Efficient Relaxation Mechanism Research Articles

The role of symmetric functionalisation on photoisomerisation of a UV commercial chemical filter.

Photoisomerisation has been shown to be an efficient excited-state relaxation mechanism for a variety of nature-based and artificial-based molecular systems. Here we report on the excited-state relaxation dynamics and consequent photostability of a symmetrically functionalised cinnamate by transient electronic absorption spectroscopy, along with complementary computational and steady-state spectroscopy methods. The findings are then discussed in comparison to 2-ethylhexyl-E-4-methoxycinnamate, a structurally related 'off the shelf' chemical filter present in commercial sunscreens with a similar absorption profile. The present study allows for a like-for-like comparison beween 2-ethylhexyl-E-4-methoxycinnamate and the functionalised cinnamate, driven by the need to enhance solar protection across both the UVA and UVB regions of the electromagnetic spectrum.

Substituent position effects on sunscreen photodynamics: A closer look at methyl anthranilate

Towards the development of a bottom-up rationale for sunscreen design, the effects of substituent position on the ultrafast photodynamics of the sunscreen precursor methyl anthranilate (MA, an ortho compound) were evaluated by studying para- and meta-MA in vacuum. Time-resolved ion yield (TR-IY) measurements reveal a long-lived S1 excited state (≫1.2 ns) for para-MA, proposed to be the result of a weakly fluorescent, bound excited state. In the case of meta-MA, TR-IY transients reveal a much faster (∼2 ns) excited state relaxation, possibly due to multiple low-lying S1/S0 conical intersections of prefulvenic character. While meta-MA may not be an ideal sunscreen ingredient due to a low ultraviolet absorbance, its comparatively efficient relaxation mechanism may constitute an alternative to common sunscreen relaxation pathways. Thus, our results should prompt further studies of prefulvenic relaxation pathways in potential sunscreen agents.

Frohlich Interaction in Compound Semiconductors: A Comparative Study

In polar semiconductor materials, LO phonons produce a macroscopic electric field, which interacts with the electrons. This coupling of long range is known as Frohlich interaction. Due to the interaction of an electron with LO phonons, a quasi-particle is formed known as polaron. The strength of this coupling is expressed by a dimensionless Frohlich coupling constant. Due to the polar coupling to LO phonons, the carriers lose their initial kinetic energy to the lattice by a very efficient relaxation mechanism in III-V bulk semiconductors. Therefore it would be interesting to examine the effect of Frohlich coupling constant on the operational characteristics of optical parametric interaction in compound semiconductor medium. Using hydrodynamic model and coupled mode theory, a theoretical model is developed to determine threshold pump field for the onset of parametric process. For the sake of numerical estimations, data of ZnS and GaAs have been used. These prominent materials are assumed to be shined by a 10.6 μm CO2 laser so that nonlinearity could be induced in the medium. It is found that material having lower coupling constant is beneficial for a cost effective parametric interaction. It is hoped that result of this paper may be useful for various industrial applications.

Superradiant Emission from a Collective Excitation in a Semiconductor.

We report an anomalous wide broadening of the emission spectra of an electronic excitation confined in a two-dimensional potential. We attribute these results to an extremely fast radiative decay rate associated with superradiant emission from the ensemble of confined electrons. Lifetimes extracted from the spectra are below 100 fs and, thus, 6 orders of magnitude faster than for single particle transitions at similar wavelength. Moreover, the spontaneous emission rate increases with the electronic density, as expected for superradiant emission. The data, all taken at 300 K, are in excellent agreement with our theoretical model, which takes into account dipole-dipole Coulomb interaction between electronic excitations. Our experimental results demonstrate that the interaction with infrared light, which is usually considered a weak perturbation, can be a very efficient relaxation mechanism for collective electronic excitations in solids.

Nonlinear relaxation and selective polychromatic lasing of confined polaritons

Polaritons have often been proposed as promising candidates for the realization of all-optical devices due to the easy manipulation and control of their density and spin. In this paper we present a relaxation mechanism for confined polaritons and its application in a device based on polariton lasing in which the incoming monochromatic beam can be channeled into several polariton lasers at different wavelengths. Polaritons are injected in a 3 $\ensuremath{\mu}$m diameter trap by an incoming beam slightly detuned with respect to a confined state, and the resulting optical bistability is then imprinted on the lower confined states through an efficient relaxation mechanism which combines phonon interaction and bosonic stimulation. Above threshold, all the confined states behave like lasers and the onset of coherence is demonstrated by spectral narrowing. Moreover, the initial polarization of the laser is conserved during the relaxation process allowing for spin logic operations. In this paper we demonstrate that, due to the nonlinear behavior, it is possible to switch ON and OFF the lasing from the confined states of the trap by tailoring the input beam. All the experimental findings are validated and very well reproduced in the framework of the generalized Gross-Pitaevskii equation.

