Asymmetric Fragmentation Research Articles

Catalytic asymmetric fragmentation of cyclopropanes.

The stereoselective activation of alkanes constitutes a long-standing and grand challenge for chemistry. Although metal-containing enzymes oxidize alkanes with remarkable ease and selectivity, chemical approaches have largely been limited to transition metal-based catalytic carbon-hydrogen functionalizations. Alkanes can be protonated to form pentacoordinated carbonium ions and fragmented into smaller hydrocarbons in the presence of strong Brønsted acids. However, catalytic stereocontrol over such reactions has not previously been accomplished. We show here that strong and confined acids catalyze highly enantioselective fragmentations of a variety of cyclopropanes into the corresponding alkenes, expanding the boundaries of catalytic selective alkane activation. Computational studies suggest the involvement of the long-debated cycloproponium ions.

Fission fragment spectroscopy of 235U(nth,f)

Simultaneous measurements of the relative fission fragment charge and mass yield distributions have been performed for the even-even fission fragments produced from the reaction, 235U(nth,f). The measurements have been carried out using the conventional fission fragment spectroscopic technique. The extracted results are interpreted on the basis of the Multi-Modal Random Neck Rupture Model (MM-RNRM). The results from the analysis bring out the necessary experimental evidence for the influence of the deformed proton shell closures at Z = 52 and ∼ 56 along with the neutron shell closures at N = 82 (spherical) and 88 (deformed) in controlling the respective S1 and S2 fission modes occurring in the heavier group of asymmetric fission fragments. The new findings from the present investigation provide the crucial inputs for understanding the features of different fission modes that persist in the low-energy fission dynamics of the lighter actinides. The pair wise neutron multiplicity distribution profiles for the five correlated fission fragment pairs have been presented, and the corresponding extracted neutron multiplicity values are also reported.

Charge-symmetric and -asymmetric fragmentation dynamics of argon dimers in slow Ar8+–Ar2 collisions

We present an experimental study of multiple-electron capture-induced fragmentation dynamics of Ar2m+ (4 ≤m≤ 7) dimer ions in 4 keV/u Ar8+–Ar2 collisions. The fragment recoil ion pairs and the charge-changing projectiles are coincidentally measured using a double coincidence technique. The branching ratios between the different charge-sharing fragmentation channels show an inherent enhancement of the asymmetric channels. The kinetic energy release (KER) distributions for the associated electron capture process show a shift in the mean KER values toward the higher side with increasing capture stabilization. The interplay between the different projectile autoionization processes sheds light on the energy depositions to the system during collisions. The Coulomb potential energy curves give a physical insight into the role of the projectile final states in the dimer fragmentation dynamics. The dimer-axis orientation-dependent cross sections for the asymmetric fragmentation channels reveal a forward–backward asymmetry that arises from the geometry of the collision system. Our findings thus give insight into the impact parameter-controlled fragmentation dynamics of multiply charged Ar2m+ dimer ions in highly charged ion–dimer slow collisions.

Study on Radial Distribution of Fragments Velocity of D-shaped Fragmentation Warhead

For the asymmetric D-shaped fragmentation warhead with high charging coefficient and space efficiency, we simulate the driving process of fragments under typical eccentric initiation by using the fluid-structure interaction algorithm, and obtained the change rule of the fragment velocity radial distribution with the central angel, the thickness and material of the upper end cap. The results of numerical simulation show that the fragment velocity of D-shaped warhead increases gradually from the reverse to the forward direction, the velocity of fragment in forward direction increases by about 10% under single-point eccentric initiation compared with the cylindrical casing under center initiation, and the fragment velocity under double-point eccentric initiations is higher than single-point eccentric initiation; The initial velocity of the fragment has a positive correlation with the thickness and density parameters of the upper end cap; the mass of the warhead and the total kinetic energy of the fragments in the forward area increase with the increase of the central angle, but their ratio increases first and then decreases with the increase of the central angle. The results can be applied to structural optimization design and power evaluation of the warhead.

