Population Of Metastable States Research Articles

Energy landscapes of homopolymeric RNAs revealed by deep unsupervised learning

Conformational dynamics of RNA plays important roles in a variety of cellular functions such as transcriptional regulation, catalysis, scaffolding, and sensing. Recently, RNAs with low-complexity sequences have been shown to phase separate and form condensate phases similar to lowcomplexity protein domains. The affinity for phase separation and the material characteristics of RNA condensates are strongly dependent on sequence composition and patterning. We hypothesize that differences in the affinities for RNA phase separation can be uncovered by studying sequence-dependent conformational dynamics of single RNA chains. To this end, we have employed atomistic simulations and deep dimensionality reduction techniques to map temperature-dependent conformational free energy landscapes for 20 base-long homopolymeric RNA sequences: poly(U), poly(G), poly(C), and poly(A). The energy landscapes of homopolymeric RNAs reveal a plethora of metastable states with qualitatively different populations stemming from differences in base chemistry. Through detailed analysis of base, phosphate, and sugar interactions, we show that experimentally observed temperature-driven shifts in metastable state populations align with experiments on RNA phase transitions. Specifically, we find that the thermodynamics of unfolding of homopolymeric RNA follows the poly(G) > poly(A) > poly(C) > poly(U) order of stability, mirroring the propensity of RNA to form condensates. To conclude, this work shows that at least for homopolymeric RNA sequences the single-chain conformational dynamics contains sufficient information for predicting and quantifying condensate forming affinities of RNAs. Thus, we anticipate that atomically detailed studies of temeprature -dependent energy landscapes of RNAs will be a useful guide for understanding the propensity of various RNA molecules to form condensates.

Protonated acetylene in the z = 0.89 molecular absorber toward PKS 1830-211

We report the first interstellar identification of protonated acetylene, C2H3+, a fundamental hydrocarbon, in the z = 0.89 molecular absorber toward the gravitationally lensed quasar PKS 1830–211. The molecular species is identified from clear absorption features corresponding to the 212–101 (rest frequency 494.034 GHz) and 111–000 (431.316 GHz) ground-state transitions of ortho and para forms of C2H3+, respectively, in ALMA spectra toward the southwestern image of PKS 1830-211, where numerous molecules, including other hydrocarbons, have already been detected. From the simple assumption of local thermodynamic equilibrium (LTE) with cosmic microwave background photons and an ortho-to-para ratio of three, we estimate a total C2H3+ column density of 2 × 1012 cm−2 and an abundance of 10−10 compared to H2. However, formation pumping could affect the population of metastable states, yielding a C2H3+ column density higher than the LTE value by a factor of a few. We explore possible routes to the formation of C2H3+, mainly connected to acetylene and methane, and find that the methane route is more likely in PDR environment. As one of the initial hydrocarbon building blocks, C2H3+ is thought to play an important role in astrochemistry, in particular in the formation of more complex organic molecules.

Study of the evolution of pulsed plasma under an external longitudinal magnetic field

An experimental study on the role of an external longitudinal magnetic field on the characteristics changes of pulsed plasma stream, produced in argon medium, is carried out at different time spans of its evolution. The spectroscopic observations are time integrated and are carried out at different times of plasma formation for pulsed discharge. This study gives insights into the recombination and diffusion phase of the plasma species in the presence of the magnetic field. The transition of plasma species from a dominant recombination phase to a diffusional phase is well revealed by the density profile during the time evolution. Moreover, the decrease in electron temperature and the increase in electron excitation temperature explain the energy transfer to electrons due to metastable quenching, and the system gradually approaches equilibrium. The magnetic field also affects the transitions of the ionized argon population between different energy levels. It is found that faster decay occurs for transitions of different plasma species to non-metastable states, while the populations of metastable states exist for a longer time. In addition, the time-resolved morphology changes of the plasma stream are also observed by high speed imaging, which shows the flow structure of the plasma stream at different time frames. The imaging of the plasma stream evolution shows the initial ejection of the plasma sheet from the electrode assembly, its detachment, the steady flow, and gradually its nature of dying out.

