Showing papers on "Ionization published in 1994"
TL;DR: A simple, analytic, and fully quantum theory of high-harmonic generation by low-frequency laser fields is presented and the exact quantum-mechanical formula for the harmonic cutoff that differs from the phenomenological law Ip+3.17Up is presented.
Abstract: We present a simple, analytic, and fully quantum theory of high-harmonic generation by low-frequency laser fields. The theory recovers the classical interpretation of Kulander et al. in Proceedings of the SILAP III Works hop, edited by B. Piraux (Plenum, New York, 1993) and Corkum [Phys. Rev. Lett. 71, 1994 (1993)] and clearly explains why the single-atom harmonic-generation spectra fall off at an energy approximately equal to the ionization energy plus about three times the oscillation energy of a free electron in the field. The theory is valid for arbitrary atomic potentials and can be generalized to describe laser fields of arbitrary ellipticity and spectrum. We discuss the role of atomic dipole matrix elements, electron rescattering processes, and of depletion of the ground state. We present the exact quantum-mechanical formula for the harmonic cutoff that differs from the phenomenological law Ip+3.17Up, where Ip is the atomic ionization potential and Up is the ponderomotive energy, due to the account for quantum tunneling and diffusion effects.
3,007 citations
TL;DR: In this article, the results of laser-induced breakdown experiments in fused silica (SiO2) employing 150 fs −7 ns, 780 nm laser pulses are reported and the avalanche ionization mechanism is found to dominate over the entire pulse width range.
Abstract: Results of laser‐induced breakdown experiments in fused silica (SiO2) employing 150 fs–7 ns, 780 nm laser pulses are reported. The avalanche ionization mechanism is found to dominate over the entire pulse‐width range. Fluence breakdown threshold does not follow the scaling of Fth∼ √τp, when pulses are shorter than 10 ps. The impact ionization coefficient of SiO2 is measured up to ∼3×108 V/cm. The relative role of photoionization in breakdown for ultrashort pulses is discussed.
848 citations
TL;DR: In this article, a theoretical model for calculating electron-impact ionization cross sections for atoms and molecules is presented, which combines the binary-encounter theory with the dipole interaction of the Bethe theory for fast incident electrons.
Abstract: A theoretical model, which is free of adjustable or fitted parameters, for calculating electron-impact ionization cross sections for atoms and molecules is presented. This model combines the binary-encounter theory with the dipole interaction of the Bethe theory for fast incident electrons. The ratios of the contributions from distant and close collisions and interference between the direct and exchange terms are determined by using the asymptotic behaviors predicted by the Bethe theory for ionization and for stopping cross sections. Our model prescribes procedures to calculate the singly differential cross section (energy distribution) for each subshell using the binding energy, average kinetic energy, and the differential dipole oscillator strengths for that subshell. Then the singly differential cross section is integrated over the ejected electron energy to obtain the total ionization cross section. The resulting total ionization cross section near the threshold is proportional to the excess energy of the projectile electron. We found that this model yields total ionization cross sections for a variety of atoms and molecules from threshold to several keV which are in good agreement (\ensuremath{\sim}10% or better on average) with known experimental results. The energy distributions also exhibit the expected shapes and magnitudes. We offer a simpler version of the model that can be used when differential oscillator strengths are not known. For the ionization of ions with an open-shell configuration, we found that a minor modification of our theory greatly improves agreement with experiment.
799 citations
TL;DR: The production of He[sup +] and He[Sup 2+] by a 160 fs, 780 nm laser has been measured over an unprecedented 12 orders of magnitude in counting range and the NS yield is found to scale with the ac-tunneling rate for the neutral, even when tunneling is not the dominant ionization pathway.
Abstract: The production of ${\mathrm{He}}^{+}$ and ${\mathrm{He}}^{2+}$ by a 160 fs, 780 nm laser has been measured over an unprecedented 12 orders of magnitude in counting range. Enhanced double electron emission, called nonsequential (NS) ionization, was observed over an intensity range where the single ionization dynamics is evolving from multiphoton to pure tunneling. The NS yield is found to scale with the ac-tunneling rate for the neutral, even when tunneling is not the dominant ionization pathway. A rescattering mechanism fails to predict the observed NS threshold or magnitude.
