Mechanism For Selective Excitation Research Articles

Resonant laser excitation and time-domain imaging of chiral topological polariton edge states

We investigate the dynamics of chiral edge states in topological polariton systems under laser driving. Using a model system comprised of topolgically trivial excitons and photons with a chiral coupling proposed by Karzig et al. [Phys. Rev. X 5, 031001 (2015)], we investigate the real-time dynamics of a lattice version of this model driven by a laser pulse. By analyzing the time- and momentum-resolved spectral function, measured by time- and angle-resolved photoluminescence in analogy with time- and angle-resolved photoemission spectroscopy in electronic systems, we find that polaritonic states in a ribbon geometry are selectively excited via their resonance with the pump laser photon frequency. This selective excitation mechanism is independent of the necessity of strong laser pumping and polariton condensation. Our work highlights the potential of time-resolved spectroscopy as a complementary tool to real-space imaging for the investigation of topological edge state engineering in devices.

Photocatalytic E → Z Isomerization of Alkenes

The current renaissance of organic photochemistry and photocatalysis has emboldened non-specialists to explore this rich landscape in search of solutions to long-standing challenges in synthesis. Many iconic stoichiometric photochemical transformations that were often platforms for mechanistic explorations have found limited application in contemporary synthesis, likely due to a perceived difficulty in reaction execution and the dearth of scope. Fortunately, many of these valuable processes are now being translated into robust methods, often with the aid of catalysis. The selective E → Z photochemical isomerization of olefins exemplifies this growing trend. Whilst examples of photochemical isomerization reactions are ubiquitous in physical organic textbooks, their intricate mechanistic delineations often contrast starkly with highly restricted substrate scopes: In the case of carbogenic scaffolds, electron-rich styrenes and stilbenes continue to dominate despite their limited synthetic potential. In this Account , our recent development of a photocatalytic isomerization of activated olefins is discussed. Inspired by the antipodal Z → E process with retinal that constitutes the basis of the mammalian visual system, and a qualitative report describing the reverse E → Z directionality enabled by crystalline (–)-riboflavin localized in the eye, critical noncovalent interactions have been distilled allowing this latter biological process to be reengineered. The result is an expansive, operationally trivial isomerization of substrates containing the cinnamyl motif using an inexpensive, readily available photocatalyst. Reliant on a selective excitation mechanism, this process complements the pioneering work of Hammond, Arai, and others by expanding the scope of olefin isomerization. In the case of carboxylic acids, it was possible to generate the coumarin scaffold directly by exploiting the two discrete activation modes of riboflavin sequentially, namely energy transfer (E T ) followed by single electron transfer (SET). This first example of a “ one catalyst, two activation modes ” strategy constitutes a photocatalytic variant of covalent cascade catalysis and provides a new entry to an important class of pharmaceutical building blocks. Reaction development is placed in an historical context, and key findings from other laboratories that facilitated the study are highlighted. 1 Introduction 2 A Brief Introduction to Photochemistry 3 Photochemical Isomerization of Olefins 4 The Bioinspired Isomerization of Activated Olefins 5 Emulating Coumarin Biosynthesis with (–)-Riboflavin 6 Conclusions

Damping filter method for obtaining spatially localized solutions.

Spatially localized structures are key components of turbulence and other spatiotemporally chaotic systems. From a dynamical systems viewpoint, it is desirable to obtain corresponding exact solutions, though their existence is not guaranteed. A damping filter method is introduced to obtain variously localized solutions and adapted in two typical cases. This method introduces a spatially selective damping effect to make a good guess at the exact solution, and we can obtain an exact solution through a continuation with the damping amplitude. The first target is a steady solution to the Swift-Hohenberg equation, which is a representative of bistable systems in which localized solutions coexist and a model for spanwise-localized cases. Not only solutions belonging to the well-known snaking branches but also those belonging to isolated branches known as "isolas" are found with continuation paths between them in phase space extended with the damping amplitude. This indicates that this spatially selective excitation mechanism has an advantage in searching spatially localized solutions. The second target is a spatially localized traveling-wave solution to the Kuramoto-Sivashinsky equation, which is a model for streamwise-localized cases. Since the spatially selective damping effect breaks Galilean and translational invariances, the propagation velocity cannot be determined uniquely while the damping is active, and a singularity arises when these invariances are recovered. We demonstrate that this singularity can be avoided by imposing a simple condition, and a localized traveling-wave solution is obtained with a specific propagation speed.

