Anisotropic Angular Distribution Research Articles

H-Atom Product Channel in the Ultraviolet Photodissociation of the Thiomethoxy Radical (CH3S) via the B̃2A2 State.

The photodissociation dynamics of jet-cooled thiomethoxy radical (CH3S) via the B̃2A2 ← X̃2E transition was studied in the ultraviolet region of 216-225 nm using the high-n Rydberg H-atom time-of-flight (HRTOF) technique. The H-atom product channel was directly observed from the H-atom TOF spectra (using both dimethyl disulfide and dimethyl sulfide precursors). The H-atom photofragment yield spectrum showed a broad feature in the region of 216-225 nm and three B̃2A2 vibronic peaks at 217.7, 220.3, and 221.5 nm. Several H-atom dissociation pathways were identified. The excited-state CH3S had a repulsive, prompt dissociation pathway to the ground-state H2CS(X̃1A1) + H products, with the product translational energy peaking near the maximum available energy, a predominant C-S stretch vibrational excitation in H2CS(X̃1A1), and an anisotropic angular distribution. The main pathway was the H2CS(X̃1A1) + H product channel via the unimolecular dissociation of internally hot CH3S radical in the ground electronic state after internal conversion from the electronic excited state, with a modest translational energy release (peaking at a low translational energy of ∼11 kcal/mol and extending near the maximum available energy) and a nearly isotropic angular distribution. The H + H2CS(Ã1A2) and H + H2CS(ã3A2) product channels were also observed but were minor channels. The C-H bond dissociation energy of CH3S to the H + H2CS(X̃1A1) products was determined to be 48.8 ± 0.7 kcal/mol.

Real-Time GSE Mapping of Muon-Flux Directional Variation with the URAGAN Hodoscope

Continuous monitoring of the muon flux over wide ranges of zenith and azimuth angles with the URAGAN detector (Moscow, 55.7° N, 37.7° E, 173 m AMSL) allows detection of variations in its intensity and angular distribution. A way of locating domains of anisotropy in the muon angular distribution is described, and they are mapped in matrix form in the GSE coordinate system. The maximum depth (height) of deformation, the zenith and azimuth angles of the center in the local system, and its GSE longitude and latitude are determined for each observed domain. The GSE longitudes of the detected domains of anisotropy are available on the Internet as functions of time and can be used to monitor the heliosphere’s activity.

A stratified jet model for AGN emission in the two-flow paradigm

Context.High-energy emission of extragalactic objects is known to take place in relativistic jets, but the nature, the location, and the emission processes of the emitting particles are still unknown. One of the models proposed to explain the formation of relativistic ejections and their associated non-thermal emission is the two-flow model, where the jets are supposed to be composed of two different flows, a mildly relativistic baryonic jet surrounding a fast, relativistically moving electron positron plasma. Here we present the simulation of the emission of such a structure taking into account the main sources of photons that are present in active galactic nuclei (AGNs).Aims.We try to reproduce the broadband spectra of radio-loud AGNs with a detailed model of emission taking into account synchrotron and inverse-Compton emission by a relativistically moving beam of electron positron, heated by a surrounding turbulent baryonic jet.Methods.We compute the density and energy distribution of a relativistic pair plasma all along a jet, taking into account the synchrotron and inverse-Compton process on the various photon sources present in the core of the AGN, as well as the pair creation and annihilation processes. We use semi-analytical approximations to quickly compute the inverse-Compton process on a thermal photon distribution with any anisotropic angular distribution. The anisotropy of the photon field is also responsible for the bulk acceleration of the pair plasma through the “Compton rocket” effect, thus imposing the plasma velocity along the jet. As an example, the simulated emerging spectrum is compared to the broadband emission of 3C 273.Results.In the case of 3C 273, we obtain an excellent fit of the average broadband energy distribution by assuming physical parameters compatible with known estimates. The asymptotic bulk Lorentz factor is lower than what is observed by superluminal motion, but the discrepancy could be solved by assuming different acceleration profiles along the jet.