711 citations
TL;DR: All rare gas atoms investigated show similar behavior, indicating that the plateau in above threshold ionization is a universal phenomenon.
Abstract: We present photoelectron energy spectra for the rare gas atoms in strong 40 fs, 630 nm laser pulses. A new property in the above threshold ionization distribution is described, namely, a plateau. Numerical calculations using one- and three-dimensional models suggest that at least in part this is a one-electron effect. All rare gas atoms investigated show similar behavior, indicating that the plateau in above threshold ionization is a universal phenomenon. We discuss a simple mechanism possibly responsible for the plateau.
453 citations
TL;DR: In this article, a technique was developed for highly efficient postionization of sputtered metal atoms from a magnetron cathode, based on conventional magnetron sputtering with the addition of a high density, inductively coupled rf (RFI) plasma in the region between the sputtering cathode and the sample.
Abstract: A technique has been developed for highly efficient postionization of sputtered metal atoms from a magnetron cathode. The process is based on conventional magnetron sputtering with the addition of a high density, inductively coupled rf (RFI) plasma in the region between the sputtering cathode and the sample. Metal atoms sputtered from the cathode due to inert gas ion bombardment transit the rf plasma and can be ionized. The metal ions can then be accelerated to the sample by means of a low voltage dc bias, such that the metal ions arrive at the sample at normal incidence and at a specified energy. The ionization fraction, measured with a gridded mass‐sensitive energy analyzer is low at 5 mTorr and can reach 85% at 30 mTorr. Optical emission measurements show scaling of the relative ionization to higher discharge powers. The addition of large fluxes of metal atoms tends to cool the Ar RFI plasma, although this effect depends on the chamber pressure and probably the pressure response of the electron tempera...
376 citations
TL;DR: In this article, the authors extend the analysis of axisymmetric magnetic shear instabilities from ideal magnetohydrodynamic (MHD) flows to weakly ionized plasmas with coupling between ions and neutrals caused by collisions, ionization, and recombination.
Abstract: We extend the analysis of axisymmetric magnetic shear instabilities from ideal magnetohydrodynamic (MHD) flows to weakly ionized plasmas with coupling between ions and neutrals caused by collisions, ionization, and recombination. As part of the analysis, we derive the single-fluid MHD dispersion relation without invoking the Boussinesq approximation. This work expands the range of applications of these instabilities from fully ionized accretion disks to molecular disks in galaxies and, with somewhat more uncertainty, to protostellar disks. Instability generally requires the angular velocity to decrease outward, the magnetic field strengths to be subthermal, and the ions and neutrals to be sufficiently well coupled. If ionization and recombination processes can be neglected on an orbital timescale, adequate coupling is achieved when the collision frequency of a given neutral with the ions exceeds the local epicyclic freqency. When ionization equilibrium is maintained on an orbital timescale, a new feature is present in the disk dynamics: in contrast to a single-fluid system, subthermal azimuthal fields can affect the axisymmetric stability of weakly ionized two-fluid systems. We discuss the underlying causes for this behavior. Azimuthal fields tend to be stabilizing under these circumstances, and good coupling between the neutrals and ions requires the collision frequency to exceed the epicyclic frequency by a potentially large secant factor related to the magnetic field geometry. When the instability is present, subthermal azimuthal fields may also reduce the growth rate unless the collision frequency is high, but this is important only if the field strengths are very subthermal and/or the azimuthal field is the dominant field component. We briefly discuss our results in the context of the Galactic center circumnuclear disk, and suggest that the shear instability might be present there, and be responsible for the observed turbulent motions.
372 citations
TL;DR: The existence and angular position of these side lobes are consequences of the classical kinematics of electrons in laser fields as discussed by the authors, and it is shown that the existence of these lobes is a consequence of the intrinsic properties of the laser electric field.