A potential-controlled switch on/off mechanism for selective excitation in mixed electrochemiluminescent systems

We demonstrate the complete, rapid, and reversible switching between the emissions from two electrogenerated chemiluminescence (ECL) systems contained within the same solution, controlled by simple modification of the applied potential. The fundamental bases of the approach are the ability to selectively ‘switch on’ luminophores at distinct oxidation potentials, and an intriguing observation that the emission from the well-known electrochemiluminescent complex, fac-Ir(ppy)3, (where ppy is 2-phenylpyridinato), can be selectively ‘switched-off’ at high overpotentials. The dependence of this phenomenon on high concentrations of the co-reactant implicates quenching of the excited [Ir(ppy)3]* state by electron transfer. Rapid spectral scanning during modulation of the applied potential reveals well resolved maxima for mixtures comprising either green and red or green and blue luminophores, illustrating the vast potential of this approach for multiplexed ECL detection.

Scintillation properties and anomalousCe3+ emissionof Cs2NaREBr6:Ce3+ (RE = La,Y,Lu)

We report the optical and scintillation properties of theCe3+-dopedbromoelpasolites Cs2NaREBr6 (RE = La,Y,Lu).The γ-ray scintillation light yield of these materials varies from 6000 to 17 000 photons per MeV absorbedγ-ray energy. At roomtemperature (RT), the γ-ray scintillation decay curves for all compounds show a fast component of 61 ns, whereas the intrinsicCe3+ decay time is 30 ns. The scintillation mechanism in elpasolites is addressed. InCs2NaLuBr6:Ce3+ andCs2NaYBr6:Ce3+, we observe for thefirst time the so-called Ce3+ anomalous emission in bromide compounds. This emission previously observed for chloride compounds is anultrafast Ce3+ emission with a selective excitation mechanism. The decaytime of the anomalous emission at 10 K in bromide compounds(∼7.80 ns) is faster than that in chloride compounds(∼9.90 ns). Two bands of the anomalous emission are resolved for the first time. The mechanismbehind this emission is discussed.

Asymmetric charge transfer with hydrogen ions — An important factor in the “hydrogen effect” in glow discharge optical emission spectroscopy

It is now well known that traces of hydrogen or nitrogen in the argon plasma gas in glow discharge optical emission spectrometry (GD-OES) may affect the sputtering rate. More seriously, such traces can also selectively affect the absolute and relative intensities of individual lines, and thereby have a major effect on the accuracy of analytical results. This problem is becoming more severe as the potential of GD-OES as an analytical tool is steadily increasing, and the technique is now used for the analysis of more complex samples. The results presented form part of an extensive study of the effects of hydrogen and nitrogen on the spectra of a number of elements — Fe, Ti, V, Ni, Zn etc. Two systems have been used to record the spectra for the present results — high resolution Fourier transform spectroscopy at Imperial College, London, and the LECO GDS500A, with CCD spectrometer at LECO Instrumente, Plzeň. The two approaches have yielded consistent results. During the analysis of the data for Fe II and Ti II spectra, it has become clear that asymmetric charge transfer involving hydrogen ions (H-ACT) is a very important selective excitation mechanism for spectral lines with a total excitation energy close to 13.6 eV, the ionisation potential of hydrogen. Detailed evidence for this mechanism is presented. The magnitude of the effect varies for different elements and spectral lines but great care must be taken before choosing ionic lines with a total excitation energy of between 12.5–14 eV for analytical use.

