown to be sensitive to spectral diffusion to a different degree. By making standard assumptions about glass dynamics, the main experimental results can be accounted for. A long range chromophore–glass interaction and a 1/R distribution of relaxation rates R at short times are indicated.

Homogeneous optical dephasing and line broadening processes in an organic glass: comparison of the temperature dependences of picosecond photon echo and hole burning experiments

The homogeneous optical dephasing times ( T 2) and the non-photochemical hole burning (NPHB) linewidths of resorufin in glycerol glass are reported from 1.1 to 25.5 K. At low temperature, the linewidths obtained from NPHB are substantially wider than the linewidths (1/π T 2) obtained from photon echo measurements, but the two become equal at high temperatures. The extra broadening of the NPHB is found to increase as T 0.8. A model of homogeneous dephasing including both two-level system and pseudolocal mode contributions is successful over the entire temperature range.

Quantum dynamics of the vibrational relaxation following the electronic transition in centers with quadratic vibronic interaction

The temporal evolution of the distribution of the configurational coordinate in the excited electronic state of an impurity centre of a crystal, allowing for the process of pulse photoexcitation, is considered. Resonance absorption band is supposed to be broad (in comparison with the mean phonon frequency). Lattice vibrations are regarded in harmonic approximation and the quadratic vibronic interaction is taken into account. In this model relaxation is caused by the dephasing of phonons conditioned by their dispersion. Formulas have been obtained relating the relaxation characteristics regarded with the dynamical Green's function. The present solution takes exactly into consideration the effect of mixing normal modes at electronic transition on the behaviour of mean value of configurational coordinate and its fluctuation characteristics. The results of the theory are applied to the relaxation of vibrations described by one pseudolocal mode whose equilibrium position and frequency change in electronic transition.

Transient Spectrum, Intensity, and Intensity Correlation of the Resonance Fluorescence from an Impurity Atom in a Solid

AbstractThe influence of the linear electron‐phonon coupling on the resonance fluorescence light of an impurity atom in solids is studied in the limits of low temperature and resonant excitation of the zero‐phonon transition. It is shown that with respect to the total intensity the electron‐phonon coupling gives rise only to a Debye‐Waller factor. The transient spectrum is studied taking into account the effect of the spectral apparatus. Further, it is shown that the intensity correlation can be modified substantially provided that the electron is coupled to a pseudolocal phonon mode of very narrow bandwidth.

EPR Investigation of Mo5+ in CaMoO4

AbstractIn single crystals of CaMoO4 a Mo5+ centre is investigated. The g‐tensor values are gx = 1.973, gy = 1.946, and gz = 1.845. Using the g‐values and results of optical measurements the ground state is derived and a centre model is suggested. The temperature‐dependent line broadening is caused by spin—lattice relaxation which is determined by a pseudolocal mode.

Role of pseudomolecules in vibrational relaxation in solidsa)

A quantum-mechanical theory of the participation of pseudolocal modes in vibrational energy relaxation (VER) of molecules in solids is developed. A simple closed expression for the VER rate constant is derived in terms of the microscopic properties of the system. This expression is used to explore the role of local translational modes in a one-dimensional model. The results of model calculations are consistent with experimental findings for the system Cl2 isolated in rare-gas matrices.

Structure of the infrared absorption bands and the phonon sidebands of SH‐impurity molecules in potassium halide crystals

AbstractThe structure of the SH− stretching hand and its phonon sidebands in KCl, KBr, and KI crystals from 2 to 300 K has been studied. A splitting of the stretching bands was observed at low temperatures into three or more components that may be attributed to the formation of the SH− clusters. The intensity distribution of the sidebands corresponds, in general, to the phonon density of states of the pure host lattice. A local gap mode was observed in the phonon spectrum gap region at the frequencies 95 and 78 cm−1 in KBr and KI, respectively. The sharp maxima at 136 cm−1 in KI and at 158 cm−1 in KBr, which are situated in the lattice vibration region, were related with the pseudolocal modes.