X-ray-initiated photodissociation of the glycine molecule

We investigated the charge distribution and dissociation dynamics of glycine (${\mathrm{NH}}_{2}{\mathrm{CH}}_{2}\mathrm{COOH}$) molecules irradiated with 310-eV x rays from the Advanced Light Source synchrotron. With simultaneous measurements of the fragment ion yield, dissociation angle, and kinetic energy, we were able to reconstruct a three-dimensional image of the x-ray-initiated molecular dissociation. Using coincidence and correlated analysis and applying a systematic comparison of properties of ion species, we partially disentangled the fragmentation pathways and identified the most probable fragmentation channels that lead to the observed fragment ions. In addition, we showed anisotropic angular distributions of dissociation subsequent to core-level photoionization and Auger decay and found an association between the initial bond-breaking sites and the kinetic energies of the final fragment ions.

Study of mass yield and kinetic energy distribution of fission fragments from fission of the excited compound nucleus 254Fm produced in fusion reaction

A stochastic approach based on four-dimensional Langevin fission dynamics has been used to calculate the anisotropy of fission fragment angular distribution, the mass and energy distribution of fission fragments, the yield and variance of the mass distribution of fission fragments, the fission cross section and the pre-scission neutron multiplicity for the compound nucleus 254Fm produced in the reaction 16O + 238U. In the dynamical calculations dissipation coefficient of K , \( \gamma_k\) , was considered as a free parameter and its magnitude inferred by fitting measured data on the anisotropy of fission fragments angular distribution. It was shown that the results of the anisotropy of fission fragments angular distribution for the compound nucleus 254Fm are in good agreement with the experimental data by using values of dissipation coefficient of K, equal to \(\gamma_{k}=(0.076-0.088)\) (MeV zs)-1/2. We also calculated the above-mentioned experimental data for the compound nucleus 254Fm by using \(\gamma_{k}=(0.076-0.088)\) (MeV zs)-1/2 and results of the calculations compared with the experimental data. A comparison of the theoretical results with the experimental data showed that the results of calculations are in good agreement with the experimental data, although at high energy the difference between the results of calculations with the experimental data for the variance of the mass distribution of fission fragments and the pre-scission neutron multiplicity is slightly high. It was also shown that at high energy the variance of the mass distribution of fission fragments and the pre-scission neutron multiplicity can be reproduced for the compound nucleus 254Fm by using values of dissipation coefficient of K equal to \(\gamma_{k}=0.098\) (MeV zs)-1/2 and \(\gamma_{k}=0.065\) (MeV zs)-1/2, respectively.

Time‐domain measurement of pure rotational Raman collisional linewidths of ethane C2H6

AbstractThe pure rotational Raman linewidths of the symmetric‐top molecule C2H6 are determined from a time‐resolved femtosecond experiment at room temperature and moderate pressure in a pump‐probe scheme. After excitation by a strong 100 fs‐duration nonresonant near infrared pump pulse, the molecular sample exhibits periodic post‐pulse alignment revivals that produce an anisotropic angular distribution of the molecular axis. Such molecular motion induces in turn a time‐dependent optical birefringence, which can be probed using a weak time‐delayed probe pulse. The Fourier transform of the temporal signal, recorded over a wide time delay range (several hundred picoseconds) for high frequency resolution, is directly related to the rotational Raman spectrum of ethane molecules. The Raman lines are analyzed with a frequency lineshape taking into account the collisional relaxation and the apparatus function. The linewidths are measured for rotational quantum numbers J=3 to J=30 in the pressure range 0.69–1.06 bar. A decrease of the collisional broadening coefficient is observed as J increases. These first measurements of pure rotational Raman linewidths of ethane are compared with previously reported infrared and far‐infrared values.