Electron loss of CH− and [formula omitted] induced by interactions with N2 and O2 at keV energies

Relative electron detachment cross sections for the projectiles CH− and CH2− colliding with N2 and O2 target gases were measured. Two different methods to measure the cross sections were used, the signal growth rate and the beam attenuation. The cross sections measured with the beam attenuation technique were higher than those measured with the signal growth rate method. An analysis based on the differences of the cross sections measured with the two methods showed consistency with the time-dependent processes of metastable auto-detaching state populations undergoing decay as a function of its time of flight. These metastable processes seem to be target dependent. When the target was O2, a decaying process characterized by a lifetime of the order of 0.40 ± 0.16 μs was found. For the case of the N2 as a target, it was not possible to derive a characteristic lifetime.

Radiative Emissions in Visible–IR of Krypton Excilamp: Experimental and Theoretical Interpretations

The paper presents an experimental and theoretical study of the radiative emission in the pure krypton lamp excited by dielectric barrier discharge. Parametric studies were made in the aim to observe the influence of the applied voltage, the gas pressure and the frequency on the radiation intensity emission from visible to infrared range. Stable dielectric barrier discharges were achieved between two identical insulated electrodes separated by 2 mm gap containing pure krypton at pressure from 129 to 460 mbar. The discharge was powered with a sinusoidal supply at frequencies from 10 to 50 kHz. Electrical characteristics of the DBD were studied. A spectroscopic and kinetic analysis of a pure krypton dielectric barrier discharge is reported here. The discharge emits radiations in UV, visible and IR range. In order to understand the emission process a theoretical interpretation of the results is given in the IR range by describing the chemical properties of the lamp. IR emission and a detailed study of kinetics allow to follow the population of metastable excited states of Krypton in (Kr*(3P0,2)). This population is indirectly responsible for UV emission.

Complexity at simple interfaces: Dynamically generated deep trap states at the noble-metal/alkali-halide interface

The trapping of delocalized electrons at ultrathin layers of NaCl on Ag(100) is investigated in real time using two-photon photoemission. Above 79 K, electrons localize to deep trap states assigned to low-coordinated sites on the NaCl surface. Below 79 K, these low-coordinated trap states serve as precursors for a new deep trap state at increased binding energies. The observation that the emerging trap state requires a localized precursor state suggests that a dynamic polarization of the NaCl lattice may be required for its formation. This behavior is consistent with the expectations for the formation of a interfacial (electron) small polaron which is theoretically unstable in bulk NaCl. The alkali-halide/noble-metal interface shows the sequential population of multiple metastable deep trap states in two dimensions in real time.

Short time dynamics determine glass forming ability in a glass transition two-level model: A stochastic approach using Kramers’ escape formula

The relationship between short and long time relaxation dynamics is obtained for a simple solvable two-level energy landscape model of a glass. This is done through means of the Kramers’ transition theory, which arises in a very natural manner to calculate transition rates between wells. Then the corresponding stochastic master equation is analytically solved to find the population of metastable states. A relation between the cooling rate, the characteristic relaxation time, and the population of metastable states is found from the solution of such equation. From this, a relationship between the relaxation times and the frequency of oscillation at the metastable states, i.e., the short time dynamics, is obtained. Since the model is able to capture either a glass transition or a crystallization depending on the cooling rate, this gives a conceptual framework in which to discuss some aspects of rigidity theory, for example.

