Search Results (106 total)

Double ionization of helium by electron impact is studied in a kinematical complete experiment in the threshold regime at 5 eV excess energy. As expected the recoil ion carries the full initial projectile momentum and the emitted electrons’ sum momentum in average is zero. The electron emission is revealed to be completely dominated by the symmetric 120° configuration predicted by many threshold theories but never observed experimentally before. Fully differential cross sections show a more complex structure than expected for a pure threshold collision dynamics.

Recoil-ion momentum distributions for two-photon double ionization of He and Ne (ℏω=44  eV) have been recorded with a reaction microscope at FLASH (the free-electron laser at Hamburg) at an intensity of ∼1×10¹⁴  W/cm² exploring the dynamics of the two fundamental two-photon–two-electron reaction pathways, namely, sequential and direct (or nonsequential) absorption of the photons. We find strong differences in the recoil-ion momentum patterns for the two mechanisms pointing to the significantly different two-electron emission dynamics and thus provide serious constraints for theoretical models.

Nonsequential double ionization of Ar by 45 fs laser pulses (800 nm) at (4–7)×10¹³  W/cm² was explored in fully differential measurements. Well below the field-modified recollision threshold we enter the multiphoton regime. Strongly correlated back-to-back emission of the electrons along the polarization direction is observed to dominate in striking contrast to all previous data. No effect of Coulomb repulsion can be found, the predicted cutoff in the sum-energy spectra of two emitted electrons is confirmed, and the potential importance of multiple recollisions is discussed.

Momentum distributions of Ne²⁺ and Ar²⁺ ions created by linearly polarized 795  nm, 25  fs laser pulses have been traced at intensities from 10¹⁴  to  3×10¹⁵  W∕cm² using a “reaction microscope.” Apart from the transition from nonsequential to sequential ionization, characterized by significant changes in longitudinal momentum distributions developing from a double hump, over a triple-peak structure to a narrow single Gaussian observed for both ions, for Ar²⁺ we find a similar behavior but reversed in its intensity dependence in the purely nonsequential regime, pointing to contributions of recollision excitation plus subsequent field ionization, or to the role of “Z trajectories” recently predicted within classical calculations.

We have performed experimental and theoretical studies of double ionization of helium by 6 MeV proton impact using a recently developed tool, four-particle Dalitz plots [Schulz , J. Phys. B 22, 3091 (2007)] which enable the representation of multiple differential cross sections as a function of all four fragments in a single spectrum without loss of any part of the total cross section. As a result, the relative importance of the various interactions between the fragments can be studied in great detail. Comparisons of experimental data with theoretical first-order calculations and simulations for the higher-order (TS-2) process show that elastic scattering between the heavy particles is surprisingly strong. For a large fraction of collision events, the final-state electron momenta are small compared to the momenta of the heavy particles. Our results suggest that an uncorrelated double ionization mechanism, involving two independent interactions of the projectile with both electrons, is significantly more important than previously expected for such fast collisions.

The double ionization of helium by electron impact for 106  eV incident energy was studied in a kinematically complete experiment by using a reaction microscope. The pattern of the angular correlation of the three emitted electrons was analyzed by selecting different values of the recoil ion longitudinal momentum. The Wannier predicted geometry appears when the recoil ion carries the full initial projectile momentum. It was found that at this low impact energy, the outgoing electrons still remember the initial-state collision information.

We present experimental and theoretical fully differential cross sections for single ionization by fast, 1-keV (v=8.6  a.u.) electron impact. The cross sections were measured using a momentum imaging technique for electrons and ions (reaction microscope), which covers a large fraction of the emission angles for emitted low-energy electrons (E<15  eV) and a wide range of scattering angles. Therefore comprehensive data sets are obtained for ionizing collisions at small relative momentum and energy transfer from the projectile to the target system. The experimental data are compared with predictions from several state-of-the-art theoretical calculations. At this high impact energy the calculated cross section for electron emission out of the scattering plane appears to be particularly sensitive to the treatment of higher orders in the projectile-target interaction within perturbative models.

The two-electron QED contributions to the ground-state binding energy of Kr³⁴⁺ ions have been determined in two independent experiments performed with electron beam ion traps (EBIT) in Heidelberg (HD) and Tokyo (BT, Belfast-Tokyo collaboration). X rays arising from radiative recombination (RR) of free electrons to the ground state of initially bare Kr³⁶⁺ and hydrogenlike Kr³⁵⁺ ions were observed as a function of the interacting electron energy. The K edge absorption by thin Eu and W foils provided fixed photon energy references used to measure the difference in binding energy ΔE_2e between the H- and He-like Kr ions (Kr³⁵⁺ and Kr³⁴⁺, respectively). The two values agree well, yielding a final result of ΔE_2e=641.8±1.7  eV, confirming recent results of rigorous QED calculations. This accuracy is just of the order required to access screened radiative QED contributions.