Abstract: Recent experimental investigations of the high-order above-threshold ionization peaks near the onset of the plateau have exhibited anomalous angular distributions of the emitted photoelectrons with pronounced side lobes surrounding emission in the direction of the laser electric field. It is shown that the existence and angular position of these side lobes are consequences of the classical kinematics of electrons in laser fields.
367 citations
TL;DR: In this article, it was shown that hollow atoms can become the dominant excitation mode for such systems, making their exploitation in an X-ray laser a real possibility, which could in principle provide an imaging system capable of molecular resolution.
Abstract: SEVERAL recent experimental findings1–3 have pointed to a possible route for making an X-ray laser, which could in principle provide an imaging system capable of molecular resolution4. The method involves the multiphoton excitation of atoms in van der Waals clusters or in molecules to yield ions with core-electron vacancies1,2, which can then decay by emission of X-rays, in conjunction with a self-chanelling propagation mode of electromagnetic radiation3. The multiphoton excitation may be stimulated by ultrahigh-brightness, subpicosecond pulses of laser light5. We have previously observed2 emission of X-rays from L-shell transitions in core-excited krypton atoms using this approach. Here we report the multiphoton production of X-rays of wavelength 2–3 A from highly ionized xenon atoms which possess a large number of innershell vacancies while retaining several electrons in relatively weakly bound outer orbitals. Atoms with this ‘inverted’ electronic configuration are designated ‘hollow atoms’6,7. We find that generation of hollow atoms can become the dominant excitation mode for such systems, making their exploitation in an X-ray laser a real possibility.
324 citations
TL;DR: In this paper, a new method to preferentially sense the collection of single-polarity charge carriers in ionization detectors is presented, which uses coplanar electrodes to closely emulate the function of Frisch grids commonly employed in gas and liquid ionisation detectors.
Abstract: A new method to preferentially sense the collection of single‐polarity charge carriers in ionization detectors is presented. It uses coplanar electrodes to closely emulate the function of Frisch grids commonly employed in gas and liquid ionization detectors. The coplanar electrode structure allows for easy implementation on semiconductor detectors. This method can be used to obtain good energy resolution from detectors in which only one polarity type of carriers is efficiently collected. Experimental results using a CdZnTe detector demonstrate the effectiveness of this method.
289 citations
TL;DR: In this article, a series of planar radiative shock models that cover a wide range of shock velocities, preshock densities, and magnetic fields are presented.
Abstract: In this paper we calculate emission-line ratios from a series of planar radiative shock models that cover a wide range of shock velocities, preshock densities, and magnetic fields The models cover the initial conditions relevant to stellar jets, and we show how to estimate the ionization fractions and shock velocities in jets directly from observations of the strong emission lines in these flows The ionization fractions in the HH 34, HH 47, and HH 111 jets are approximately 2%, considerably smaller than previous estimates, and the shock velocities are approximately 30 km/s For each jet the ionization fractions were found from five different line ratios, and the estimates agree to within a factor of approximately 2 The scatter in the estimates of the shock velocities is also small (+/- 4 km/s) The low ionization fractions of stellar jets imply that the observed electron densities are much lower than the total densities, so the mass-loss rates in these flows are correspondingly higher (approximately greater than 2 x 10(exp -7) solar mass/yr) The mass-loss rates in jets are a significant fraction (1%-10%) of the disk accretion rates onto young stellar objects that drive the outflows The momentum and energy supplied by the visible portion of a typical stellar jet are sufficient to drive a weak molecular outflow Magnetic fields in stellar jets are difficult to measure because the line ratios from a radiative shock with a magnetic field resemble those of a lower velocity shock without a field The observed line fluxes can in principle indicate the strength of the field if the geometry of the shocks in the jet is well known
TL;DR: In this article, the results of a systematic study of the first transition series using a spin restricted Kohn-Sham density-functional method with the gradient-corrected functionals of Becke and Lee, Yang and Parr are discussed.