Excitation and relaxation of metastable atomic states in an active medium of a repetitively pulsed copper vapour laser

The influence of a pre-pulse population of copper atom metastable states and their sub-population at a current pulse edge on the copper vapour laser pulse energy is studied under optimal temperature conditions. Experiments have been performed with active elements of a commercial laser having an internal diameter of a discharge channel of 14 and . It is found that at a pulse repetition frequency of , corresponding to a maximal output power, the reduction of the energy due to a residual population of metastable states is by an order of magnitude less than due to their sub-population at a current pulse edge. The modelling based on the experimental results obtained has shown that in the case of an active element with an internal diameter of , a decrease in the pulse leading edge from to does not reduce the laser pulse energy up to the repetition frequency of at an average output power of and efficiency of .

Photo-induced linkage NO isomers in the dinitrosyl complex [Ru(NO)2(PCy3)2Cl](BF4) identified by photocrystallography and IR-spectroscopy

Photoinduced metastable linkage NO isomers have been known for a long time in mono-nitrosyl ruthenium or iron transition metal complexes [MLxLy(NO)]n for which appropriate temperature and excitation conditions allow a controlled generation of different metastable states. Dinitrosyl complexes offer attractive possibilities owing to the potential switching of the two nitrosyl ligands, possibly in a selective manner, leading to a complicated and intricated population of metastable states. [Ru(NO)2(PCy3)2Cl]BF4 belongs to a complete series of bis-phosphane complex salts of general formula [Ru(NO)2(PR3)2X](BF4) (X = Cl, Br, I). It presents a square pyramidal geometry with both a linear and a bent NO ligand, and an Ru(d6) electron configuration. Multiple light-induced reversible metastable NO linkage isomers (PLIs) are detected by a combination of photocrystallographic and infrared (IR) analysis. The IR signature of three PLI states has been clearly identified. The structural configuration of the major component (PLI-1) has been derived by X-ray photocrystallography. After irradiation with 405 nm, the orientation of the bent nitrosyl is changed by rotation towards the Cl ligand, opposite to the linear NO. The minor second metastable state (PLI-2) is proposed to result from a side-bonded (similar to MS2 in mono-nitrosyl complexes) state of the linear nitrosyl.

Photoexcited states of the harmonic honeycomb iridateγ−Li2IrO3

We report equilibrium and nonequilibrium optical measurements on the recently synthesized "harmonic" honeycomb iridate \gamma-Li$_2$IrO$_3$ (LIO), as well as the layered honeycomb iridate Na$_2$IrO$_3$ (NIO). Using Fourier transform infrared microscopy we performed reflectance measurements on LIO, from which we obtained the optical conductivity below 2 eV. In addition we measured the photoinduced changed in reflectance, \Delta R, as a function of time, t, temperature, T, and probe field polarization in both LIO and NIO. In LIO, \Delta R(t,T) is anisotropic and comprised of three T dependent components. Two of these components are related to the onset of magnetic order and the third is related to a photoinduced population of metastable electronic excited states. In NIO, \Delta R(t,T) has a single T dependent component that is strikingly similar to the electronic excitation component of \Delta R in LIO. Through analysis and comparison of \Delta R(t,T) for two compounds, we extract information on the onset of magnetic correlations at and above the transition temperature in LIO, the bare spin-flip scattering rate in equilibrium, the lifetime of low-lying quasiparticle excitations, and the polarization dependence of optical transitions that are sensitive to magnetic order.

Population distribution subsequent to charge exchange of 29.85 keV Ni+ on sodium vapor

The population of low-energy metastable states in Ni I subsequent to charge-exchange reactions and cascade decay was studied for 29.85keV Ni ions impinging on a sodium vapor. The charge-exchange cross sections were calculated using a semi-classic time-dependent perturbation theory and redistribution of the population via rapid spontaneous decays was simulated. Collinear laser spectroscopy experiments were performed on four emission lines in Ni I to determine populations in low-energy metastable states: 3d94s3D3 (204.787cm−1), 3d94s3D2 (879.816cm−1), 3d94s3D1 (1713.087cm−1) and 3d94s1D2 (3409.937cm−1). Fair agreement was obtained between the simulation and experimental results. The low-energy metastable states were preferentially populated after the cascade decay in contrast to weak populations in near-resonant high-energy states in Ni I.

