The Role of Stepwise Photoionization in Measurements of the Ionization Potentials in Dense Plasma

The interaction of high-contrast high-intensity laser radiation with solids allows us to create hot or warm plasma of solid or even over-solid density, such as in the case of inertial fusion particularly. The multicharged ions contained in it can no longer be considered isolated. As a result, this leads to a decrease in the ionization potentials and to the disappearance of a number of bound ionic states. To describe the ionization potential depression, two major approaches are now used predominantly, where the key parameter is either average interelectronic or interionic distance. Since neither of the approaches can be substantiated purely theoretically, their applicability can only be established by comparison with experimental results. In recent experiments with X-ray free-electron lasers, it was concluded that the ionization potential depression rather depends on the interelectronic distance. However, when measuring ionization potentials, it was assumed that the main role in ionization processes is played by the direct photoionization of the ion ground state. In the present paper, we show that stepwise photoionization processes should play a significant role in dense plasma, disrupting a straight correspondence between the threshold in direct photoionization by X-ray laser photons and the actual ionization potential of multicharged ions. It means that the measurement results mentioned above are not correct, and the main conclusion about the importance of the interelectronic distance for depression of the ionization potential is not correct

Analysis of Lyα dielectronic satellites to characterize temporal profile of intense femtosecond laser pulses

In the paper, an X-ray spectroscopy-based approach on laser pulse temporal profile characterization is described. The structure of dielectronic satellites to H-like Lyα lines strongly depends on a plasma electron density, so it can be applied for diagnostics. These spectral lines are mainly emitted during initial stage of laser plasma expansion. It means that plasma parameters obtained via them characterizes matter conditions in a region surrounding a spot of laser-matter interaction. In the case when a laser contrast is high enough, the radiation interacts with cold matter, which had not been preliminary perturbed by a laser prepulse, and the satellites structure shape corresponding to high densities should be observed. It allows us to consider the satellites as a diagnostic tool for the laser temporal profile quality. In the paper dependencies of the dielectronic satellites structure on electron densities obtained from detailed kinetic calculations in the wide range of plasma parameter for different elements are under discussion. Fundamental theoretical aspects of plasma diagnostic based on the feature of satellite structures shape in hot dense plasma, which led to development of the proposed method, are also explained

Understanding the Regioselectivity of Aromatic Hydroxylation over Divanadium-Substituted γ‑Keggin Polyoxotungstate

The aromatic hydroxylation of pseudocumene (PC) with aqueous hydrogen peroxide catalyzed by the divanadium-substituted γ-Keggin polyoxotungstate TBA₄[γ-PW₁₀O₃₈V₂(μ-O)­(μ-OH)] (TBA-<b>1H</b>, TBA = tetrabutylammonium) has been studied using kinetic modeling and DFT calculations. This reaction features high chemoselectivity and unusual regioselectivity, affording 2,4,5-trimethylphenol (TMP) as the main product. Then the computational study was extended to the analysis of the regioselectivity for other alkoxy- and alkylarene substrates. The protonation/deprotonation of TBA-<b>1H</b> in MeCN/<i>t</i>BuOH (1:1) was investigated by ³¹P NMR spectroscopy. Forms with different protonation states, [γ-PV₂W₁₀O₄₀]^5– (<b>1</b>), [γ-HPV₂W₁₀O₄₀]^4– (<b>1H</b>), and [γ-H₂PV₂W₁₀O₄₀]^3– (<b>1H</b>_<b>2</b>), have been identified, and the protonation equilibrium constants were estimated on the basis of the ³¹P NMR data. DFT calculations were used to investigate the oxygen transfer process from hydroperoxo species, [γ-PW₁₀O₃₈V₂(μ-O)­(μ-OOH)]^4– (<b>2</b>) and [γ-PW₁₀O₃₈V₂(μ-OH)­(μ-OOH)]^3– (<b>2H</b>), and peroxo complex [γ-PW₁₀O₃₈V₂(μ-η²:η²-O₂)]^3– (<b>3</b>) toward the different positions in the aromatic ring of PC, anisole, and toluene substrates. Product, kinetic, and computational studies on the PC hydroxylation strongly support a mechanism of electrophilic oxygen atom transfer from peroxo complex <b>3</b> to the aromatic ring of PC. The kinetic modeling revealed that the contribution of <b>3</b> into the initial reaction rate is, on average, about 70%, but it may depend on the reaction conditions. DFT calculations showed that the steric hindrance exerted by peroxo complex <b>3</b> is responsible for the origin of the unusual regioselectivity observed in PC hydroxylation, while for anisole and toluene the regioselective <i>para</i>-hydroxylation is due to electronic preference during the oxygen transfer from the active peroxo species <b>3</b>

Study of damage structure formation on aluminum film targets by picosecond soft X-ray laser ablation around threshold region

We irradiated soft X-ray laser pulses onto aluminum film targets, which were evaporated on silicon and lithium fluoride substrates. After laser irradiation, we observed irradiated surfaces and confirmed damage with nanometer scale-modified structures. These damage structures were the same as those formed on the aluminum bulk surface. In this article, we discuss a possibility of the growth process of damage structures on the Al surface based on the experimental results

Absolute keV X-ray yield and conversion efficiency in over dense Si sub-petawatt laser plasma

Laser-produced plasmas are bright, short sources of X-rays commonly used for time-resolved imaging and spectroscopy. Their usage implies accurate knowledge of laser-to-X-ray conversion efficiency, spectrum, photon yield and angular distribution. Here we report on soft X-ray emission in the direction close to the target normal from a thin Si foil irradiated by a sub-PW picosecond laser pulse. These absolute measurements cover a continuous and broad spectral range that extends from 4.75 to 7.5 Å(1.7–2.6 keV). The X-ray spectrum consists of spectral line transitions from highly charged ions and broadband emission with contributions from recombination and free-free processes that occur when electrons decelerate in plasma electromagnetic fields. Angular distribution of the emission was investigated via PIC simulations, which allowed to estimate the yield into the full solid angle. We found that experiment and simulation estimations of laser to free-free emission conversion efficiency are in a good agreement

Absolute keV X-ray yield and conversion efficiency in over dense Si sub-petawatt laser plasma

Laser-produced plasmas are bright, short sources of x-rays commonly used for time-resolved imaging and spectroscopy. Their usage implies accurate knowledge of laser-to-x-ray conversion efficiency (CE), spectrum, photon yield and angular distribution. Here, we report on soft x-ray emission in the direction close to the target normal from a thin Si foil irradiated by a sub-PW picosecond laser pulse. These absolute measurements cover a continuous and broad spectral range that extends from 4.75 to 7.3 Å (1.7-2.6 keV). The x-ray spectrum consists of spectral line transitions from highly charged ions and broadband emission with contributions from recombination and free-free processes that occur when electrons decelerate in plasma electromagnetic fields. Angular distribution of the emission was investigated via particle-in-cell simulations, which allowed to estimate the yield into the full solid angle. We find that experimental and simulation estimations of laser to free-free emission CE are in good agreement