Abstract: The excitation energies and ionization potentials of the atoms in the first transition series are notoriously difficult to compute accurately. Errors in calculated excitation energies can range from 1 to 4 eV at the Hartree–Fock level, and errors as high as 1.5 eV are encountered for ionization energies. In the current work we present and discuss the results of a systematic study of the first transition series using a spin‐restricted Kohn–Sham density‐functional method with the gradient‐corrected functionals of Becke and Lee, Yang and Parr. Ionization energies are observed to be in good agreement with experiment, with a mean absolute error of approximately 0.15 eV; these results are comparable to the most accurate calculations to date, the quadratic configuration interaction single, double (triple) [QCISD(T)] calculations of Raghavachari and Trucks. Excitation energies are calculated with a mean error of approximately 0.5 eV, compared with ∼1 eV for the local density approximation and 0.1 eV for QCISD(T)....
TL;DR: In this paper, it has been shown that the grounding resistance of a concentrated electrode drops when it is subjected to a high current charge, and that the degree of the resistance depends on the magnitude of the ionization gradient of the soil E/sub o/.
Abstract: It has been known for many years that the grounding resistance of a concentrated electrode drops when it is subjected to a high current charge. This helps reduce the ground potential rise. The degree of the resistance depends on the magnitude of the ionization gradient of the soil E/sub o/. Based on both a theoretical analysis and a critical review of the large number of available measurements, this paper recommends that E/sub o/ be taken equal to 300 kV/m for typical soils. This is significantly less than the 1000 kV/m value used by some authors. Graphs are also given describing the behaviour of the rod electrodes which are used in many field installations. >
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TL;DR: In this article, the conditions necessary to extract momentum-density information from the ionization experiments are examined and related to general theories of electron scattering, and different experimental arrangements are reviewed and momentum density results from selected examples are discussed.
Abstract: Most of our knowledge of the electronic structure of atoms and molecules is derived from excitation energies and transition probabilities. These observable quantities are related to the electronic wave functions by integrals over unmeasured variables. Another observable more directly related to the wave function than energy or transition probability is the single-electron momentum density, the probability that an electron in a well-defined orbital has a given value of momentum. Over the last twenty years a technique has been developed for measuring momentum densities in atoms and molecules. The technique, ([ital e],2[ital e]) spectroscopy, is based on electron-impact ionization with complete determination of the momenta of both incoming and outgoing electrons. The conditions necessary to extract momentum-density information from the ionization experiments are examined and related to general theories of electron scattering. Different experimental arrangements are reviewed and momentum-density results from selected examples are discussed.
TL;DR: The strong ellipticity dependence of correlated two-electron multiphoton ionization of neon and of the high-harmonic emission from argon and neon are reported, suggesting a common underlying mechanism and quantitatively consistent with a recently developed Keldysh-like model of high harmonic generation.
Abstract: The strong ellipticity dependence of correlated two-electron multiphoton ionization of neon and of the high-harmonic emission from argon (harmonic N=21) and neon (N=41) are reported. These measurements suggest a common underlying mechanism and are quantitatively consistent with a recently developed Keldysh-like model of high harmonic generation which treats the interaction between a newly freed electron and the ion core.
TL;DR: With proper adjustment of pH and other conditions in the source, the ionization is very soft, enabling injection of weakly bound complexes; these have been accelerated and measured in the spectrometer without observable fragmentation.
Abstract: An electrospray source has been coupled to a reflecting time-of-flight mass spectrometer. The ions enter as a continuous beam in a direction perpendicular to the spectrometer axis and are formed into short bursts of ions with velocities parallel to the axis by electrical pulses applied to injection electrodes. The instrument may be operated either in the linear mode, with ions detected behind the electrostatic mirror, or in the reflecting mode, with ions detected after reflection. In the latter case the time resolution is < or = 8 ns for m/z approximately 600, enabling observation of individual isotopic peaks for masses up to about 4000 u. The sensitivity is adequate to enable measurement of mass spectra for 10 fmol of cytochrome c (approximately 12,000 u). The spectrometer does not limit the range of m/z values, and ions have been observed up to m/z approximately 6000. With proper adjustment of pH and other conditions in the source, the ionization is very soft, enabling injection of weakly bound complexes; these have been accelerated and measured in the spectrometer without observable fragmentation.