Lowest Singlet State of p-Chloroaniline in Pure and Mixed Crystals

The lowest 1B2 state of p-chloroaniline has been examined in neat and mixed crystals at 4.2°K. The coupling of the p-chloroaniline electronic state to host phonon modes varies greatly depending on the host. Weak coupling has been observed for a durene host, and stronger coupling with the excitation of pseudolocal phonon modes has been observed with a biphenyl host. The change in dipole moment on excitation to the 1B2 state was +1.26 D. In the neat-crystal spectra large vibronic effects, not present in the mixed-crystal spectra, have been found. These effects, along with a strong depolarization of the vibrational bands in the spectra, indicate a possible vibronic interaction among the factor group states.

On the Theory of Hot Luminescence and Resonant Raman Effect of Impurity Centres

AbstractFor an impurity centre the resonant secondary radiation spectrum, consisting of luminescence, Raman scattering, hot luminescence, and interference terms, is studied at low temperatures using the model of one local or pseudolocal mode, active in the electronic transition and taking into account the change of the equilibrium position and frequency of this mode. It is shown that the latter plays an essential role in the problem of the classification of the resonant Raman scattering and hot luminescence. The anti‐Stokes region of the spectrum is analysed in detail for three cases of excitation: monochromatic, “white”, and intermediate excitation. The influence of the interference of different relaxation channels on the intensity, width, and shape of the lines is considered.

Vibrational Structures Accompanying the Optic Transitions of Bound Electrons in Crystals

A description of the phonon processes involved in the optic transitions of a bound electron in crystals is presented, with the specific properties of the electron-phonon interactions considered in detail. The set of forces that the "hole-electron pair" exerts on the surrounding lattice are used as coupling constants, and the projected density of states for the perturbed phonon field is fully explained. The orbit radii for the optic electron are assumed to be of the order of, or smaller than, the lattice parameter, and both dipole-allowed and electric-dipole phonon-forced transitions are considered. Short- and long-range forces are examined. The connection between short-range forces and the stress coefficients of the absorption (emission) band is found. The symmetry properties of the electron wave functions are further taken into account in the analysis of electrostatic types of interactions. It is shown that the electric dipole forces activate essentially the normal modes of the perfect lattice, while the short-range forces activate more easily the possible local and pseudolocal modes. An explanation is also suggested for the intraconfiguration transition of rare-earth ions not activating the pseudolocal modes. The Huang-Rhys parameter is split into local (pseudolocal) and continuum-mode terms. It is shown that the continuum-mode term prescribes how the total intensity of the band is shared among multiple-phonon lines (if present) and broad background absorption. The configuration-diagram description of the many-phonon process is subsequently considered, including the exciton absorption. An interpretation is presented of the Stokes shift between absorption and emission bands: The whole Stokes shift is split into a purely Stokes term and a stored-energy term, the latter being related to the infrared component P(r) of the polarization field. This purely Stokes term, as well as the phonon contribution to the peak position, is evaluated for the $F$ band, within the linear and quadratic approximations for the electron-phonon interaction. Finally, a qualitative explanation is suggested for the Urbach rule in Perovskite-type crystals.

Theory of the vibrational structure of the Raman spectra of impure crystals

1. If the frequency of the incident light is considerably closer to the frequencies of impurity centres than to those absorbed by the pure crystal, the vibrational structure of the Raman spectra may be appreciably determined by the local dynamics of the lattice and the vibronic interaction in the impurity centre. For lack of the first order Raman spectrum of the pure crystal the matrix conditions of observing this vibrational structure are favourable in systems such as impure alkali-halide crystals. 2. It is shown that the Raman spectra would have in the Born-Oppenheimer adiabatic approximation quasi linear vibrational structure analogous to that of the absorption (luminescence) spectra : in the absence of local modes the spectrum consists of a sharp no-phonon line (the Raman analogue of the Mossbauer line) and of a considerably wider phonon band (the Raman analogue of the phonon wing of the Mossbauer line) ; a local (or pseudo-local) mode causes in addition to this aspect a series of vibrational lines. 3. For various models of the impurity centre the intensity distribution and temperature dependence of the vibrational structure are calculated. If the frequency of the incident light is close to the maximum of the impurity absorption band and the Stokes-shift is appreciable, the distribution of the intensity in a vibrational series is no longer monotonic and the maximum intensity will now belong to a vibrational line whose frequency differs from the frequency of the incident light by nω, where ω is the frequency of the local mode and n is an integer, n > 1. 4. The theoretical conclusions are compared with the experimental results of Stekhanov and Eliashberg on the Raman spectra of impure alkali-halide crystals.