Showing papers on "Ionization published in 1993"

Plasma perspective on strong field multiphoton ionization.

Abstract: During strong-field multiphoton ionization, a wave packet is formed each time the laser field passes its maximum value Within the first laser period after ionization there is a significant probability that the electron will return to the vicinity of the ion with very high kinetic energy High-harmonic generation, multiphoton two-electron ejection, and very high energy above-threshold-ionization electrons are all conssequences of this electron-ion interaction One important parameter which determines the strength of these effects is the rate at which the wave packet spreads in the direction perpendicular to the laser electric field; another is the polarization of the laser It will be essential for experimentalists to be aware of these crucial parameters in future experiments

The physics of simple metal clusters: experimental aspects and simple models

Abstract: The study of simple metal clusters has burgeoned in the last decade, motivated by the growing interest in the evolution of physical properties from the atom to the bulk solid, a progression passing through the domain of atomic clusters. On the experimental side, the rapid development of new techniques for producing the clusters and for probing and detecting them has resulted in a phenomenal increase in our knowledge of these systems. For clusters of the simplest metals, the alkali and noble metals, the electronic structure is dominated by the number of valence electrons, and the ionic cores are of secondary importance. These electrons are delocalized, and the electronic system exhibits a shell structure that is closely related to the well-known nuclear shell structure. In this article the results from a broad range of experiments are reviewed and compared with theory. Included are the behavior of the mass-abundance spectra, polarizabilities, ionization potentials, photoelectron spectra, optical spectra, and fragmentation phenomena.

Review of chemical-kinetic problems of future NASA missions, II: Mars entries

Abstract: A number of chemical-kinetic problems related to phenomena occurring behind a shock wave surrounding an object flying in the earth atmosphere are discussed, including the nonequilibrium thermochemical relaxation phenomena occurring behind a shock wave surrounding the flying object, problems related to aerobraking maneuver, the radiation phenomena for shock velocities of up to 12 km/sec, and the determination of rate coefficients for ionization reactions and associated electron-impact ionization reactions. Results of experiments are presented in form of graphs and tables, giving data on the reaction rate coefficients for air, the ionization distances, thermodynamic properties behind a shock wave, radiative heat flux calculations, Damkoehler numbers for the ablation-product layer, together with conclusions.

Above threshold ionization beyond the high harmonic cutoff.

Abstract: We present high sensitivity electron energy spectra for xenon in a strong 50 ps, 1.053 \ensuremath{\mu}m laser field. The above threshold ionization distribution is smoothly decreasing over the entire kinetic energy range (0--30 eV), with no abrupt changes in the slope. This is in direct contrast to the sharp cutoff observed in xenon optical harmonic generation spectra. Calculations using the single active electron approximation show excellent agreement with the observed electron distributions. These results directly address the unresolved relationship between the electron and photon emission from an atom in an intense field.

Impact ionization, trap creation, degradation, and breakdown in silicon dioxide films on silicon

Abstract: Degradation of silicon dioxide films is shown to occur primarily near interfaces with contacting metals or semiconductors. This deterioration is shown to be accountable through two mechanisms triggered by electron heating in the oxide conduction band. These mechanisms are trap creation and band‐gap ionization by carriers with energies exceeding 2 and 9 eV with respect to the bottom of the oxide conduction band, respectively. The relationship of band‐gap ionization to defect production and subsequent degradation is emphasized. The dependence of the generated sites on electric field, oxide thickness, temperature, voltage polarity, and processing for each mechanism is discussed. A procedure for separating and studying these two generation modes is also discussed. A unified model from simple kinetic relationships is developed and compared to the experimental results. Destructive breakdown of the oxide is shown to be correlated with ‘‘effective’’ interface softening due to the total defect generation caused by both mechanisms.

Ground-state correlation energies for atomic ions with 3 to 18 electrons.

Abstract: Recently Davidson [ital et] [ital al]. [Phys. Rev. A 44, 7071 (1991)] have estimated nonrelativistic correlation energies and relativistic corrections to ionization potentials for atomic ions with up to 10 electrons. In this work, this approach is extended to atomic ions with 11 to 18 electrons. The correlation energies for 3- to 10-electron atomic ions are also recomputed using more recent experimental and theoretical data. Unlike other work the method focuses on the correlation contribution to the individual ionization energies which are obtained by comparing experimental data with relativistic complete-valence-space energies. [ital Ab] [ital initio] estimates of correlation contributions to the ionization energies with extensive configuration-interaction calculations of 3- to 10-electron atomic ions with nuclear charge from 4 through 10 and 18, 36, 50, 72, 100, and 144 have been obtained. The correlation energies obtained from some density-functional models are also compared to these correlation energy data.

Particle detection with drift chambers

Abstract: Gas Ionization by Charged Particles and by Laser Rays.- The Drift of Electrons and Ions in Gases.- Electrostatics of Tubes, Wire Grids and Field Cages.- Amplification of Ionization.- Creation of the Signal.- Electronics for Drift Chambers.- Coordinate Measurement and Fundamental Limits of Accuracy.- Geometrical Track Parameters and their Errors.- Ion Gates.- Particle Identification by Measurement of Ionization.- Existing Drift Chambers #x2013 An Overview.- Drift-Chamber Gases.

