Stopping of protons Improved accuracy of the UCA model

Recent theoretical developments in the unitary convolution approximation UCA for electronic energy losses of bare and screened ions are presented. Examples are given for proton beams and rare gas targets. For gas targets there exists a sufficient amount of experimental data on charge exchange, for pinpointing the largely unknown stopping power contribution of electron capture processes at low and intermediate energie

Impact parameter dependence of the electronic energy loss of fast cluster projectiles

Electronic energy loss of molecular clusters as a function of impact parameter is less understood than atomic energy loss. Vicinage effects due to mutual interference between cluster fragments play a key role in the determination of the cluster electronic energy loss. In this work, we describe a molecular extension of the perturbative convolution approximation PCA energy loss model, namely MPCA molecular PCA , which yields remarkable agreement with first order Born semiclassical approximation SCA results. The physical inputs of the model are the oscillators strengths of the target atoms and the projectile electron density. A very good agreement is obtained with time consuming full first order calculations for bare incident molecular clusters for several angles between cluster axis and velocity directio

The retarding Bessel Box an electron spectrometer designed for pump probe experiments

A new type of electrostatic electron spectrometer is developed, capable of particular sensitive measurements of energy spectra and time of flight distributions. This instrument is specifically designed and optimized for laser pump X ray probe measurements, where photo electrons or Auger electrons from surfaces, clusters, molecular or atomic targets are being measured with high time resolution at an extremely low detection noise level. The compact and robust cylinder symmetrical system is a strongly improved Bessel Box design, featuring electron retardation, a large detection solid angle, about 100 electron transmission gridless design and excellent time resolution. In this paper we describe the principle of operation of this type of spectrometer and various tests. We present quantitative results for electron measurements with different solid state targets and two different electron detection systems in comparison to electron trajectory simulations inside the electrostatic spectrometer fields. Picosecond pump probe operation has been tested with high laser power and even the ability to work under femtosecond pump probe conditions with electron detection at the BESSY II slicing facility has been prove

Evidence for enhanced desorption of hydrogen atoms from a Si 100 surface induced by slow highly charged ions

We report evidence for an enhanced desorption of hydrogen atoms from a Si 100 surface bombarded by 30 keV Xeq q 6?22 ions. The measured desorption yield amounts to 0.76 and 2.2 hydrogen atoms per incident Xe10 and Xe18 ion, respectively. For understanding the behaviour of hydrogen desorption from Si, another experiment was carried out to see the hydrogen signals as a function of time for about 140 min after deliberately introducing hydrogen into the target chamber and then shut off the valve. The results are discussed in the light of potential sputtering which essentially dominates for ions at higher charge states and the interpretation is supported by theoretical estimates

An analytical energy loss line shape for high depth resolution in ion beam analysis

The knowledge of the energy loss distribution in a single ion atom collision is a prerequisite for subnanometric resolution in depthprofiling techniques such as nuclear reaction profiling NRP and medium energy ion scattering MEIS . The usual Gaussian approximation specified by the stopping power and energy straggling is not valid for near surface regions of solids, where subnanometric or monolayer resolution can be achieved. In this work we propose an analytical formula for the line shape to replace the usual Gaussian distribution widely used in low resolution ion beam analysis. Furthermore, we provide a simple physical method to derive the corresponding shape parameters. We also present a comparison with full coupled channel calculations as well as with experimental data at nearly single collision condition

Femtosecond dynamics snapshots of the early ion track evolution

The energy dissipation and femtosecond dynamics due to fast heavy ions in matter is critically reviewed with emphasis on possible mechanisms that lead to materials modi cations. Starting from a discussion of the initial electronic energy deposition processes, three basic mechanisms for the conversion of electronic into atomic energy are investigated by means of Auger electron spectroscopy. Results for amorphous Si, amorphous C and polypropylene are presented and discussed. Experimental evidence for a highly charged track region as well as for hot electrons inside tracks is shown. As follows mainly from Auger electron spectroscopy, there are strong indications for di erent track production mechanisms in di erent material

Laser pump X ray probe experiments with electrons ejected from a Cu 111 target space charge acceleration

A comprehensive investigation of the emission characteristics for electrons induced by X rays of a few hundred eV at grazing incidence angles on an atomically clean Cu 111 sample during laser excitation is presented. Electron energy spectra due to intense infrared laser irradiation are investigated at the BESSY II slicing facility. Furthermore, the influence of the corresponding high degree of target excitation high peak current of photoemission on the properties of Auger and photoelectrons liberated by a probe X ray beam is investigated in time resolved pump and probe measurements. Strong electron energy shifts have been found and assigned to space charge acceleration. The variation of the shift with laser power and electron energy is investigated and discussed on the basis of experimental as well as new theoretical result

Asymmetric line shapes for medium energy H and He ions undergoing a large angle collision

Asymmetric line shapes for medium energy H and He ions backscattered from topmost adatoms such as Si 111 3x 3 Sb and Ni 111 2x2 O are measured by a toroidal electrostatic analyzer with an excellent energy resolution. The spectra exhibit a pronounced asymmetric nature and are well fitted by an exponentially modified Gaussian profile. It is found that the nonperturbative coupled channel calculations reproduce well the observed asymmetric line shapes for He impact on different materials, although slightly overestimate the asymmetry for H impact on Au. On the other hand, the CASP 3.2 program involving additional approximations gives large underestimates for He ions and overestimates for H ions. This problem has been partially solved by modifying the order of the implementation of the shell corrections and higher order effects in the CASP mode

Search for short time phase effects in the electronic damage evolution A case study with silicon

This work focusses on the production and decay properties of inner shell vacancies and valence band excitations induced by swift highly charged ions interacting with amorphous and crystalline Si. High resolution electron spectra have been taken for fast heavy ions at 1.78 5 MeV u as well as for electrons of similar velocity incident on atomically clean Si targets of well defined phase. Various Augerelectron structures are analyzed concerning their width, their intensity and exact peak position. All measured peaks show a small shift towards lower energy when the charge of the projectile is increased. This finding is an indication for a nuclear track potential inside the ion track. A detailed analysis of the Auger electron spectra for amorphous Si and crystalline Si 111 7 x 7 points to a small but significant phase effect in the short time dynamics of ion track