Search Results (289 total)

Angular fragment distributions from the dissociative recombination (DR) of HD⁺ were measured with well directed monochromatic low-energy electrons over a dense grid of collision energies from 7 to 35 meV, where pronounced rovibrational Feshbach resonances occur. Significant higher-order anisotropies are found in the distributions, whose size varies along energy in a partial correlation with the relative DR rate from fast-rotating molecules. This may indicate a breakdown of the nonrotation assumption so far applied to predict angular DR fragment distributions.

Excited low-spin, nonyrast states in ^170,172,174Hf were populated in β⁺/ε decay and studied through off-beam γ-ray spectroscopy. New coincidence data allowed for a substantial revision of the level schemes of ^170,172Hf and a confirmation of the level scheme of ¹⁷⁴Hf. The Hf isotopes represent a unique situation in which a crossing of collective intrinsic excitations occurs, enhancing significantly the effects of mixing. Using branching ratios from excited 2⁺ states, this mixing is followed and studied. The resulting mixing matrix elements are found to be ~30 keV—an order of magnitude larger than estimated previously for nearby nuclei. In the case of ¹⁷⁰Hf, the 2_β⁺ and 2_γ⁺ level are shown to be completely mixed.

Rate coefficients for dissociative recombination (DR), dissociative excitation (DE), and vibrational excitation between the helium dimer ion ⁴He₂⁺ and electrons from a few meV up to 40 eV were measured using fast (3.8 and 8.3 MeV) ion beams stored for up to 85 s. Vibrational relaxation to greater than 95% in the v=0 level was achieved by collisions with cold electrons over 50 s. Low-energy, strongly v-dependent DR rate coefficients are given for v=0, 1, and ≥2. The rate coefficients at higher energies for v=0, with DR and DE given on an absolute scale, are compared to results from recent wave-packet calculations on the fast dissociation dynamics of the doubly excited helium dimer, where the three processes occur as competing reaction channels. Also given are rate coefficients for vibrationally superelastic electron collisions at near 10 meV average energy and the approximate vibrational excitation cross section for fast collisions with the residual gas.

Isotope shifts in dielectronic recombination spectra were studied for Li-like ^ANd⁵⁷⁺ ions with A=142 and A=150. From the displacement of resonance positions energy shifts δE^142 150(2s-2p_1/2)=40.2(3)(6)  meV [(stat)(sys)] and δE^142 150(2s-2p_3/2)=42.3(12)(20)  meV of 2s-2p_j transitions were deduced. An evaluation of these values within a full QED treatment yields a change in the mean-square charge radius of ^142 150δ⟨r²⟩=-1.36(1)(3)  fm². The approach is conceptually new and combines the advantage of a simple atomic structure with high sensitivity to nuclear size.

Term energies for dielectronic-recombination Rydberg resonances below 0.07 eV are determined for Sc¹⁸⁺ with absolute accuracies below 0.0002 eV by electron collision spectroscopy in an ion storage ring, using the twin-electron-beam technique and a cryogenic photocathode. The lithiumlike 2s_1/2-2p_3/2 transition energy for Z=21 is determined to 4.6 ppm, less than 1% of the few-body effects on radiative corrections. Features from the hyperfine structure of the 2s state could be resolved in the dielectronic-recombination spectrum.

Excited low-spin, nonyrast states of ¹⁶⁸Hf were populated in β⁺/ε decay and studied through γ-ray spectroscopy to assess the nature of low-lying K^π=0⁺,2⁺ intrinsic excitations. Coincidence data provided improved measurements of the decay properties of the low-lying states, and γ-γ angular correlation measurements yielded spin assignments for several levels as well as E2/M1 mixing ratios. The resulting level scheme of ¹⁶⁸Hf is compared with the predictions of new simple geometrical models, including the confined beta soft model and the Davidson potential. It is found that the predictions for most observables are similar for all the models and in agreement with the data on ¹⁶⁸Hf. However, large differences exist in the predictions for the excited K=0⁺ sequence, with the Davidson potential best reproducing the data in ¹⁶⁸Hf.

