Spectral analysis of K-shell X-ray emission of magnesium plasma produced by ultrashort high-intensity laser pulse irradiation

Spectral analysis of K-shell X-ray emission of magnesium plasma, produced by laser pulses of 45 fs duration, focussed up to an intensity of ～10¹⁸ W cm^?2, is carried out. The plasma conditions prevalent during the emission of X-ray spectrum were identified by comparing the experimental spectra with the synthetic spectra generated using the spectroscopic code PrismSPECT. It is observed that He-like resonance line emission occurs from the plasma region having sub-critical density, whereas K-α emission arises from the bulk solid heated to a temperature of 10 eV by the impact of hot electrons. K-α line from Be-like ions was used to estimate the hot electron temperature. A power law fit to the electron temperature showed a scaling of I ^0.47 with laser intensity.     

Short wavelength X-ray emission generated by highly excited cluster beams

X-ray radiation is studied for large clusters consisting of 10⁷–10¹⁰ atoms and irradiated by an intense laser pulse with an intensity ranged from (10¹⁴ up to 10¹⁸ W/cm²). The model is developed for such a laser plasma that includes the radiative transitions and the processes of excitation and quenching of multicharged ions of this plasma by electron impact. Due to interaction of a radiating multicharged ion with a surrounding plasma, spectral lines of emission are broaden and neighboring spectral lines are overlapped. As a result, the spectrum of radiation of multicharged ions is transformed into a continuous spectral band. The model under consideration includes important plasma processes including dielectronic recombination, spontaneous radiation, excitation, quenching and ionization of multicharged ions by electron impact. On the basis of the model developed the X-ray spectrum and spectral power are evaluated. In the range of laser intensities under consideration a laser plasma formed contains multicharged ions with charges Z = 26?36 that corresponds to the 3d-electron shell in the xenon case.     

Fast ion beams from intense,femtosecond laser irradiated nanostructured surfaces

We present results on hot electron and energetic ion (keV–MeV) generation from polished and nanostructured metallic surfaces excited by p-polarized, femtosecond laser pulses in the intensity range of 1×10¹⁵–1.5×10¹⁷ W cm^-2. A clear enhancement in the hard X-ray spectrum from nanoparticle-coated surfaces is observed, indicating ‘hotter’ electron production in nanoparticle-produced plasma until the intensity of 2×10¹⁶ W cm^-2 is reached. Contrary to the existing perception, we find that the hotter electrons do not lead to hotter ion emission. The total ion flux and the ion energy integrated over the 4–1400 keV energy range are found to be enhanced by 50% and 16%, respectively, for nanostructured targets in comparison to those from polished targets. 55% enhancement in yield is observed for ions at the lower end of the energy range, while hotter ions are actually found to be suppressed by ∼40%. The surface modulations present on the nanoparticle-coated targets are observed to reduce the maximum energy of the ions and showed an intensity-dependent increase in the divergence of the ion beam. PACS 79.20.Ds; 68.47.De; 61.80.Ba; 61.82.Bg; 42.65.Re     

A new comparison between experiment and theory for the X-ray K absorption edge of nickel

New structure has been resolved in the X-ray K absorption edge of nickel. There is rough qualitative agreement between our spectrum and the calculations of Nagel et al., for which the presence of the core hole is neglected in the computation of the final state wave function. However, the two lowest energy experimental peaks are significantly shifted to lower energies relative to the corresponding theoretical peaks. In addition, the theoretical spectrum is much sharper than the experimental curve even after accounting for instrumental broadening and broadening due to the natural width of the K level, indicating that hot electron broadening effects strongly influence the final ejected electron states.     

EBIT as a versatile experimental facility

The Electron Beam Ion Trap (EBIT) produces ions, confined within the electron beam, with charges ranging up to U⁹²⁺ at near rest energies. This allows to study the interaction of a monoenergetic electron beam with any trapped ion to a high degree of precision via X-ray spectroscopy. The development of the EBIT into an ion (trap) source enables the possibility to perform for the first time studies of the interaction dynamics in strong fields of ions with matter where the ions carry hundreds of keV potential energy at very low kinetic energies (eV).     