Deliberative democracy and the digital public sphere: Asymmetrical fragmentation as a political not a technological problem

Deliberative democracy and the digital public sphere: Asymmetrical fragmentation as a political not a technological problem

Mechanism and contact time of off-center impacts at relatively high Weber numbers

When a droplet off-center impacts a superhydrophobic surface with macro-ridges, it may split into two asymmetric fragments that then exhibit asymmetric spreading and retraction phenomena. The contact time depends on the volume of the larger fragment, which is determined by the off-center distance. However, it is difficult to predict the contact time in practical applications. To establish a quantitative law of the contact time based on the volume of the larger fragment, the dynamics of off-center impacts at relatively high Weber numbers are investigated using lattice Boltzmann method simulations. The relationship between the volume of the larger fragment and the off-center distance is established to elucidate the mechanism of the redistribution of liquid volume. On this basis, another mechanism, i.e., the asymmetry of the liquid film, is studied to derive the relationship between the contact time and the length of the larger liquid film. Combining the two mechanisms, a quantitative relationship of the contact time for off-center impacts, which is proved to be applied in a wide range of Weber numbers, is finally established.

Debris cloud structure and hazardous fragments distribution under hypervelocity yaw impact

This study investigates how the debris cloud structure and hazardous fragment distribution vary with attack angle by simulating a circular cylinder projectile hypervelocity impinging on a thin plate using the finite element–smoothed particle hydrodynamics (FE-SPH) adaptive method. Based on the comparison and analysis of the experimental and simulation results, the FE-SPH adaptive method was applied to address the hypervelocity yaw impact problem, and the variation law of the debris cloud structure with the attack angle was obtained. The screening criterion of the hazardous fragment at yaw impact is given by analyzing the debris formation obtained by the FE-SPH adaptive method, and the distribution characteristics of hazardous fragments and their relationship with the attack angle are given. Moreover, the velocity space was used to evaluate the distribution range and damage capability of asymmetric hazardous fragments. The maximum velocity angle was extended from fully symmetrical working conditions to asymmetrical cases to describe the asymmetrical debris cloud distribution range. In this range, the energy density was calculated to quantitatively analyze how much damage hazardous fragments inflict on the rear plate. The results showed that the number of hazardous fragments generated by the case near the 35° attack angle was the largest, the distribution range was the smallest, and the energy density was the largest. These results suggest that in this case, debris cloud generated by the impact had the strongest damage to the rear plate.

Sol-gel transition programmed self-propulsion of chitosan hydrogel.

Active soft materials exhibit various dynamics ranging from boat pulsation to thin membrane deformation. In the present work, in situ prepared ethanol-containing chitosan gels propel in continuous and intermittent motion. The active life of the organic material loaded to the constant fuel level follows a linear scaling, and its maximal velocity and projection area decrease steeply with chitosan concentration. A thin propelling platelet forms at low polymer content, leading to the suppression of intermittent motion. Moreover, the fast accelerating thin gels can split into a crescent and circular-like shape or fission into multiple asymmetric fragments.

Asymmetric Dissociative Tunneling Ionization of Tetrafluoromethane in ω - 2ω Intense Laser Fields.

Dissociative ionization of tetrafluoromethane (CF4) in linearly polarized ω-2ω ultrashort intense laser fields (1.4 × 1014 W/cm2, 800 and 400 nm) has been investigated by three-dimensional momentum ion imaging. The spatial distribution of produced by CF4 → + F + e− exhibited a clear asymmetry with respect to the laser polarization direction. The degree of the asymmetry varies by the relative phase of the ω and 2ω laser fields, showing that 1) the breaking of the four equivalent C-F bonds can be manipulated by the laser pulse shape and 2) the C-F bond directed along the larger amplitude side of the ω-2ω electric fields tends to be broken. Weak-field asymptotic theory (WFAT) shows that the tunneling ionization from the 4t 2 second highest-occupied molecular orbital (HOMO-1) surpasses that from the 1t 1 HOMO. This predicts the enhancement of the tunneling ionization with electric fields pointing from F to C, in the direction opposite to that observed for the asymmetric fragment ejection. Possible mechanisms involved in the asymmetric dissociative ionization, such as post-ionization interactions, are discussed.