Imaging Isocyanic Acid Photodissociation at 193 nm: the NH(a1Δ)+CO(X1Σ+) Channel

The photodissociation dynamics of isocyanic acid (HNCO) has been studied by the time-sliced velocity map ion imaging technique at 193 nm. The NH(a1Δ) products were measured via (2+1) resonance enhanced multiphoton ionization. Images have been accumulated for the NH(a1Δ) rotational states in the ground and vibrational excited state (v=0 and 1). The center-of-mass translational energy distribution derived from the NH(a1Δ) images implies that the CO vibrational distributions are inverted for most of the measured 1NH(v|j) internal states. The anisotropic product angular distribution observed indicates a rapid dissociation process for the N−C bond cleavage. A bimodal rotational state distribution of CO(v) has been observed, this result implies that isocyanic acid dissociates in the S1 state in two different pathways.

Statistical and dynamical modeling of heavy-ion fusion–fission reactions

A modified statistical model and a four dimensional dynamical model based on Langevin equations have been used to simulate the fission process of the excited compound nuclei 207At and 216Ra produced in the fusion 19F + 188Os and 19F + 197Au reactions. The evaporation residue cross section, the fission cross section, the pre-scission neutron, proton and alpha multiplicities and the anisotropy of fission fragments angular distribution have been calculated for the excited compound nuclei 207At and 216Ra. In the modified statistical model the effects of spin K about the symmetry axis and temperature have been considered in calculations of the fission widths and the potential energy surfaces. It was shown that the modified statistical model can reproduce the above mentioned experimental data by using appropriate values of the temperature coefficient of the effective potential equal to λ=0.0180±0.0055, 0.0080±0.0030 MeV−2 and the scaling factor of the fission barrier height equal to rs=1.0015±0.0025, 1.0040±0.0020 for the compound nuclei 207At and 216Ra, respectively. Three collective shape coordinates plus the projection of total spin of the compound nucleus on the symmetry axis, K, were considered in the four dimensional dynamical model. In the dynamical calculations, dissipation was generated through the chaos weighted wall and window friction formula. Comparison of the theoretical results with the experimental data showed that two models make it possible to reproduce satisfactorily the above mentioned experimental data for the excited compound nuclei 207At and 216Ra.

Pu242 neutron-induced fission cross-section measurement from 1 to 2 MeV neutron energy

Relative values of the neutron-induced fission cross section $\ensuremath{\sigma}(n,f)$ of $^{242}\mathrm{Pu}$ have been measured with respect to the standard $^{1}\mathrm{H}(n,p)$ elastic scattering cross section, at average energies of 1.0, 1.4, and $1.9\phantom{\rule{4pt}{0ex}}\mathrm{MeV}$. The measurements are part of an international effort to reduce uncertainties and provide independent nuclear data relevant for fast-spectrum reactors. The shape of the measured cross section is in good agreement with data from Tovesson et al. [Phys. Rev. C 79, 014613 (2009)] and with the most recent data from Matei et al. [Phys. Rev. C 95, 024606 (2017)], but disagrees with the shapes of ENDF/B-VII.1 and JEFF3.2 evaluations. Absolute values of $\ensuremath{\sigma}(n,f)$, obtained under some assumptions, indicate an overestimation of $\ensuremath{\sigma}(n,f)$ in the evaluated libraries at 1.0 and $1.4\phantom{\rule{4pt}{0ex}}\mathrm{MeV}$, while a good agreement is found with ENDF/B-VII.1 at $1.9\phantom{\rule{4pt}{0ex}}\mathrm{MeV}$. A careful analysis of the impact of scattered neutrons and anisotropy of the fission fragment angular distribution has been performed. The measurement of the neutron flux by means of a proton-recoil detector is discussed. A comprehensive study of corrections applied to the data, of associated uncertainties, and of correlations between the measurements at the different energies is presented.

Finite slice analysis (FINA) of sliced and velocity mapped images on a Cartesian grid.

