Ionization Of Atomic Hydrogen Research Articles

Formation of Quantum Vortices at the Ionization of an Atom by an Ultrashort Laser Pulse: Two- and Three-Dimensional Cases

We theoretically analyze the quantum vortices - the zeros of electron wave function which are formed through atom ionization by ultrashort laser pulse. In 2D space we consider the case of above-threshold ionization of hydrogen atom and in 3D space the ionization of the atom in the zero-radius-potential approximation. With the help of obtained analytical results we make the assumption that localization of quantum vortices in 3D space can be predicted by analysis of 2D model.

Design and comparison of the Cs ovens for the test facilities ELISE and SPIDER.

Negative ion sources for fusion rely on the formation of negative hydrogen (or deuterium) ions by conversion of atomic hydrogen and positive hydrogen ions at a low work function caesiated surface. Cs is thus evaporated into the source to decrease the surface work function, which may change due to the removal and redistribution of Cs during plasma phases. To maintain a temporarily stable low work function during 1 h plasma, continuous evaporation of caesium is required, and this is performed by temperature controlled Cs ovens. The Cs ovens for ELISE (IPP Garching) and SPIDER (Consorzio RFX) are based on the evaporation of liquid Cs from a reservoir located at one end of the oven, which is controlled by the reservoir temperature. The ampoule Cs oven of ELISE is in operation since 2015, allowing for controllable and stable evaporation. The SPIDER oven is based on the ELISE oven although it required significant changes due to the vacuum environment and the oven location (at the back-plate instead of the sidewalls), leading to a different design of the oven and the nozzle. First investigations on the SPIDER oven in a dedicated test stand show that Cs evaporation is controllable, stable, and reproducible.

Quantum chaos in strong field ionization of hydrogen

We present a study of quantum chaos in the strong field ionization of hydrogen atoms. We use the Bohmian approach to quantum mechanics which allows us to use classical tools such as the Lyapunov exponents to characterize chaotic dynamics. We consider the hydrogen atom in a monochromatic electromagnetic field for the tunneling and intermediate regimes of ionization (corresponding to the values γ ∼ 1 of the Keldysh parameter). We show that with increasing frequency the quantum dynamics of this system becomes increasingly chaotic.

The mechanism of negative and positive hydrogen ions production on the Ni surface

The studies about surface production of hydrogen negative ions play an important role in many technologies. In the present paper, a low energy molecular hydrogen ion beam was irradiated to the porous Ni sheet. It was found that negative atomic hydrogen ions were detected at the back of the porous Ni sheet when the energy of incident ion beam was lower than 180 eV. When the energy of the incident ions increased, the positive ions dominated at the back of the porous Ni sheet. The production of negative hydrogen on Ni surface can't be explained by comparing the electron affinity of Ni and H. The charge transferring during the process of H atoms reflected from Ni surface was calculated by using constrained DFT (cDFT). The results show the production of negative hydrogen on Ni surface may well be due to heterogeneous electron density on the Ni surface. However, the influence of incident energy on the production of negative and positive hydrogen ions is unable to be explained by cDFT. The problem may be solved by using TDDFT which is able to capture the transfer of energy to electrons during the collision between atoms.

Calculation of the triple and double cross sections in the proton-hydrogen impact at the ionization channel by removing singularity in the scattering amplitudes

In the present work, the triple and double cross sections of atomic hydrogen ionization in collision with the proton are calculated. The potential of the interaction is considered as a Coulombic; also, the first and second order Born approximations have been used. The singularity of the Green operator in the second order approximation has been removed for the analytical solution of the scattering amplitude and the triple cross section calculation. Finally, the results for triple and double cross sections have been compared with the available theoretical and experimental results. The first and the second Born approximations are usually applied at intermediate and high impact energy ranges. However, it is shown that it could be successfully applied at lower energies as well.

FORMATION OF LIGHT IMPURITIES IN A HYDROGEN PLASMA AT INITIAL STAGE OF A DISCHARGE

Analyzing the dynamics of density for atomic and molecular hydrogen ions, the values of atomic hydrogen and UV radiation fluxes to the walls of the plasma chamber were obtained, resulting in light impurities of carbon and oxygen at plasma start-up during the process of desorption from the walls under irradiation. The fluxes of impurity atoms associated with the fluxes of photons and hydrogen atoms in a discharge are determined. Recommendations are given to reduce the amount of impurities at the initial stage of discharge.

Manipulating dynamical Rabi-splitting with two-color laser pulses.