We report on a kinematically complete experiment on nonsequential double ionization of He by 25 fs 800 nm laser pulses at 1.5  PW/cm². The suppression of the recollision-induced excitation at this high intensity allows us to address in a clean way direct (e,2e) ionization by the recolliding electron. In contrast with earlier experimental results, but in agreement with various theoretical predictions, the two-electron momentum distributions along the laser polarization axis exhibit a pronounced V-shaped structure, which can be explained by the role of Coulomb repulsion and typical (e,2e) kinematics.

We consider relativistic collisions of heavy hydrogenlike ions with hydrogen and helium atoms in which the ion-atom interaction causes both colliding particles to change their internal states. Concentrating on the study of the longitudinal momentum spectrum of the atomic recoil ions, we discuss the role of relativistic and higher order effects, predict a surprisingly strong influence of the projectile’s electron on the momentum transfer, and show that the important information about the doubly inelastic collisions could be obtained in experiment merely by measuring the recoil momentum spectrum.

The Zeeman line components of the magnetic-dipole (M1) 1s²2s²2p ²P_1∕2–²P_3∕2 transition in boronlike Ar¹³⁺ were experimentally resolved by high-precision emission spectroscopy using the Heidelberg electron beam ion trap. We determined the gyromagnetic (g) factors of the ground and first-excited levels to be g_1∕2=0.663(7) and g_3∕2=1.333(2), respectively. This corresponds to a measurement of the g factor of a relativistic electron in a bound non-S state of a multielectron ion with a 1.5 parts-per-thousand accuracy. The results are compared to theoretical calculations by means of the configuration interaction Dirac-Fock-Sturmian method including electron correlation effects and additional quantum electrodynamic corrections. Our measurements show that the classical Landé g factor formula is sufficiently accurate to the present level of accuracy in few-electron ions of medium nuclear charge number Z.

We have measured the continuum momentum distribution for radiative electron capture to the continuum (RECC) cusp electrons in 90A MeV U⁸⁸⁺+N₂→U⁸⁸⁺+{N₂^+*}+e_cusp(0°)+hν (RECC) collisions. We demonstrate that x rays coincident with RECC cusp electrons originate from the short-wavelength limit of the electron-nucleus bremsstrahlung and explain the asymmetric cusp shape by comparison with theory within the relativistic impulse approximation.

Reaction Microscope-based, complete, and time-resolved Coulomb explosion imaging of vibrating and dissociating D₂⁺ molecules with femtosecond time-resolution allowed us to perform an internuclear distance (R-)dependent Fourier analysis of the corresponding wave packets. Calculations demonstrate that the obtained two-dimensional R-dependent frequency spectra enable the complete characterization of the wave packet dynamics and directly visualize the field-modified molecular potential curves in intense, ultrashort laser pulses.

We have performed a kinematically complete experiment for target ionization with simultaneous projectile detachment (TIPD) in 200-keV H⁻+He collisions. From the data we extracted triple-differential cross sections (TDCSs) for each electron separately. These TDCSs closely resemble corresponding data for single ionization by charged-particle impact. Surprisingly, the contributions from higher-order processes to TIPD, proceeding through two independent interactions of each electron with the core of the respective other collision partner, are found to be somewhat larger than the first-order process proceeding through the electron-electron interaction.

An analysis of experimental fully differential data for single ionization in 100  MeV∕amu C⁶⁺+He collisions is reported. We present a convolution of the first Born approximation with elastic scattering by using an event generator technique. Furthermore, the calculation is convoluted with all known experimental resolutions. Our analysis shows that elastic scattering is a viable explanation for surprising structures observed in the fully differential cross sections outside the scattering plane. Furthermore, it may even explain discrepancies in the “recoil peak” frequently observed for both ion and electron impact.

We report wavelength measurements of H-like and He-like ions obtained with a novel x-ray spectrometer at the Heidelberg Electron Beam Ion Trap. The experimental uncertainty for the Lyman-α₁ wavelength in Cl¹⁶⁺ is reduced by a factor of 3 and, as expected, excellent agreement with theory is maintained. For the resonance line in He-like Ar¹⁶⁺, an uncertainty of only δλ/λ=2×10^-6 was achieved. This is the most precise x-ray wavelength reported for highly charged ions to date, and allows to test recent predictions on QED two-electron and two-photon radiative corrections for He-like ions. The results also point to the advantages of establishing absolute x-ray wavelength standards using Lyman-α transitions (in the present case Ar¹⁷⁺ Lyman-α₁) to supersede the current ones.

