Isotopic Mass Effects Research Articles

Inelastic neutron scattering investigations of the quantum molecular dynamics of a H2molecule entrapped inside a fullerene cage

The quantum dynamics of dihydrogen molecules entrapped inside fullerene cages has been investigated using inelastic neutron scattering (INS). For the endofullerene H${}_{2}$@C${}_{60}$ the low-lying energy levels of the manifold of coupled translational and rotational states have been accurately determined by studying INS spectra recorded in the temperature range $1.5\ensuremath{\leqslant}T\ensuremath{\leqslant}240\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. The majority of transitions observed in the INS spectra interconvert the nuclear spin isomers orthohydrogen and parahydrogen. The cage potential has icosahedral symmetry and splittings observed in the INS spectra reveal the coupling of translational and rotational angular momentum of the H${}_{2}$ molecules. The effects of nuclear spin symmetry, isotope mass effects, and cage anisotropy have been further investigated by studying HD@C${}_{60}$ and H${}_{2}$ inside an open cage endofullerene. The momentum transfer $\ensuremath{\kappa}$ arising from the neutron scattering event has also been investigated. The $\ensuremath{\kappa}$-dependence spectra reflect the physical dimensions of the dihydrogen molecule and its confinement in its cage. We show how this may be used as a tool for assigning the INS transitions.

Isotope mass and charge effects in tokamak plasmas

The effect of primary ion species of differing charge and mass—specifically, deuterium, hydrogen, and helium—on instabilities and transport is studied in DIII-D plasmas through gyrokinetic simulations with gyro [J. Candy and E. Belli, General Atomics Technical Report No. GA-A26818, 2010]. In linear simulations under imposed similarity of the profiles, there is an isomorphism between the linear growth rates of hydrogen isotopes, but the growth rates are higher for Z > 1 main ions due to the appearance of the charge in the Poisson equation. On ion scales the most significant effect of the different electron-to-ion mass ratio appears through collisions stabilizing trapped electron modes. In nonlinear simulations, significant favorable deviations from pure gyro-Bohm scaling are found due to electron-to-ion mass ratio effects and collisions. The presence of any non-trace impurity species cannot be neglected in a comprehensive simulation of the transport; including carbon impurity in the simulations caused a dramatic reduction of energy fluxes. The transport in the analyzed deuterium and helium discharges could be well reproduced in gyrokinetic and gyrofluid simulations while the significant hydrogen discrepancy is the subject of ongoing investigation.

Isotopic mass effects for low-energy channeling in a silicon crystal

Monte Carlo (MC) simulations have been used to study the low-energy channeling of 10B and 11B ions along the [100] axis in Si crystal. MC simulations show that the critical angle ΨC ≈ 15.3(keV/E)1/2 (in degrees) for the channeling of isotopic 10B ions and ΨC=14.5(keV/E)1/2 (in degrees) for the channeling of isotopic 11B ions, where E is the incident energy. This means that (ΨC for 10B ions/ΨC for 11B ions)≈ (15.3/14.5)≈ (11/10)1/2.

Comment on “Isotopic mass effects for low-energy channeling in a silicon crystal”

In April, 2010, after reading “An improved critical angle equation for ion channeling” (Zheng theory) proposed by Zheng et al. (1) in Nuclear Instruments and Methods in Physics Research Section B (NIM-B), we felt very strange, because the theory contradicts the well-known Lindhard theory (2) and many of the experimental results published during the last few decades (3). Therefore, we submitted our first comments (4) to NIM-B for publication on 15 April 2010. In September 2010, we had found a new paper published by Zheng et al. (5) in Radiation Effects and Defects in Solids. In the latter paper (5), Zheng et al. studied the isotopic mass effects for low-energy channeling in a silicon crystal by using Monte Carlo (MC) simulations. Immediately, we remembered that a similar work had been published by Zheng et al. two years before for low-energy ion channeling in single-wall carbon nanotubes (SWCNT) (6). The aim of the two papers (5, 6) is finding simulation evidence to support the Zheng theory (1) by studying isotopic ion channeling.