Although time-sliced imaging yields improved signal-to-noise and resolution compared with unsliced velocity mapped ion images, for finite slice widths as encountered in real experiments there is a loss of resolution and recovered intensities for the slow fragments. Recently, we reported a new approach that permits correction of these effects for an arbitrarily sliced distribution of a 3D charged particle cloud. This finite slice analysis (FinA) method utilizes basis functions that model the out-of-plane contribution of a given velocity component to the image for sequential subtraction in a spherical polar coordinate system. However, the original approach suffers from a slow processing time due to the weighting procedure needed to accurately model the out-of-plane projection of an anisotropic angular distribution. To overcome this issue we present a variant of the method in which the FinA approach is performed in a cylindrical coordinate system (Cartesian in the image plane) rather than a spherical polar coordinate system. Dubbed C-FinA, we show how this method is applied in much the same manner. We compare this variant to the polar FinA method and find that the processing time (of a 510 × 510 pixel image) in its most extreme case improves by a factor of 100. We also show that although the resulting velocity resolution is not quite as high as the polar version, this new approach shows superior resolution for fine structure in the differential cross sections. We demonstrate the method on a range of experimental and synthetic data at different effective slice widths.

Dissociative ionization and Coulomb explosion of CH3I in intense femto second laser fields

The interaction of CH3I molecules with 100 fs 800 nm linearly polarized laser fields has been investigated at the intensity region from 2.6 × 1014 to 5.8 × 1014 W/cm2 by means of a velocity map imaging method. The kinetic energy distribution of the various atomic fragment ions Iq+ (q = 1–3) has been measured and reproduced by a fit of multiple Gaussian functions. Several dissociative ionization and Coulomb explosion channels were identified for Iq+ (q = 1–3). As expected for a geometric alignment dominated interaction process the anisotropic angular recoil distributions of the atomic ion fragments are peaked in the laser polarization direction. The kinetic energy release (KER) of Iq+ (q = 1–3) depending upon the laser intensity has been investigated. The relative weight of the various contributions from the identified dissociative ionization (DI) and Coulomb explosion (CE) channels is found to depend strongly on the laser intensity.Graphical abstract

Dissociative Ionization of Argon Dimer by Intense Femtosecond Laser Pulses.

We experimentally and theoretically studied dissociative ionization of argon dimer driven by intense femtosecond laser pulses. In the experiment, we measured the ion yield and the angular distribution of fragmental ions generated from the dissociative ionization channels of (1,1) (Ar22+ → Ar+ + Ar+) and (2,1) (Ar23+ → Ar2+ + Ar+) using a cold target recoil ion momentum spectroscopy. The channel ratio of (2,1)/(1,1) is 4.5-7.5 times of the yield ratio of double ionization to single ionization of argon monomer depending on the laser intensity. The measurement verified that the ionization of Ar+ is greatly enhanced if there exists a neighboring Ar+ separated by a critical distance. In addition, the fragmental ions exhibit an anisotropic angular distribution with the peak along the laser polarization direction and the full width at half maximum becomes broader with increasing laser intensity. Using a full three-dimensional classical ensemble model, we calculated the angle-dependent multiple ionization probability of argon dimer in intense laser fields. The results show that the experimentally observed anisotropic angular distribution of fragmental ions can be attributed to the angle-dependent enhanced ionization of the argon dimer in intense laser fields.

Fluorimetric oxygen sensor with an efficient optical read-out for in vitro cell models