We consider the (single) electron loss from hydrogenlike, heliumlike, and lithiumlike uranium ions in collisions with neutral atoms in the domain of the low-relativistic impact energies where the collision velocity is already a substantial fraction of the speed of light but still does not exceed the typical electron velocities in the K shell of the uranium ions. In collisions with many-electron atoms at these impact energies the presence of the atomic electrons is of minor importance for the electron loss process which occurs predominantly via the interaction with the unscreened atomic nucleus. This interaction can be effectively very strong if the atoms have large atomic numbers which leads to a tremendous failure of the first Born approximation. We show that experimental data for the loss cross sections can be well described using an eikonal amplitude proposed recently [Phys. Rev. A 75, 062716 (2007)].

We study the excitation of heavy hydrogenlike ions occurring in high-energy collisions with many-electron atoms by considering three theoretical approaches. In all of them the initial and final undistorted states of the electron in the ion are described by relativistic Coulomb-Dirac wave functions. In two of these approaches the interaction between the electron of the ion and the atom is described within the first order perturbation theory. In the first approach the presence of the atomic electrons is neglected whereas the second approach takes them into account. The comparison of results of these two approaches allows one to establish the range of collision energies where the effect of the electrons of the atom on the excitation process is weak and can be neglected. At these energies, however, the interaction between the electron of the ion and the nucleus of the atom may become too strong for the first order theory to be a good approximation. In order to deal with this point we present the third approach which is based on the symmetric eikonal approximation. Theoretical results are compared with available experimental data.

Recent multiply differential experimental data taken with reaction microscopes severely challenge predictions of quantum mechanical few-body models. Here, we report on a thorough analysis of all known possible experimental resolution effects and their influence on the extracted cross sections. Using a Monte Carlo event generator to simulate true events on the basis of quantum calculations allows us to consistently incorporate all aspects of the experimental resolution of the reaction microscope. We study the effect of the instrumental function in single ionization of helium by 3.6  MeV∕u Au⁵³⁺ and 100  MeV∕u C⁶⁺ ions and find it to significantly modify the simulated theoretical predictions. Nevertheless strong discrepancies between simulated and experimental data persist. Structures in the measured cross sections reported earlier, which could not be reproduced by theory, are thus not solely due to the experimental resolution. Our study suggests that the method using event generators, as routinely done in high-energy physics, provides the ultimate pathway of benchmarking calculations against experimental data on few-particle reactions studied with modern imaging techniques.

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.

Few-photon multiple ionization of Ne and Ar atoms by strong vacuum ultraviolet laser pulses from the free-electron laser at Hamburg was investigated differentially with the Heidelberg reaction microscope. The light-intensity dependence of Ne²⁺ production reveals the dominance of nonsequential two-photon double ionization at intensities of I<6×10¹²  W/cm² and significant contributions of three-photon ionization as I increases. Ne²⁺ recoil-ion-momentum distributions suggest that two electrons absorbing “instantaneously” two photons are ejected most likely into opposite hemispheres with similar energies.

Double ionization of the helium atom by slow electron impact (E₀=106  eV) is studied in a kinematically complete experiment. Because of a low excess energy E_exc=27  eV above the double ionization threshold, a strongly correlated three-electron continuum is realized. This is demonstrated by measuring and calculating the fully differential cross sections for equal energy sharing of the final-state electrons. While the electron emission is dominated by a strong Coulomb repulsion, also signatures of more complex dynamics of the full four-body system are identified.

In a proof-of-principle experiment, we demonstrate high-resolution resonant laser excitation in the soft x-ray region at 48.6 eV of the 2 ²S_1/2 to 2 ²P_1/2 transition of Li-like Fe²³⁺ ions trapped in an electron beam ion trap by using ultrabrilliant light from Free Electron Laser in Hamburg (FLASH). High precision spectroscopic studies of highly charged ions at this and upcoming x-ray lasers with an expected accuracy gain up to a factor of a thousand, become possible with our technique, thus potentially yielding fundamental insights, e.g., into basic aspects of QED.

The lifetime of the 3s²3p  ²P_3∕2^o first excited energy level of Fe  XIV (Al like) was measured at the Heidelberg electron beam ion trap by monitoring its optical decay to the ground state by a magnetic dipole (M1) forbidden transition at λ=530.29  nm (green coronal line), a well-known line in stellar spectra. A new trapping scheme has been applied. Possible systematic effects were investigated by studying the dependence of the decay curves on various trapping conditions with high statistical significance. The result of 16.726_−0.010^+0.020  ms shows an unexplained discrepancy in the average value of existing theoretical predictions. The inclusion of the electron anomalous magnetic moment within the theoretical calculations increases this disagreement, thus pointing at other possible origins of this discrepancy.