TL;DR: The equilibrium ionization balance, determined from the intensities of radiative recombination x rays, gives values of 1.55[plus minus]0.27 and 2.35 b for the electron impact ionization cross sections of hydrogenlike and heliumlike uranium, respectively.
Abstract: An equilibrium population of approximately 500 hydrogenlike U[sup 91+] ions and ten fully stripped U[sup 92+] ions has been produced and trapped in an electron beam ion trap at 198-keV electron energy. The equilibrium ionization balance, determined from the intensities of radiative recombination x rays, gives values of 1.55[plus minus]0.27 and 2.82[plus minus]0.35 b for the electron impact ionization cross sections of hydrogenlike and heliumlike uranium, respectively. These values are somewhat larger than relativistic distorted wave calculations, and much smaller than previous values inferred from stripping of accelerator beams.
TL;DR: A self-consistent, two-dimensional hybrid fluid-particle model is presented and used to describe the electrical behavior of dc low-pressure discharges, resulting in a unified physical picture of discharge behavior in the cathode fall, negative glow, and positive column regions over a wide range of discharge currents.
Abstract: A self-consistent, two-dimensional hybrid fluid-particle model is presented and used to describe the electrical behavior of dc low-pressure discharges in the current range ${10}^{\mathrm{\ensuremath{-}}7}$--${10}^{\mathrm{\ensuremath{-}}2}$ A in argon for products of the gas pressure and the gap spacing (pd) from 1 to 33 cm torr The two-dimensional distributions of the potential, charged particle densities and ionization source term at steady state are shown to illustrate the discharge behavior during the transition from the normal to the abnormal regimes For the larger values of pd, a positive column region as well as the cathode regions are clearly apparent The model used here consists of Poisson's equation for the electric field coupled to the continuity equations for the electrons and ions with the important feature that the ionization source term appearing in the continuity equations is nonlocal and determined from a Monte Carlo simulation This description yields a unified physical picture of discharge behavior in the cathode fall, negative glow, and positive column regions over a wide range of discharge currents
TL;DR: In this paper, a numerical solution to the transport and energy degradation problem was proposed, verified by reproducing simplified problems to which analytic solutions exist, internal self-consistency tests, comparison with laboratory experiments of electron beams penetrating a collision chamber, and by comparison with auroral observations, particularly the emission ratio of the N2 second positive to N+2 first negative emissions.
Abstract: . Auroral electron transport calculations are a critical part of auroral models. We evaluate a numerical solution to the transport and energy degradation problem. The numerical solution is verified by reproducing simplified problems to which analytic solutions exist, internal self-consistency tests, comparison with laboratory experiments of electron beams penetrating a collision chamber, and by comparison with auroral observations, particularly the emission ratio of the N2 second positive to N+2 first negative emissions. Our numerical solutions agree with range measurements in collision chambers. The calculated N22P to N+21N emission ratio is independent of the spectral characteristics of the incident electrons, and agrees with the value observed in aurora. Using different sets of energy loss cross sections and different functions to describe the energy distribution of secondary electrons that emerge from ionization collisions, we discuss the uncertainties of the solutions to the electron transport equation resulting from the uncertainties of these input parameters.
TL;DR: In this paper, radio mapping at three frequencies of the Seyfert 2 galaxy NGC 5252 is reported, which is known to exhibit a spectacular pair of ionization cones in optical emission-line images.
