Ionization Of Atomic Hydrogen Research Articles

Above threshold ionization of atomic hydrogen in ns states with up to four excess photons

In a high-intensity laser field an atom can absorb more photons than the minimum necessary for ionization. It is known as above threshold ionization (ATI). Theoretically it is the most difficult case to handle as we have to consider transitions in continuum. To study ATI we use the perturbation theory and Green's function formalism. We have derived the modified two-term Coulomb Green's function (CGF) Sturmian expansion. In each term explicit summation over all intermediate states is carried out. The transition amplitude may be obtained in a closed form. The generalized cross sections are evaluated for the photoionization of atomic hydrogen in ns states with up to four excess photons. Calculations are performed in a wide range of wavelengths for linear and circular polarization. In the cases for which data are available, our results agree very well with the previous ones.

1D Modeling of LHD Divertor Plasma and Hydrogen Recycling

One dimensional plasma and neutral model of the divertor plasma in Large Helical Device is presented. The plasma is described by stationary fluid equations for electron and ion. The atomic processes such as dissociation of hydrogen molecules released from the divertor plate, ionization of hydrogen atoms, charge exchange and recombination are included in equations of neutrals. This model is intended to be employed in an integrated simulation where an equilibrium of the upstream plasma and plasma-surface interactions at the divertor plate are solved in different numerical codes separately. From the computational point of view, the numerical code for the divertor plasma is developed for 1D flux tube where the boundary conditions of both ends are specified. The calculation time is less than one second and reasonably short to use in future integrated simulations. In the results, interactions between plasma and neutrals and dependence of the energy loss on the plasma density are studied. In low density case, the energy is lost through ionization and charge exchange but the total amount of the loss is small and the impurity loss is negligibly small. In high density case, the ionization loss and impurity cooling become much larger than the charge exchange loss and causes a drop of the heat flux.

Ionization of atomic hydrogen by an intense resonant laser pulse

The ionization of atomic hydrogen by an intense laser field resonant with the transition ls-Spo is studied. The intensity of the field is strong enough to make the Rabi oscillations important. The theoretical treatment of this work is based on a modified form of the Coulomb-Volkov approximation. The decay to the continuum is accounted by coupling the initial and the excited resonant states to the continuum part of the atomic spectrum. A system of integro-differential equations numerically solved is obtained. The results obtained in this work have been tested against the numerical solution of the time-dependent Schrodinger equation. Good agreement has been obtained. ATI peaks splitting related to the Autler-Townes effect is discussed.

Double differential projectile angular distributions for single ionization of atomic hydrogen by 75 keV proton impact

Doubly differential cross sections (DDCSp) for single ionization of atomic hydrogen by 75-keV proton impact as a function of the projectile scattering angle and energy loss have been measured.

Fully differential cross sections for heavy particle impact ionization

We describe a procedure for extracting fully differential ionization cross sections from an impact parameter coupled pseudostate treatment of the collision. Some examples from antiproton impact ionization of atomic Hydrogen are given.

Angular distribution in multi-photon ionization of hydrogen in intense laser fields

We have studied the multi-photon ionization of atomic hydrogen by a strong laser pulse. Particular emphasis is placed on the angular distribution of the ejected electrons. Rapid changes in the β parameters across the photolines may complicate the comparison of theory and experiment in practical scenarios.

Atomic ionization by ultrashort half-cycle pulses

A theoretical study of the ionization of hydrogen atoms as a result of the interaction with ultrashort external half-cycle pulses is presented. Total ionization probability and energy distributions of ejected electrons are calculated in the framework of the single distorted Coulomb-Volkov (CVA) and the doubly-distorted impulsive Coulomb-Volkov (ICV) approximations. Quantum and classical results are compared to the exact solution of the time dependent Schrödinger equation. We show that the ICV bears a considerable improvement to the CVA energy distribution in the low-energy region, where CVA fails.

Nonequilibrium Calculation of High-Temperature Radiating H2-He Flowfield

Chemical kinetics of high-temperature hydrogen-helium gas mixture behind a shock wave is numerically investigated by integrating rate equations for species concentration in time. State-to-state transition rates are used to determine quasi-steady-state rate coefficients for atomic hydrogen ionization. The electron concentrations in front of the shock wave are deduced by solving the radiative heat transfer equation of molecular hydrogen. The computed incubation time of avalanche ionization is compared with the experimental data and those appeared in past studies. It is found that the precursor photoionization and the associative ionization of the molecular hydrogen are important to determine the ionization time behind the shock wave. The present chemical kinetic models are found to reproduce the shock tube experimental data of the ionization time reasonably well.

