38 research outputs found
Antiproton and proton collisions with the alkali metal atoms Li, Na, and K
Single-electron ionization and excitation cross sections as well as cross
sections for excitation into the first excited p state of the alkali metal
atoms Li(2s), Na(3s) and K(4s) colliding with antiprotons and protons were
calculated using a time-dependent channel-coupling approach. For antiprotons an
impact-energy range from 0.25 to 1000 keV and for protons from 2 to 1000 keV
was considered. The target atoms are treated as effective one-electron systems
using a model potential. The results are compared with theoretical and
experimental data from literature and calculated cross sections for
antiproton-hydrogen collisions. For proton collisions a good overall agreement
is found which confirms the present numerical approach, whereas discrepancies
are found between the present antiproton cross sections and those calculated by
Stary et al., J.Phys.B 23, 263 (1990)
Full two-electron calculations of antiproton collisions with molecular hydrogen
Total cross sections for single ionization and excitation of molecular
hydrogen by antiproton impact are presented over a wide range of impact energy
from 1 keV to 6.5 MeV. A nonpertubative time-dependent close-coupling method is
applied to fully treat the correlated dynamics of the electrons. Good agreement
is obtained between the present calculations and experimental measurements of
single-ionization cross sections at high energies, whereas some discrepancies
with the experiment are found around the maximum. The importance of the
molecular geometry and a full two-electron description is demonstrated. The
present findings provide benchmark results which might be useful for the
development of molecular models.Comment: 4 pages, 3 figure
Collisions of antiprotons with hydrogen molecular ions
Time-dependent close-coupling calculations of the ionization and excitation
cross section for antiproton collisions with molecular hydrogen ions are
performed in an impact-energy range from 0.5 keV to 10 MeV. The
Born-Oppenheimer and Franck-Condon approximations as well as the impact
parameter method are applied in order to describe the target molecule and the
collision process. It is shown that three perpendicular orientations of the
molecular axis with respect to the trajectory are sufficient to accurately
reproduce the ionization cross section calculated by [Sakimoto, Phys. Rev. A
71, 062704 (2005)] reducing the numerical effort drastically. The
independent-event model is employed to approximate the cross section for double
ionization and H+ production in antiproton collisions with H2.Comment: 12 pages, 5 figures, 4 table
A simple parameter-free one-center model potential for an effective one-electron description of molecular hydrogen
For the description of an H2 molecule an effective one-electron model
potential is proposed which is fully determined by the exact ionization
potential of the H2 molecule. In order to test the model potential and examine
its properties it is employed to determine excitation energies, transition
moments, and oscillator strengths in a range of the internuclear distances, 0.8
< R < 2.5 a.u. In addition, it is used as a description of an H2 target in
calculations of the cross sections for photoionization and for partial
excitation in collisions with singly-charged ions. The comparison of the
results obtained with the model potential with literature data for H2 molecules
yields a good agreement and encourages therefore an extended usage of the
potential in various other applications or in order to consider the importance
of two-electron and anisotropy effects.Comment: 8 pages, 6 figure
Collisions of low-energy antiprotons with molecular hydrogen: ionization, excitation and stopping power
A time-dependent coupled-channel approach was used to calculate ionization,
excitation, and energy-loss cross sections as well as energy spectra for
antiproton and proton collisions with molecular hydrogen for impact energies 8
keV < E < 4000 keV.Comment: 4 pages, 4 figures, conference LEAP0
Stopping power of antiprotons in H, H2, and He targets
The stopping power of antiprotons in atomic and molecular hydrogen as well as
helium was calculated in an impact-energy range from 1 keV to 6.4 MeV. In the
case of H2 and He the targets were described with a single-active electron
model centered on the target. The collision process was treated with the
close-coupling formulation of the impact-parameter method. An extensive
comparison of the present results with theoretical and experimental literature
data was performed in order to evaluate which of the partly disagreeing
theoretical and experimental data are most reliable. Furthermore, the size of
the corrections to the first-order stopping number, the average energy
transferred to the target electrons, and the relative importance of the
excitation and the ionization process for the energy loss of the projectile was
determined. Finally, the stopping power of the H, H2, and He targets were
directly compared revealing specific similarities and differences of the three
targets.Comment: v1: 12 pages, 8 figures, and 1 table v2: 15 pages, 9 figures, and 2
tables; extended discussion on IPM in Method; influence of double ionization
on stopping power discussed in Result
Dosimetric evidence confirms computational model for magnetic field induced dose distortions of therapeutic proton beams
Given the sensitivity of proton therapy to anatomical variations, this cancer
treatment modality is expected to benefit greatly from integration with
magnetic resonance (MR) imaging. One of the obstacles hindering such an
integration are strong magnetic field induced dose distortions. These have been
predicted in simulation studies, but no experimental validation has been
performed so far. Here we show the first measurement of planar distributions of
dose deposited by therapeutic proton pencil beams traversing a one-Tesla
transversal magnetic field while depositing energy in a tissue-like phantom
using film dosimetry. The lateral beam deflection ranges from one millimeter to
one centimeter for 80 to 180 MeV beams. Simulated and measured deflection agree
within one millimeter for all studied energies. These results proof that the
magnetic field induced proton beam deflection is both measurable and accurately
predictable. This demonstrates the feasibility of accurate dose measurement and
hence validates dose predictions for the framework of MR-integrated proton
therapy
Synthesis of C2-Symmetric Diphosphormonoamidites and Their Use as Ligands in Rh-Catalyzed Hydroformylation: Relationships between Activity and Hydrolysis Stability
A series of diphosphoramidites has been synthetized with a piperazine, homopiperazine, and an acyclic 1,2-diamine unit in the backbone. New compounds were tested alongside related N-acyl phosphoramidites as ligands in the Rh-catalyzed hydroformylation of n-octenes to investigate their influence on the activity and regioselectivity. A subsequent study of their hydrolysis stability revealed that the most stable ligands induced the highest activity in the catalytic reaction