Burning Kinetics Research Articles

Nonexponential hole burning in Sm 2+ doped mixed crystals

Kinetics of nonexponential hole burning in Sm 2+ doped BaFCl 0.5Br 0.5 is investigated at 77 K. Recombination in two-photon hole burning and tunnelling in single-photon hole burning are considered to result in the decrease of the hole-burning rate with the increase of hole depth.

Temperature-annealing-cycling hole spectra of dye and dye-oligonucleotide doped in polymer films

Temperature-annealing-cycling hole spectra are introduced to examine the hole burning kinetics of dye and dye-oligonucleotide in polymer films. When a chromophore binds to a large group of oligonucleotide, the burning efficiency decreases and the electron-phonon coupling strength increases.

A relationship between the kinetics of persistent spectral hole burning and its mechanism. Experiments with porphyrin dications

Analytical results are obtained for the non-dispersive and dispersive kinetics of persistent hole burning. Model calculations are used to interpret some hole burning data and new experiments with protoporphyrin salt, a system involving electron transfer.

Investigation of dispersive kinetics of spectral hole burning

Investigation of dispersive kinetics of spectral hole burning

Modelling of unsteady combustion regimes for polydisperse fuels—I. Instability and auto-oscillations

A general theory of instability and self-oscillations accompanying combustion of polydisperse assemblages of particles is elaborated on the basis of a model including heat and mass balance equations and the kinetic equation governing the particle size distribution. At arbitrary burning kinetics of particles and the particle influx rate they are reduced; under the condition of ideal mixing, to only two functional integrodifferential equations for the dimensionless temperature and oxidant concentration. Steady combustion processes are shown to be unstable in many important situations. Depending on physical and regime parameters, the break of stability can be realized in either ‘soft’ or ‘hard’ manner giving rise to establishing a regular auto-oscillating or a chaotic pulsating regime, respectively. The properties of possible slightly non-linear auto-oscillations are studied in greater detail.

Dispersive kinetics of nonphotochemical hole burning and spontaneous hole filling: Cresyl Violet in polyvinyl films

The dispersive kinetics of nonphotochemical burning and spontaneous filling of the zero-phonon hole of Cresyl Violet in polyvinyl alcohol at 1.6 K are analyzed in terms of the standard external two-level system (TLSezt) model for probe–glass systems and a distribution function for the tunnel frequency derived from a normal distribution function for the tunnel parameter λ. Average values for the relaxation rates for burning and filling are determined. It is shown that the dominant mechanism for filling is not global spectral diffusion but rather antihole reversion. A high degree of positive correlation between the rates of burning and filling associated with the TLSext is found. A new methodology that permits a more physically reasonable interpretation of spontaneous hole-filling kinetics is described. It is based on the hypothesis that only a fraction of burned sites, on reversion to the ground state, yield sites with resonance frequencies that lie within the hole profile.

Hole burning spectroscopy of tetraphenylporphyrins

The persistent spectral hole burning technique has been applied to free base and central metal tetraphenylporphyrins with different substituents. The upper limits of the singlet-singlet homogeneous widths determined from the observed persistent holes have been correlated with the fluorescence lifetimes at low temperatures. Hole burning kinetics and mechanisms of both free base and central metal pigments are discussed.

Optical and transport nonlinearities in laser excited semiconductors

In this paper we present the progress that has been made in the theoretical description of laser excited semiconductors resting on the use of nonequilibrium Green's functions. This technique permits consideration of several peculiarities (nonequilibrium character of carrier distributions, memory effects and dephasing of the interband polarization, screening of interactions, temporal and spatial evolution of excitations) simultaneously and on an equal microscopic footing. Applications to spectral hole burning and saturation kinetics under stationary conditions as well as to the propagation of ultrafast pulses are given.