Product Rotational Polarization in the Li + HF(v = 0, j = 0) Reaction and Its Isotopic Variants

Product rotational polarization in the title reactions is investigated by employing the quasi-classical trajectory method. An accurate ground 12A′ potential energy surface [J. Chem. Phys. 107 (1997) 10085] and a collision energy of 110 meV are adopted in these calculations. The commonly used rotational alignment parameter A(2)0 and the three angular distributions P(θr), P(ϕr), and P(θr, ϕr) (θr and ϕr being the polar angles of the product angular momentum) are computed in the cm. frame to gain insight into the product rotational alignment and orientation. It is found that the product rotational angular momentum prefers not only to align perpendicular to the reagent relative velocity vector k but also orientate along the negative y axis, which is successfully interpreted by an impulsive mode. The isotope mass effect on the product rotational polarization is also revealed and discussed.

Theoretical study of stereodynamics for the reaction O(3P) + D2 (v = 0, j = 0) → OD+D and isotope effect

Quasi-classical trajectory (QCT) calculations have been performed to study the product polarization behaviours in the reaction O(3P) + D2 (v = 0, j = 0) → OD + D. By running trajectories on the 3A′ and 3A″ potential energy surfaces (PESs), vector correlations such as the distributions of the polarization-dependent differential cross sections (PDDCSs), the angular distributions of P(θr) and P(φr) are presented. Isotope effect is discussed in this work by a comprehensive comparison with the reaction O(3P) + H2 (v = 0, j = 0) → H + H. Common characteristics as well as differences are discussed in product alignment and orientation for the two reactions. The isotope mass effect differs on the two potential energy surfaces: the isotope mass effect has stronger influence on P(θr) and PDDCSs of the 3A′ PES while the opposite on P(φr) of the 3A″ potential energy surface.

Product rotational Franck–Condon oscillations in HOD (Jka , kc ) dissociation

The technique of H(D) atom photofragment translation spectroscopy has been used to investigate the dissociation of jet-cooled HOD molecules following excitation to individual rovibrational levels of its (1B1) Rydberg state near 124 nm. Spectra have been recorded for both the D + OH and H + OD dissociation channels. The branching ratios between OH/OD(X , high v, low N), OH/OD(X , low v, high N), and OH/OD(A ) channels vary between different parent rotational levels, and between the OH and OD products. The variation between the OH and OD channels can be attributed to an isotopic mass effect on the rotational axes which influences non-adiabatic coupling strengths. In addition, there is a population alternation with product rotational quantum number N for the OH/OD(X, high v, low N) product, most striking for the OH case, the sense of which is correlated with the value of the parent pseudo-quantum number . It is shown that this is a consequence of recoil forces which lead to a closing of the HOD angle to near 90° accompanying non-adiabatic transfer from the state to the (1B1) state. The oscillation in population then derives from the symmetry properties around this angle of the product rotational wavefunctions. This suggests that the symmetry-induced population oscillation can only occur for a narrowly defined angular dissociation path. Model calculations also explain the different structure of the spin-orbit doubled low N band heads for each v(OH/OD) which also alternates for even and odd . The necessary conditions to observe these phenomena are discussed.

A large iron isotope effect in SmFeAsO1 - xF x and Ba1 - xK x Fe2As2

The recent discovery of superconductivity in oxypnictides with a critical transition temperature (T(C)) higher than the McMillan limit of 39 K (the theoretical maximum predicted by Bardeen-Cooper-Schrieffer theory) has generated great excitement. Theoretical calculations indicate that the electron-phonon interaction is not strong enough to give rise to such high transition temperatures, but strong ferromagnetic/antiferromagnetic fluctuations have been proposed to be responsible. Superconductivity and magnetism in pnictide superconductors, however, show a strong sensitivity to the crystal lattice, suggesting the possibility of unconventional electron-phonon coupling. Here we report the effect of oxygen and iron isotope substitution on T(C) and the spin-density wave (SDW) transition temperature (T(SDW)) in the SmFeAsO(1 - x)F(x) and Ba(1 - x)K(x)Fe(2)As(2) systems. The oxygen isotope effect on T(C) and T(SDW) is very small, while the iron isotope exponent alpha(C) = -dlnT(C)/dlnM is about 0.35 (0.5 corresponds to the full isotope effect). Surprisingly, the iron isotope exchange shows the same effect on T(SDW) as T(C). This indicates that electron-phonon interaction plays some role in the superconducting mechanism, but a simple electron-phonon coupling mechanism seems unlikely because a strong magnon-phonon coupling is included.