The g factor of the 2₁⁺ state of ¹⁷⁰Hf was measured by perturbed γ-γ angular correlation in a static external magnetic field. The result, g(2₁⁺)=0.28(5), extends the systematics of g factors of even-even Hf isotopes to N=98 and enables a better test of theoretical models. The g(2₁⁺) experimental values of these isotopes exhibit a remarkable constancy as a function of neutron number. This phenomenon, which was also observed for other isotopic chains in the Gd–W range, is explained in terms of a recently proposed empirical model.

We report on observations of coherent, impulsive radio Cherenkov radiation from electromagnetic showers in solid ice. This is the first observation of the Askaryan effect in ice. As part of the complete validation process for the ANITA experiment, we performed an experiment at the Stanford Linear Accelerator Center in June 2006 using a 7.5 metric ton ice target. We measure for the first time the large-scale angular dependence of the radiation pattern, a major factor in determining the solid-angle acceptance of ultrahigh-energy neutrino detectors.

The electron-ion recombination rate coefficient for Si  IV forming Si  III was measured at the heavy-ion storage-ring TSR. The experimental electron-ion collision energy range of 0–186  eV encompassed the 2p⁶nln^′l^′ dielectronic recombination (DR) resonances associated with 3s→nl core excitations, 2s2p⁶3snln^′l^′ resonances associated with 2s→nl (n=3,4) core excitations, and 2p⁵3snln^′l^′ resonances associated with 2p→nl (n=3,…,∞) core excitations. The experimental DR results are compared with theoretical calculations using the multiconfiguration Dirac-Fock (MCDF) method for DR via the 3s→3pn^′l^′ and 3s→3dn^′l^′(both n^′=3,…,6) and 2p⁵3s3ln^′l^′ (n^′=3,4) capture channels. Finally, the experimental and theoretical plasma DR rate coefficients for Si  IV forming Si  III are derived and compared with previously available results.

A three-dimensional imaging technique installed at the Heidelberg Test Storage Ring (TSR) has been used to investigate the three-body breakup channels occurring in the dissociative recombination process of the methylene ion CH₂⁺. By selecting dissociation planes perpendicular to the molecular beam direction the dissociation kinematics could be measured with unprecedented momentum resolution. Release energies, the relative branching ratio, and the kinematical correlations between the three fragments were determined for the two energetically allowed channels: C(³P)+H(²S)+H(²S) and C(¹D)+H(²S)+H(²S).

Molecular photofragmentation has been studied by event imaging on HeH⁺ ions at 32 nm (38.7 eV) in a fast ion beam crossed with the free-electron laser in Hamburg (FLASH), analyzing neutral He product directions and energies. Fragmentation into He(1snl,n≥2)+H⁺ was observed to yield significant photodissociation at 32 nm with an absolute cross section of (1.4±0.7)×10^-18  cm², releasing energies of 10–20 eV. A clear dominance of photodissociation perpendicular to the laser polarization was found in contrast to the excitation paths so far emphasized in theoretical studies.

We give a scheme for a physical implementation of the programmable state discriminator that unambiguously discriminates between two unknown qubit states with optimal probability of success. One copy of each of the unknown states is provided as input, or program, for the two program registers, and the data state, which is guaranteed to be prepared in one of the program states, is fed into the data register of the device. This device will then tell us, in an optimal way, which of the templates stored in the program registers the data state matches. The implementation is based on Neumark’s theorem. We introduce a single qubit as ancilla and a unitary operator that entangles the system with the ancilla in such a way that projective measurements performed in the computational basis of the system plus ancilla transform the initial system states according to the optimal positive-operator-valued measure.

We derive explicit expressions for the Wigner function of wave functions in D dimensions which depend on the hyperradius—that is, of s waves. They are based either on the position or the momentum representation of the s wave. The corresponding Wigner function depends on three variables: the absolute value of the D-dimensional position and momentum vectors and the angle between them. We illustrate these expressions by calculating and discussing the Wigner functions of an elementary s wave and the energy eigenfunction of a free particle.