We theoretically investigate strong-field ionization of hydrogen atoms by orthogonally polarized two-color (OTC) laser pulses consisting of a fundamental field that is resonant with the 1s - 2p transition and its second harmonic. Numerical simulations are performed by solving the two-dimensional time-dependent Schrödinger equation and recording the photoelectron momentum distribution. In this strong-field resonant ionization, the atom undergoes many Rabi cycles and the electron can be emitted within a completed Rabi-cycle leading to the splitting of the localized momentum distribution. Here, the splitting is attributed to dynamic Rabi-splitting as a result of the dynamic Stark effect. The employed OTC scheme is shown to be efficient for controlling the dynamic Rabi-splitting through the control of quantum-path interferences involved in one-photon and two-photon absorption processes. The control scheme is accomplished by varying the relative ratio intensity and optical phase between the two pulses, and its footprint is mapped in the momentum distribution. This is shown to lead to an asymmetric distribution and suppression of the ionization process. The obtained results suggest the OTC scheme as a tool for coherent control of dynamic Rabi-oscillations via the controlled quantum-path interferences, thus opening new directions towards designing quantum states via the control OTC scheme.

Above-threshold ionization of hydrogen atoms in a two-color laser field

By using the method of electron wave packet interference, we investigate the above-threshold ionization (ATI) of hydrogen atoms in a linearly polarized two-color laser pulse (λ1 = 800 nm and λ2 = 400 nm). Firstly, using the strong field approximation (SFA) with the time window function, we obtain the intracycle interference and intercycle interference patterns of hydrogen atoms in a 10 fs laser pulse, and it is found that the general characteristics of two-dimensional photoelectron momentum spectrum are formed by the interplay of the intracycle interference and intercycle interference, and the side rings in two-dimensional photoelectron momentum spectra originate from intercycle interference. Subsequently, we calculate the two-dimensional photoelectron momentum spectra of hydrogen atoms by SFA in full pulse. When the relative phase ϕ = 0, the intracycle interference and intercycle interference fringes in two-dimensional momentum spectrum are symmetrical, and when the relative phase is ϕ=0.5π, the intracycle interference fringes in the two-dimensional momentum spectrum are obviously asymmetrical. The results show that ionization is vitally dependent on relative phase of two laser pulses in two-color laser filed.

Differential cross sections for ionization of atomic hydrogen by antiprotons

We consider ionization of atomic hydrogen by antiproton impact. Doubly differential cross sections for ionization calculated within a recently developed semiclassical approach by Bondarev et al. (Phys. Rev. A 95, 052709, 2017) are presented. Comparison of the obtained results with other theoretical predictions is given.

Breakdown of the nonrelativistic approximation in superintense laser-matter interactions

We study the breakdown of the nonrelativistic approximation in the multiphoton ionization of atomic hydrogen by some intense x-ray laser pulse, in a regime where the dipole approximation is no longer valid. To this end, both the time-dependent Dirac equation as well as its nonrelativistic counterpart, the time-dependent Schr\odinger equation, are solved within an ab initio numerical framework. It is demonstrated that a recently developed semirelativistic Schr\odinger equation for superintense laser fields [Lindblom et al., Phys. Rev. Lett. 121, 253202 (2018)] yields results in excellent agreement with the fully relativistic treatment. The semirelativistic equation is then used in an investigation of the role of higher-order beyond-dipole corrections to the laser-matter interaction. The result of the present study can be summarized into two main findings: (1) relativistic effects predict a blueshift of the multiphoton ionization spectrum, and (2) higher-order beyond dipole corrections (beyond the leading-order term) indicate a corresponding redshift of the photoelectron spectrum. However, the two shifts turn out to be of the same order of magnitude, effectively leading to a net cancellation of their respective contributions. This apparent cancellation effect raises an important question: Is the distinction between relativistic blueshifts and higher-order beyond dipole redshifts meaningful from an experimental point of view? The result of the present study indicates that the answer is negative because the two effects nonetheless cannot be measured separately. Therefore, instead, we suggest that the present findings should merely be taken as a demonstration that caution should be exercised when higher-order beyond-dipole and relativistic corrections are to be taken into account in approximation schemes in the modeling of superintense laser-matter interactions.

