High Electronic Excitation Research Articles

Femtosecond laser irradiation induced-high electronic excitation in band gap materials: A quantum-kinetic model based on Boltzmann equation

A quantum kinetic approach based on the Boltzmann equation is employed to describe the response of dielectric and semiconductor materials to high electronic excitation induced by laser irradiation. The formalism describes from the initial photo-ionization inter-band processes through free carrier absorption inducing additional impact ionization to the final heat up by electron–phonon coupling. Swift thermalization through electron–electron scattering, Auger recombination and formation of free excitons, their self-trapping and subsequent non-radiative decay are included. The energy exchange between the electrons and phonons are given by a separate equation for the lattice temperature where the rates of energy transfer from the electrons to the lattice per unit volume are defined quantum mechanically. As a result of our calculations the electron energy distribution function, average kinetic energy of the electron system and electron density are obtained as a function of laser intensity, laser photon energy (wavelength) and laser pulse duration. Examples of application in fs-laser irradiated-silica are discussed.

High Electronic Excitation Induced Modifications by 120 MeV Ni9+ Ions in Ag-Polypyrrole Composite Films: A Comparative Study

High Electronic Excitation Induced Modifications by 120 MeV Ni9+ Ions in Ag-Polypyrrole Composite Films: A Comparative Study

Persistence of magnetic order in a highly excited Cu2+ state in CuO

We use ultrafast resonant x-ray diffraction to study the magnetic order in CuO under conditions of high electronic excitation. By measuring changes in the spectral shape of the Cu2+ magnetic (1/2 0 −1/2) reflection we investigate how an intense optical pump pulse perturbs the electronic and magnetic states. We observe an energy shift in the magnetic resonance at short times after the pump pulse. This shift is compared with expectations from band structure calculations at different electronic temperatures. This spectral line shift indicates that although the electrons are heated to effective electron temperatures far above TN on a time scale faster than the experimental resolution, magnetic order persists in this highly excited state for several hundred femtoseconds.Received 17 September 2013Revised 24 March 2014Corrected 25 June 2014DOI:https://doi.org/10.1103/PhysRevB.89.220401©2014 American Physical Society

Combined in situ infrared and mass spectrometric analysis of high-energy heavy ion induced degradation of polyvinyl polymers

Degradation of polyvinyl polymers (polyvinyl formal, polyvinyl alcohol, and polyvinyl acetate) by high-energy heavy ions was analysed with in situ mass spectrometry and infrared spectroscopy. Both at room temperature and at cryogenic temperature, the polymers are severely damaged by scission of side chains and the polymer backbone, releasing a large number of molecular fragments. Correlation of structural analysis of the damaged polymer and mass analysis of its volatile fragments gives plausible reaction mechanisms for the ion induced degradation. These are compared to thermal degradation. The differences found between the two degradation processes are due to the non-equilibrium character of the ion based process with its high electronic excitation and ionisation. The results are of practical relevance for applications of the polymers in devices of particle accelerators and space vehicles.

Defect formation and accumulation in CeO2 irradiated with swift heavy ions

We have investigated microstructure evolution in CeO2 irradiated with 210MeV Xe ions by using transmission electron microscopy to gain the fundamental knowledge on radiation damage induced by fission fragments in nuclear fuel and transmutation target. Analysis on the accumulation of ion tracks has revealed an influence region to recover pre-existing core damage regions of ion tracks to be 8.4nm in radius. Cross section observations showed that high-density electronic excitation induces both ion tracks and dislocation loops. At high fluences of 1.5×1019 and 1×1020ionsm−2, depth-dependent microstructure was developed with radiation-induced defects of ion tracks, dislocation loops (dot-contrast) and line dislocations. Formation of sub-divided small grains was found at shallow depth at a fluence of 1×1020ionsm−2. The microstructure evolution was discussed in terms of the accumulation of interstitials due to significant overlap of high density electronic excitation.

Phase Transformations in Pyrochlores Irradiated with Swift Heavy Ions: Influence of Composition and Chemical Bonding

Gd2Ti2O7 and Gd2Zr2O7 pyrochlores were irradiated with swift heavy ions in order to investigate the effects of the chemical composition on the structural changes induced by high electronic excitation. The XRD results show that the structural modifications induced by irradiation with 93 MeV Xe ions are strongly dependent on the sample composition: Gd2Ti2O7 is readily amorphized, whereas Gd(2)Zr(2)O7 is transformed into a radiation-resistant anion-deficient fluorite structure. Atomistic simulations with the second-moment tight-binding QEq model allow us to calculate the lattice properties of both Gd2Ti2O7 and Gd2Zr2O7, and also to quantify the degree of covalency and ionicity in these compounds. These calculations clearly show that Gd2Ti2O7 is more covalent than Gd2Zr2O7, thus confirming that the amorphization resistance can be related to the covalent character of insulators.