Non-equilibrium isotopic and elemental fractionation during diffusion-controlled crystal growth under static and dynamic conditions

Rapid crystal growth can lead to disequilibrium uptake of growth-medium components whose diffusivities limit their dispersal near an advancing crystal interface. The recent documentation of an isotope mass effect on diffusion raises the possibility that even isotope ratios in crystals may be subject to this effect. Building upon existing 1-dimensional treatments, we describe a numerical modeling approach in which a spherical grain grows at the center of an infinite spherical medium of predetermined composition. Local equilibrium at the interface between the crystal and the growth medium is assumed, but the concentration of the species of interest in the growth medium is allowed to vary near the interface as a consequence of slow diffusion combined with rejection from (or incorporation within) the growing crystal. The disequilibrium uptake of elements and isotopes depends upon the ratio of crystal growth rate ( R) to diffusivity in the growth medium ( D). Conditions of fast mineral growth in a viscous magma—e.g., in lava lakes or small igneous bodies—result in accumulation of elements with K << 1 (or depletion of elements with K >> 1) near the growing mineral interface, forming a compositional boundary layer in the growth medium. In a static system, the magnitude of this compositional perturbation depends critically upon the diffusivity of the element or isotope of interest in the growth medium. If the system is dynamic—i.e., experiencing free or forced convection—then the vigor of convection also affects behavior. Significant fractionation of elements and isotopes is predicted to occur within the boundary layer during progressive crystal growth because diffusion rates of individual elements vary with size and charge and those of isotopes of the same element depend on their masses. Local equilibrium at the interface between the crystal and its growth medium means that a fast-growing crystal will record this fractionation in its resulting radial concentration profile. If the boundary-layer thickness, BL, is small (say, < 100 μm) and the equilibrium partition coefficient, K, is < 0.5, then a first-order estimate of the steady-state isotopic fractionation in a growing crystal is given by δ ( ‰ ) = 1000 ⋅ ( 1 − D A D B ) ⋅ ( R ⋅ B L D A ) ⋅ ( 1 − K ) , where D A and D B are the diffusivities of the faster and slower species in the growth medium and δ is the deviation from the equilibrium isotope ratio in parts per thousand. For isotopes of a single element, D A and D B will generally differ by < 1%, but plausible R/ D ratios can nevertheless lead to deviations from equilibrium between the crystal and the growth medium of up to ~ 3‰. The model may bear on disequilibrium crystal-growth phenomena in a variety of geologic settings—including element- and isotopic profiles in crystals of both igneous and metamorphic rocks. It is suggested that compositional core to rim profile of a crystal may be a proxy for the near surface composition of the growth medium during crystal growth. Isotopic effects are discussed in detail because these have not been addressed previously; igneous systems are emphasized because higher crystal growth rates are more conducive to disequilibrium (including in the compositions of melt inclusions).

Isotopic Mass Effects for Low-Energy Ion Channeling in Single-Wall Carbon Nanotubes

Monte Carlo (MC) simulation studies of isotopic mass effects have been conducted, for low-energy channeling of 12C and 13C ions in single-wall carbon nanotubes. It is found in MC simulations that (ΨC for 12C/ΨC for 13C) = (13/12)1/2, for the same incident energies. Here, ΨC for 12C is the critical angle for 12C channeling, andΨC for 13C is the critical angle for 13C channeling. This work shows the mass effect to be the incident momentum one, but a recent work (Moura, C. S.; Amaral, L. J. Phys. Chem. B 2005, 109, 13515) considered it to be the energy transfer effect. Our work disagrees with that of Moura and Amaral.