High performance materials

It is now well known that traces of hydrogen or nitrogen in the argon plasma gas in glow discharge optical emission spectrometry (GD-OES) may affect the sputtering rate. More seriously, such traces can also selectively affect the absolute and relative intensities of individual lines, and thereby have a major effect on the accuracy of analytical results. This problem is becoming more severe as the potential of GD-OES as an analytical tool is steadily increasing, and the technique is now used for the analysis of more complex samples.The results presented form part of an extensive study of the effects of hydrogen and nitrogen on the spectra of a number of elements — Fe, Ti, V, Ni, Zn etc. Two systems have been used to record the spectra for the present results — high resolution Fourier transform spectroscopy at Imperial College, London, and the LECO GDS500A, with CCD spectrometer at LECO Instrumente, Plzeň. The two approaches have yielded consistent results.During the analysis of the data for Fe II and Ti II spectra, it has become clear that asymmetric charge transfer involving hydrogen ions (H-ACT) is a very important selective excitation mechanism for spectral lines with a total excitation energy close to 13.6 eV, the ionisation potential of hydrogen. Detailed evidence for this mechanism is presented. The magnitude of the effect varies for different elements and spectral lines but great care must be taken before choosing ionic lines with a total excitation energy of between 12.5–14 eV for analytical use.

Endothelin-1 (ET-1) selectively enhances the activation gating of slowly inactivating tetrodotoxin-resistant sodium currents in rat sensory neurons: a mechanism for the pain-inducing actions of ET-1.

Endothelin-1 (ET-1) causes pain through activation of nociceptors, by either direct depolarization or increased excitability. Here we examined the effect of ET-1 on nociceptor-associated tetrodotoxin-resistant (TTX-R) sodium currents using whole-cell voltage clamp of acutely dissociated rat dorsal root ganglion (DRG) neurons. DRG neurons that responded had enhanced activation gating when exposed to 10 nm ET-1, as determined by significant shifts in their average activation midpoint potentials (DeltaE(0.5) = -8.0 +/- 0.5 mV) when compared with control (DeltaE(0.5) = -2.2 +/- 0.4 mV; n = 6) and ET-1 unresponsive cells (DeltaE(0.5) = -3.2 +/- 0.2 mV). ET-1 also modified the availability of TTX-R channels, as determined by negative shifts in the average midpoint potential for inactivation of ET-1 responsive cells when compared with controls. These actions of ET-1 occurred predominantly in cells with more slowly inactivating TTX-R currents. Both time-to-peak current and inactivation time constants were shortened by ET-1 in responsive cells. Previous exposure of cells to the endothelin-A (ET(A)) receptor antagonist BQ-123 (1 microm) prevented ET-1-induced shifts in TTX-R activation. In contrast to changes in TTX-R, ET-1 did not modify tetrodotoxin-sensitive currents recorded from DRG neurons. These results demonstrate that the algogenic peptide ET-1 induces ET(A) receptor-mediated, hyperpolarizing shifts in the voltage-dependent activation of TTX-R Na+ channels, a potential mechanism for selective excitation by ET-1 of nociceptors that we observed in vivo.

Endothelin-1 (ET-1) Selectively Enhances the Activation Gating of Slowly Inactivating Tetrodotoxin-Resistant Sodium Currents in Rat Sensory Neurons: A Mechanism for the Pain-Inducing Actions of ET-1