Optical radiation and ionization of hydrogen atoms in heterogeneous exothermal reactions proceeding in an electric field

Optical radiation related to the Balmer series (Hα, Hβ, Hγ) of hydrogen atoms is discovered when studying the isothermal reaction of trimeric acetone peroxide decomposition on the surface of oxidized tungsten in a static electric field with a strength of up to 4 × 106 V/cm at T = 300 K. The distance from the surface over which desorbing excited hydrogen atoms radiate is determined from the Stark splitting of the lines. Electronically excited atoms remaining on the surface ionize according to the surface ionization mechanism.

Above-threshold ionization by few-cycle nonlinear chirped pulses

We theoretically investigate the above-threshold ionization of hydrogen atom in a few-cycle nonlinear chirped laser pulse by solving one-dimensional time-dependent Schr\"odinger equation. It is shown that the cutoff of the above-threshold ionization spectrum could be extremely extended by adjusting the chirp parameter. The results can be well explained by the simple-man model.

Three-Body Dynamics in Single Ionization of Atomic Hydrogen by 75 keV Proton Impact

Doubly differential cross sections for single ionization of atomic hydrogen by 75 keV proton impact have been measured and calculated as a function of the projectile scattering angle and energy loss. This pure three-body collision system represents a fundamental test case for the study of the reaction dynamics in few-body systems. A comparison between theory and experiment reveals that three-body dynamics is important at all scattering angles and that an accurate description of the role of the projectile-target-nucleus interaction remains a major challenge to theory.

Three-body scattering theory without knowledge of exact asymptotic boundary conditions

We formulate the theory of three-body scattering without explicit reference to exact asymptotic boundary conditions on the wave function. The transition rate and amplitude are expressed as volume integrals of the resolvent, which are insensitive to the region of asymptotically large distances. The physical branch of the resolvent is selected through the arrow of time, which is required to point forward in each subchannel. This is accomplished by first expressing the resolvent as an integral over time and then making a conformal transformation of each half of the time plane onto a unit disk. The physical branch corresponds to a path of integration in the upper half of the disk. We have tested the method, using a real discrete basis, by calculating the total cross section for singlet $S$-wave electron impact ionization of atomic hydrogen; our results are in reasonable agreement overall with the landmark results of Bartlet and Stelbovics [Phys. Rev. Lett. 93, 233201 (2004)].

Initial Testing of a Compact Portable Microwave-Driven Neutron Generator

A portable, moderate yield D-D neutron generator based on permanent magnet microwave ion source is being developed at Lawrence Berkeley National Laboratory for applications such as short range SNM detection in suitcases and small parcels. Microwave power (2.45 GHz) can be coupled to a pyrex tube through a standard wave guide and generate plasma. In this source configuration, the wave guide serves as a secondary containment for sealed tube design. Hydrogen plasma has been successfully ignited in the pyrex tube at a microwave power of 200 W. The 2.45 GHz microwave signal can also be directly coupled to the wave guide through a standard coaxial cable with a N-type connection and generate plasma. Preliminary results show that over 60% of atomic hydrogen ions were generated at a microwave power of 300 W. Higher atomic fraction is expected with insertion of boron nitride liner. The current density at power of 200 W (with an extraction aperture of 2 mm in diameter, and gas flow at 0.2 sccm) was approximately 22 mA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> .

Differential and total cross sections for antiproton-impact ionization of atomic hydrogen and helium

We develop a method for extracting fully differential cross sections for ionization from an impact-parameter treatment of the collision. The approach uses pseudostates. The method is applied to antiproton-impact ionization of H and He. It is not restricted to antiprotons but can be used with other projectiles. The method automatically includes the interaction between the projectile and the target nucleus. This interaction is shown to be very important in low-energy antiproton ionization. Integrated cross sections for elastic scattering, discrete excitation, ionization, and total scattering are also calculated. For ionization, these are compared with experimental data on H and He. At impact energies greater than 30 keV there is agreement with these data. At lower energies the calculated cross sections for He are in disagreement with the trend of the older experimental data of Andersen et al. and Hvelplund et al. but are in qualitative accord with the recent measurement of Knudsen et al. However, there is no overall quantitative agreement with the new measurements. First-order Born calculations are also presented as a benchmark for checking the pseudostate approximation.