Free Particle Spin Speed

The electron and the proton are stable particles, compounds of the atoms, which in turn are the macroscopic scale components. A heated metal plate becomes the source of a cloud of electrons, which by an acceleration in an electromagnetic field becomes an electron beam. Protons are obtained by ionized hydrogen atoms; as the mass of the electron is negligibly small relative to the mass of the proton, a hydrogen reservoir is basically a proton reservoir. Many other subatomic particles (elemental or composite) may be produced in elementary processes that occur in nature or in the laboratory. In various physical applications of elementary particles, it is necessary to know the rotation speed around the own axis of an elementary particle in motion. The work determines this angular velocity of rotation of the particle with a very high accuracy depending on its known linear displacement velocity. In other words, the angular velocity of an elementary particle is in the function of its linear displacement velocity, also obtained by De Broglie's hypothesis that the impulse of the elementary particle is preserved. Applications of this new theory can be used in nuclear power, in nuclear fusion and in high-power laser physics. Of the desire to add a new brick to the physical construction of nuclear fusion reactor theory, to obtaining sustainable nuclear energy, but also to the optimal realization and to the other possible applications, one presented this original work.

Holographic interferences in photoelectron spectra: different approaches

We perform a theoretical study of the holographic structures in photoelectron spectra for ionization of hydrogen atoms induced by short laser pulses. To elucidate the nature of the holographic structures present in the momentum distributions of photoelectrons, we use several quantum approximations, such as the strong field and Coulomb–Volkov approximations up to second order, as well as semiclassical Monte Carlo simulations. In a single-cycle pulse, we eliminate the intracycle interference from the spectra isolating the holographic structure formed in the photoionization process. We probe the different approaches and analyze the role of electron–core interaction numerically by solving the time dependent Schrodinger equation. We show that the two-step semiclassical model of Shvetsov-Shilovski et al. [Phys. Rev. A 94, 013415 (2016)] fully considers the effect of the Coulomb potential on the electron dynamics and semiclassical phase reproducing the holographic structure in full quantum calculations. Contrarily, perturbative quantum (strong field and Coulomb–Volkov) and semiclassical (quantum trajectory Monte Carlo) methods account only partially for some of the characteristics of the holographic interference pattern.

氢原子在少周期强激光场中阈上电离的电子波包干涉图像

氢原子在少周期强激光场中阈上电离的电子波包干涉图像

The modeling of multiphoton ionization by path integral approach

The probability of quantum transitions of a molecule between its states under the action of an electromagnetic field is represented as an integral over trajectories of a real alternating functional. A method is proposed for computing the integral using recurrence relations. The method is used to describe the ionization of hydrogen atom.

Near-Threshold Cross Sections for Electron and Positron Impact Ionization of Atomic Hydrogen.

Using classical arguments Wannier [Phys. Rev. 90, 817 (1953)PHRVAO0031-899X10.1103/PhysRev.90.817] proposed an electron-impact ionization cross section for neutral atoms to behave as E^{1.127}, where E is the excess energy above threshold. Using similar arguments Klar [J. Phys. B 14, 4165 (1981)JPAMA40022-370010.1088/0022-3700/14/21/027] obtained E^{2.65} to be the corresponding threshold law for positron impact. Recently, Babij etal. [Phys. Rev. Lett. 120, 113401 (2018)PRLTAO0031-900710.1103/PhysRevLett.120.113401] measured near-threshold positron-impact breakup behavior to be similar to that expected for electrons. Using the convergent close-coupling method for the atomic hydrogen target, we examine cross sections at near-threshold energies for electron and positron impact. Contrary to the experiment, the calculated cross sections are found to behave differently for the two projectiles and consistently with the aforementioned threshold laws, despite the entirely quantum nature of these problems. For electron impact, the threshold behavior holds while the total electron spin asymmetry remains constant, whereas for positron scattering the threshold law holds for breakup while the positronium-formation component of the ionization cross section remains constant.

Magnetogenesis in cyclical Universe

We propose a simple solution to the magnetogenesis problem based on cyclic cosmology. It is demonstrated that magnetic fields, of sufficient strengths to account for the present observational bounds, can be created in the contracting phase preceding the beginning of the current cosmological cycle. The basic assumption of this model is that the Universe enters a contraction phase essentially empty, characterized by small seed electric fields. We discuss how such small seed electric fields can be naturally created due to charge separation of astrophysical currents during the expansion of the Universe or, alternatively, due to ionization as a consequence of the growth of the effective dark energy contribution reaching the hydrogen atom ionization potential. In this framework, there is no need for any new theoretical additions to explain magnetogenesis, such as new scalar fields or the non-minimal coupling between curvature, scalar fields and the electromagnetic sector. Moreover, the proposed model is general in the sense that it does not assume any specific modified gravity theory to enable the cosmological bounce. When compared to the inflationary magnetogenesis paradigm, the proposed model also has the advantage of not leading to the backreaction and strong coupling problems.