Endothelin-1 (ET-1) causes pain through activation of nociceptors, by either direct depolarization or increased excitability. Here we examined the effect of ET-1 on nociceptor-associated tetrodotoxin-resistant (TTX-R) sodium currents using whole-cell voltage clamp of acutely dissociated rat dorsal root ganglion (DRG) neurons. DRG neurons that responded had enhanced activation gating when exposed to 10 nm ET-1, as determined by significant shifts in their average activation midpoint potentials (DeltaE(0.5) = -8.0 +/- 0.5 mV) when compared with control (DeltaE(0.5) = -2.2 +/- 0.4 mV; n = 6) and ET-1 unresponsive cells (DeltaE(0.5) = -3.2 +/- 0.2 mV). ET-1 also modified the availability of TTX-R channels, as determined by negative shifts in the average midpoint potential for inactivation of ET-1 responsive cells when compared with controls. These actions of ET-1 occurred predominantly in cells with more slowly inactivating TTX-R currents. Both time-to-peak current and inactivation time constants were shortened by ET-1 in responsive cells. Previous exposure of cells to the endothelin-A (ET(A)) receptor antagonist BQ-123 (1 microm) prevented ET-1-induced shifts in TTX-R activation. In contrast to changes in TTX-R, ET-1 did not modify tetrodotoxin-sensitive currents recorded from DRG neurons. These results demonstrate that the algogenic peptide ET-1 induces ET(A) receptor-mediated, hyperpolarizing shifts in the voltage-dependent activation of TTX-R Na+ channels, a potential mechanism for selective excitation by ET-1 of nociceptors that we observed in vivo.

A Dynamical mechanism for Selective Excitation of the Kelvin Mode at Timescale of 30–50 Days

This work presents a dynamical mechanism for selective excitation of the equatorial Kelvin wave at timescale of 30–50 days. The setting is analytical: the basic idea involves modeling of an implicit scale interaction in the moist tropical atmosphere between large-scale flow and convection. The moist tropical atmosphere is modeled by shallow-water equations along with a time-dependent moisture equation. The internal heating is provided by precipitation, which is modeled as a relaxation process for the moisture variable with an appropriate timescale. This is unlike the usual approach adopted in earlier studies in which precipitation is parameterized through wave-CISK. Several new features emerge from the new formalism. In particular, the moist processes drive the Kelvin mode unstable. However, in sharp contrast to earlier results with similar linear models, the Kelvin mode in the present model has a maximally growing wave with period about 30–50 days. On the other hand, the high-frequency waves are damped. The associated wavelength for the maximally growing wave lies close to the planetary scale, as observed. The sensitivity of the results to various physical parameters is discussed to demonstrate that the appearance of a maximally growing Kelvin wave with a period in the range of 30–50 days is persistent over a wide range of parameters. The model also predicts a northward propagation of the maximally growing wave, with phase speed very close to the observed value. Finally, a simple dynamical picture is presented to account for the source of scale selection in the model.

Microscopic Model for the Relaxation and Excitation of Localized Spin Waves

AbstractA microscopic Hamiltonian with long‐range spin‐spin interactions showing the host‐spin‐induced energy relaxation and frequency shift of the impurity spin is studied within a Markovian process. The localization conditions and the associated dispersion laws of spin waves in ferromagnetic and antiferromagnetic systems containing one or two impurities are investigated at various domains of the exchange integrals and the spin amplitudes. The correspondence between the localized spin waves and lattice vibrational modes is analyzed. Finally, the energy absorption and the possible mechanism for selective excitation of a localized magnon are discussed based on a quantized Hamiltonian which includes the impurity‐host coupling and their interactions to an external magnetic field with frequency near resonance to that of the localized active mode.

Frequency splitting in AP stars

view Abstract Citations (96) References (9) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Frequency splitting in AP stars Dziembowski, W. ; Goode, P. R. Abstract The oblique pulsator model by Kurtz was generalized to account for the observed properties of rapidly oscillating Ap stars. In this model, which includes advection and an oblique magnetic field, the modes are represented, in general, by a superposition of all spherical harmonics having the appropriate degree, l. It is predicted that an observer will report a mode splitting into (2l + 1)-frequency components with the spacing equal to the rotation frequency of the star. The relative amplitudes at these frequencies are predicted following from a postulated selective excitation mechanism due to the field, and the diagnostic potential of the data on these Ap stars is discussed. Publication: The Astrophysical Journal Pub Date: September 1985 DOI: 10.1086/184542 Bibcode: 1985ApJ...296L..27D Keywords: A Stars; Peculiar Stars; Pulsars; Stellar Models; Stellar Oscillations; Stellar Rotation; Advection; Magnetic Field Configurations; Spherical Harmonics; Stellar Magnetic Fields; Astrophysics full text sources ADS |