Experimental Verification of Hydrogen Isotope Separation by Pressure Swing Adsorption

Focusing on synthetic zeolites that adsorb hydrogen isotopes at liquid N2 temperature with priority in the order of T2, DT, D2, HT, HD and H2, we have been developing a pressure swing adsorption process system for hydrogen isotope separation. For this purpose, we carried out fundamental experiments of adsorption and desorption of a tracer D2 in bulk H2 with zeolite packedbed columns. In this paper, the results are reported that D2 is enriched in the adsorbed phase at separation factors near 2.0, flowing through zeolite 5A and 13X packed-beds at 77.4 K. These are in agreement with values predicted from the multi-component equilibrium characteristics. In the gas samples recovered by evacuating the packed-beds, however, D2 was detected at a relative concentration of 1.20 or 1.32 to that in the feed gas. This lower range results from the isotopic mass effect in kinetic process. That suggests a highly D2-enriched residual left during evacuation. This is verified with an unusually high enrichment factor of 6.68 or 9.21 for zeolite 5A or 13X measured in the residual sample desorbed from the packed-bed by heating up to room temperature.

Diffusional behavior of tritium in V–4Cr–4Ti alloy

Tritium diffusion behavior in a V–4Cr–4Ti (NIFS-Heat-2) alloy has been examined with a tritium tracer technique. Firstly, a small amount of tritium (T) was implanted into the specimen surface, and then the specimen was diffusion-annealed at temperatures ranging from 373 K to 573 K. The diffusion depth profile of T in the specimen was measured with a tritium imaging plate (IP) technique to determine the diffusion coefficient. The obtained diffusion coefficient of tritium in V–4Cr–4Ti is expressed as D t ( cm 2 / s ) = ( 7.5 ± 0.2 ) × 10 - 4 exp ( - 0.13 ( eV ) / kT ) , which is lower than that in pure vanadium, and is comparable with literature values of protium in a V–4Ti alloy taking the isotope mass effect into consideration.

Long-range interactions forHe(nS)–He(n′S)andHe(nS)–He(n′P)

The energetically lowest five states of a helium atom are: He($1^1S$), He($2^3S$), He($2^1S$), He($2^3P$), and He($2^1P$). Long-range interaction coefficients $C_3$, $C_6$, $C_8$, $C_9$, and $C_{10}$ for all $S-S$ and $S-P$ pairs of these states are calculated precisely using correlated wave functions in Hylleraas coordinates. Finite nuclear isotope mass effects are included.

Publisher’s Note: “Effect of isotope mass on simulations of the high-mode pedestal and edge localized modes” [Phys. Plasmas 12, 112508 (2005)

First Page

Fundamentals and Applications of Isotope Effect in Modern Technology

Different crystals (semiconductors and insulators) with varying isotopic composition have been recently grown. I discuss here the effect of isotopic mass and isotopic disorder on the properties (vibrational, elastic, thermal and optical) of different crystals. The main applications of the stable isotopes are included self-diffusion, neutron transmutative doping (NTD) of different semiconductors, optical fibers, isotope-based quantum computers, etc. Because of space limitations this discussion will not exhaustive. I hope however, to give sufficient references to published work so that the interest reader can easily find the primary literature sources to this rapidly expanding field of solid state physics.