Static diode pumped alkali lasers: Model calculations of the effects of heating, ionization, high electronic excitation and chemical reactions

The effects of heating, ionization, high electronic excitation and chemical reactions on the operation of diode pumped alkali lasers (DPALs) with a static, non-flowing gain medium are calculated using a semi-analytical model. Unlike other models, assuming a three-level scheme of the laser and neglecting influence of the temperature on the lasing power, it takes into account the temperature rise and losses of neutral alkali atoms due to ionization and chemical reactions, resulting in decrease of the pump absorption and slope efficiency. Good agreement with measurements in a static DPAL [B.V. Zhdanov, J. Sell, R.J. Knize, Electron. Lett. 44 (2008) 582] is obtained. It is found that the ionization processes have a small effect on the laser operation, whereas the chemical reactions of alkali atoms with hydrocarbons strongly affect the lasing power.

High electronic excitation induced modifications by 100 MeV O7+ and 150 MeV Ni11+ ions in Makrofol KG polycarbonate film

Makrofol-KG polycarbonate (M-KG PC) films are irradiated with 150MeV Ni11+ and 100MeV O7+ ion beams at various fluences ranging from 1×1011 to 3×1012ions/cm2. We have investigated the effect of irradiation parameters such as energy and ions fluence on optical, structural and chemical properties of the studied polymer, respectively. UV–Visible, Fourier Transform Infrared (FTIR) and X-ray diffraction (XRD) spectral studies have been employed in the present investigation. UV–Visible spectra exhibit a shift towards the higher wavelength regime after irradiation. This shift clearly reflects decrease in optical band gap after irradiation. The FTIR spectrum shows a decrease in intensity of the typical bands whereas the formation of new bands indicates the degradation of the polymer after irradiation. The XRD pattern of M-KG PC shows the decreasing intensity of peak positions with increase in ions fluence, which suggests loss of crystallinity of the films due to irradiation. Observed results indicate the formation of disordered system in the irradiated films. Magnitude of effect of irradiation is greater in case of 150MeV Ni11+ rather than 100MeV O7+ ions due to high electronic energy loss of Ni11+ ions in M-KG PC. The results can be correlated on the basis of linear energy transfer (LET) of the irradiated ions.

Online in situ x-ray diffraction setup for structural modification studies during swift heavy ion irradiation

The high energy density of electronic excitations due to the impact of swift heavy ions can induce structural modifications in materials. We present an x-ray diffractometer called ALIX ("Analyse en Ligne sur IRRSUD par diffraction de rayons X"), which has been set up at the low-energy beamline (IRRadiation SUD - IRRSUD) of the Grand Accélérateur National d'Ions Lourds facility, to allow the study of structural modification kinetics as a function of the ion fluence. The x-ray setup has been modified and optimized to enable irradiation by swift heavy ions simultaneously to x-ray pattern recording. We present the capability of ALIX to perform simultaneous irradiation-diffraction by using energy discrimination between x-rays from diffraction and from ion-target interaction. To illustrate its potential, results of sequential or simultaneous irradiation-diffraction are presented in this article to show radiation effects on the structural properties of ceramics. Phase transition kinetics have been studied during xenon ion irradiation of polycrystalline MgO and SrTiO(3). We have observed that MgO oxide is radiation-resistant to high electronic excitations, contrary to the high sensitivity of SrTiO(3), which exhibits transition from the crystalline to the amorphous state during irradiation. By interpreting the amorphization kinetics of SrTiO(3), defect overlapping models are discussed as well as latent track characteristics. Together with a transmission electron microscopy study, we conclude that a single impact model describes the phase transition mechanism.

Phenomenological model for the formation of heterogeneous tracks in pyrochlores irradiated with swift heavy ions

Zirconate and titanate pyrochlores were irradiated with swift heavy ions in order to investigate the effects of the chemical composition on the structural changes induced by high electronic excitation. Both transmission electron microscopy and X-ray diffraction results show that the structural modifications induced by irradiation with 120-MeV U ions are strongly dependent on the sample composition: Gd 2Ti 2O 7 is readily amorphized, whereas Gd 2Zr 2O 7 is transformed into a radiation-resistant anion-deficient fluorite structure. For Sm 2Zr 2O 7, Eu 2Zr 2O 7 and Nd 2Zr 2O 7, more complex behavior is observed, since both pyrochlore–fluorite phase transformation and amorphization occur. A new phenomenological model (the heterogeneous track overlap model, HTOM), which assumes a direct impact mechanism coupled with a single track overlap process, is proposed to describe the formation of heterogeneous track structures consisting of mixed anion-deficient fluorite and amorphous domains. The pyrochlore composition mainly influences the structure of ion tracks (and weakly their diameter), and essentially concerns the amount of amorphous phase vs the amount of fluorite counterpart.