Cold electrons acceleration in TNSA laser-generated plasma using a low-contrast fs laser

The fs laser facility in Bordeaux, delivering an intensity of 10¹⁸ W/cm² at normal incidence on thin foils, has been used to induce forward electron and ion acceleration in target-normal-sheath-acceleration (TNSA) regime. Micrometric thin foils with different composition, thickness, and electron density, were prepared to promote the charge particle acceleration in the forward direction. The plasma electron and ion emission monitoring were performed on-line using SiC semiconductor detectors in time-of-flight (TOF) configuration and gaf-chromics films both covered by thin absorber filters. The experiment has permitted to accelerate electrons and protons. A special attention was placed to detect relativistic hot electrons escaping from the plasma and cold electrons returning to the target position. The electron energies of the order of 100 keV and of about 1 keV were detected as representative of hot and cold electrons, respectively. A high cold electron contribution was measured using low-contrast fs laser, while it is less evident using high-contrast fs lasers. The charge particle acceleration depends on the laser parameters, irradiation conditions, and target properties, as will be presented and discussed.     

Electron paramagnetic resonance and optical spectroscopy of K3Na(CrO4)2 ferroelastics activated by MnO 4 2− molecular impurity ions

Crystals of a proper ferroelastic K₃Na(CrO₄)₂ containing molecular impurity ions MnO ₄ ^2? are studied using electron paramagnetic resonance (EPR) and optical spectroscopy. The EPR spectrum of the Mn⁶⁺ ion contained in the molecular impurity ion MnO ₄ ^2? is identified at low temperatures (T ≤ 20 K). The intensity of this spectrum decreases unusually fast as the temperature increases. A broad IR luminescence band with a vibronic structure well resolved at a temperature of 8 K is revealed. Theoretical treatment of the Mn⁶⁺ ion involved in the molecular impurity ions MnO ₄ ^2? of the K₃Na(CrO₄)₂ ferroelastic crystal suggests that an important role in this case is played by the pseudo-Jahn-Teller. The pseudo-Jahn-Teller effect offers an explanation for the appearance of a fine structure in the vibronic replicas in the luminescence spectrum, on the one hand, and accounts for the fast decrease in the intensity of the EPR spectrum of K₃Na(CrO₄)₂: MnO ₄ ^2? with increasing temperature, on the other.     

1-萘酚团簇的紫外光电离质谱研究

用飞行时间质谱仪和超声速脉冲分子束技术研究了紫外激光对1-萘酚(1HN)团簇的电离质谱.观测到(1HN)_n~+系列的团簇离子,且离子强度随团簇尺寸的增大而减小.电离激光的强度(在5μJ/pulse～100μJ/pulse范围内)对团簇离子强度的相对分布影响较小,说明软电离为产生团簇离子的主要过程,团簇离子的强度分布反映出电离前中性团簇的分布特征.增大电离区的进样气压可以产生更大尺寸的团簇离子,同时在(1HN)_n~+后面观测到新系列的团簇离子.这些新生离子与(H_2O)_m有关,考虑到1-萘酚团簇可以通过OH形成H键,推测该新生团簇离子通过团簇内的反应而产生.     

Development of a new experimental setup for studying collisions of keV-electrons with thick and thin targets

Development of a new lectron-recoil ion/photon coincidence setup for investigating some of the electron induced collision processes, such as electron bremsstrahlung, electron backscattering, innershell excitation and multiple ionization of target atoms/molecules in bombardment of electrons having energies from 2.0 keV to 30.0 keV with solid and gaseous targets is described. The new features include the use of a compact multipurpose scattering chamber, a time-of-flight spectrometer for detection of multiply charged target ions, a 45°-parallel plate electrostatic analyzer for measuring energy and angle of the ejected electrons, a room temperature high resolution Si-PIN photo diode X-ray detector for counting the collisionally induced photons, a coincidence data acquisition system consisting of a 200 MHz Pentium based 8K-multichannel analyzer and a standard network of a fast/slow coincidence electronics. In particular, the details of design, fabrication and assembly of indigenous components employed in the setup are presented. Selected experiments planned with the setup are mentioned and briefly discussed. A report on performance, optimization, efficiency, time resolution etc. of the time-of-flight (TOF) spectrometer and that of the 45°-parallel plate electrostatic analyzer (PPEA) is presented. Test spectra of electron-recoil ion coincidences, energy distribution of ejected electrons and characteristic plus non-characteristic X-ray spectrum are illustrated to exhibit the satisfactory performance of the developed setup.     

Generation of fast ions in femto-and picosecond laser plasmas at low intensities of the heating radiation

X-ray spectra from Teflon targets irradiated by laser pulses with a duration of 60 fs to 1 ps have been investigated experimentally. It is shown that, when the contrast of the laser pulse is sufficiently low, the effect of self-focusing of the main laser pulse in the plasma produced by the prepulse can significantly enhance the generation efficiency of fast particles. In this case, ions with energies as high as ～1 MeV are observed at relatively low laser intensities, q _las ≈ (4–6) × 10¹⁶ W/cm².     