Trap losses due to near-resonant Rydberg dressing of cold atomic gases

SynopsisStudies of near-resonant dressing of cold strontium gases and BECs held in an optical dipole trap (ODT) with the 303S1 Rydberg state reveal that, even when well detuned from resonance, such dressing can lead to a rapid decrease in the ground state atom population in the trap. This decrease is attributed to Rydberg atom excitation, which can trigger direct escape from the trap and population of long-lived metastable states.

Influence of magnetic field on laser-produced barium plasmas: Spectral and dynamic behaviour of neutral and ionic species

The expansion dynamics and spectral behaviour of plasma produced by a Nd:YAG laser (λ = 1.064 μm, pulse width: 8 ns) from barium target and expanding in 0.45 T transverse magnetic field in vacuum (10−5 Torr pressure) are investigated using time-of-flight optical emission spectroscopy. The experiments are carried out at various laser fluences from 12 to 31 J/cm2. The temporal profiles of neutral (Ba I 553.5 and 577.7 nm) lines are temporally broadened, while that of ionic (Ba II 413.0 and 455.4 nm) lines show strong confinement in the presence of a magnetic field. In the absence of magnetic field, the temporal profile of Ba I 553.5 nm is exactly reproduced by fitting with two Shifted Maxwell Boltzmann (SMB) Distribution components, while in the presence of a magnetic field the profile could only be fitted with three components. The field enhanced and field induced SMB components of neutral profile are correlated with populations of ground state, metastable states, and long-lived Rydberg states present in the barium plasma, while SMB components of ionic lines are explained on the basis of the presence of super-elastic collisions among the excited species in the plasma. The spatial variation of electron temperature and temporal variation of electron density are deduced and correlated to the different collisional processes in the barium plasma. The ionic profiles show efficient confinement in the presence of a magnetic field at higher fluences.

Statistical analysis of the dynamics of secondary electrons in the flare of a high-voltage beam-type discharge

In the present work, results of investigations into the dynamics of secondary electrons with helium atoms in the presence of the reverse electric field arising in the flare of a high-voltage pulsed beam-type discharge and leading to degradation of the primary electron beam are presented. The electric field in the discharge of this type at moderate pressures can reach several hundred V/cm and leads to considerable changes in the kinetics of secondary electrons created in the process of propagation of the electron beam generated in the accelerating gap with a grid anode. Moving in the accelerating electric field toward the anode, secondary electrons create the so-called compensating current to the anode. The character of electron motion and the compensating current itself are determined by the ratio of the field strength to the concentration of atoms (Е/n). The energy and angular spectra of secondary electrons are calculated by the Monte Carlo method for different ratios E/n of the electric field strength to the helium atom concentration. The motion of secondary electrons with threshold energy is studied for inelastic collisions of helium atoms and differential analysis is carried out of the collisional processes causing energy losses of electrons in helium for different E/n values. The mechanism of creation and accumulation of slow electrons as a result of inelastic collisions of secondary electrons with helium atoms and selective population of metastable states of helium atoms is considered. It is demonstrated that in a wide range of E/n values the motion of secondary electrons in the beam-type discharge flare has the character of drift. At E/n values characteristic for the discharge of the given type, the drift velocity of these electrons is calculated and compared with the available experimental data.

On the Question of the Existence of High-Frequency Oscillations in the Power Supply Circuits of a Copper Vapor Laser and Their Influence on the Lasing Mechanism

The statement of the problem and conclusions in the publication by N. A. Yudin, M. R. Tret’yakova, and N. N. Yudin “Influence of electrophysical processes in the discharge circuit on the energy characteristics of a copper vapor laser” (Russ. Phys. J., 55, No. 9, 1080 − 1090 (2013)) is considered. It is shown that the main positions of the publication touching on relaxation of the populations of metastable states in the afterglow and their influence on the frequency-energy characteristics of lasers are mistaken.