Enhancement of LPG sensing properties in nanocrystalline zinc oxide thin film by high electronic excitation

We present an efficient detection of liquefied petroleum gas (LPG) using zinc oxide (ZnO) thin films irradiated with swift heavy ions (SHI) of 120 MeV Au 9+ ions. The nanostructured ZnO thin films on ITO glass substrates were synthesized using a simple, low cost, and efficient successive ionic layer adsorption and reaction (SILAR) technique. The as-synthesized thin films were annealed at 250 °C (pristine) in vacuum atmosphere (10 −2 Torr) for 1 h and these films were further irradiated using SHI of 120 MeV Au 9+ ions with different fluence 3 × 10 12 and 5 × 10 12 ions/cm 2, respectively. SHI induced modification in structural, morphological, optical and electrical properties of ZnO thin films were studied. We observed significant enhancement in LPG gas sensing properties in the ZnO thin films on irradiation with fluence 5 × 10 12 ions/cm 2, especially showing maximum sensitivity to LPG at (200 °C) with LPG concentration of 200 ppm. In addition, irradiated ZnO thin film sensors displayed fast response and recovery time. The observed improvement in gas sensing properties is mainly attributed to the formation of cabbage leaf-like porous ZnO nanostructures throughout the film surface after irradiation. These results along with a simple fabrication process demonstrate that the ZnO thin films irradiated using 120 MeV Au 9+ ions with fluence 5 × 10 12 ions/cm 2 is promising for developing low cost and high performance LPG sensors.

Effect of composition on the behavior of pyrochlores irradiated with swift heavy ions

A 2B 2O 7 pyrochlores (zirconates with A = Gd, Nd, Sm or Eu, and a titanate Gd 2Ti 2O 7) were irradiated with swift heavy ions in order to investigate the effects of the chemical composition on the structural changes induced by high electronic excitation. XRD results show that the structural modifications induced by irradiation with 119 MeV U ions are strongly dependent on the sample composition: Gd 2Ti 2O 7 is readily amorphized, whereas Gd 2Zr 2O 7 is transformed into a radiation-resistant anion-deficient fluorite structure. For Sm 2Zr 2O 7, Eu 2Zr 2O 7 and Nd 2Zr 2O 7, a more complex behavior is observed since both a pyrochlore–fluorite phase transformation and amorphization occur. The results suggest that an ion track is composed of an amorphous track core surrounded by an anion-deficient fluorite shell. The diameters of different track regions were estimated by using a direct impact mechanism coupled with a single overlap process to fit the transformation build-ups. These diameters depend on the sample composition, the susceptibility to amorphization by high electronic excitation increasing with the cation radius ratio r A/ r B.

Phase transformations induced by high electronic excitation in ion-irradiated Gd2(ZrxTi1−x)2O7 pyrochlores

The pyrochlore oxides (A2B2O7) exhibit a remarkable range of structural, physical, and magnetic properties related to their various chemical compositions. This article reports the phase transformations induced by high electronic excitation in pyrochlores of the Gd2(ZrxTi1−x)2O7 family irradiated with swift ions. The structural changes, investigated by using several analytical techniques (x-ray diffraction, Raman spectroscopy, and transmission electron microscopy), strongly depend on the chemical composition. The high electronic excitation along the ion trajectory results in the amorphization of ion tracks for Gd2Ti2O7 and Gd2TiZrO7, whereas a defective fluorite structure is formed in Gd2Zr2O7. Moreover, the results underline the existence of an electronic stopping power threshold of 6 keV/nm for amorphizable compounds and 10 keV/nm for Gd2Zr2O7, below which phase transformations do not occur. Finally, the study of the thermal recovery of irradiated pyrochlores provides the recrystallization temperature for amorphized samples and reveals differences in the recovery process which are related to the chemical composition.

Surface nanostructuring of SrTiO 3 single crystals by slow highly charged ions and swift heavy ions

Single crystals of strontium titanate have been irradiated with both slow highly charged Xe ions extracted from an Electron Beam Ion Trap and swift heavy Xe ions. After irradiation, the crystals were investigated by scanning force microscopy in air. In both cases nanohillocks due to impact of individual projectiles were observed. This similarity originates from the fact that both swift heavy ions and slow highly charged ions initially transfer their energy to the electronic system of the target, leading to a localized region of high electronic excitation. This electronic excitation is subsequently transferred to the lattice atoms by electron–phonon coupling, leading to pronounced lattice heating. The formation of surface hillocks can then be ascribed to a melting process. We also present first evidence for the existence of a potential energy threshold for nanohillock formation on strontium titanate surfaces by slow highly charged ions.