Studying the influence of nitrogen seeding in a detached-like hydrogen plasma by means of numerical simulations

The leading candidate for impurity seeding in ITER is currently nitrogen. To date, there have only been a few studies on the plasma chemistry driven by N2/H2 seeding and its effect on the molecular-activated recombination of incoming atomic hydrogen ions in a detached-like scenario. Numerical simulations are needed to provide insights into such mechanisms. The numerous plasma chemical reactions that may occur in such an environment cannot be entirely included in a 2- or 3-dimensional code such as Eirene. A complete global plasma model, implemented with more than 100 plasma chemical equations and 20 species, has been set up on the basis of the Plasimo code. This study shows that two main nitrogen-including recombination reaction paths are dominant, i.e. the ion conversion of NH followed by dissociative recombination, and the proton transfer between and N2, producing N2H+. These two processes are referred to as N-MAR (nitrogen molecular-activated recombination) and have subsequently been implemented in Eunomia, which is a spatially resolved Monte Carlo code designed to simulate the neutral inventory in linear plasma machines such as Pilot-PSI and Magnum-PSI. To study the effect of N2 on the overall recombination, three studies have been set up, and from a defined puffing location with a constant total seeding rate of H2 + N2, three N2 ratios were simulated, i.e. 0%, 5% and 10%. The parameter monitored is the density of atomic hydrogen, being the final hydrogenic product of any recombination mechanism in the scenario considered. The difference in H density between the 0% case and the 10% case is about a factor of three. The importance of NH as an electron donor is highlighted, and the N-MAR reaction routes are confirmed to enhance the conversion of ions to neutrals, making the heat loads to the divertor plate more tolerable. This work is a further step towards a full understanding of the role of N2-H2 molecules in a detached divertor plasma.

Discharge Characteristics of the Penning Plasma Source

The results of investigation of discharge characteristics of the Penning plasma source are presented. Volt-ampere characteristics (VAC), energy distribution, and mass-charge composition of ions emitted from the discharge under different modes of its combustion were measured. The connection between the discharge current bursts and increase in the potential drop (up to 50% of the anode voltage) is established. The measured VACs agree well with the theoretical dependencies. It is shown that the content of atomic hydrogen ions increases from 5 to 10% with an increase in the anode voltage from 1 to 3.5 kV and the power applied in the discharge (from 0.2 to 3 W).

Phase-dependent ionization of hydrogen by intense sub-cycle pulses.

We have performed simulations and analytic calculations that show strong carrier-envelope phase dependence in the ionization of hydrogen atoms using intense sub-cycle sub-femtosecond laser pulses. When the pulse width is comparable to the classical orbit time of the initial bound state, sine-like pulses can ionize more than cosine-like pulses that have the same fluence. This result is the opposite of what is expected from a tunneling-like model, where the ionization probability primarily depends on the peak amplitude of the electric field during the pulse.

Moiré patterns in doubly differential electron-momentum distributions in atomic ionization by mid-infrared lasers

We analyze the doubly differential electron momentum distribution in above-threshold ionization of atomic hydrogen by a linearly-polarized mid-infrared laser pulse. We reproduce side rings in the momentum distribution with forward-backward symmetry previously observed by Lemell \textit{et al.} in Phys. Rev. A \textbf{87}, 013421(2013), whose origin, as far as we know, has not been explained so far. By developing a Fourier theory of moir\'{e} patterns, we demonstrate that such structures stems from the interplay between intra- and intercycle interference patterns which work as two separate grids in the two-dimensional momentum domain. We use a three dimensional (3D) description based on the saddle-point approximation (SPA) to unravel the nature of these structures. When the periods of the two grids (intra- and intercycle) are similar, principal moir\'{e} patterns arise as concentric rings symmetrically in the forward and backward directions at high electron kinetic energy. Higher order moir\'{e} patterns are observed and characterized when the period of one grid is multiple of the other. We find a scale law for the position (in momentum space) of the center of the moir\'{e} rings in the tunneling regime. We verify the SPA predictions by comparison with time-dependent distorted wave strong-field approximation (SFA) calculations and the solutions of the full 3D time-dependent Schr\"{o}dinger equation (TDSE).