Low Initial Energy Research Articles

Electron-mediated vibration–electronic (V–E) energy transfer in optically pumped plasmas

The paper discusses experiments on vibration-to-electronic energy transfer in CO laser pumped CO–Ar and CO–N 2 plasmas. Ionization in these strongly nonequilibrium plasmas occurs by an associative mechanism, in collisions of two highly vibrationally excited CO molecules. The experiments show that removal of the electrons from the optically pumped plasmas using a saturated Thomson discharge results in considerable reduction of the UV/visible radiation from the plasma (CO 4th positive bands, NO γ bands, CN violet bands, and C 2 Swan bands). At some conditions, the removal of electrons results in a nearly complete extinguishing of the UV/visible glow of the plasma. This effect occurs even though electron removal results in an increase of the high vibrational level populations of the ground electronic state CO(X 1 Σ, v∼15–35). On the other hand, deliberate electron density increase by adding small amounts of O 2 or NO to the optically pumped CO–Ar plasmas produced a substantial increase of the UV/visible radiation intensity, which strongly correlates with the electron density. The results of the present experiments indicate that the vibration-to-electronic (V–E) energy transfer process CO( X 1Σ→ A 1Π ), and, possibly, analogous processes populating radiating excited electronic states of NO, CN, and C 2, in optically pumped plasmas, may be mediated by the presence of electrons which are created in the absence of an electric field, with low initial energies. Most importantly, this effect occurs at ionization fractions as low as n e/ N∼10 −9–10 −7.

RF focusing of ion beams in the axisymmetric periodic structure of a linac

A new approach to the analysis of charge particle motion in periodic resonant RF accelerators is suggested. A three-dimensional equation of motion in the Hamiltonian form is derived. This equation makes it possible to carefully study the relationship between the transverse and the longitudinal dynamics of the ion beams at low initial energies. General conditions for RF focusing in ion linacs are formulated. Basic results are compared with numerical simulation data for the beam dynamics in the polyharmonic field of the accelerating cavity. A version of an RF-focusing proton accelerator in which the current transmission coefficient is close to that in an accelerator with radio frequency quadrupole is described.

Instability versus nonlinearity in certain nonautonomous oscillators: A critical dynamical transition driven by the initial energy.

The equation of motion q+Omega(2)(t)q+alpha/q/(gamma-2)q=0 (gamma>2) for the real coordinate q(t) is studied, as an example of the interplay between nonlinearity and instability. Two contrasting mechanisms determine the behavior of q(t), when the time-varying frequency Omega(t) does produce exponential instability in the linear equation q (lin)+Omega(2)(t)q(lin)=0. At low energy, the exponential instability is the dominant effect, while at high energy the bounding effect of the autonomous nonlinear term prevails. Starting from low initial energies, the result of this competition is a time-varying energy characterized by quasiperiodic peaks, with an average recurrence time T(peak). A closed critical curve S(omega) exists in the initial phase space, whose crossing corresponds to a divergence of the recurrence time T(peak). The divergence of T(peak) has a universal character, expressed by a critical exponent a=1. The critical curve S(omega) is the initial locus of the solutions that vanish asymptotically. A close relationship exists between this dynamical transition and the transition from mobile to self-trapped polarons in one spatial dimension. The application to a number of physical problems is addressed, with special attention to the Fermi-Pasta-Ulam problem and to transitions to chaos.

Generation of long, laminar plasma jets at atmospheric pressure and effects of flow turbulence

Long, laminar plasma jets at atmospheric pressure of pure argon and a mixture of argon and nitrogen with jet length up to 45 fi,Hes its diameter could be generated with a DC are torch by! restricting the movement of arc root in the torch channel. Effects of torch structure, gas feeding, and characteristics of power supply on the length of plasma jets were experimentally examined. Plasma jets of considerable length and excellent stability could be obtained by regulating the generating parameters, including are channel geometry gas flow I ate, and feeding methods, etc. Influence of flow turbulence at the torch,nozzle exit on the temperature distribution of plasma jets was numerically simulated. The analysis indicated that laminar flow plasma with very low initial turbulent kinetic energy will produce a long jet, with low axial temperature gradient. This kind of long laminar plasma jet could greatly improve the controllability for materials processing, compared with a short turbulent are let.