Reaction of a hydrogen-terminated Si(100) surface in UHV with ion-pump generated radicals

The authors present scanning tunneling microscopy images and mass spectra that show that dosing gases at pressures in the range of 10−6 Torr in an ion-pumped ultrahigh vacuum (UHV) chamber results in a measurable concentration of reactive molecular radicals and atomic hydrogen ions being created. One source of radicals is the fragmentation of the dosed molecule, while another is atomic hydrogen that is re-emitted from the ion pump itself. The dosing of noble gases such as helium also results in harmful radicals escaping the ion pump. These radicals are able to create new reactive sites on a hydrogen-terminated Si(100) surface; they show that these new dangling bonds result in extra molecular line growth in a 2,3-dimethyl-1,3-butadiene dosing experiment. These results serve as a cautionary note to experimenters working with ion-pumped UHV systems and surfaces that are sensitive to radicals, such as hydrogen-terminated Si.

Limitations of the strong field approximation in ionization of the hydrogen atom by ultrashort pulses

We present a theoretical study of the ionization of hydrogen atoms as a result of the interaction with an ultrashort external electric field. Doubly-differential momentum distributions and angular momentum distributions of ejected electrons calculated in the framework of the Coulomb-Volkov and strong field approximations, as well as classical calculations are compared with the exact solution of the time dependent Schr odinger equation. We show that in the impulsive limit, the Coulomb-Volkov distorted wave theory reproduces the exact solution. The validity of the strong field approximation is probed both classically and quantum mechanically. We found that classical mechanics describes the proper quantum momentum distributions of the ejected electrons right after a sudden momentum transfer, however pronounced the differences at latter stages that arise during the subsequent electron-nucleus interaction. Although the classical calculations reproduce the quantum momentum distributions, it fails to describe properly the angular momentum distributions, even in the limit of strong fields. The origin of this failure can be attributed to the difference between quantum and classical initial spatial distributions.

Ellipticity dependence of the validity of the saddle-point method in strong-field ionization by few-cycle laser pulses

For a three-cycle arbitrarily polarized laser pulse propagating in the $z$ direction, we calculate the $({q}_{x},{q}_{y})$ momentum distribution for strong-field ionization of atomic hydrogen. We consider the Keldysh theory for a few-cycle pulse and compare the results obtained by evaluating the transition amplitude numerically with the results obtained by evaluating the transition amplitude using the saddle-point method. We show that the validity of the latter depends highly on the ellipticity for fixed peak intensity. This dependence and the range of applicability of the saddle-point method is qualitatively explained by a semiclassical model.

Multiphoton resonant ionization of hydrogen atom exposed to two-colour laser pulses

This paper studies the multiphoton resonant ionization by two-colour laser pulses in the hydrogen atom by solving the time-dependent Schrodinger equation. By fixing the parameters of fundamental laser field and scanning the frequency of second laser field, it finds that the ionization probability shows several resonance peaks and is also much larger than the linear superposition of probabilities by applying two lasers separately. The enhancement of the ionization happens when the system is resonantly pumped to the excited states by absorbing two or more colour photons non-sequentially.

Rotational de-excitation of hydrogen–helium muonic molecules in non-ionizing collisions with hydrogen atoms

Rotational J = 1 → J = 0 transitions in muonic molecules (3Heμh)++J, in the process (3Heμh)++1 + H → (3Heμh)++0 + H, where h = p, d, t; H is the respective hydrogen isotope atom (H, D, T) and μ = μ− (the negative muon), are investigated. The corresponding cross sections and reaction rates are calculated for collision energies 0.001–0.5 eV in the frame of the distorted-wave Born approximation method using the Born–Oppenheimer approximation for a two-centre wavefunction of the hydrogen atom electron. The results obtained for transition rates, normalized to the liquid hydrogen density, are relatively small and range between 102 s−1 and 103 s−1 for (3Heμp)++1 − H and (3Heμd)++1 − D, whereas for (3Heμt)++1 − T they range between 103 s−1 and 104 s−1. They are about 6–8 orders of magnitude smaller than reaction rates for the 1 → 0 de-excitation induced by the hydrogen atom ionization considered in our previous paper (Czapliński et al 2008 J. Phys. B: At. Mol. Phys. 41 035101).

Theory of catalytic dissociation of hydrogen atoms on a metal surface

The model of hydrogen atom ionization near a metal surface is discussed on the basis of a comparison between the metal work function and the atom ionization energy. In the theoretical calculation, it is shown that the hydrogen atom ionization energy decreases when the atom approaches the metal surface. The ionization energy vanishes when the distance between proton and the metal surface is somewhat less than the Bohr radius.