High resolution probe of coherence in low-energy charge exchange collisions with oriented targets

The trapping lasers of a magneto-optical trap (MOT) are used to bring Rb atoms into well defined oriented states. Coupled to recoil-ion momentum spectroscopy (RIMS), this yields a unique MOTRIMS setup which is able to probe scattering dynamics, including their coherence features, with unprecedented resolution. This technique is applied to the low-energy charge exchange processes Na$^+$+Rb($5p_{\pm 1}$) $\rightarrow$ Na($3p,4s$)+Rb$^+$. The measurements reveal detailed features of the collisional interaction which are employed to improve the theoretical description. All of this enables to gauge the reliability of intuitive pictures predicting the most likely capture transitions

Atomic site sensitive processes in low energy ion-dimer collisions

Electron capture processes for low energy Ar9+ ions colliding on Ar2 dimer targets are investigated, focusing attention on charge sharing as a function of molecule orientation and impact parameter. A preference in charge-asymmetric dissociation channels is observed, with a strong correlation between the projectile scattering angle and the molecular ion orientation. The measurements provide here clear evidences that projectiles distinguish each atom in the target and, that electron capture from near-site atom is favored. Monte Carlo calculations based on the classical over-the-barrier model, with dimer targets represented as two independent atoms, are compared to the data. They give a new insight into the dynamics of the collision by providing, for the di erent electron capture channels, the two-dimensional probability maps p(~b), where ~b is the impact parameter vector in the molecular frame

Molecular frame photoelectron angular distribution for oxygen 1s photoemission from CO_2 molecules

We have measured photoelectron angular distributions in the molecular frame (MF-PADs) for O 1s photoemission from CO2, using photoelectron-O+–CO+ coincidence momentum imaging. Results for the molecular axis at 0, 45 and 90° to the electric vector of the light are reported. The major features of the MF-PADs are fairly well reproduced by calculations employing a relaxed-core Hartree–Fock approach. Weak asymmetric features are seen through a plane perpendicular to the molecular axis and attributed to symmetry lowering by anti-symmetric stretching motion

SPORT: A new sub-nanosecond time-resolved instrument to study swift heavy ion-beam induced luminescence - Application to luminescence degradation of a fast plastic scintillator

We developed a new sub-nanosecond time-resolved instrument to study the dynamics of UV-visible luminescence under high stopping power heavy ion irradiation. We applied our instrument, called SPORT, on a fast plastic scintillator (BC-400) irradiated with 27-MeV Ar ions having high mean electronic stopping power of 2.6 MeV/\mu m. As a consequence of increasing permanent radiation damages with increasing ion fluence, our investigations reveal a degradation of scintillation intensity together with, thanks to the time-resolved measurement, a decrease in the decay constant of the scintillator. This combination indicates that luminescence degradation processes by both dynamic and static quenching, the latter mechanism being predominant. Under such high density excitation, the scintillation deterioration of BC-400 is significantly enhanced compared to that observed in previous investigations, mainly performed using light ions. The observed non-linear behaviour implies that the dose at which luminescence starts deteriorating is not independent on particles' stopping power, thus illustrating that the radiation hardness of plastic scintillators can be strongly weakened under high excitation density in heavy ion environments.Comment: 5 figures, accepted in Nucl. Instrum. Methods

Breakdown of the Two-Step Model in K-Shell Photoemission and Subsequent Decay Probed by the Molecular-Frame Photoelectron Angular Distributions of CO_2

We report results of measurements and of Hartree-Fock level calculations of molecular-frame photoelectron angular distributions (MFPADs) for C 1s photoemission from CO2. The agreement between the measured and calculated MFPADs is on average reasonable. The measured MFPADs display a weak but definite asymmetry with respect to the O+ and CO+ fragment ions at certain energies, providing evidence for an overlap of gerade and ungerade final ionic states giving rise to a partial breakdown of the two-step model of core-level photoionization and its subsequent Auger decay

Carbon K-shell photoelectron angular distribution from fixed-in-space CO2 molecules

Measurements of photoelectron angular distributions for carbon K-shell ionization of fixed-in-space CO2 molecules with the molecular axis oriented along, perpendicular and at 45 degrees to the electric vector of the light are reported. The major features of these measured spectra are fairly well reproduced by calculations employing a relaxed-core Hartree-Fock approach. In contrast to the angular distribution for K-shell ionization of N-2, which exhibits a rich structure dominated by the f-wave (l = 3) at the shape resonance, the angular distribution for carbon K-shell photoionization of CO2 is quite unstructured over the entire observed range across the shape resonance

Ion impact induced Interatomic Coulombic Decay in neon and argon dimers

We investigate the contribution of Interatomic Coulombic Decay induced by ion impact in neon and argon dimers (Ne$_2$ and Ar$_2$) to the production of low energy electrons. Our experiments cover a broad range of perturbation strengths and reaction channels. We use 11.37 MeV/u S$^{14+}$, 0.125 MeV/u He$^{1+}$, 0.1625 MeV/u He$^{1+}$ and 0.150 MeV/u He$^{2+}$ as projectiles and study ionization, single and double electron transfer to the projectile as well as projectile electron loss processes. The application of a COLTRIMS reaction microscope enables us to retrieve the three-dimensional momentum vectors of the ion pairs of the fragmenting dimer into Ne$^{q+}$/Ne$^{1+}$ and Ar$^{q+}$/Ar$^{1+}$ (q = 1, 2, 3) in coincidence with at least one emitted electron

Interatomic Coulombic Decay as a New Source of Low Energy Electrons in slow Ion-Dimer Collisions

We provide the experimental evidence that the single electron capture process in slow collisions between O$^{3+}$ ions and neon dimer targets leads to an unexpected production of low-energy electrons. This production results from the interatomic Coulombic decay process, subsequent to inner shell single electron capture from one site of the neon dimer. Although pure one-electron capture from inner shell is expected to be negligible in the low collision energy regime investigated here, the electron production due to this process overtakes by one order of magnitude the emission of Auger electrons by the scattered projectiles after double-electron capture. This feature is specific to low charge states of the projectile: similar studies with Xe$^{20+}$ and Ar$^{9+}$ projectiles show no evidence of inner shell single-electron capture. The dependence of the process on the projectile charge state is interpreted using simple calculations based on the classical over the barrier model

Energy deposition by heavy ions: Additivity of kinetic and potential energy contributions in hillock formation on CaF2

The formation of nano-hillocks on CaF2 crystal surfaces by individual ion impact has been studied using medium energy (3 and 5 MeV) highly charged ions (Xe19+ to Xe30+) as well as swift (kinetic energies between 12 and 58 MeV) heavy ions. For very slow highly charged ions the appearance of hillocks is known to be linked to a threshold in potential energy while for swift heavy ions a minimum electronic energy loss is necessary. With our results we bridge the gap between these two extreme cases and demonstrate, that with increasing energy deposition via electronic energy loss the potential energy threshold for hillock production can be substantially lowered. Surprisingly, both mechanisms of energy deposition in the target surface seem to contribute in an additive way, as demonstrated when plotting the results in a phase diagram. We show that the inelastic thermal spike model, originally developed to describe such material modifications for swift heavy ions, can be extended to case where kinetic and potential energies are deposited into the surface.Comment: 12 pages, 4 figure