Dynamics of Ar+CH4/Ni{111} collision-induced desorption

Classical trajectory simulations were used to study Ar+CH4/Ni{111} collision-induced desorption and compared with experiment. To perform the simulations, analytic potentials were determined for Ar/CH4 and CH4/Ni{111}. An accurate form for the former potential was derived by carrying out a series of ab initio calculations at various levels of theory, while previously published ab initio calculations were used to develop the latter CH4/Ni{111} potential. Overall the simulation and experimental desorption cross sections are in excellent agreement, except at small incident angles θi (with respect to the surface normal) and low initial Ar translational energies, Ei, where the simulation cross sections are approximately a factor of 2 too large. Most of the desorption occurs by trajectories in which Ar first strikes CH4, but for both large θi and Ei, a small fraction of the desorption occurs by trajectories in which Ar first strikes the Ni surface. Excitation of the CH4 vibrational modes is negligible and CH4 rotation receives less than 10% of the available energy. Most of the available product energy is partitioned to CH4 translation and to the Ni surface and Ar atom. At low Ei, CH4 translation receives the majority of the available energy, with the effect greater for large θi. At high Ei, approximately 40% of the available energy goes to CH4 translation, independent of θi. The CH4 translational energy distribution is multimodal and its peaks may be associated with trajectories in which the Ar atom rebounds off or sticks to the Ni surface and collisions in which Ar strikes CH4 with small and large impact parameters.

Origin of the Boltzmann translational energy distribution in the scattering of hyperthermal Ne atoms off a self-assembled monolayer

A classical trajectory simulation is performed to study the dynamics of Ne-atom scattering off an n-hexyl thiolate self-assembled monolayer (SAM). The energy distribution of the scattered Ne-atoms may be deconvoluted into a Boltzmann component based on the surface temperature and a remaining non-Boltzmann high energy component. The former component becomes as large as 100% at low Ne-atom incident energies and appears to assume a high incident energy asymptotic value of ∽0.2–0.3 for incident angles less than 50°. This Boltzmann component does not arise from a Ne–SAM intermediate, since the vast majority of the collisions are direct with only one inner turning point in the Ne+SAM motion. Instead, it has varying contributions from trajectories which directly scatter from and those that penetrate the SAM. The translation (T)→vibration (V) energy transfer mechanism for the former class of trajectories may have similarities to exit-channel T→V coupling in models of unimolecular dissociation. SAM penetration becomes more important as the initial translational energy Ei is increased, resulting in both efficient and inefficient energy transfer. The latter results from repulsive forces as the Ne-atom is ‘‘ expelled’’ from the SAM. For incident angles less than 50° the size of the Boltzmann component scales with the total incident energy Ei reflecting equivalent energy transfer probabilities from the normal and parallel components of Ei. Such a result is consistent with the corrugated SAM surface. Penetration of the SAM scales with the normal component of Ei for small incident angles θi and Ei . Both the Boltzmann component to the transitional energy distribution of the scattered Ne atom, P(Ei) and SAM penetration become negligible for an increase in θi from 45° to 60°, suggesting an abrupt transition in the collision dynamics. The angular distributions for the scattered Ne-atoms are random for low initial energies Ei and θi, reflecting the surface corrugation and not the presence of an actual Ne–SAM intermediate. At high Ei and θi the scattering is non-random, preferring to remain in the incident plane with the parallel component of Ei conserved. This study shows there are ambiguities in associating a trapping desorption intermediate with statistical gas–surface scattering attributes, such as a Boltzmann component, in the translational energy distribution and a random angular distribution.

Threshold phenomena in ultracold atom–molecule collisions

Vibrational relaxation cross sections for an atom–molecule collision are calculated by explicit numerical solution of the coupled equations of scattering theory and shown to follow an inverse velocity dependence at very low initial kinetic energies of the incident atom, in accordance with Wigner's threshold law. Rate constants for H+H 2( v, j=0)→H+H 2( v′< v, j=0) are calculated for all the vibrational levels of H 2 and found to vary by seven orders of magnitude between the deepest level v=0 and the vibrational level v=12.

Role of migration in the dynamics of bromine atoms reacting with iodine molecules

Abstract Differential cross sections have been measured for the reactive scattering of Br ( 2 P 3 2 ) atoms with I 2 molecules at initial translational energies E ≈ 87 and 52 kJ mol −1 using a supersonic beam of Br atoms seeded in He and Ne buffer gases generated from a high pressure microwave discharge source. The centre-of-mass angular distributions of IBr scattering peak sharply in the forward direction with subsidiary backward peaks of relative height ∼ 0.55 and 0.60. The product translational energy distributions are broad, peaking at a fraction ƒ′ ≈ 0.30 ± 0.05 of the total available energy but with the forward scattering being shifted to lower translational energy than the wide angle scattering. An extended phase space theory is proposed to model the observed reactive scattering without prior assumptions as to the geometry of the collision complex or its mode of dissociation. This provides a good description of the observed reactive scattering only when the effects of a potential energy barrier in the exit valley of the potential energy surface are absent from the calculation. This is attributed to the contribution of migratory trajectories at large impact parameters sampling the less stable IBrI configuration and yielding scattering which is sharply peaked in the forward direction, while smaller impact parameter collisions sample the more stable BrII configuration and yield scattering over a wide angular range. The photoinitiated reaction samples only the BrII configuration due to its constrained geometry and low initial translational energy.