This paper presents a phase fluorimetric sensor for the monitoring of the oxygen concentration in in vitro cell models. The sensing surface of the sensor consists of oxygen sensitive fluorescent dyes (platinum(II) octaethylporphyrinketone) embedded in a thin polystyrene film. In order to optimize the optical read-out scheme of the sensor, we carried out electromagnetic simulations of a fluorescently doped polystyrene film deposited on a glass-water interface. The simulation results showed highly anisotropic angular emission distribution with the maximum irradiance being at super critical angles, which attracts tailored optical designs to maximize the fluorescence collection efficiency. For this purpose, we applied an efficient optical read-out scheme based on an in-contact parabolic lens. The use of parabolic lens also facilitates confocal total internal reflection excitation from the substrate side. This makes the excitation effective and insensitive to biofouling or other optical changes in the sensing surface and, more importantly, greatly reduces the amount of excitation power radiated into the cell culture chamber. Experimental results show that when applied together with phase fluorimetric lifetime sensing, this optical scheme allows one to use thin films (<500nm), dilute dye-polymer ratios (0.025%), low power LED excitation (<2mW), and yet achieve over 40dB signal-to-noise ratio at 10Hz data rate at physiologically relevant oxygen concentrations. These features are important in cell studies, as the potential cytotoxicity of the dyes and the sensing method (i.e. the production of singlet oxygen) are mitigated by the low dye content and excitation power. In addition, thin and dilute polystyrene films are highly transparent and facilitate optical microscopy. We conclude the study by presenting experimental results where the device is applied together with on-line microscopy to an oxygen tension stress study of beating cardiomyocyte cultures.

Investigation of the 16O+194Pt reaction: One- and two-dimensional dynamical interpretation

In this paper, we applied the Langevin dynamical model to investigate the different aspects of the [Formula: see text]O+[Formula: see text]Pt reaction. Elongation and orientation degree of freedom ([Formula: see text] coordinate) which are the first and second dimensions of dynamical calculations, are presented here. Fission time, fission cross-section, pre-scission neutron multiplicity, and fission probability were calculated using one- and two-dimensional Langevin equations. Also, anisotropy of fission-fragments angular distribution has been investigated based on the transition state model, one- and two-dimensional Langevin dynamical models. It was found that by adding the orientation degree of freedom to calculations, the fission time and pre-scission neutrons multiplicity increases whereas fission cross-section, and fission probability decreases. The two-dimensional dynamical calculations are a better match to the experimental data than the one-dimensional dynamical calculations, when using nominal values for the reduced dissipation coefficient and shape-dependent level density parameter. However, if model parameters are adjusted to reproduce the fission cross-section data, then both the one- and two-dimensional models give a satisfactory match to the fission fragment anisotropy data. Nonequilibrium [Formula: see text] distributions in the dynamical model can better explain the experimental anisotropy of the angular distribution of fission-fragments with respect to the equilibrium [Formula: see text] distribution in saddle and scission point transition state models.

Prompt release of O 1D products upon UV excitation of CH2OO Criegee intermediates

Velocity map imaging has been used to characterize the angular and velocity distributions of O 1D photofragments arising from UV excitation of the simplest Criegee intermediate CH2OO in the long wavelength tail region (364 to 417 nm) of the B1A'-X1A' spectrum. The O 1D images exhibit anisotropic angular distributions indicative of rapid dissociation to H2CO X1A1 + O 1D products, which occurs faster than the rotational period (ps) of CH2OO. As a result, the broad oscillatory structure reported previously in the long wavelength region of the UV absorption spectrum is attributed to short-lived resonances associated with the excited B1A' state of CH2OO, which decay by nonadiabatic coupling to repulsive singlet states. The total kinetic energy distributions show that nearly half of the available energy, on average, is partitioned to product translation. The balance results in significant internal excitation of the H2CO co-fragments. The product anisotropy and energy partitioning are unchanged across the UV spectrum and consistent with previously reported experimental and theoretical findings of the CH2OO B-X transition moment and dissociation energy to H2CO X1A1 + O 1D products.

Propagation of galactic cosmic rays in the outer heliosphere

The propagation of galactic cosmic rays in heliospheric magnetic fields is studied. An approximate solution to the cosmic ray transport equation has been derived on the basis of a method that takes into account the small value of anisotropy of particle angular distribution. The spatial and energy distributions of the cosmic ray intensity and anisotropy have been investigated, and estimates of cosmic ray energy flux have been carried out.