Dispersive kinetics of nonphotochemical hole growth for oxazine 720 in glycerol, polyvinyl alcohol and their deuterated analogues

Dispersive hole growth kinetic data obtained with burn intensities in the nW to μW/cm 2 range are reported for the laser dye oxazine 720 in glycerol glasses and polyvinyl alcohol polymer films. The data are well described by a theory that employs a distribution function for the hole burning rate constant derived from a Gaussian distribution for the tunnel parameter of extrinsic two-level systems. The linear electron-phonon coupling is taken into account in the analysis. An approach for calculating the nonphotochemical hole burning quantum yield as a function of the hole depth is described. Deuteration of the hydroxyl proton of the amorphous hosts reduces the average hole burning quantum yield by a factor of ≈ 20, indicating that large amplitude motion of this proton is an important component of the tunneling coordinate. Temperature-dependent data are used to prove that the dispersive hole burning kinetics are independent of temperature between 1.6 and 7 K (the range studied).

Spectral hole burning of model photosynthetic systems

Basic problems in the primary processes of photosynthesis, interactions between pigments, proteins and quinones, as well as processes of energy transfer and electron transport in the primary events were studied using low temperature optical spectroscopy (hole burning technique). The obtained results (homogeneous widths of optical transitions and hole burning kinetics) were correlated on both model and in vivo photosynthetic systems.

Kinetics of photochemical hole burning in the absorption spectra of molecules in matrices

The relation between the homogeneous absorption spectrum of impurity centers and the hole photochemically burned by monochromatic laser radiation in an inhomogeneous absorption spectrum is studied. The changes, due to the burning, of the zero-phonon is studied. The changes, due to the burning, of the zero-phonon hole contour are analytically calculated for any ratio of the laser and the zero-phonon lines. The model calculations are confirmed by experiments with the protoporphyrin dictation in an amorphous matrix at helium temperature.

FCC Regenerator flow model

Catalyst flow patterns in FCC cross-flow and swirl-type dense-bed regenerators affect regenerator efficiency. A two-dimensional, pseudo-homogeneous flow model describing such flow patterns is developed. The predicted catalyst flow patterns for swirl regenerators indicate catalyst bypassing and recirculating zones. More severe bypassing is predicted for cross-flow regenerators, the severity of bypassing increases with increasing Reynolds number. The calculated flow patterns are combined with a radial mixing-cell model to account for radial mixing in fluidized beds. Radial mixing rates in commercial size regenerators counteract somewhat the effect of bypassing. Model calculations coupling coke burning kinetics with the flow model, however, show that bypassing leads to non-uniform coke levels and oxygen breakthrough. This modeling technology is used to optimize FCC regenerator operation.

Model for burning kinetics and shape of fluence-saturated spectral holes

We propose a simple model for the study of fluence-broadened holes. The model is based on a strong dispersion of the burning rates and the burning process takes place over several decades of fluence. In this so-called semi-saturated regime, the hole depth varies almost linearly with the logarithm of the fluence and the hole width increases as the power 0.25 of the fluence, i.e. exponentially with the hole depth. A careful study of semi-saturated holes could allow us in principle to determine the homogeneous width free from the contribution of spectral diffusion. The hole shape is also discussed and can often be fitted either by Lorentzian or by logarithm of Lorentzian profiles. Several examples of semi-saturated holes can be found in previous works in the literature. As a further instance, we present our results on free-base tetraazaporphin in a Langmuir-Blodgett film. The reasonable agreement with the model allows us to evaluate the rate dispersion in this system and to ascribe it to barrier height fluctuation induced by the disordered molecular surrounding.

Time dependence of hole spectrum due to dispersive burning kinetics in dye-doped polymers

Time evolution of a persistent hole spectrum has been investigated for dyes doped in polyvinyl alcohol films at 2 K. The zero-phonon hole profile measured for resorufin using a single-mode dye laser has been found to be nearly Lorentzian even in the case of relatively deep holes. The observed fluence-dependent depth and width of the zero-phonon hole are reproduced well by the numerical simulation based on a dispersive burning-kinetics model. It is pointed out that a large distribution of the burning rate is responsible for the fluence-dependent hole broadening. Time evolution of the phonon-sideband hole simulated using the same model is also compared with the experimental data. It is shown that the low-energy sideband hole is enhanced when the burning kinetics is dispersive.