Role of Intersystem Crossing in the Dynamics of the O(3P) + C2H5I Reaction

Reactive scattering of O(3P) atoms with C2H5I molecules has been studied at initial translational energies E ∼ 51 and 16 kJ mol-1 using a supersonic beam of O atoms seeded in He and Ne buffer gas generated from a microwave discharge source. A mildly forward- and backward-peaked angular distribution of IO product observed at lower initial translational energy becomes backward-scattered at higher initial translational energy. An isotropic angular distribution is observed for HOI product at lower initial translational energy which becomes forward- and backward-peaked, slightly favoring the backward direction at higher initial translational energy. Extended phase space calculations indicate that reaction leading to IO product proceeds via intersystem crossing from the lowest triplet 3A‘‘ potential energy surface to a bound OIC2H5 intermediate on the singlet 1A‘ potential energy surface in small impact parameter collisions at low initial translational energy. However, the emergence of backward-scattered IO product indicates the onset of direct reaction over the triplet 3A‘‘ potential energy surface at higher initial translational energy. The formation of HOI reaction product arises from the singlet OIC2H5 complex via a five-membered ring transition state where the H atom is abstracted from the terminal CH3 group at the top of a barrier leading to vibrational excitation of HOI and repulsion between the reaction products in this strongly exoergic exit valley of the potential energy surface.

The role of DNA repair on cell killing by charged particles

It can be noted that it is not simple double strand breaks (dsb) but the non-reparable breaks that are associated with high biological effectiveness in the cell killing effect for high LET radiation. Here, we have examined the effectiveness of fast neutrons and low (initial energy = 12 MeV/u) or high (135 MeV/u) energy charged particles on cell death in 19 mammalian cell lines including radiosensitive mutants. Some of the radiosensitive lines were deficient in DNA dsb repair such as LX830, M10, V3, and L5178Y-S cells and showed lower values of relative biological effectiveness (RBE) for fast neutrons if compared with their parent cell lines. The other lines of human ataxia-telangiectasia fibroblasts, irs 1, irs 2, irs 3 and irs1SF cells, which were also radiosensitive but known as proficient in dsb repair, showed moderate RBEs. Dsb repair deficient mutants showed low RBE values for heavy ions. There experimental findings suggest that the DNA repair system does not play a major role against the attack of high linear energy transfer (LET) radiations. Therefore, we hypothesize that a main cause of cell death induced by high LET radiations is due to non-reparable dsb, which are produced at a higher rate compared to low LET radiations.

Relaxation process of the velocity distribution and transport coefficients of electron swarms in krypton

The relaxation process of the one-dimensional electron swarm in krypton gas from an initial Maxwellian velocity distribution to the hydrodynamic regime is calculated by solving an initial value problem of the Boltzmann equation, which was rewritten in matrix form using the Burnett basis function. The matrix analysis using an eigenvalue and eigenfunction procedure is adopted for calculation of the spatial moments of the velocity distribution and the number density. The time evolution of the isotropic and anisotropic components of the velocity distribution function as well as the time-dependent transport coefficients and the mean energy have been obtained under several conditions of initial mean energy and values of the electric field upon the gas number density E/N. It is shown that, when electrons are released with lower mean energy than that of the hydrodynamic regime, the development of the perpendicular components of the mean energy epsilon a perpendicular to is delayed after the increase of the parallel components epsilon a//. The drift velocity of the centroid and the longitudinal diffusion coefficient of swarms in the earlier time period are found to overshoot the hydrodynamic values in the case of low initial energy in the low E/N region. This can be understood as the result of rapid acceleration of electrons with a smaller collision loss from elastic scattering with neutral gas molecules, which was predicted in the previous work as a negative relaxation time.

Simple calculation of the electron-backscatter factor.