Above-threshold ionization in neon produced by combining optical and bichromatic XUV femtosecond laser pulses

We consider the ionization of neon induced by a femtosecond laser pulse composed of overlapping, linearly polarized bichromatic extreme ultraviolet and infrared fields. In particular, we study the effects of the infrared light on a two-pathway ionization scheme for which Ne 2s22p53s1P is used as intermediate state. Using time-dependent calculations, supported by a theoretical approach based on the strong-field approximation, we analyze the ionization probability and the photoelectron angular distributions associated with the different sidebands of the ionization spectrum. Complex oscillations of the angular distribution anisotropy parameters as a function of the infrared light intensity are revealed. Finally, we demonstrate that coherent control of the asymmetry is achievable by tuning the infrared frequency to a nearby electronic transition.

Theoretical study of different features of the fission process of excited nuclei in the framework of the modified statistical model and four-dimensional dynamical model

Evaporation residue cross section, fission probability, anisotropy of fission fragment angular distribution, mass and energy distributions of fission fragments and the pre-scission neutron multiplicity for the excited compound nuclei and produced in fusion reactions have been calculated in the framework of the modified statistical model and multidimensional dynamical model. In the dynamical calculations, the dynamics of fission of excited nuclei has been studied by solving three- and four-dimensional Langevin equations with dissipation generated through the chaos-weighted wall and window friction formula. Three collective shape coordinates plus the projection of total spin of the compound nucleus to the symmetry axis, were considered in the four-dimensional dynamical model. A non-constant dissipation coefficient of was applied in the four-dimensional dynamical calculations. A comparison of the results of the three- and four-dimensional dynamical models with the experimental data showed that the results of the four-dimensional dynamical model for the evaporation residue cross section, fission probability, anisotropy of fission fragment angular distribution, mass and energy distributions of fission fragments and the pre-scission neutron multiplicity are in better agreement with the experimental data. It was also shown that the modified statistical model can reproduce the above-mentioned experimental data by choosing appropriate values of the temperature coefficient of the effective potential, and the scaling factor of the fission-barrier height, .

Spectral dependence of photoemission in multiphoton ionization of NO2 by femtosecond pulses in the 375-430 nm range.

We investigate the multiphoton ionization of NO2 using tunable (430-375 nm) femtosecond pulses and photoelectron-photoion coincidence momentum spectroscopy. In order to understand the complex electronic and nuclear photodynamics at play following absorption of three to five photons, we also report extended photoionization calculations using correlated targets and coupled channels. Exploring the multiphoton dissociative ionization (MPDI) and multiphoton ionization (MPI) processes over such a broad energy range enables us to lend further support to our work carried out around 400 nm of a femtosecond laser [S. Marggi Poullain et al., J. Phys. B: At., Mol. Opt. Phys., 2014, 47, 124024]. Two excitation energy regions are identified and discussed in terms of the proposed reaction pathways, highlighting the significant role of Rydberg states, such as the [R*(6a1)-1, 3pσ] Rydberg state, in the NO2 multiphoton excitation and photoionization. These new results support our previous assumption that different bent and linear geometries of the NO2+(X1Σg) ionic state contribute to the MPDI and MPI, consistent with the reported calculations which reveal an important vibronic coupling characterizing the photoemission. Remarkably, the strong anisotropy of the recoil frame photoelectron angular distribution (RFPAD) previously observed at 400 nm appears as a fingerprint across the whole explored photon energy range.

Calculation of Neutron Production Rates and Spectra from Compounds of Actinides and Light Elements

The code NEDIS allows the calculation of neutron production rate and continuous energy spectra due to (α,n) reaction on Li, Be, B, C, O, F, Ne, Na Mg, Al, Si, P, S, Cl, Ar, K, and Ca. It accounts for anisotropic angular distribution of neutrons of (α,n) reaction in centre-of- mass system and dimensions of alpha emitting source material particles. Spontaneous fission spectra are calculated with evaluated half-life, spontaneous fission branching, ν- averaged per fission, and Watt spectrum parameters. The results of calculations by NEDIS can be used as input for Monte Carlo simulation for materials that will be used in radiation shielding and for underground neutron experiments