Probing the interface of doped isotopically mixed helium droplets by the directional anisotropy of interatomic Coulombic decay

Interatomic Coulombic decay (ICD) represents an efficient electronic relaxation mechanism of an ionized or an excited system embedded in an environment. The type of this environment and its size have a great impact on the ICD performance. It is stressed that ICD is sensitive to the arrangement of neighboring atoms when the initially created vacancy has a polarization direction. This is demonstrated in the present paper for the case of a 3p-ionized Ca surrounded by He atoms. Useful explicit expressions are derived for the ICD widths which show that the neighbors located along the polarization direction of the ionized orbital have the largest contribution to the ICD rate. By comparison with ab initio results for small clusters, we also show that in a helium environment, the pairwise approximation represents a reliable approach for computing ICD widths. Using this approximation and the density distribution of the helium atoms obtained within density functional theory, we explore ICD in large isotopically mixed helium droplets doped with Ca. A special emphasis is given to the difference between the ICD widths for the Ca3p orbitals directed perpendicular and parallel to the droplet surface. Depending on the size and isotopic composition of the droplet, Ca resides in the interfacial layer between the (4)He core and the (3)He outer shell. Hence, ICD studies in these droplets may provide valuable information on the properties of this interface.

Full dimension Rb2He ground triplet potential energy surface and quantum scattering calculations

We have developed a three-dimensional potential energy surface for the lowest triplet state of the Rb(2)He complex. A global analytic fit is provided as in the supplementary material [see supplementary material at http://dx.doi.org/10.1063/1.4709433 for the corresponding Fortran code]. This surface is used to perform quantum scattering calculations of (4)He and (3)He colliding with (87)Rb(2) in the partial wave J = 0 at low and ultralow energies. For the heavier helium isotope, the computed vibrational relaxation probabilities show a broad and strong shape resonance for a collisional energy of 0.15 K and a narrow Feshbach resonance at about 17 K for all initial Rb(2) vibrational states studied. The broad resonance corresponds to an efficient relaxation mechanism that does not occur when (3)He is the colliding partner. The Feshbach resonance observed at higher collisional energy is robust with respect to the isotopic substitution. However, its effect on the vibrational relaxation mechanism is faint for both isotopes.

Spin dynamics of electrons and holes in p-doped InAs/GaAs quantum dots

We have investigated the electron and hole spin dynamics in p-doped semiconductor InAs/GaAs quantum dots by time resolved photoluminescence. We observe a decay of the average electron spin polarisation down to 1/3 of its initial value with a characteristic time of T<FONT FACE=Symbol>D »</FONT> 500ps. We attribute this decay to the hyperfine interaction of the electron spin with randomly orientated nuclear spins. Magnetic field dependent studies reveal that this efficient spin relaxation mechanism can be suppressed by a field of the order of 100mT. In pump-probe like experiments we demonstrate that the resident hole spin, "written" with a first pulse, remains stable long enough to be "read" 15ns later with a second pulse.

Electron scattering spectroscopy by a high magnetic field in quantum cascade lasers

We present a detailed experimental and theoretical study of a $\mathrm{GaAs}∕\mathrm{AlGaAs}$ mid-infrared quantum cascade laser (QCL) under a strong magnetic field. The emitted power and the magnetoresistance show strong oscillations as a function of $B$, due to a magnetic field modulation of the lifetime of the upper state of the laser transition. The analysis of these oscillations shows a modulation of the electron lifetime due to inelastic (LO-phonon emission) and elastic scattering mechanisms. We show that at low temperature, the interface roughness scattering is the most efficient elastic relaxation mechanism; it becomes dominant whenever LO-phonon emission is inhibited by the magnetic field. By comparing experimental and theoretical results, we are able to estimate the relative weight of elastic and inelastic scattering. The magnetic field is thus a powerful spectroscopic tool to study the scattering mechanisms in QCL active regions.