Investigation of the Dependence of Argon Metastable State Population on the Conditions of High-Frequency Glow Discharge Excitation

The population of metastable states of argon atoms in high-frequency glow discharge plasma is calculated. Dependences of the population of metastable states of argon atoms in the high-frequency glow discharge plasma on the standard gas flow rate (14–30 sccm) and vibration generator power (10–50 W) in the atmospheres of pure argon and argon-silane (95% Ar + 5% SiH4) mixture are investigated together with the dependence of the population of metastable states of argon atoms in the high-frequency glow discharge plasma on the silane content (0.5–5%) in the argon-silane mixture.

The population of metastable states as a probe of relativistic-energy fragmentation reactions

Isomeric ratios have been measured for high-spin states in Po84198,200,206,208, At85208,209,210,211, Rn86210,211,212,213,214, Fr87208,211,212,213,214, Ra88210,211,212,214,215, and Ac89215 following the projectile fragmentation of a 1 A GeV 238U beam by a 9Be target at GSI Helmholtzzentrum für Schwerionenforschung. The fragments were separated in the fragment separator (FRS) and identified by means of energy loss and time-of-flight techniques. They were brought to rest at the centre of the RISING gamma-ray detector array and intensities of gamma rays emitted in the decay of isomeric states with half-lives between 100 ns and 40 μs and spin values up to 55/2 ℏ were used to obtain the corresponding isomeric ratios. The data are compared to theoretical isomeric ratios calculated in the framework of the abrasion–ablation model. Large experimental enhancements are obtained for high-spin isomers in comparison to expected values.

Influence of the electrophysical processes in the discharge circuit on the energy characteristics of a copper vapor laser

The influence of the electrode arrangement in the gas-discharge tube (GDT) on the electrophysical processes in the discharge circuit is considered. The equivalent GDT circuit and the mechanism of its transformation depending on the electrode arrangement in the cold buffer or hot discharge channel zones are designed. The breakdown mechanism at the GDT channel ends is considered. It is demonstrated that the breakdown can be considered as the time moment since which the voltage is redistributed between the GDT ends and the active medium in favor of the last. A high relaxation rate of metastable copper atom states in the near afterglow is caused by ionization electron cooling in the active medium. In this case, the energy stored in the reactive components of the discharge circuit impedance during this time period is dissipated in the cold buffer zones. The energy is dissipated in the cold buffer zones until the time when the plasma resistance at the discharge channel ends reaches the value starting from which the duct capacitive components of the GDT shunt the cold buffer zones. As a result, a high-frequency circuit is formed, and the energy is further dissipated in the active medium. This reduces the relaxation rate of the metastable states during the time period between pulses and determines the characteristic bend in the time dependence of the metastable state population density determining the time moment of forming the high-frequency circuit.

Study on the relationship between the population of metastable state and time delay in an erbium-doped optical fiber

The slow light propagation in optical fiber has attracted much attention because of its practical applications. Use of coherent population oscillation to produce slow light propagation has many advantages over other methods. We investigate the reduction of the group velocity propagation by the coherent population oscillation in erbium-doped optical fiber, and also the relationship between the population of metastable state and the time delay by changing the doping concentration, the length of the fiber, the input power, and the pump power. The results show that we can select a high doping concentration of erbium in the fiber, increase the length of the fiber, decrease the pump power and select the appropriate power of the signal light to increases the time delay.

Bayesian analysis of the evidence for minor components in crystallographic models: an alternative to the Hamilton {\cal R} test

A simple test based on Bayesian statistics for the presence of minority populations in single-crystal structure refinement is presented. The test is illustrated by analysis of photocrystallographic experiments on single crystals of ruthenium-sulfur-dioxide-based complexes. In data sets collected after irradiation with light, conventional refinements of the populations of different metastable states to values below 4% are shown to be statistically significant. The results also confirm that the photo-induced states are absent from data collected in the dark.