Ultralow Energies Research Articles

Extrapolation of astrophysical S factors for the reaction 14N((p, γ) 15O to near-zero energies

The astrophysical S factors for the radiative-capture reaction 14N(p, γ)15O in the region of ultralow energies were calculated on the basis of the R-matrix approach. The values of the radiative and protonic widths were fitted to new experimental data. The contribution of direct radiative capture to bound states of the 15O nucleus was determined with the aid of asymptotic normalization coefficients, whose values were refined in the present study on the basis of the results obtained from an analysis of the reaction 14N(3He, d)15O at three different energies of incident helium ions. A value of S(0) = 1.79 ± 0.31 keV b was obtained for the total astrophysical S factor, and the reaction rate was determined for the process 14N(p, γ)15O.

An ultra-low energy (30–200 eV) ion-atomic beam source for ion-beam-assisted deposition in ultrahigh vacuum

The paper describes the design and construction of an ion-atomic beam source with an optimized generation of ions for ion-beam-assisted deposition under ultrahigh vacuum (UHV) conditions. The source combines an effusion cell and an electron impact ion source and produces ion beams with ultra-low energies in the range from 30 eV to 200 eV. Decreasing ion beam energy to hyperthermal values (≈10(1) eV) without loosing optimum ionization conditions has been mainly achieved by the incorporation of an ionization chamber with a grid transparent enough for electron and ion beams. In this way the energy and current density of nitrogen ion beams in the order of 10(1) eV and 10(1) nA/cm(2), respectively, have been achieved. The source is capable of growing ultrathin layers or nanostructures at ultra-low energies with a growth rate of several MLs/h. The ion-atomic beam source will be preferentially applied for the synthesis of GaN under UHV conditions.

Cold Chemistry with Ionic Partners: Quantum Features of HeH+(1Σ) with H(1S) at Ultralow Energies

Quantum reactive calculations are presented for an ion-atom reaction involving the HeH(+)cation and its destruction via a barrierless interaction with H atoms. The range of collision energies considered is that of a cold trap regime (around and below millikelvin) where the ionic partner could be spatially confined. Specific resonant features caused by the interplay of the strong ionic interaction with the very slow partners' dynamics are found and analyzed. Indications are also given on the consequences of the abstraction mechanism that acts for this reaction at low energies.

Zeeman relaxation of MnH (X7Σ+) in collisions withHe3: Mechanism and comparison with experiment

We present a theoretical study of the Zeeman relaxation of the magnetically trappable lowest field seeking state of MnH (${}^{7}\ensuremath{\Sigma}$) in collisions with $^{3}\mathrm{He}$. We analyze the collisional Zeeman transition mechanism as a function of the final diatomic state and its variation as a function of an applied magnetic field. We show that as a result of this mechanism the levels with $\ensuremath{\Delta}{M}_{j}>2$ give negligible contributions to the Zeemam relaxation cross section. We also compare our results to the experimental cross sections obtained from the buffer-gas cooling and magnetic trapping of this molecule and investigate the dependence of the Zeeman relaxation cross section on the accuracy of the three-body interaction at ultralow energies.

Study of slow and fast extraction for the ultralow energy storage ring (USR)

The Facility for Antiproton and Ion Research (FAIR, GSI) will not only provide future users with high fluxes of antiprotons in the high-energy range, but it is also intended to include a dedicated program for low-energy antiproton research in the keV regime, realized with the FLAIR project. The deceleration of antiprotons with an initial energy of 30 MeV down to ultralow energies of 20 keV will be realized in two steps. First, the beam is cooled and slowed down to an energy of 300 keV in a conventional magnetic ring, the Low energy Storage Ring (LSR) before being transferred into the electrostatic Ultralow energy Storage Ring (USR). In this synchrotron the deceleration to a final energy of 20 keV will be realized. This paper describes the ion-beam optical and mechanical layout of the beam extraction from the USR and summarizes the expected beam qualities of extracted beams.

Computing charge-exchange cross sections for Ca+ collisions with Rb at low and ultralow energies

Collisions between Ca cations and Rb atoms are computed within a quantum approach that generates the most relevant potential energy curves from accurate ab initio methods and carries out the low-energy scattering calculations by including nonadiabatic and spin-orbit coupling terms. The cross sections are obtained at relative energies typical for the likely arrangements of Rb atoms in a Magneto-Optical Trap overlapped with a Coulomb Crystal of Ca cations. The dominant nonadiabatic process is clearly identified and the efficiency of the nonadiabatic coupling terms which lead to the charge-exchange process is discussed.