The IMPACT satellite fragmentation model

IMPACT is a hypervelocity collision and explosion model developed by The Aerospace Corporation to predict the characteristics of debris generated by fragmentation events. Used for over 30 years, it is a semi-empirical model that has supported a wide range of orbital debris analyses including satellite risk assessment, on-orbit event analyses, flight test and mission planning, debris mitigation and spacecraft design studies, and long-term debris environment evolution and space safety analyses. The model interconnects empirical expressions with conservation laws and boundary conditions to model debris generation from individual fragmentation events.A series of efforts to study available on-orbit data and newly-generated ground-test data during the past decade has added to the understanding of on-orbit fragmentation events, particularly with regard to explosion events, sub-catastrophic breakups, smaller fragment characteristics, and more diverse modern satellite construction materials. These insights have led to several upgrades of the IMPACT collision and explosion model. The model's capabilities have been expanded to represent sub-catastrophic and asymmetric fragmentation events, more accurately model fragment material differences and their relationships to area-to-mass ratios, model smaller fragments, and relate ground-based and on-orbit data. Additional insights have been gained through the integration of individual model improvements while maintaining overall model consistency.This paper provides an overview of the current IMPACT model, discussing the design and expressions used in the current collision and explosion algorithms, changes over the past decade, and implications of insights from both on-orbit and ground test data. Modelling techniques discussed include representation of fragmenting object material differences, sub-catastrophic fragmentations, and small fragment modelling. Interactions between different model improvements are discussed, including small versus large fragment characteristics and sub-catastrophic versus catastrophic fragmentation events. Model updates are considered in the context of fragmentation event data. Future directions for development also are considered based on the insights from the aggregated data and model updates.

High Energy Inner Shell Photoelectron Diffraction in CO2

Photoelectron diffraction is an effective tool to probe the structures of molecules. The higher the photoelectron kinetic energy is, the higher order the diffraction pattern is disclosed in. Up to date, either the multi-atomic molecule with the photoelectron kinetic energy below 150 eV or the diatomic molecule with 735 eV photoelectron has been experimentally reported. In this study, we measured the diffraction pattern of C 1s and O 1s photoelectrons in CO2 with 319.7 and 433.5 eV kinetic energies, respectively. The extracted C–O bond lengths are longer than the C–O bond length at the ground state, which is attributed to the asymmetric fragmentation that preferentially occurs at the longer chemical bond side during the zero-energy asymmetric vibration.

Experimental study of fast fission and quasifission in the Ca40+Pb208 reaction leading to the formation of the transfermium nucleus No248

Background: The stability of the transfermium nucleus against fission is mainly determined by the shell correction depending on its angular momentum and excitation energy.Purpose: The study of the fast fission process of the transfermium nucleus $^{248}\mathrm{No}$ and its dependence on the interaction energy and introduced angular momentum.Methods: Mass-energy distributions of the $^{248}\mathrm{No}$ fission fragments formed in the $^{40}\mathrm{Ca}$ + $^{208}\mathrm{Pb}$ reaction at energies above the Coulomb barrier have been measured using the double-arm time-of-flight spectrometer CORSET at the $^{40}\mathrm{Ca}$-beam energies of 223, 250, and 284 MeV.Results: The contribution of the fast fission process is determined from the calculations of the driving potential, taking into account shell effects and rotational energy and amounts to 39% and 61% at 250 and 284 MeV, respectively. The mass-energy distributions of the quasifission and fast fission fragments have been extracted by subtracting the mass-energy matrices associated with compound nucleus fission from those of all measured fissionlike events. The asymmetric fragments with masses 97 and 151 u were found to be the most probable in the fast fission of $^{248}\mathrm{No}$. With increasing $^{40}\mathrm{Ca}$ energy from 250 to 284 MeV the mass distributions of the fast fission fragments change slightly.Conclusions: Contrary to quasifission in which the fragments are focused mainly around the closed neutron or proton shells, the influence of known proton or neutron shells on the asymmetric mass distribution in the fast fission process was not observed.

Isomerization Dynamics in the Symmetric and Asymmetric Fragmentation of Ethane Dications.