Abstract: We report radio mapping at three frequencies of the Seyfert 2 galaxy NGC 5252, which is known to exhibit a spectacular pair of 'ionization cones' in optical emission-line images. The radio structure of the galaxy comprises an unresolved (less than 50 pc) source coincident with the optical nucleus, weak, narrow features extending approximately equal to 900 pc to north and south from the nucleus, and an unresolved radio source some 10 kpc from the nucleus. The inner parts of the extended radio structure and the off-nuclear source align well with the axis of the ionization cones. There are currently 11 Seyfert galaxies known to possess an ionization cone or a bi-cone; 8 of these galaxies also contain a linear (double, triple, or jet-like) nuclear radio structure. For this limited, incomplete sample, there is a tight alignment between cone and radi axes: the formal mean difference between the measured projections of these axes on the sky is only 6 deg, and the alignment may well be better than this at the location(s) closer to the nucleus where the collimation occurs. Although the degree of collimation is much worse for the ionizing photons than for the radio plasma, it is clear that they are collimated by the same, or coplanar, nulcear disks or tori. In particular, if the ionization cones result from absorption by dusty tori on the pc scale and the radio ejecta from accretion disks around the central black hole, the absence of differential precession indicates that either the gravitating mass distribution is close to spherical or the dusty torus has settled into a preferred plane. The cones currently known in late-type (but not early-type) spirals show a trend to align with the axis of the galaxy stellar disk. We argue that this alignment is either an observational selection effect or indicates that the gas accreted to power the nuclear activity has an internal origin in late-type spirals, but may have an external origin (e.g., a galaxy merger) in early-types. .
TL;DR: In this article, a table of X-ray scattering factors for naturally occurring elements and ions was calculated using a multiconfiguration Dirac-Fock code and the results showed that the mean inner potential can be very sensitive to ionization state.
Abstract: Tables of X-ray scattering factors for naturally occurring elements and ions have been calculated using a multiconfiguration Dirac–Fock code. The complete data set takes up about 250 kbytes of space. For accurate values, it is preferable to interpolate directly. For consistency with previous work, parametric fits are also presented with error estimates and a range of validity. The limiting values of the electron scattering factor as sinθ/λ tends to zero, which are needed to calculate the mean inner potential for electron scattering, are also tabulated. The results show that the mean inner potential can be very sensitive to ionization state.
TL;DR: In this paper, a hydrodynamic model of the presheath is investigated accounting for an oblique magnetic field and for collisions, and it is shown that the main effect of a strong magnetic field is to "compress" the collisional presheaths into a thin layer with a characteristic extension of the ion gyroradius ρi.
Abstract: In the limit of a small Debye length (λD→0), the plasma boundary layer in front of a negative absorbing wall is split up into a collision‐free planar space charge sheath and a quasineutral presheath, where the ions are accelerated to ion sound speed (Bohm criterion) Usually the presheath mechanism depends decisively on collisional friction of the ions, on ionization, or on geometric ion current concentration If the ion dynamics in the presheath is dominated by a magnetic field (nearly) parallel to the wall, an additional effect must be considered to provide an ion transport to the wall The special cases (a) of an ion transport by field lines intersecting the wall at a finite angle and (b) of an ion transport by collisions result in somewhat contradictory conclusions To get a coherent picture, a hydrodynamic model of the presheath is investigated accounting for an oblique magnetic field and for collisions The limiting cases (a) and (b) are discussed, and it is shown that (in plane geometry) the presheath ion acceleration depends always on elementary processes The main effect of a strong magnetic field is to ‘‘compress’’ the collisional presheath into a thin layer with a characteristic extension of the ion gyroradius ρi
Proceedings Article•
08 May 1994TL;DR: In this article, photoassociative spectroscopy involves illuminating a collection of laser cooled and trapped atoms with a tunable probe laser when the probe laser is resonant with transition to an excited molecular state, molecules are formed and detected either by ionizing the molecules or by monitoring changes in the number of trapped atoms.