Photophysics and excited state relaxation dynamics of p-hydroxy and p-amino-substituted benzophenones: a review

Very short lifetimes (a few picoseconds) of the lowest excited singlet (S1) state of benzophenone (BP) and p-hydroxybenzophenone (HOBP) suggest that they have π* character in all kinds of solvents. However, in the case of the p-amino-substituted benzophenones (ABPs), the nature of the S1 state, whether it is n–π* or π–π*, depends on the nature of the solvent and the identity of the substituent groups. The lowest excited triplet (T1) state of BP has nπ* character in all kinds of solvents and hence phosphorescence decay is short (a few ms). The T1 state character of HOBP and ABP’s are solvent dependent. In a few cases dual exponential decay of phosphorescence indicates that n–π* and π–π* triplet states lie close to each other. The π–π* type of T1 state has longer phosphorescence lifetime (a few tens of ms). Investigations of the ultrafast relaxation dynamics of 4-N,N-dimethylaminobenzophenoene (DMABP) in different kinds of solvents of varying polarities and viscosities reveal that conversion of the locally excited (LE) state to the twisted intramolecular charge transfer (TICT) state by twisting of the dimethylanilino group is the major process, which is responsible for the efficient non-radiative relaxation mechanism of the S1 state of this molecule and solvation dynamics plays a minor role in it. In self-quenching interaction between the triplet state and its ground state, the n–π* kind of T1 state plays the major role and no triplet exciplex is involved as an intermediate. The T1 state of BP is equally reactive towards hydrogen atom abstraction reaction with the ground state of each of the ABPs either by direct hydrogen atom transfer or by a chargetransfer-coupled proton-transfer mechanism. Exciplex formation has been observed only in the case of the reaction between the T1 state of BP and DMABP.

Synthesis and characterization of a macrocyclic Schiff base Gd III complex as a relaxation agent for a faster acquisition of 2H NMR spectra of ethanol

Quantitative 2H NMR analysis at the natural abundance represents a well-recognized and efficient method for the identification of the origin of ethanol from different sources. An intrinsic limitation of the protocol used is the long time required, about 8 h, because of the long T 1 values of the 2H resonances. In this work we propose the use a paramagnetic relaxation agent that significantly catalyzes the relaxation times and reduces the total time of the analysis. This agent is the macrocyclic Schiff base complex [Gd(H 2L)(H 2O) 3(EtOH)](Cl) 3 · 2 EtOH (H 2L is the [1 + 1] macrocycle derived from the condensation of 3,3 ′-(3-oxapentane-1,5-diyldioxy)bis(2-hydroxybenzaldehyde) with 1,5-diamino-3-azamethylpentane), which is highly stable and soluble in the alcoholic solution used. Elemental analysis, IR and mass spectrometry have characterized this complex. The homogeneity of the complex and the correct Gd:Cl=1:3 ratio was established by SEM-EDS measurements. Further characterization of the paramagnetic complex has been achieved by measuring the magnetic field dependence of the 1H longitudinal nuclear magnetic relaxation time of a 1 mM solution in CH 3OD with a field-cycling relaxometer. The Gd III ion accommodates up to four methanol molecules in its inner coordination sphere, whose rapid exchange with the bulk provides an efficient relaxation mechanism. The addition of about 37 mg of the complex to a solution of ethanol (3.0 g) and tetramethylurea (TMU) (1.5 g) results in the reduction of the experimental time of more than 50% with a S/N ratio compatible with that required for this application.

Anomalous ESR behavior of carbon nanofilaments grown from palladium seeds

We report on the anomalous spin relaxation behavior of conduction electrons in carbon nanofilaments produced by decomposition of acetylene on Pd nanoparticles. Thanks to first-principle calculations, we found that adsorption of Pd above the center of a hexagon in the carbon lattice is energetically favorable, and that π–d orbital interactions probably lead to an efficient spin relaxation mechanism involving spin–orbit coupling. Incorporation of Pd impurities during carbon growth implies that nanofilament length depends on initial particle size, as actually observed.

Molecular dynamics simulations for the Ag/Cu (111) system: from segregated to constitutive interfacial vacancies

The occurrence of an Ag layer on top of a Cu (111) substrate can result from either deposition (Ag/Cu) or strong segregation at the surface of a dilute Cu(Ag) alloy. Molecular dynamics simulations within many-body tight-binding potentials show that two structures compete: one corresponding to a rather uniform Ag (111) layer on top of a Cu (111) substrate leading to a Moiré structure and the other involving constitutive vacancies forming triangular loops of partial dislocations in the first Cu substrate layer. A useful guide to predict the most efficient relaxation mechanism is to characterize the regions which undergo the largest stress. Thus, the local pressure map in the first Cu underlayer allows to predict a complementarity between the segregation of vacancies in compressive sites and the Ag enrichment, due to the larger atomic radius of the Ag atoms, in the tensile ones. Segregation energy maps, both for vacancies and Ag atoms, confirm this prediction and lead to several perspectives for the kinetics of dissolution of an Ag deposit over a Cu (111) substrate.

