In this study highly charged ions produced in Electron Beam Ion Traps are used to
investigate electron capture from surfaces and gases. The experiments with gas targets
focus on spectroscopic measurements of the K-shell x-rays emitted at the end of radiative
cascades following electron capture into Rydberg states of Ar^{17+} and Ar^{18+}
ions as a function of collision energy. The ions are extracted from an Electron Beam
Ion Trap at an energy of 2 keVu^{-1}, charge-selected and then decelerated
down to 5 eVu^{-1} for interaction with an argon gas target. For decreasing
collision energies a shift to electron capture into low orbital angular momentum capture
states is observed. Comparative measurements of the K-shell x-ray emission following
electron capture by Ar^{17+} and Ar^{18+} ions from background gas
in the trap are made and a discrepancy in the results compared with those from the
extraction experiments is found. Possible explanations are discussed. For the investigation
of electron capture from surfaces, highly charged ions are extracted from an Electron
Beam Ion Trap at energies of 2 to 3 keVu^{-1}, charge-selected and directed
onto targets comprising arrays of nanoscale apertures in silicon nitride membranes.
The highly charged ions implemented are Ar^{16+} and Xe^{44+} and
the aperture targets are formed by focused ion beam drilling in combination with ion
beam assisted thin film deposition, achieving hole diameters of 50 to 300 nm and aspect
ratios of 1:5 to 3:2. After transport through the nanoscale apertures the ions pass
through an electrostatic charge state analyzer and are detected. The percentage of
electron capture from the aperture walls is found to be much lower than model predictions
and the results are discussed in terms of a capillary guiding mechanism. (orig.)$$$$
Total cross sections for the positive and negative fragments resulting from dissociative
collisions with He of vibrationally relaxed H_{3}^{+}, D_{3}^{+},
and HD_{2}^{+} molecular ions have been measured in the energy range
3-9.8 keV. The measured absolute total-cross-section values are more than one order
of magnitude smaller than those previously reported with the molecular ions without
vibrational relaxation. When the cross sections are plotted as a function of the projectile
speed and normalized to compensate for the relative fragment yield, the values for
the production of deuterium fragments are higher than those for hydrogen ions in the
energy range of the present study. These results are consistent with the theoretical
predictions for the behavior of triatomic molecular ions with high rovibrational excitation$$$$
We experimentally investigate various processes present in the photoassociative interaction
of an ultracold atomic sample with shaped femtosecond laser pulses as an detailed
extension of previous work [W. Salzmann et al., Phys. Rev. Lett. 100, 233003 (2008)].
We demonstrate the photoassociation of pairs of rubidium atoms into electronically
excited, bound molecular states using spectrally cut femtosecond laser pulses tuned
below the rubidium D_{1} or D_{2} asymptote. Time-resolved pump-probe
spectra reveal oscillations of the molecular formation rate, which are due to coherent
transient dynamics in the electronic excitation. The oscillation frequency corresponds
to the detuning of the spectral cut position to the asymptotic transition frequency
of the rubidium D_{1} or D_{2} lines, respectively. Measurements of
the molecular photoassociation signal as a function of the pulse energy reveal a nonlinear
dependence and indicate a nonperturbative excitation process. Chirping the association
laser pulse allowed us to change the phase of the coherent transients. Furthermore,
a signature for molecules in the electronic ground state is found, which is attributed
to molecule formation by femtosecond photoassociation followed by spontaneous decay.
In a subsequent article [A. Merli et al., Phys. Rev. A 80, 063417 (2009)] quantum
mechanical calculations are presented, which compare well with the experimental data
and reveal further details about the data and reveal further details about the observed
coherent transient dynamics.$$$$
We present in this communication the theoretical differential and total cross section
for electron-positron pair creation by intermediate energy photons (5.0-10.0 MeV)
on different targets (Z=1, 30, 50, 68, 82 and 92). The computed cross sections are
in distorted wave Born approximation (DWBA) in point Coulomb potential. The database
of the differential and total pair production cross sections is presented in tabulated
as well as in graphical form and the interpolation of differential cross sections
for different atomic numbers, positron and photon energies is discussed$$$$
The interaction of intense extreme ultraviolet (XUV) laser pulses (λ=32 nm, I=10^{11}-10^{14}
W/cm^{2}) with small rare-gas clusters (Ar_{147}) is studied by quasiclassical
molecular dynamics simulations. Our analysis supports a very general picture of the
charging and heating dynamics in finite samples under short-wavelength radiation that
is of relevance for several applications of free-electron lasers. First, up to a certain
photon flux, ionization proceeds as a series of direct photoemission events producing
a jellium-like cluster potential and a characteristic plateau in the photoelectron
spectrum as observed in Bostedt et al. [Phys. Rev. Lett. 100, 133401 (2008)]. Second,
beyond the onset of photoelectron trapping, nanoplasma formation leads to evaporative
electron emission with a characteristic thermal tail in the electron spectrum. A detailed
analysis of this transition is presented. Third, in contrast to the behavior in the
infrared or low vacuum ultraviolet range, the nanoplasma energy capture proceeds via
ionization heating, i.e., inner photoionization of localized electrons, whereas collisional
heating of conduction electrons is negligible up to high laser intensities. A direct
consequence of the latter is a surprising evolution of the mean energy of emitted
electrons as function of laser intensity.$$$$
Precision methods for measuring the hyperfine splitting E_{HFS}(2S) of the
metastable level in light hydrogen-like systems such as hydrogen, deuterium, and the
^{3}He^{+} ion are considered. These measurements open up a new possibility
for precise testing of quantum electrodynamics (QED) of bound states in hadronic systems.