X-Rays emission by high-intensity pulsed lasers irradiating thin foils at PALS laboratory

X-rays and forward ion emission from laser-generated plasma in the Target Normal Sheath Acceleration regime of different targets with 10-μm thickness, irradiated at Prague Asterix Laser System (PALS) laboratory at about 10¹⁶ W/cm² intensity, employing a 1,315 nm-wavelength laser with a 300-ps pulse duration, are investigated. The photon and ion emissions were mainly measured using Silicon Carbide (SiC) detectors in time-of-flight configuration and X-ray streak camera imaging. The results show that the maximum proton acceleration value and the X-ray emission yield growth are proportional to the atomic number of the irradiated targets. The X-ray emission is not isotropic, with energies increasing from 1 keV for light atomic targets to about 2.5 keV for heavy atomic targets. The laser focal position significantly influences the X-ray emission from light and heavy irradiated targets, indicating the possible induction of self-focusing effects when the laser beam is focalized in front of the light target surface and of electron density enhancement for focalization inside the target.     

HITRAP: A Facility for Experiments with Trapped Highly Charged Ions

HITRAP is a planned ion trap facility for capturing and cooling of highly charged ions produced at GSI in the heavy-ion complex of the UNILAC-SIS accelerators and the ESR storage ring. In this facility heavy highly charged ions up to uranium will be available as bare nuclei, hydrogen-like ions or few-electron systems at low temperatures. The trap for receiving and studying these ions is designed for operation at extremely high vacuum by cooling to cryogenic temperatures. The stored highly charged ions can be investigated in the trap itself or can be extracted from the trap at energies up to about 10 keV/q. The proposed physics experiments are collision studies with highly charged ions at well-defined low energies (eV/u), high-accuracy measurements to determine the g-factor of the electron bound in a hydrogen-like heavy ion and the atomic binding energies of few-electron systems, laser spectroscopy of HFS transitions and X-ray spectroscopy. This revised version was published online in July 2006 with corrections to the Cover Date.     

Atomic collisions involving C60 and collective excitation

Here we review and discuss some of our recent investigations on collective excitation in a free C₆₀ molecule and its influence on the atomic collisions. In particular, emphasis has been given for collisions with fast highly charged ions. It is demonstrated, from the charge-state-dependence studies of recoil-ion spectra, that the plasmon excitation plays a dominant role in the single and double ionization process. The observed linear charge-state-dependence is in contrast to the expected behavior predicted by ion-atom collisions models. This behavior was observed for different projectiles and at different energies. The time-of-flight recoil-ion mass spectroscopy experiments involve 1–5 MeV/u C, O, F and Si ion beams with different charge states, ranging between 4⁺ and 14⁺. In addition, the influence of the collective excitation on the electron capture process was also investigated. The wake-field induced Stark-mixing and splitting of sub-levels of projectile-ions following electron capture from C60 carries signature of the collective plasmon excitation. For the electron capture studies X-ray spectroscopic technique was used for collisions with bare and dressed S and Cl ion beams. The results on the TOF data on fullerene target obtained in last few years will be summarized.     

Spectroscopic diagnostics of carbon and aluminum laser-produced plasmas

VUV emission spectra of plasmas produced by focusing laser radiation with intensity of 10¹⁰–10¹¹ W/cm² on carbon and aluminum targets were studied. Using the partial local thermodynamic equilibrium model for an electron density exceeding 10¹⁷ cm^?3, the spectroscopic diagnostics and the analysis of ion composition of plasmas were carried out. The electron temperatures determined for carbon and aluminum plasmas from the ratio of intensities of ionic lines were found to be 8±3 eV and 11±4 eV, respectively. Stark broadening of aluminum lines was measured and parameters of electron broadening were determined. Using the spatially resolved measurement of Stark line broadening, the spatial density distribution and the law of electron gas expansion were found. The electron gas in the hot region of size 5 mm with an average density of (5±2) 10₁₇cm ^?3 experienced one-dimensional expansion according to the law 1/z ^1.1 with increasing distance z from the target.     