The hyperfine induced 2s2p ³P₀→2s^2 1S₀ transition rate A_HFI in berylliumlike ⁴⁷Ti¹⁸⁺ is measured. Resonant electron-ion recombination in a heavy-ion storage ring is employed to monitor the time dependent population of the ³P₀ state. The experimental value A_HFI=0.56(3)  s^-1 is almost 60% larger than theoretically predicted.

Electron-ion recombination observed in storage ring experiments shows a strong enhancement of the recombination rate for highly charged ions with low-energy electrons relative to what standard radiative recombination rates predict. We present detailed simulations of the toroid and solenoid regions of the electron cooler, analyzing the classical dynamics of an electron in the presence of the Coulomb field of the ion, the homogeneous magnetic field inside the cooler, and the transient electric field in the merging section. Both bound and continuum electron dynamics display partially chaotic motion. For bound states we observe stochastic and quasiperiodic l mixing while for continuum electrons we find transient chaos characterized by a fractal generalized reflection function. We determine the modified radiative and field-induced recombination of the electron with a highly charged ion in a storage ring. The obtained absolute excess recombination rates are compared with the experimental data and, overall, reasonable agreement is found. The scaling of the rate with the average relative energy, the ion charge, the magnetic guiding field, and the electron-beam temperatures is analyzed.

We show that s waves, that is wave functions that only depend on a hyperradius, are entangled if and only if the corresponding Wigner functions exhibit negative domains. We illustrate this feature using a special class of s waves which allows us to perform the calculations analytically. This class includes a Gaussian, a maximally entangled as well as a “shell” state.

A simple phenomenological model is discussed that simultaneously accounts for the saturation of B(E2; 0₁⁺→ 2₁⁺) values and the newly recognized near constancy of g(2₁⁺) factor values in deformed nuclei. The model invokes reduced effective contributions to these observables from the valence neutrons and protons. Empirical evidence supporting this ansatz comes from recently extracted proton-neutron interaction strengths.

The energy-resolved rate coefficient for the dissociative recombination (DR) of H₃⁺ with slow electrons has been measured by the storage-ring method using an ion beam produced from a radiofrequency multipole ion trap, employing buffer-gas cooling at 13 K. The electron energy spread of the merged-beams measurement is reduced to 500  μeV by using a cryogenic GaAs photocathode. This and a previous cold-H₃⁺ measurement jointly confirm the capability of ion storage rings, with suitable ion sources, to store and investigate H₃⁺ in the two lowest, (J,G)=(1,1) and (1,0) rotational states prevailing also in cold interstellar matter. The use of para-H₂ in the ion source, expected to enhance para-H₃⁺ in the stored ion beam, is found to increase the DR rate coefficient at meV electron energies.

Electron-ion recombination observed in storage ring experiments shows a strong enhancement relative to what standard radiative recombination rates predict. We simulate the effect of a transient motional electric field induced by the merging of an electron and an ion beam in the electron cooler which opens an additional pathway for free-bound transitions of electrons. We show that the measured rate contains a significant contribution from radiative stabilization of Rydberg states formed by this transient motional electric field. The absolute excess recombination rates obtained are in good agreement with the experimental data. The scaling of the rate with the ion charge and the magnetic guiding field is analyzed.

The g factor of the 2₁⁺ state of ¹⁶⁰Er was measured by perturbed γ-γ angular correlation in a static external magnetic field of 5.82 T. The result, g(2₁⁺)=0.33(6), is discussed within the systematics of g factors of even-even isotopes for the Ba-Pt region.