The authors have studied the dependence of the electron-backscatter factor (EBF) on mean electron energy and on backscatterer atomic number by using the semiempirical depth-dose code EDMULT. A plane-parallel electron beam is assumed to be normally incident on a polystyrene slab, which is backed with a layer (backscatterer) of different materials of effectively semi-infinite thickness. A small air cavity to measure ionization is embedded in the polystyrene slab at the boundary facing the backscatterer. The EBF is defined as the ratio of the ionization with the backscatterer to the ionization with a full polystyrene medium, and is approximated by the ratio of the doses computed at the depth of the cavity. Values of EBF obtained show trends similar to the experimental data of Klevenhagen et al. [Phys. Med. Biol. 27, 263-373 (1982)], although the former are generally lower than the latter. When the typical energy spread of clinical electron beams is taken into account, the difference between the experimental and the calculated values is reduced. The present results also show the same trend of increase of the backscatter factor with increasing energy as observed by Klevenhagen et al. in some series of measurements for the lead backscatterer at the lowest energies. This is explained by the rapid buildup of the dose with depth for electrons of low initial energies incident on the full polystyrene medium.

Effect of solvation on the dynamics of H + CH3 association

The gas phase association of CH3 with the HAr2 cluster to form a vibrationally/rotationally excited CH 4 * molecule is used as a model to study microscopic solvation dynamics. A potential energy surface for the reactive system is constructed from a previously fitted H + CH3 ab initio potential and 12-6 Lennard-Jones Ar-Ar, Ar-C, and Ar-H potentials. Classical trajectory calculations performed with the chemical dynamics computer program VENUS are used to investigate the CH3 + HAr2 → CH 4 * + Ar2 reaction dynamics. Reaction is dominated by a mechanism in which the CH3 “strips” the H-atom from HAr2 during large impact parameter collisions. For a large initial relative translational energy the CH3 + HAr2 → CH 4 * + Ar2 cross section is the same as that for H + CH3 association, so that HAr2 acts like a “heavy” H-atom. However, at a low initial relative translational energy, the long-range Ar2—CH3 attractive potential apparently makes the CH3 + HAr2 association cross section larger than that for H + CH3. Partitioning of energy to the CH 4 * and Ar2 products is consistent with a stripping mechanism. The initial and final relative translational energies are nearly identical and the CH 4 * rotational energy is controlled by the initial CH3 rotational energy. The velocity and orbital tilt scattering angles, θ(v i ,v f ) and θ(l i ,l f ), respectively, are consistent with the stripping mechanism. On average only a small amount of the product energy is partitioned to Ar2 vibration/rotation and CH 4 * + Ar2 relative translation.

Determination of absolute strengths of N2 quadrupole lines from high‐resolution ground‐based IR solar observations

High‐resolution, high signal‐to‐noise ratio, solar absorption spectra recorded with a Fourier transform spectrometer at the International Scientific Station of the Jungfraujoch, Switzerland, have been analyzed to determine the strengths of several lines belonging to the S branch of the N2 (1–0) electric quadrupole vibration‐rotation band centered at 2329.9168 cm−1. The method which was applied here was based on equivalent width measurements of lines observed over a broad range of air masses; extrapolation of these measurements to zero air mass gave the line strengths for the transitions S7 to, S10, independent of half widths, an ambiguity unavoidable with the use of curve‐fitting techniques. The resulting absolute accuracies of the line strengths derived here, estimated to be better than ±2.5% for S8, ±2.6% for S10, ±3.4% for S9, and ±5.1% for S7, are due largely to the high quality and quantity of the spectra retained in this analysis and the accuracy with which the observation conditions are known. An important application of the improved values for these N2 transitions, which have low initial ground state energies, is the direct determination of the line‐of‐sight atmospheric air masses associated with remotely sensed infrared spectroscopic observations. Positions of the N2 transitions studied here have further been redetermined with an absolute accuracy better than 0.0002 cm−1.

Elimination of cluster interferences in secondary ion mass spectrometry using extreme energy filtering

A Cameca IMS-3f secondary ion microanalyzer has been modified to allow analysis using secondary ions emitted with initial kinetic energies as high as 4500 eV. Cluster ion signals including hydrides are essentially absent when ions with initial energies < 400 eV are rejected, while atomic ion signals are sufficiently intense to allow trace element analysis. The useful ion yield for 31P − sputtered from a phosphorus-implanted silicon wafer is comparable under these conditions to the useful ion yield in a depth profile obtained at high mass resolution. Because the mass spectrometer is operated at low mass resolution, peak switching is possible to monitor several ion signals in the course of a single depth profile, and the energy acceptance of the mass spectrometer can also be adjusted for individual species so that interference-free ions can be sampled at low initial energy and consequently high sensitivity.