Clarification of high density electronic excitation effects on the microstructural evolution in UO 2

In order to understand the properties of ion tracks and the microstructural evolution under accumulation of ion tracks in UO 2, 100 MeV Zr 10+ and 210 MeV Xe 14+ ions irradiation examinations have been done at a tandem accelerator facility of JAEA-Tokai, and it has been observed the microstructure by means of a transmission electron microscope (TEM) and a scanning electron microscope (SEM) in CRIEPI. Comparison of the diameter of ion tracks between UO 2 and CeO 2 under irradiation with 100 MeV Zr 10+ and 210 MeV Xe 14+ ions at room temperature clarify that the sensitivity on high density electronic excitation of UO 2 is much less than that of CeO 2. By the cross-sectional observation of UO 2 under irradiation with 210 MeV Xe 14+ ions at 300 °C, elliptical changes of fabricated pores that exist till ∼6 μm depth and the formation of dislocations have been observed in the ion fluence over 5 × 10 14 ions/cm 2. The drastic changes of surface morphology and inner structure in UO 2 indicate that the overlapping of ion tracks will cause the point defects, enhance the diffusion of point defects and dislocations, and form the sub-grains at relatively low temperature.

Complete phase transformation in zirconia induced by very high electronic excitations

It is now well established that the irradiation of pure zirconia (ZrO 2) with swift heavy ions can lead to a transformation from the monoclinic to the tetragonal phase when the deposited electronic energy loss exceeds an effective threshold ∼12 keV nm −1. Previous swift heavy ion irradiations carried out with electronic energy losses less than ∼30 keV nm −1 evidenced that this phase transition is characterized by two constant features: (i) it is driven by a double ion impact process, and (ii) it is never complete even at extremely high fluences. It is then quite interesting to check whether this phase transformation does not depart from this behaviour at very high electronic energy losses. For this purpose, zirconia was irradiated with 0.6-GeV Pb ions giving rise to an electronic energy loss ∼42 keV nm −1 at the sample surface. Despite this huge value, the experimental data revealed that the phase change is here again still characterized by a double ion impact process. However, contrary to previous cases, it was found that this phase transformation not only occurs very rapidly but achieves full completion at rather low fluences.

Study of H loss from hydrogenated Hg 1−xCd xTe under high electronic excitation by elastic recoil detection analysis (ERDA)

Large erosion (∼1.1 × 10 4 atoms/ion) of H from hydrogenated MCT wafers is observed due to the bombardment with 80 MeV Ni 9+ ions. The initial H areal concentration and hydrogen depletion rate is monitored by elastic recoil detection analysis. The ion-damaged zones from where depletion of H takes place have been calculated from fluence-dependent hydrogen areal content analysis. The results are explained on the basis of the thermal spike model of ion–solid interaction.

高密度電子励起により誘起されるスピネル化合物中の不規則化領域の評価

高密度電子励起により誘起されるスピネル化合物中の不規則化領域の評価

Nano size effects in the high resolution electron energy loss spectra and excitation function of hydrogenated diamond films

The present work studies the high resolution electron energy loss spectra (HR-EELS) and excitation function of hydrogenated diamond films composed of diamond grains of varying size of ∼ 300, ∼ 20 nm and ∼ 5 nm. It is reported that HR-EELS modes associated with C–C lattice phonons are comparably more intense than modes associated with the C–H surface vibrational modes in the case of nano-crystalline compared to micro-crystalline diamond films. Excitation function measurements of the different modes show a pronounced maximum at ∼ 8 eV for the nano-diamond films, whereas for micro-crystalline films similar modes showed maximum intensities at 13 eV with a smaller contribution at 8 eV. It is reported that the elastic electron reflectivity is substantially larger for the nano-diamond films, showing broad maxima at ∼ 8 and ∼ 13 eV in both cases. It is suggested that these effects are associated with the larger probability for inelastic scattering of electrons by lattice phonons mediated by scattering with internal grain boundaries and defects in the case of nano-crystalline diamond films.

Synthesis of Nanodimensional TiO2 Thin Films

Nanodimensional TiO2 has wide application in the field of photocatalysis, photovoltaic and photochromic devices. In present investigation TiO2 thin films deposited by pulsed laser deposition method are irradiated by 100 MeV Ag ion beam to achieve growth of nanophases. The nanostructure evolution is characterized by atomic force microscopy (AFM). The phases of TiO2 formed after irradiation are identified by glancing angle X-ray diffraction and Raman spectroscopy. The particle radius estimated by AFM varies from 10-13 nm. Anatase phase of TiO2 is formed after irradiation. The blue shift observed in UV-VIS absorption spectra indicates the nanostructure formation. The shape and size of nanoparticles formed due to high electronic excitation depend upon thickness of the film.