Effect of isotope mass on simulations of the high-mode pedestal and edge localized modes

Simulations of Joint European Torus (JET) [P. H. Rebut et al., Nucl. Fusion 25, 1011 (1985)] type I high-mode (H-mode) discharges with edge localized modes (ELMs) are used to study the effect of isotope mass on the height of the pedestal and the frequency of ELMs. A dynamic model for the H-mode pedestal and ELM cycles is employed in the JETTO integrated modeling code [M. Erba et al., Plasma Phys. Control. Fusion 39, 261 (1997)]. The stability criteria that are used to trigger ELM crashes in the JETTO simulations are calibrated with the HELENA and MISHKA ideal magnetohydrodynamic (MHD) stability codes [A. B. Mikhailovskii et al., Plasma Phys. Rep. 23, 713 (1997)]. Results obtained using a pedestal model in which the pedestal width increases with isotope mass are compared with those obtained using a fixed, prescribed pedestal width. In JET type I ELMy H-mode discharges, it is found that the height of the pressure pedestal increases and that the frequency of the ELMs decreases as the isotope mass is increased. Both of these experimentally observed trends are obtained simultaneously in the simulations only if the pedestal width increases with isotope mass. The physical processes that play a significant role in these simulations are described.

Effects of pressure on the phonon–phonon and electron–phonon interactions in semiconductors

AbstractWe discuss the dependence of phonon frequencies on hydrostatic pressure for tetrahedrally bonded semiconductors, in particular the linear term represented by mode Grüneisen parameters. The relation between these parameters and the thermal expansion is also elucidated. Emphasis is placed on recent results that concern the zero‐point renormalization of lattice parameters and its dependence on isotopic masses. The pressure dependence of transverse charges of ir‐active phonons is also touched upon. We also discuss strong pressure and isotopic mass effects on phonon widths and line shapes that are occasionally observed for selected materials. Finally, we present recently observed effects of pressure on the luminescence of isotopically pure silicon which are strong enough to be easily detected in the 0–1 bar range. (© 2004 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Effect of isotopic mass on the photoluminescence spectra of zinc oxide

ZnO is a wurtzite-type semiconductor at ambient conditions whose natural composition consists of almost pure 16O and a mixture of 48.6% 64Zn, 27.9% 66Zn, 4.1% 67Zn, 18.8% 68Zn, and 0.6% 70Zn. In this work, we report a photoluminescence study of different samples of isotopically pure and natural zinc oxide between 15 and 300 K. The isotopic coefficients of the bound exciton energy have been obtained and compared with previous values for the shift of the free A exciton energy. The isotopic coefficients of the bound exciton energy have allowed us to estimate the contribution of the zinc and the oxygen vibrations to the bandgap renormalization by electron–phonon interaction. A two-oscillator model based on the zinc and oxygen renormalization energies has been used to account for the temperature dependence of the bandgap in ZnO.

Adsorption and isotope effects by ion exchange with 1-aza-12-crown-4 bonded Merrifield peptide resin

Magnesium isotope effects were investigated by chemical ion exchange with synthesized 1-aza-12-crown-4 bonded Merrifield peptide resin using elution chromatography. The capacity of azacrown ion exchanger was 0.89 meq/g dry resin. The heavier isotopes of magnesium were enriched in the resin phase, while the lighter isotopes were enriched in the solution phase. The hydration effect is less than the complexation and isotope mass effects. The single stage separation factor was determined according to the method of Glueckauf from the elution curve and isotopic assays. The separation factors of 24Mg 2+– 25Mg 2+, 24Mg 2+– 26Mg 2+, and 25Mg 2+– 26Mg 2+ were 1.012, 1.023, and 1.011, respectively.

Adsorption and Isotope Effects by Ion Exchange with Aqueous-2-Aminomethyl-18-Crown-6 Bonded Merrifield Resin

Magnesium isotopes effects were investigated by chemical ion exchange using synthesized 2-aminomethyl-18-crown-6 (AM18C6) bonded Merrifield peptide resin. It was found that separation factors larger those reported previously were obtained, and the hydration and isotope mass effects are more significant than that of the complexation. The capacity of the crown ion exchanger was 2.3 meq/g dry resin. The adsorption capacity of the resin for magnesium ion was 26.8 mg/g dry resin at pH 7. The heavier isotopes of magnesium were enriched in the solution phase, while the lighter isotopes were enriched in the resin phase. The separation factors of 24Mg– 25Mg, 24Mg– 26Mg, and 25Mg– 26Mg were 1.0085, 1.0162, and 1.0081, respectively.