Ab initio pseudopotential calculations for electronic and geometric structure of mixed Si–Ge dimers on the Si(001)-(1×2) and Si(001)-(2×4) surfaces

We report on ab initio pseudopotential calculations for electronic and geometric structure of mixed Si–Ge dimers on the Si(001)-(1×2) and Si(001)-(2×4) surfaces. The structural parameters obtained from our calculations provide some agreement with the recently proposed structure by Chen et al. based on adjacent substitutional Si–Ge dimers buckled in opposite directions. Dimer bonding and electronic states in the silicon band gap are also studied. It is noted that the c-(2×4) reconstruction provides a more efficient substrate relaxation mechanism than does the simple (1×2) reconstruction.

Efficient Carrier Relaxation Mechanism in InGaAs/GaAs Self-Assembled Quantum Dots Based on the Existence of Continuum States

Comparison of near-field and far-field photoluminescence excitation (PLE) spectra gives new insight into the carrier relaxation process in InGaAs/GaAs self-assembled quantum dots. The near-field PLE spectra of single quantum dots clearly show 2D-like continuum states and a number of sharp lines, between a large zero-absorption region due to the quasi-0D density of states and the 2D wetting layer absorption edge. The results reveal an efficient intradot relaxation mechanism, proceeding as follows: The carriers can relax easily within continuum states, and make transitions to the excitonic ground state by resonant emission of localized phonons.

Ag/Cu(111) structure revisited through an extended mechanism for stress relaxation

The Ag/Cu(111) system can be considered as a model one concerning the atomic structure of one monolayer deposited on a substrate in the case of strong size mismatch. Thus, it has been the subject of many experimental [Auger electron spectroscopy, low-energy electron diffraction and scanning tunneling microscopy (STM)] and theoretical studies, in particular within N-body potentials. Although most results agreed both with the existence of an $n\ifmmode\times\else\texttimes\fi{}n$ superstructure accommodating the size mismatch and with a strong corrugation of the Ag adlayer, the morphologies---derived from STM on the one hand and numerical simulations on the other hand---were not found to be consistent. Here we revisit the previous theoretical study, taking into account different additional mechanisms (Ag and Cu vacancy formation, partial dislocation loop) to relax the interfacial stress. As a result, we obtain that the most efficient relaxation mechanism is the formation of partial dislocation loops in the first Cu substrate layer, requiring the formation of four or five Cu vacancies per unit cell in this plane. This leads to a strong damping of the corrugation in the Cu underlayers, and a perfect agreement is reached between observed and calculated surface morphology.

Dynamic Hydrogen Disorder in Solid Tropolone. A Single-Crystal NMR Study of the Hydroxyl Deuterons

The orientation and temperature dependences of the deuterium NMR spectrum and spin–lattice relaxation time of the hydroxyl deuterons in single crystals of tropolone-d1are reported. The results are interpreted in terms of a dynamic hydrogen disorder model in which the hydrogen nuclei move in an asymmetric double well potential. According to this model, the hydrogen-bonded dimer structure as determined by X-ray diffraction constitutes a majority species in the tropolone crystal, comprising more than 98% of the molecules at room temperature. However, there also exists a tautomeric minority species formed by a concerted back and forth shifting of the hydroxyl hydrogens (deuterons) along the hydrogen bonds to the nearby carbonyl oxygens. This process results in a modulation of the electric field gradient tensor at the site of the deuterons, thus providing an efficient relaxation mechanism. The concentration of the minority species is too low and its lifetime is too short to make its direct observation possible. Still, structural information about this species and kinetic and thermodynamic parameters of the hydrogen shift process can be derived by fitting the measuredT1values to the above model.