Kinetics of growth and thermally induced recovery of spectral holes in a cresylviolet/glycerol glass

Kinetics of burning and recovery of spectral holes in a cresylviolet/glycerol glass have been investigated in the temperature range 4.2 to 60 K. Thermally induced hole filling has been studied by probing hole profiles after exposing the sample to a sequence of heat pulses of variable intensity. The data can be fitted on the basis of a two-site model invoking a dispersive non-thermal forward reaction and an activated recovery reaction, the activation energy being subject to a Gaussian distribution function of width 1500 cm−1 and centered at E=0. The model is in accord with a general framework proposed recently for rationalizing dynamic glass properties.

Saturation effects on the hole shapes produced by photochemical hole burning: Application to the anthracene–tetracene photoadduct

The kinetics of photochemical spectral hole burning are modeled considering low temperature photochemical mechanisms requiring both one and two photons. The calculated results are compared with experimental results on photochemical hole burning in the anthracene–tetracene photoadduct (AT) using single color irradiation. Better agreement is obtained for the two photon mechanism while assuming the second photon is absorbed by the lowest triplet state of AT. The effects of irradiation source bandwidth and polarization are included in the analysis. Photochemical and transient saturation effects are evident in the calculated hole shapes.

Effects of dispersive burning kinetics on hole-burning spectrum in dye-doped polymers

The profile of the burnt zero-phonon hole, as well as the time dependence of the hole depth, has been measured at 2 K for rhodamine 640 in a polyvinyl alcohol film by means of a highly sensitive spatial-modulation technique. The result is well explained by a dispersive first-order reaction model in which the homogeneous width of the zero-phonon line and the finite spectral width of the laser light are taken into account. It is shown that the dispersive burning kinetics causes not only the non-exponential hole growth but also on the time-dependent spectral broadening of the burnt hole.

Temperature-dependent kinetics of photophysical hole burning in a tetracene-doped mthf glass

Kinetics of photophysical hole burning in a MTHF(TC) glass follows the concept of decoupled first-order reactions with Gaussian distribution of the tunnelling parameter. The decrease of the hole-burning yield with increasing temperature signals thermally activated hole refilling in the excited state. The ratio of TLS states responsible for dephasing and hole burning, respectively, is estimated to be ≈ 10 7.

Explosive He4 burning: I. Kinetics of burning at constant temperature and density

The kinetics of He/sup 4/ burning at a constant temperature T> 10/sup 9/ /sup 0/K and a density rho> 10/sup 5/ g/cm/sup 3/ is considered. The regions of formation of iron group elements and lighter nuclides during He/sup 4/ burning are indicated in the rho, T plane. The dependence of the mean atomic number of the nuclides formed on the temperature and the density is determined. For the temperature T greater than or equal to4.10/sup 9/ /sup 0/K a ''neutron flash'' is found, which can lead to a change in the isotopic composition of the r- and s-process elements present. The range of rho and T in which the formation of an excess number of the elements beyond the iron peak is found.

Smoke formation from burning polystyrene and its suppression

AbstractStudies have been made of the influence on smoke generation from polystyrene of the incorporation of a number of apparently inert solids, the smoke‐producing tendency being expressed in terms of the maximum specific optical density of smoke per unit weight of polymer consumed. Of the compounds investigated, by far the most efficient smoke suppressant is pyrogenic silica, the effect of which is, over a wide range, directly proportional to the total surface area of the material introduced. Examination of partially burned polystyrene‐pyrogenic silica composites reveals the presence at the surface of a hard rigid skin, which is not observer with polystyrene alone of with polymer samples containing other additives. The formation of a protective skin is also indicated by the kinetics of forced burning of these composites where the weight loss is a linear function of log time and by measurements of the rate of change of thickness of polymer samples during combustion. Thermogravimetric experiments in which polystyrene and pyrogenic silica are heated in contact with one another show that the silica encourages the formation of a carbon residue from the polymer. A mechanism of skin production is proposed which involves the build‐up of layers of tangential spherical silica particles cemented together by cross‐linked polystyrene.