Hydrogen- or proton-migration-induced isomerization has recently been of concern for its critical role in the dissociation of organic molecules of astrophysical or biological relevance. Herein we present a combined experimental and theoretical study of the two-body C-C bond breakdown dissociation of ethane dication. For the asymmetric fragmentation channel CH2+ + CH4+, the kinetic energy release measurements and ab initio quantum chemical calculations demonstrate that the reaction pathway involving hydrogen-migration-induced isomerization of [CH3-CH3]2+ to [CH2-CH4]2+ can be accessed via the lowest triplet state rather than the ground singlet state of ethane dication. Interestingly, it is found that a considerable proportion of the yield of symmetric fragmentation CH3+ + CH3+, which is usually considered from a direct Coulomb explosion and seemingly independent of isomerization, could come from the dissociation of ethane dication in the ground singlet state with the involvement of [CH3-CH3]2+ isomerization to intermediate [H2C(H2)CH2]2+ of the diborane-like double-bridged structure.

Research on the Reasonable Coal Pillar Width and Surrounding Rock Supporting Optimization of Gob-Side Entry under Inclined Seam Condition

In order to study the optimal coal pillar width and surrounding rock control mechanism of gob-side entry under inclined seam condition, the 130205 return air entry adjacent to 130203 gob in Yangchangwan No. 1 well is taken as a typical engineering background. By means of engineering background analysis, theoretical analysis based on inside and outside stress field, numerical simulation by FLAC3D software, and in situ industrial test and relevant monitoring methods, the optimal coal pillar width and surrounding rock control technology are obtained. The results show that the influence range of inside stress field is about 12.2∼12.8 m based on theoretical calculation result; under the influence of 10 m coal column, the overall deformation of the roadway is relatively small and within the reasonable range of engineering construction, so the width of the coal pillar along the return air roadway is set to 10 m which is more reasonable; the cross-section characteristics of special-shaped roadway lead to asymmetric stress distribution and fragmentation of surrounding rock, and then the asymmetric surrounding rock control technology under the coupling effect of roof prestressed anchor + high-strength single anchor cable + truss anchor cable support is proposed. The monitoring results of this support method are effective for the maintenance of gob-side entry, and the study conclusions provide new guidance for the surrounding rock control mechanism of gob-side entry under inclined seam conditions.

Impact of different hydrodynamical mass transfer approaches in the spontaneous fission of Cf isotopes

Theoretical possibilities for the spontaneous emission of fission fragments from [Formula: see text]Cf parents are investigated within the framework of preformed cluster decay model (PCM). The fragmentation potential exhibits a modification from dominated asymmetric fission profile to symmetric splitting with the rise in the N/Z ratio of parent nuclei. The calculated spontaneous fission (SF) half-life times of Cf isotopes find nice agreement with the experimental data, except for [Formula: see text]Cf nucleus. Within the PCM, the hydrodynamical mass transfer among the outgoing binary fragments occurs through a cylindrical vessel connecting them. For the overlapping configuration [Formula: see text]), the two classical models namely Model A and Model B (differ in the way the radius of the connecting cylinder is controlled) are used to estimate mass transfer flow of binary fragmentation. It is observed that with change in the overlapping distance, the radius of the cylindrical vessel changes in Model A, whereas the same remains fixed in Model B. In case of Model B, the effect of cylindrical radii parameter ([Formula: see text]) is also analyzed for [Formula: see text]Cf parents at optimum neck-length ([Formula: see text]R) in view of different observable such as most probable SF fragments, preformation probability, mass transfer, the SF half-lives and the results are compared with Model A calculations. The magnitude of mass transfer, preformation probability, and hence the SF half-lives gets significantly modified on switching from Model A to Model B. Further, a large amount of mass is transferred between the asymmetric fragments as compared to the symmetric ones. The SF half-lives are shown to depend strongly on the choice of classical models as well as on the cylindrical radius parameter, [Formula: see text]. The study infers the importance of classical models to spread further light in the understanding of the dynamical behavior of fragment formation in the fission process.

CRASY: correlated rotational alignment spectroscopy of pyridine. The rotational Raman spectrum of pyridine and asymmetric fragmentation of pyridine dimer cations.

We investigated the rotational Raman spectrum of pyridine monomers and pyridine dimers with mass-correlated rotational alignment spectroscopy (mass-CRASY) and ab initio calculations. The mass spectrum showed a strong signal for the protonated pyridine cation, which we assigned to asymmetric fragmentation of the dimer: ab initio calculations revealed facile proton transfer in the dimer cation and thermodynamically favorable asymmetric fragmentation. In the rotational spectrum correlated to the monomer mass channel, we assigned up to 40 lines for rotational states J ≤ 8. No spectrum could be assigned for the dimer, possibly due to the theoretically predicted presence of multiple dimer structures.