Damage spreading in the Q2R Ising model

We find evidence of metastability in the spreading of damage in the Q2R cellular automaton approximation for a 2D Ising system which is first equilibrated with a standard Metropolis simulation. A subsequent study of both single-site and whole-line damage spreading in Metropolis/Q2R systems as well as in pure Q2R systems shows a logarithmic dependence of the damage-spreading threshold on time, and saturation effects in single-site damage suggest a transition in the damage-spreading threshold at low initial energy values. Examination of the magnetization as a function of energy in pure Q2R shows extremely long relaxation times for p < p c , which may bear some relation to the extremely slow convergence of the damage-spreading threshold.

A classical trajectory study of the O( 3P)+I 2 reaction

A trajectory study of the O( 3P)+ I 2 reaction at three initial translational energies, using a LEPS potential energy function is described. Comparison is made with the experimental measurements of Grice and co-workers. It is found that the symmetry of forward-backward scattering begins to deviate at high collision energies. This may be interpreted in terms of a gradual transition from a long-lived collision complex mechanism at very low initial translational energies to a two-component mechanism with prevailing contribution from backward scattering as the collision energy is increased.

Summary: Our 50-year odyssey with fission

On the occasion of this International Conference on Fifty Years Research in Nuclear Fission, we summarize our present understanding of the fission process and the challenges that lie ahead. The basic properties of fission arise from a delicate competition between disruptive Coulomb forces, cohesive nuclear forces and fluctuating shell and pairing forces. These static forces are primarily responsible for such experimental phenomena as deformed ground-state nuclear shapes, fission into fragments of unequal size, sawtooth neutron yields, spontaneously fissioning isomers, broad resonances and narrow intermediate structure in fission cross sections and cluster radioactivity. However, inertial and dissipative forces also play decisive roles in the dynamical evolution of a fissioning nucleus. The energy dissipated between the saddle and scission points is small for low initial excitation energy at the saddle point and increases with increasing excitation energy. At moderate excitation energies, the dissipation of collective energy into internal single-particle excitation energy proceeds largely through the interaction of nucleons with the mean field and with each other in the vicinity of the nuclear surface, as well as through the transfer of nucleons between the two portions of the evolving dumbbell-like system. These unique dissipation mechanisms arise from the Pauli exclusion principle for fermions and the details of the nucleon-nucleon interaction, which make the mean free path of a nucleon near the Fermi surface at low excitation energy longer than the nuclear radius. With its inverse process of heavy-ion fusion reactions, fission continues to yield surprises in the study of large-amplitude collective nuclear motion. Future challenges include devising experiments to unambiguously distinguish dissipative effects from analogous effects caused by collective degrees of freedom and computing fission directly from the underlying hadronic interaction.

Trajectory studies and sensitivity analysis of rotational energy transfer in gas–surface collisions

The stochastic trajectory method has been applied to the scattering of CO from an LiF(100) surface. At low surface temperature TS, the trajectories of the gas molecule exhibited multiple collisions with the surface. The degree of rotational and translational energy accommodation could be related to the residence time at the surface. The residence time of the molecule on the surface was in turn related to a desorption rate constant which had an Arrhenius form with an activation energy which was about one third of the interaction potential well depth. At high TS most of the trajectories exhibited only one gas–surface collision. In this scattering regime we used stochastic sensitivity analysis (SSA) to obtain first and second order sensitivity coefficients which described how the final rotational and translational energies were coupled to TS and to the initial rotational and translational energies. At low initial translational energies EiT, we found that the most important effect on the final rotational energy of increasing EiT was the increase of the accommodation of energy between the surface modes and the rotational modes of the molecule. The direct coupling of the translational to rotational modes became dominant only at higher EiT. The energy parameters found at high TS with the SSA were also found to yield the per collision rate of energy accommodation at low TS.

Collisional effects on coherent nonlinear wave–particle interactions at cyclotron harmonics

When particle orbits are trapped very near a cyclotron harmonic resonance, the quasilinear concept of weakly perturbed, uncorrelated passages through resonance breaks down, and nonlinear effects become important. In numerical as well as analytic studies, it is demonstrated that relativistic detuning of the resonance can be important for electrons even at low initial energies (∼20 eV) and that coupling to perturbed parallel motion can lead to strong interactions for values of the turning point where the wave frequency differs from a harmonic multiple of the bounce-averaged gyrofrequency by an integral multiple of the bounce frequency. The resultant motion is described by large periodic energy excursions for which small-angle Coulomb collisions or other randomization processes are required to realize net heating. Analytic formulas are derived describing the energy excursion behavior and heating in a mildly relativistic limit. Also, a Monte Carlo numerical model of the collisional effects on the orbits has been employed to study electron heating at the second-harmonic cyclotron resonance and to test the analytic results. In certain regimes of collisionality, a strong enhancement over quasilinear heating has been found.