Abstract: Recently, several groups,1-4 have used photoassociation of ultracold atoms to study transitions from free atoms to bound molecules. Photoassociative spectroscopy involves illuminating a collection of laser cooled and trapped atoms with a tunable probe laser. When the probe laser is resonant with transition to an excited molecular state, molecules are formed and detected either by ionizing the molecules or by monitoring changes in the number of trapped atoms.
TL;DR: In this article, the convergent-close-coupling method was extended for the calculation of electron-hydrogen scattering to hydrogen-like targets, atoms, or ions.
Abstract: We extend the convergent-close-coupling method for the calculation of electron-hydrogen scattering to hydrogenlike targets, atoms, or ions. These include H, Li, Na, and K, atoms, as well as the multitude of ions which have the same isoelectronic sequence as any of these atoms. The reliability of the method is independent of the projectile energy, and we demonstrate its applicability by achieving excellent agreement with a large set of measurements for electron scattering on sodium at projectile energies ranging from 1 to 54.4 eV. These measurements include spin asymmetries, singlet and triplet ${\mathit{L}}_{\mathrm{\ensuremath{\perp}}}$, reduced Stokes parameters, differential, integrated, and total cross sections, as well as the total ionization spin asymmetry. The method is found to give better agreement with experiment than any other over this entire energy range.
TL;DR: Two-color phase control in the total ion yield of ionization is successfully demonstrated in a tunneling regime by using a 100-fs Ti:sapphire laser and its third harmonic.
Abstract: Two-color phase control in the total ion yield of ionization is successfully demonstrated in a tunneling regime by using a 100-fs Ti:sapphire laser and its third harmonic. Adding the third harmonic with an intensity of only 10% enhances the ion yield by a factor of 7. In photoelectron spectra, above threshold ionization peaks due to the third harmonic disappear when two colors are superimposed, resulting in a continuum spectrum. This shows two-color interference clearly. The intensities of high-order harmonics in the plateau region are enhanced by an order of magnitude.
TL;DR: In this article, the ionization rate from above-threshold ionization (ATI) in an optical field whose phase dependence is known and controllable was measured using a harmonic two-color light field with a calibrated phase difference.
Abstract: We present the first measurement of the ionization rate from above-threshold ionization (ATI) in an optical field whose phase dependence is known and controllable. Our method uses a harmonic two-color light field with a calibrated phase difference between the $1\ensuremath{\omega}$ and $2\ensuremath{\omega}$ fields. Our measurements confirm many of the predictions of recent semiclassical theories, but show an important discrepancy: An asymmetry in the ATI rates relative to the sign of the two-color phase difference supports recently suggested modifications to the models to include electron-core rescattering during ionization.
TL;DR: The CCSD approximation is implemented, which includes single and double virtual excitations in a self-consistent manner, incorporating therefore the effects of the Coulomb and Breit interactions to all orders in these excitations.
Abstract: The relativistic Fock-space coupled cluster method for the direct calculation of ionization potentials and excitation energies (including fine structure) is presented and applied to atomic Au and its ions. The no-pair Dirac-Coulomb-Breit Hamiltonian is taken as the starting point. The CCSD approximation is implemented (where CCSD indicates coupled cluster with single and double excitations), which includes single and double virtual excitations in a self-consistent manner, incorporating therefore the effects of the Coulomb and Breit interactions to all orders in these excitations. A rather large basis set (21s17p11d7f) of kinetically balanced Gaussian spinors is used to span the atomic orbitals. All calculated energies (ionization potential and electron affinity of Au, excitation energies of Au and ${\mathrm{Au}}^{+}$) agree with experiment to 0.1 eV or better, with an average error of 0.06 eV. Fine-structure splittings are accurate to better than 0.01 eV.
TL;DR: In this article, low-density, low-excitation photoionization models were proposed to explain the observed spectrum of the diffuse ionized gas (DIG), which is strong in low stages of ionization (N II, SII), weak in (O III, and very weak in O I) lambda 6300.