Asymmetric information risk in FX markets

This work studies the information content of trades in the world’s largest over-the-counter (OTC) market, the foreign exchange (FX) market. It analyzes a novel, comprehensive order flow data set, distinguishing among different groups of market participants and covering a large cross-section of currency pairs. We find compelling evidence of heterogeneous superior information across agents, time, and currency pairs, consistent with the asymmetric information theory and OTC market fragmentation. A trading strategy based on the permanent price impact, capturing asymmetric information risk, generates high returns even after accounting for risk, transaction cost, and other common risk factors shown in the FX literature.

Investigation of fusion probabilities in the reactions with Cr52,54, Ni64 , and Zn68 ions leading to the formation of Z=120 superheavy composite systems

Background: The formation of superheavy nuclei in fusion reactions is suppressed by a competing quasifission process. The competition between the formation of the compound nucleus and the quasifission depends strongly on the reaction entrance channel. Purpose: The investigation of fission and quasifission processes in the formation of Z=120 superheavy composite systems in the Cr52,54+Cm248 and Zn68+Th232 reactions, and their comparison with the Ni64+U238 reaction at energies in the vicinity of the Coulomb barrier. Methods: Mass-energy distributions of fissionlike fragments formed in the reactions Cr52,54+Cm248 and Zn68+Th232 at energies near the Coulomb barrier were measured using the double-arm time-of-flight spectrometer CORSET. Results: Capture cross sections for the reactions under investigation were measured. The most probable fragment masses and total kinetic energies as well as their variances in dependence on the interaction energy were studied for asymmetric and symmetric fragments. The fusion probabilities were estimated from the analysis of mass-energy distributions. Conclusions: The estimated fusion probability drops down by a factor of 103 in the Cr54+Cm248 reaction compared to the reactions of Ca48 ions with actinides. Among the studied reactions, the Cr54+Cm248 is the most favorable one for the production of the superheavy element with Z=120.1 MoreReceived 3 August 2020Accepted 2 September 2020DOI:https://doi.org/10.1103/PhysRevC.102.044605©2020 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasFissionLow & intermediate energy heavy-ion reactionsNuclear fusionPropertiesA ≥ 220Nuclear Physics

Fission partition a reflection of shell closures: Decay of 220,224U⁎ at eight excitation energies

Decay modes of the compound systems formed in 86Kr + 134,138Ba → 220,224U⁎ reactions at eight excitation energies have been compared to explore the influence of shell closures and excitation energy. The mass distributions of the cross-sections have been calculated for the two compound systems by keeping their excitation energies the same. The production cross-section for the light particles, intermediate mass fragments, symmetric and asymmetric fission fragments, total fission fragments, percentage of the cross-section for symmetric and asymmetric fission fragments, the ratio of the cross-sections of the symmetric and asymmetric fission fragments and fission barrier as a function of the incident energy have been compared. The range of the incident energy which corresponds to the high probability of the compound nucleus process has been obtained. Three angular momentum windows have been realized from the variation of the production cross-section for the light particles, intermediate mass fragments, fission fragments and the total fragments as a function of the angular momentum which correspond to the dominance of a reaction mechanism in that window. The shell closures have been reflected in the fission partition and weaken with the increase of the incident energy along with the loss of odd-even effects. The mode of the reaction depends upon the excitation energy and angular momentum of the compound system.

Gondwanan fragments in the southern Appalachians

Abstract Upper-plate and lower-plate asymmetric passive margin fragments are preserved within Carolinia, one of several terranes that rifted from Gondwana in the Furongian (late Cambrian) to form the Rheic Ocean. In the upper plate, 1–2 km of preserved rocks are middle Cambrian (Drumian, Ptychagnostus atavus zone) trilobite-bearing mudstones that lie above an angular unconformity and are the youngest stratified rocks in Carolinia. In the lower plate, 4–5 km of stratigraphy preserved in the Kings Mountain terrane are particularly interesting, because a 4 km thick Cambrian Series 2 clastic sedimentary section increasingly dominated by western Amazonian detritus lies above a Carolinian volcanic arc basement. Here, we describe for the first time the origin and setting of the youngest rocks in the Appalachians of wholly Gondwanan origin.