Ultrasonography characterization of heart morphology and blood flow of lower vertebrates

Echocardiography is an noninvasive technique that utilizes high frequency ultrasound to evaluate structural, functional, and hemodynamic status of the cardiovascular system. With all the inherent potentials of this technique, its application has not been explored on lower vertebrates. The objective of our work was to apply ultrasonography technique, with an available commercial equipment (Echo camera—Aloka SSD 830 and Toshiba using probes of 5 and 7.5 MHz frequency emission) and to identify and characterize cardiac structures and blood flow in lower vertebrates. Specimens studied were Lophius piscatorius, Halobatrachus didactylus, and Bufo bufo. The probe was applied on the ventral middle line over heart chambers. Two-dimensional images were obtained from long and short axis incidences at various levels of cardiac chambers. During each cardiac cycle and for every specimen it was possible to identify the relation between each structure, to measure longitudinal and transversal diameters, to identify blood fluxes, and measure their velocities. The work we describe and discuss in our preliminary experience and data concerning the ultrasound study of some lower vertebrates. The cardiovascular apparatus of the different species we studied was a dynamic, phasic, and efficient contraction and relaxation mechanism. The phenomena could be well documented with two-dimensional echocardiography. The effect of the coupling phenomena of contraction and relaxation was also studied through the application of pulsed and color Doppler ultrasound. Two-dimensional and Doppler echocardiography can be easily applied to the study of the anatomy, morphology, and physiology of lower vertebrates. Well defined structures and cardiac chambers were displayed with this noninvasive imaging technique. Echo-Doppler ultrasonography is a reliable technique in these lower vertebrate specimens, to study cardiac structure and kinetics, and to identify and measure blood flow velocity. It can be used as an experimental model for cardiac function. © 1996 Wiley-Liss, Inc.

Ultrasonography characterization of heart morphology and blood flow of lower vertebrates

Echocardiography is an noninvasive technique that utilizes high frequency ultrasound to evaluate structural, functional, and hemodynamic status of the cardiovascular system. With all the inherent potentials of this technique, its application has not been explored on lower vertebrates. The objective of our work was to apply ultrasonography technique, with an available commercial equipment (Echo camera--Aloka SSD 830 and Toshiba using probes of 5 and 7.5 MHz frequency emission) and to identify and characterize cardiac structures and blood flow in lower vertebrates. Specimens studied were Lophius piscatorius, Halobatrachus didactylus, and Bufo bufo. The probe was applied on the ventral middle line over heart chambers. Two-dimensional images were obtained from long and short axis incidences at various levels of cardiac chambers. During each cardiac cycle and for every specimen it was possible to identify the relation between each structure, to measure longitudinal and transversal diameters, to identify blood fluxes, and measure their velocities. The work we describe and discuss in our preliminary experience and data concerning the ultrasound study of some lower vertebrates. The cardiovascular apparatus of the different species we studied was a dynamic, phasic, and efficient contraction and relaxation mechanism. The phenomena could be well documented with two-dimensional echocardiography. The effect of the coupling phenomena of contraction and relaxation was also studied through the application of pulsed and color Doppler ultrasound. Two-dimensional and Doppler echocardiography can be easily applied to the study of the anatomy, morphology, and physiology of lower vertebrates. Well defined structures and cardiac chambers were displayed with this noninvasive imaging technique. Echo-Doppler ultrasonography is a reliable technique in these lower vertebrate specimens, to study cardiac structure and kinetics, and to identify and measure blood flow velocity. It can be used as an experimental model for cardiac function.

Use of 1H longitudinal relaxation times in the solution structure of paramagnetic proteins. Application to [4Fe-4S] proteins.

The accuracy of the solution structures determined by NMR is often poor around paramagnetic centers because the properties of the near protons are strongly perturbed by the electronic spin. The structural information contained in the relaxation rates of these protons has been extracted here by measuring the longitudinal relaxation times with the inversion-recovery total correlation spectroscopy (IR-TOCSY) sequence based on the recovery of cross peaks. In addition to measurements with nonselective inversion-recovery for nonoverlapping signals, reliable data have been obtained for a majority of main-chain protons from Chromatium vinosum high-potential ferredoxin. When a small and constant contribution from diamagnetism as well as the electronic spin distribution over the [4Fe-4S] cluster are taken into account, the shortest longitudinal relaxation times depend directly on the distance separating the protons from the paramagnetic center. This indicates that electron-nuclei dipolar interactions are the most efficient relaxation mechanism for these protons. However, the expected dependence of the relaxation rates as the sixth power of the distance has to be corrected because of induced relaxation among fast relaxing protons. This approach reveals that the solution structure of the protein is significantly different from the crystal structure around Phe-48. In addition, it provides an independent confirmation of the actual electronic structure of the [4Fe-4S]3+ cluster in the protein. The method devised in this work, which does not rely on specific enrichment, should be useful to improve the determination of NMR-derived solution structures of paramagnetic macromolecules.