Electron radiography using hot electron jets from sub-millijoule femtosecond laser pulses

Electron jets produced in the intermediate intensity range of 10¹⁵ to 10¹⁷ W/cm² from submillijoule 120 fs Ti:Sapphire laser pulses focused to spots of a few microns in diameter have been characterized. The experimental results show strong emission of hot electrons with energies from 80 keV to above 250 keV from microplasmas created with both p- and s-polarized 250 μJ laser pulses. The electron jets with energies above 250 keV are observed to be highly directional. The electron jets are observed in the plane of polarization of the laser electric field for both p- and s-polarized laser pulses. The hot electrons emitted from these femtosecond laser plasmas have also been used for radiographic imaging. It is expected that the short initial duration of these electron pulses would make them useful for time resolved applications. PACS 41.75. Fr; 52.38.Kd; 52.70.Nc     

Control parameters for ion heating and X-ray emission from laser induced cluster explosion

The highest energies of the ions obtained from the explosion of an atomic cluster in an intense femtosecond laser field can vary greatly depending on the cluster size, atomic species and the peak intensity, duration and shape of the laser pulse. By careful choice of these parameters the ion energies, electron energies or X-ray emission can be optimised. A relationship is described that allows for rapid determination of the optimum experimental parameters. We present experimental data of keV X-ray emission from Argon clusters, which investigate intensity and pulse duration effects. In addition we present the first results from closed-loop optimal control, pulse-shaping experiments that optimise X-ray emission and show a significant enhancement in the X-ray yield. PACS 36.40.Gk; 52.50.Jm     

Energy balance measurements on a nanosecond CO2 laser created plasma

Interaction of nanosecond CO₂ laser radiation with a solid deuterium target has been investigated with incident laser fluxes up to several times 10¹²W/cm². Reflection, X-ray and ion measurements were performed at different angles in the relevant half-space around the target. The energy balance deduced from reflection and ion time-of-flight measurements led to a total integrated reflectivity of 60 to 80%. Fast D⁺ ions with kinetic energies as high as 40 keV were detected and identified. Hard X-rays were observed in the range of 1 to 10 keV. A discussion of these results is presented.     

Production of ion and electron streams by pulsed-laser ablation of Ta and Cu

A Nd:YAG laser with 10⁹ W/cm ² pulse intensity, operating at 532 nm wavelength, is used to ablate Ta and Cu targets placed in vacuum. The ablation process generates a plasma in front of the target surface, which expands along the normal to target surface. The ion and electron emissions from the plasma were measured by Faraday cups placed at different angles with respect to the normal to target surface. In the range of laser intensities from 10⁷ to 10⁹ W/cm², the fast electron yield is lower than the ion yield and it increases at higher laser intensities. The ablation threshold, the emission yield, the ion and electron average energies and the plasma ion and electron temperatures were measured for ion and fast electron streams.     

Using of sonochemically prepared components for vapor phase growing of SbI3·3S8

The using of sonochemically prepared components for growth of SbI₃·3S₈ single crystals from the vapor phase is presented for the first time. The good optical quality of the obtained crystals is important because this material is valuable for optoelectronics due to its non-linear optical properties. The products were characterized by using techniques such as X-ray crystallography, powder X-ray diffraction, scanning electron microscopy, energy dispersive X-ray analysis, high-resolution transmission electron microscopy, selected area electron diffraction, optical diffuse reflection spectroscopy and optical transmittance spectroscopy. The direct and indirect forbidden energy gaps of SbI₃·3S₈ illuminated with plane polarized light with electric field parallel and perpendicular to the c-axis of the crystal have been determined. The second harmonic generation of light in the grown crystals was observed.     

Dissociation of alkane ionized molecules

The subject of investigation is the fragmentation of variously charged molecular ions arising in col-lisions of several kiloelectronvolt H⁺, He²⁺, and Ar⁶⁺ ions with molecules of the simplest alkanes (from methane to butane). Using the method of time-of-flight mass spectrometry, the formation cross sections of dissociation-induced fragment ions are measured. The dissociation takes place when an incident ion captures an electron from a methane, ethane, or propane molecule. The role of additional ionization of the molecule, which accompanies the electron capture by the incident ion, is elucidated. The kinetic energy spectrum for protons resulting from the fragmentation of multiply charged alkane ions is determined. The most plausible kinetic energies of protons depending on the degree of ionization and molecule size fall into the range 1–25 eV. It is shown that, when the molecule loses several electrons, the kinetic energies of protons are governed by Coulomb interaction between all fragment ions and are determined by their flying apart from the relative spatial arrangement of corresponding atoms in a parent molecule.