Abstract: The diffuse ionized gas (DIG), responsible for producing the faint but pervasive H(sub alpha), requires 15% of the power of all Galactic O stars for its ionization. No other source of ionization seems practical. The spectrum is strong in low stages of ionization (N II, SII), weak in (O III), and very weak in (O I) lambda 6300, at least in one well-observed direction, all significantly different from denser H II regions. We give low-density, low-excitation photoionization models that explain the observed spectrum. The lambda 6300 observation eliminates the simplest models for the DIG (Mathis 1986), in which neutral H extends beyond the edge of the very dilute stellar radiation field. Our present models include two components. One, representing the edges of interstellar H I clouds, extends to the point where H becomes neutral. In the second, the fraction of H(sup o) is not allowed to exceed 0.05 to 0.1. Both have very low values of the ionization parameter, or ratio of the number densities of ionizing photons to electrons. The ionization parameter required by our models is shown to be compatible with observed values of photon and electron densities in the diffuse interstellar medium. Interstellar dust is not important in either model. Predictions of the model are that the Galaxy is leaking about 4% of the ionizing radiation of at least the local O stars into the intergalactic medium, that (O II) lambda 3727 approximately equals 1.1 H(sub alpha), and that He(sup +)/H (sup +)) approximately equals 0.6 He/H. A major difficul ty of this picture is having ionizing radiation propagate from the ionizing stars in the plane of the Galaxy to the DIG. We suggest, as do Miller & Cox (1993), that the ionizing radiation from O stars produces holes or tunnels in the distribution of neutral H as seen from their positions, while we see a sky covered with H I sheets or filaments because we are not near a source of ionizing radiation. With our models, several interfaces between ionized and neutral gas are allowed before the observational limit on lambda 6300 is exceeded. Another difficulty is the gas-phase N(S(sup +))/N(H(sup o)) ratio toward the high-latitude star HD 93521 (Spitzer & Fitzpatrick 1993). We require (S/H) to be somewhat lower than solar, either because of depletion onto grains or a Galactic abundance gradient, so that the S(sup +) can be produced in each of separate H(sup o) and H(sup +) regions.
TL;DR: The origin of the soft X-ray background radiation has remained mysterious since its discovery, although it is clear from the lack of absorption of the low-energy X-rays that there must be a strong local contribution as mentioned in this paper.
Abstract: THE origin of the soft X-ray background radiation has remained mysterious1 since its discovery2, although it is clear from the lack of absorption of the low-energy X-rays that there must be a strong local contribution3 Recent results4,5 demonstrate, however, that there are significant more distant contributions, whose origins are also unclear Here we propose an explanation for both the local and more distant contributions to the soft X-ray background—they seem to arise from the rapid adiabatic expansion of hot gas, driven by the explosions of massive stars This hot gas cools quickly, 'freezing in' highly ionized atomic states The X-ray emission arises from the delayed recombination of ions and electrons at relatively low temperatures, and is therefore distinct from the more usual line emission excited by collisions with electrons The X-ray flux is thus relatively insensitive to the local gas kinetic temperature, as the gas is far from ionization equilibrium
TL;DR: In this article, a simple model was proposed to evaluate the degree of l and ml mixing in high Rydberg states that results from perturbations caused by weak, homogeneous dc electric fields and static ions.
Abstract: We present a simple model to evaluate the degree of l and ml mixing in high Rydberg states that results from perturbations caused by weak, homogeneous dc electric fields and static ions. This model predicts the lifetime of these states qualitatively and explains several seemingly contradictory observations obtained using zero‐kinetic‐energy (ZEKE) photoelectron spectroscopy. The presence of a small homogeneous dc electric field and a few ions in the sample volume causes ml mixing in general as well as l mixing, both of which contribute to the lengthening of the lifetimes. Consequently, the lifetime lengthening appears to be insensitive to the sample pressure. The effect of the dc electric field on the lifetime is complex. Although the electric field results in l mixing, with increasing field strength it inhibits ml mixing, and, at still higher field strength, induces ionization. The variation of the lifetimes with ion concentration is also complicated. At low ion concentration, the ml mixing varies across...