Cold collisions of an open-shell S-state atom with a 2Π molecule: N(4S) colliding with OH in a magnetic field

We present quantum-theoretical studies of collisions between an open-shell S-state atom and a (2)Π-state molecule in the presence of a magnetic field. We analyze the collisional Hamiltonian and discuss possible mechanisms for inelastic collisions in such systems. The theory is applied to the collisions of the nitrogen atom ((4)S) with the OH molecule, with both collision partners initially in fully spin-stretched (magnetically trappable) states, assuming that the interaction takes place exclusively on the two high-spin (quintet) potential energy surfaces. The surfaces for the quintet states are obtained from spin-unrestricted coupled-cluster calculations with single, double, and noniterative triple excitations. We find substantial inelasticity, arising from strong couplings due to the anisotropy of the interaction potential and the anisotropic spin-spin dipolar interaction. The mechanism involving the dipolar interaction dominates for small magnetic field strengths and ultralow collision energies, while the mechanism involving the potential anisotropy prevails when the field strength is larger (above 100 G) or the collision energy is higher (above 1 mK). The numerical results suggest that sympathetic cooling of magnetically trapped OH by collisions with ultracold N atoms will not be successful at higher temperatures.

Experimental determination of the electron screening potential energy for the d(d, n)3He Reaction in ZrD2 and D2O in the ultralow energy region

This work is devoted to measuring of the values of the astrophysical S-factors and electron screening potential energy for a d(d,n)3He reaction occurring at ultralow energies in zirconium deuteride ZrD2 (3.5–7.0 keV) and heavy water D2O (2.2–6.0 keV). The experiment was performed on the Hall pulsed plasma accelerator at the TPU Nuclear Physics Institute (Tomsk) with ZrD2 and D2O targets produced by the magnetron sputtering of zirconium in a deuterium environment and heavy water freezing-out on a copper support, respectively. A χ2 analysis of the dependence of the neutron yields and astrophysical S-factors for the dd reaction on the deuteron collision energy E revealed that the upper bounds of the electron screening potential energy for interacting deuterons in ZrD2 and D2O and of the astrophysical S-factors at the deuteron collision energy E = 0 were Ue(ZrD2) < 30 eV, Ue(D2O) < 25 eV, S(0) = (57.2 ± 3.9) keV · b (ZrD2), S(0) = (58.6 ± 3.6) keV · b (D2O) at the 90% confidence level.

Dispersion interactions and reactive collisions of ultracold polar molecules

Progress in ultracold experiments with polar molecules requires a clear understanding of their interactions and reactivity at ultralow collisional energies. Two important theoretical steps in this process are the characterization of interaction potentials between molecules and the modeling of the reactive scattering mechanism. Here, we report on the ab initio calculation of isotropic and anisotropic van der Waals interaction potentials for polar KRb and RbCs colliding with each other or with ultracold atoms. Based on these potentials and two short-range scattering parameters, we then develop a single-channel scattering model with flexible boundary conditions. Our calculations show that, at low temperatures (and in the absence of an external electric field), the reaction rates between molecules or between molecules and atoms have a resonant character as a function of the short-range parameters. We also find that both the isotropic and anisotropic van der Waals coefficients have significant contributions from dipole coupling to excited electronic states. Their values can differ dramatically from those solely obtained from the permanent dipole moment. A comparison with recently obtained reaction rates of fermionic 40K87Rb shows that the experimental data cannot be explained by a model where the short-range scattering parameters are independent of the relative orbital angular momentum or partial wave.

Time-resolved photoelectron spectroscopy of bulk liquids at ultra-low kinetic energy

Time-resolved photoelectron spectroscopy (TR-PES) of ultrafast dynamics in solution is presented. To measure the photoelectron kinetic energy distribution (PKED) that is free from inelastic scattering in solution, photoelectrons were generated with ultra-low kinetic energies (ULKE: <5 eV). Time constants of the elementary processes in the charge-transfer-to-solvent (CTTS) reaction from I − to bulk water were in excellent agreement with those obtained by transient absorption spectroscopy, demonstrating the bulk-sensitivity of TR-PES-ULKE. The analysis suggests that the CTTS reaction proceeds via two intermediates, and that 30% of the first intermediate and 70% of the second intermediate respectively are quenched by geminate recombination between the electron and the neutral iodine atom.

