Effect of interaction of atomic electrons on ionization of an insulator in swift heavy ion tracks

2015

Appl. Phys. B

regime of photon energy up to 24-25 keV with the pulse duration of a few tens of femtoseconds, possibly reduced further down to a few femtoseconds. The advent of such light sources stimulated rapid advances in many scientific disciplines ranging from atomic physics [6,7], study of molecules and chemical reactions [8,9], to clusters [10,11] and macroscopic objects [12-14] exposed to intense laser fields. They are actively used for creating and probing plasmas [15,16], hot dense matter [16][17][18] and warm dense matter [19,20]. Lower fluences of FELs are used to investigate structural changes within solid-state matter [12,[21][22][23][24]. These new light sources have also found a broad interest in the fields of nanophysics [25], molecular physics and biophysics [26].Insulators irradiated with femtosecond X-ray FEL radiation are observed to undergo a sequence of processes [12,27]. During the pulse, the photoabsorption excites electrons from the valence band, or deep atomic shells, to unoccupied conduction band states. The deep shell atomic holes then decay via Auger processes, which are the dominant relaxation channel for light elements (low-Z) [28]. An Auger decay promotes one more electron from the valence to the conduction band, following the relaxation of the deeper shell hole into the upper shells or the valence band. In case of heavy elements, with multiple shells, an intraatomic Auger cascade involving many steps over different shells is possible, ultimately ending up with multiple holes in the valence band. This process occurs typically on the few femtosecond timescale. The released photo-and Auger electrons scatter inelastically (so-called impact ionization of valence band or deep shell electrons of the material), or elastically (scattering on atoms or phonons without ionization of the target). Each impact ionization event excites one more electron into the conduction band. The impact ionization cascading typically occurs on the femtosecond Abstract Modern X-ray free-electron lasers (FELs) provide pulses with photon energies from a few tens of eV up to the tens of keV and durations as short as only a few femtoseconds. Experimental pump-probe scheme with a FEL pump and a visible light probe of a solid-state target can be used for the pulse-duration monitor on a shot-to-shot basis.To study the electron cascading in different materials used for pulse-duration monitor, XCASCADE, a Monte Carlo model of the X-ray-induced electron cascading within an irradiated target is developed. It is shown here that the electron cascade duration is sensitive to a choice of material. An appropriately selected target can significantly shorten the electron relaxation times. The grounds, upon which such a choice of the material can be made, are discussed. The results suggest that for photon energies of 24 keV, one could achieve direct monitoring of the pulse duration of 40 fs. Further deconvolution of the electron density into the contribution from the pulse itself and from the secondary cascading can increase the resolution up ...

Section: Materials Previously Used For Pulse-duration Monitoringmentioning

confidence: 99%

Femtosecond X-ray induced electron kinetics in dielectrics: application for FEL-pulse-duration monitor

2015

Appl. Phys. B

“…No defects or impurities in the material are included in these MC simulations. Target electrons are considered as uniformly distributed particles occupying either the deep atomic energy levels 26 or the states in the valence or conduction bands according to the density of states (DOS) of a material. Taking into account large velocities of projectiles, we assume these electrons as point-like particles at fixed positions during their energy and momentum exchange with an SHI (instant collisions).…”

Section: Modelmentioning

confidence: 99%

Recrystallization as the governing mechanism of ion track formation

Rymzhanov

O’Connell

et al. 2019

Sci Rep

Response of dielectric crystals: MgO, Al 2 O 3 and Y 3 Al 5 O 12 (YAG) to irradiation with 167 MeV Xe ions decelerating in the electronic stopping regime is studied. Comprehensive simulations demonstrated that despite similar ion energy losses and the initial excitation kinetics of the electronic systems and lattices, significant differences occur among final structures of ion tracks in these materials, supported by experiments. No ion tracks appeared in MgO, whereas discontinuous distorted crystalline tracks of ~2 nm in diameter were observed in Al 2 O 3 and continuous amorphous tracks were detected in YAG. These track structures in Al 2 O 3 and YAG were confirmed by high resolution TEM data. The simulations enabled us to identify recrystallization as the dominant mechanism governing formation of detected tracks in these oxides. We analyzed effects of the viscosity in molten state, lattice structure and difference in the kinetics of metallic and oxygen sublattices at the crystallization surface on damage recovery in tracks.

“…The mutual interaction of atomic electrons during the ionization process is disregarded. The model of independent atomic electrons is applied since the electron–electron interaction during the ultrafast ionization by a swift projectile influences only weakly the spectra of appeared delocalized electrons 20.…”

Section: Modelmentioning

confidence: 99%

Formation of the defect halo of swift heavy ion tracks in LiF due to spatial redistribution of valence holes

Physica Status Solidi (b)

Schwartz

Trautmann

et al. 2013

The fundamental effect of the relaxation kinetics of the ensemble of generated valence holes on structure transformations in swift heavy ions tracks in alkali halides is demonstrated. The parameters of the spatial distributions of F‐color centers formed in the nanometric vicinities of the trajectories of different ions (from C to U) decelerated in LiF in the electronic stopping regime were extracted from the in situ spectroscopy experiments. Formation of these defects results from self‐trapping of valence holes appearing during material ionizations followed by creation and decay of self‐trapped excitons. The Monte‐Carlo model is applied in order to determine the initial radial distributions of valence holes in tracks of different ions. The detected large difference between the hole and defect distributions indicates fast and long‐range diffusion of valence holes from the ion trajectory before their self‐trapping. The averaged diffusion coefficient (Dh) of hot holes at the subpicosecond time scale after the projectile passage as well as the conversion efficiencies of valence holes into color centers (β) in relaxing tracks of different ions are estimated (Dh = 0.016–1.4 cm2 s−1, β = 0.01–0.04).