The dissociative recombination (DR) of ³He  ⁴He⁺ has been investigated at the heavy-ion Test Storage Ring (TSR) in Heidelberg by observing neutral products from electron-ion collisions in a merged beams configuration at relative energies from near-zero (thermal electron energy about 10 meV) up to 40 eV. After storage and electron cooling for 35 s, an effective DR rate coefficient at near-zero energy of 3×10⁻⁹  cm³s⁻¹ is found. The temporal evolution of the neutral product rates and fragment imaging spectra reveals that the populations of vibrational levels in the stored ion beam are nonthermal with fractions of ∼0.1–1  % in excited levels up to at least v=4, having a significant effect on the observed DR signals. With a pump-probe-type technique using DR fragment imaging while switching the properties of the electron beam, the vibrational excitation of the ions is found to originate mostly from ion collisions with the residual gas. Also, the temporal evolution of the DR signals suggests that a strong electron induced rotational cooling occurs in the vibrational ground state, reaching a rotational temperature near or below 300 K. From the absolute rate coefficient and the shape of the fragment imaging spectrum observed under stationary conditions, the DR rate coefficient from the vibrational ground state is determined; converted to a thermal electron gas at 300 K it amounts to (3.3±0.9)×10⁻¹⁰  cm³s⁻¹. The corresponding branching ratios from v=0 to the atomic final states are found to be (3.7±1.2)  % for 1s2s  ³S,(37.4±4.0)  % for 1s2s  ¹S,(58.6±5.2)  % for 1s2p  ³P, and (2.9±3.0)  % for 1s2p  ¹P. A DR rate coefficient in the range of 2×10⁻⁷  cm³s⁻¹ or above is inferred for vibrational levels v=3 and higher. As a function of the collision energy, the measured DR rate coefficient displays a structure around 0.2 eV. At higher energies, it has one smooth peak around 7.3 eV and a highly structured appearance at 15–40 eV. The small size of the observed effective DR rate coefficient at near-zero energy indicates that the electron induced rotational cooling is due to inelastic electron-ion collisions and not due to selective depletion of rotational levels by DR.

The electron-ion recombination spectrum of the Li-like Na⁸⁺ ion in the energy range 0.0–0.5  eV is presented. Experimental results obtained by storage-ring techniques are compared with a calculated spectrum, based on a combination of relativistic many-body methods and complex rotation, and the agreement is found to be very good. The deviations between measured and calculated dielectronic recombination resonance energies are usually below about 2  meV with a maximum difference at 5.5  meV, while the theoretical cross sections deviate by at most 20% from the experiment. The recombination spectrum in the investigated energy region is determined by the 2p_j7ℓ_j^′ Rydberg manifold of dielectronic recombination resonances, comprising 61 states within half an eV above the ground state of Na⁸⁺. The theoretical resonance parameters of all contributing states are provided.

We describe a real-valued and periodic representation of quantum states. This representation can be defined operationally using generalized position and momentum measurements on coupled systems. It turns out that the emerging quantum interference terms encode the complete state information and also allow us to formulate quantum dynamics. We discuss the close connection to the theory of analytic functions.

High-resolution spectroscopy of doubly excited states produced by dielectronic recombination (DR) of lithiumlike Sc¹⁸⁺ ions was performed by employing the electron-ion merged-beam technique at the heavy-ion storage ring TSR. The experimental procedure for measuring DR resonances with high precision is thoroughly described with an emphasis on the uncertainties of the experimental energy scale. Absolute measurements of recombination rate coefficients were carried out over the center-of-mass energy range 0–50  eV that comprises all DR resonances associated with 2s_1∕2→2p_1∕2,3∕2 excitations. At relative energies below 300  meV resonances due to DR via Sc¹⁷⁺ (1s²  2p_3∕2  10l_j) intermediate states were found. Their positions could be measured with an uncertainty of only ±1.8  meV. The results are compared with theoretical calculations within the framework of relativistic many-body perturbation theory. By combining the precision of the experimental and theoretical results we derive a value for the 2s_1∕2→2p_3∕2 excitation energy, 44.3107(19)  eV, which is by more than an order of magnitude more accurate than the hitherto most precise value obtained from optical spectroscopy.

The cross sections for electron detachment of internally cold C_n^- and Al_n^- clusters were measured using an electrostatic ion beam trap fitted with an internal electron target. The measured electron-impact detachment cross sections for the C_n^- (n=1–9) clusters exhibit even-odd oscillations reflecting the binding energy trend, namely, higher cross sections for weaker binding. Surprisingly, however, these cross sections increase on the average with cluster size, n, in spite of the increase in electron binding. In contrast, the Al_n^- (n=2–5) clusters follow the known scaling laws for electron detachment. We suggest that the size-dependent polarizability of these clusters is responsible for the observed behavior.