Quenching of internal rotations versus collisional cooling at ultralow energies for weakly interacting partners: Cs2(3Σu+)withHe3,4

Quantum-scattering calculations at ultralow (close to 10{sup -6} cm{sup -1}) collision energies are carried out for the Cs dimer in its spin-stretched triplet state, interacting with helium. An ab initio potential energy surface is computed and employed, while the target molecule is kept in its ground vibrational state and several excitated initial rotational states are considered in the quantum dynamics. The highly anisotropic interaction is seen to cause, in spite of its weakness, internal energy quenching rates comparable with the efficiency of the collisional cooling of relative kinetic energies. The rates of spin-flip processes are also analyzed and compared with pure rotational quenching events.

Indirect measurement of 17O(p,α)14N cross section at ultra-low energies

The indirect measurement of 17O(p,α)14N cross section was performed by means of the Trojan Horse Method. This approach allowed to investigate the ultra-low energy range (Ec.m. = 0–300 keV) relevant for several astrophysics environments, where two resonant levels of 18F at ERc.m. = 65 keV and ERc.m. = 183 keV play a significant role in the reaction rate determination.

LiH-(X2Σ+)+3,4He rotational quenching at ultralow energies: spin-flip and isotopic effects from quantum dynamics on an ionic system

We study in this work the quantum scattering between a helium atom and a LiH- molecule at low and ultralow energies. For the noble gas we have considered the two natural isotopes 3He, 4He, plus a “mock” 3.5He. Our aim is to clarify the role played by the isotopic changes of the mass in the buffer gas (in this case, helium) during collision events where the interaction is described through a newly computed ab initio rigid rotor potential between He and LiH-. The main conclusion of the present study is that this anionic species could be a good candidate for He-driven sympathetic cooling in traps since its elastic cross sections are always found to be larger than the inelastic ones in a relevant range of nearly vanishing energies. The 3He isotope is found to be more efficient than the other examples, yielding larger quenching rates in comparison with its heavier counterparts. Spin-flip inelastic channels are also analysed and discussed in terms of their dependence on the present, weak, interaction potential between partners and found to provide a further, interesting feature of the present mixture. The new results are also compared with those found by us in earlier, similar, quantum calculations on different systems.

Scattering of Yb andYb+

We calculate the cross sections for scattering of a neutral ytterbium atom with its positive ion at energies up to 30 eV. We identify peaks in the forward direction as arising from elastic collisions and those in the backward direction as from charge transfer. We show that the total cross section follows a semiclassical expression over a large range of energies and that the resonant charge transfer may be characterized by four energy regimes: an $s$-wave regime at ultralow energies, a modified Langevin regime at low energy, a Langevin regime at medium energy, and an ``exchange'' regime at high energy. We find large variations between the different isotopes for the two lowest-energy regimes and very little variation for the two highest-energy regimes. Our results are consistent with recent measurements.

Translational cooling versus vibrational quenching in ultracold OH−–Rb collisions: A quantum assessment

A new three-dimensional potential energy surface (PES), which describes the interaction between the OH(-)(X (1)Sigma(+)) anionic molecule and the Rb((2)S) atomic gas has been obtained by using a highly correlated ab initio model of the electronic structure and a large atomic basis set supplemented with a set of midbond Gaussian functions. This accurate PES is being employed to evaluate the OH(-)(X (1)Sigma(+)) vibrational quenching efficiency induced by collisions with Rb((2)S) at low and ultralow energies. Calculations employ the coupled-channel ansatz to describe the nuclear quantum dynamics and finally show a very marked dependence of the inelastic cross sections and rates, at vanishing collision energies, on the initial vibrational energy content of the partner molecules. The present findings allow us to make specific suggestions on the likely outcomes from energy interplay between the title species within a mixed MOT/Paul Trap environment.