While the direct calculation of the energy levels in such systems runs into the problem
of the uncertainty of the nuclear charge radius, the contribution of energy corrections
appearing in the calculation of the parameter D_{21} = 8E_{HFS}(2S)
- E_{HFS}(1S) taking the finite size of the nucleus into account becomes smaller.
The results of calculations of D_{21} are presented and compared with experimental
values. The approach considered in the paper allows the testing of some bound-state
QED corrections to the hyperfine splitting energy at a level of 10^{-7}, which
is comparable with the results of precision QED tests based on the study of the hyperfine
splitting in leptonic systems. (invited paper)$$$$
In this paper we explore the possibility of fundamental tests for coherent-state optical
quantum computing gates [T. C. Ralph et al., Phys. Rev. A 68, 042319 (2003)] using
sophisticated but not unrealistic quantum states. The major resource required in these
gates is a state diagonal to the basis states. We use the recent observation that
a squeezed single-photon state [S(r) vertical bar 1>] approximates well an odd superposition
of coherent states (vertical bar α>- vertical bar -α>) to address the diagonal resource
problem. The approximation only holds for relatively small α, and hence these gates
cannot be used in a scalable scheme. We explore the effects on fidelities and probabilities
in teleportation and a rotated Hadamard gate$$$$
Single ionization of Ar(2p) by antiproton and electron impact by Toekesi, K.; Gulyas, L. (Hungarian Academy of Sciences, Debrecen (Hungary). Inst.
of Nuclear Research); Paripas, B.; Vitez, G. (Miskolc Univ. (Hungary). Physics Dept.);
Vikor, Gy. (Stocholm Univ. (Sweden). Dept. of Atomic Physics) from8. Workshop on Fast Ion - Atom Collisions. Program and Abstracts Read MoreCollapse
[en]
Full text: Doubly differential cross sections for ionization of Ar(2p) are presented
within the framework of a three-body classical trajectory Monte Carlo method and the
Continuum-Distorted- Wave approximation. As a projectile we consider an antiproton
with an energy of 3.67 MeV. We also present classical cross sections at the electron
energy (2 keV) where the electron velocity is equal that of the investigated antiproton
velocity. We show analogy between the cross sections calculated by antiproton and
electron impact$$$$
The dynamics of a small quantum system coupled to condensed phase bath is considered.
Such dynamics is important in vibrational and spin relaxation of probe molecules in
condensed phase media. Adapting and generalizing the condensed phase electron transfer
analysis of Gayen et al. [J. Chem. Phys. 112 (2000) 4310], we show how to compute
the reduced system density matrix exactly for a large class of Hamiltonians, namely
those for which the system Hamiltonian and the system factor in the system-bath coupling
term commute. For this class of problems, several approximate second order relaxation
theory equations of motion for the reduced system density matrix also have special
properties. In particular, the Markovian limit of these equations of motion forms
a positive semigroup. Also, if the bath is a collection of harmonic oscillators, and
the bath coupling operator is linear in these oscillator coordinates, then local second
order relaxation theory is exact, even for strong system-bath coupling. The case of
a degenerate two-level system coupled off-diagonally to the bath is among those that
can be solved exactly. In order to treat the nondegenerate two-level analog, we show
that the Hamiltonian describing population relaxation of such a system coupled linearly
to a harmonic bath can be mapped to the canonical Spin-Boson Hamiltonian, albeit with
nonstandard initial state conditions. Nevertheless, Path Integral methods can be utilized
to compute numerically exact time-evolzed to compute numerically exact time-evolution
of the equivalent Spin-Boson problem, from which the desired population relaxation
dynamics can be extracted. Extensive comparisons to commonly utilized second order
relaxation theory approximations are presented$$$$
Quantum tunneling in the adiabatic Dicke model http://dx.doi.org/10.1103/PhysRevA.76.045801 by Chen Gang (Department of Physics, Shaoxing College of Arts and Sciences, Shaoxing
312000 (China); Institute of Theoretical Physics, Shanxi University, Taiyuan 030006
(China)); Chen Zidong (Department of Physics, Shaoxing College of Arts and Sciences,
Shaoxing 312000 (China)); Liang Jiuqing (Institute of Theoretical Physics, Shanxi
University, Taiyuan 030006 (China)) Read MoreCollapse
[en]
The Dicke model describes N two-level atoms interacting with a single-mode bosonic
field and exhibits a second-order phase transition from the normal to the superradiant
phase. The energy levels are not degenerate in the normal phase but have degeneracy
in the superradiant phase, where quantum tunneling occurs. By means of the Born-Oppenheimer
approximation and the instanton method in quantum field theory, the tunneling splitting,
inversely proportional to the tunneling rate for the adiabatic Dicke model, in the
superradiant phase can be evaluated explicitly. It is shown that the tunneling splitting
vanishes as exp(-N) for large N, whereas for small N it disappears as √(N)/exp(N).
The dependence of the tunneling splitting on the relevant parameters, especially on
the atom-field coupling strength, is also discussed$$$$