Preliminary design studies of an extraction of the USR

The next-generation Facility for Low energy Antiproton and Ion Research (FLAIR) at GSI is going to be a dedicated research facility for ion research in the keV range. These ion beams will allow to explore fundamental properties in matter-antimatter research at ultra-low energies of only 20 keV/q in hitherto impossible experiments. To provide these very low energy beams, the Ultra-low energy Storage Ring (USR), an electrostatic synchrotron, will play a major role within the FLAIR complex. It combines the electrostatic storage mode with deceleration from an initial energy of 300 keV/q down to 20 keV/q—as well as an efficient beam cooling. To fulfill its role as a multi-purpose experimental facility, the design of the USR has not only to cover in-ring experiments, but needs to include a highly flexible beam extraction for serving different external experiments as well.

Medium energy ion scattering analysis of the evolution and annealing of damage and associated dopant redistribution of ultra shallow implants in Si

As junction depths in advanced semiconductor devices move to below 20 nm, the process of disorder evolution during ion implantation at ultra low energies becomes increasingly influenced by the surface. This may also hold for shallow regrowth and dopant redistribution processes during subsequent thermal annealing of the substrate. The investigation of these near-surface processes requires analytical techniques with a depth resolution of≤1 nm. Medium energy ion scattering (MEIS) has the unique capability of simultaneously providing quantitative, high-resolution depth distributions of implant disorder (displaced Si lattice atoms) and of implanted atoms, albeit not of light species. We report here a comparative MEIS investigation into the growth mode of shallow disordered/amorphised layers during≤1 keV B+ and 2.5 keV As+ ion implantation into Si. In both cases the growth of the damage depth profiles differs significantly from the energy deposition function, as it is strongly determined on the one hand by the proximity of the surface acting as a nucleation site for migrating point defects formed during implantation, which results in planar growth of the amorphous layer, and on the other by the dynamic annealing processes operating at room temperature. When such defect recombination processes are inhibited, e.g. for low dose, ultra shallow 200 eV B+ implants, MEIS shows that defect production yields exceeding the Kinchin–Pease model predictions are achieved. For As implants, a correlation is observed between the movement of the As and the depth of the growing, planar amorphous layer. Thermal annealing of As implanted samples at different temperatures and durations leads to solid phase epitaxial regrowth. During regrowth, MEIS shows that there is a close correlation between damage dissolution, the movement of nearly half of the As dopant into substitutional sites and the snowploughing of a fraction of the As in front of the advancing amorphous/crystalline interface leading to the formation of a less than 1 nm wide As pile-up layer trapped under the oxide.

Astrophysical S-factors for р 2Н and р 3Н radiative capture

The feasibility of description of new experimental data on the astrophysical S-factors for р 2Н and р 3Н radiative capture is studied at ultralow energies within the framework of the potential cluster model with forbidden states. It is demonstrated that the examined approach describes correctly the experimental S-factors for р 2Н and р 3Н systems at energies down to 10 and 50 keV.

Formation of Ge nanocrystals in a silicon dioxide layer using pulsed plasma-immersion ion implantation

Pulsed plasma-immersion ion implantation (PIII) or Pulsed PLAsma Doping (P 2LAD) is known as a cost effective solution for ultra shallow junction formation due to its capability to implant doping species at ultra-low energies (0.05–5 keV), the advantages of P 2LAD, high concentration and sharp distribution of the implanted species, also make this technique a good candidate to fabricate nanocrystals (NCs) within silicon dioxide (SiO 2) layer. In this work, we report Ge NC fabrication within a SiO 2 layer by using the pulsed PIII technique for the first time. GeH 4 (4 sccm) and He (100 sccm) gases were flown to the plasma chamber, and a voltage of 4.5 kV was applied. After pulsed PIII process, furnace annealing was performed at 900 °C in nitrogen atmosphere for Ge agglomeration. By using such a process, we fabricated non-volatile memory devices and obtained relevant program/erase, retention, and cycling characteristics.

A dipolar gas of ultracold molecules

Ultracold polar molecular gases promise new directions and exciting applications in collisions and chemical reactions at ultralow energies, precision measurements, novel quantum phase transitions, and quantum information science. Here we briefly discuss key experimental requirements for observing strong dipole-dipole interactions in an ultracold dipolar gas of molecules. We then survey current experimental progress in the field with a focus on our recent work creating a near quantum degenerate gas of KRb polar molecules [Ni et al., Science, 2008, 322, 231].