Calculation of Photon Mass Energy-Transfer and Mass Energy-Absorption Coefficients'

Abstract: Calculations of mass energy-transfer and mass energy-absorption coefficients for photon energies from 1 keV to 100 MeV have been developed, based on a re-examination of the processes involved after the initial photon interaction. The probabilities for the initial interaction are from the current photon interaction cross-section database at the National Institute of Standards and Technology. The calculations then take into account (1) electron binding effects on the Compton-scattered photon distribution; (2) the complete cascade of fluorescence emission after ionization events in any atomic subshell, including those associated with incoherent scattering and triplet production; and (3) the radiative energy losses of the secondary electrons and positrons slowing down in the medium, including the emission of bremsstrahlung, characteristic X rays from impact ionization, and positron in-flight as well as at-rest annihilation quanta. Consideration of the processes in (3) goes beyond the continuous-slowing-down approximation and includes the effects of energy-loss straggling. Results for the mass energy-absorption coefficient are compared with those from recent tabulations.

Fluid simulations of glow discharges: Effect of metastable atoms in argon

Abstract: A one‐dimensional fluid simulation of a 13.56 MHz argon glow discharge including metastable species was performed as an example of a coupled glow‐discharge/neutral‐transport‐reaction system. Due to the slow response time of metastables (∼10 ms) direct time integration of the coupled system requires ∼105 rf cycles to converge. This translates to prohibitively long computation time. An ‘‘acceleration’’ scheme was employed using the Newton–Raphson method to speed up convergence, thereby reducing the computation time by orders of magnitude. For a pressure of 1 Torr, metastables were found to play a major role in the discharge despite the fact that their mole fraction was less than 10−5. In particular, metastable (two‐step) ionization was the main mechanism for electron production to sustain the discharge. Bulk electric field and electron energy were lower, and a smaller fraction of power was dissipated in the bulk plasma when compared to the case without metastables. These results suggest that neutral transport and reaction must be considered in a self‐consistent manner in glow discharge simulations, even in noble gas discharges.

Theoretical models of the electrical discharge machining process. III. The variable mass, cylindrical plasma model

Abstract: A variable mass, cylindrical plasma model (VMCPM) is developed for sparks created by electrical discharge in a liquid media. The model consist of three differential equations—one each from fluid dynamics, an energy balance, and the radiation equation—combined with a plasma equation of state. A thermophysical property subroutine allows realistic estimation of plasma enthalpy, mass density, and particle fractions by inclusion of the heats of dissociation and ionization for a plasma created from deionized water. Problems with the zero‐time boundary conditions are overcome by an electron balance procedure. Numerical solution of the model provides plasma radius, temperature, pressure, and mass as a function of pulse time for fixed current, electrode gap, and power fraction remaining in the plasma. Moderately high temperatures (≳5000 K) and pressures (≳4 bar) persist in the sparks even after long pulse times (to ∼500 μs). Quantitative proof that superheating is the dominant mechanism for electrical discharge ma...

Quantum efficiencies exceeding unity due to impact ionization in silicon solar cells

Abstract: Absolute measurements demonstrate internal quantum efficiencies in silicon solar cells to exceed unity for photon energies above the first direct band gap and to show distinct spectral features that correspond to specific points in the Brillouin zone. Ultraviolet radiation can generate hot carriers with sufficient energy to cause impact ionization which results in two electron hole pairs per incident photon.

Magnetron sputter deposition with high levels of metal ionization

Abstract: A new deposition technique has been developed which combines conventional magnetron sputter deposition with a rf inductively coupled plasma (RFI). The RFI plasma is located in the region between the magnetron cathode and the sample position, and is set up by a metal coil immersed in the plasma. A large fraction of the metal atoms sputtered from the magnetron cathode are ionized in the RFI plasma. By placing a negative bias on the sample, metal ions are then accelerated across the sample sheath and deposited at normal incidence. Results from a gridded energy analyzer configured with a microbalance collector and located at the sample position indicate the level of ionization is low at a few mTorr and rises to ≳80% at pressures in the 25–35 mTorr range. Optical measurements of metal ion and neutral emission lines show scaling of the relative ionization to higher discharge powers. Significant cooling of the plasma electron temperature is observed when high concentrations of metal atoms were sputtered into the...

Dynamics of Short-Pulse Excitation, Ionization and Harmonic Conversion

Abstract: In recent years there have been very significant advances in short pulse, high intensity laser technology. Lasers with pulse lengths of 0.1–1 ps and wavelengths from 0.2–1 μm can be focused to produce intensities from 1012 to above 1018 W/cm2. One major use of these systems has been for studies of the response of atoms and molecules to such intense, well characterized electromagnetic fields. Because these pulses are very short, neutral atoms can survive to experience intensities where theoretical treatments based on the traditional perturbation expansion of the wave function in terms of the field-free states will fail completely to describe the dynamics of the system. An explicit, non-perturbative time-dependent calculation is one approach which can represent these strong field effects.

Super-Intense Laser-Atom Physics

Abstract: Preface. SILAP 2000: an overview. Part I: Double Ionization of Complex Atoms. Energy distribution of two-electron ionization of helium in an intense field R. Lafon, et al. Double ionization in strong fields: ion momenta and correlated electron momenta A. Staudte, et al. Mechanism of the non sequential double ionization of helium D.G. Lappas, et al. Electron momentum distributions for double ionization in the strong field limit S.P. Goreslavski, S.V. Popruzhenko. S-matrix theory of 'recoil-ion' momentum distribution for double ionization in femtosecond laser fields F.H.M. Faisal, A. Becker. Non-sequential double ionization: a minimal correlation approach R. Kopold, W. Becker. Non-sequential double ionization of atoms in strong fields K. Sacha, B. Eckhardt. Double-electron ionization of two-electron system in strong laser field D.V. Tikhonova. Calculation of double ionization of helium H.G. Muller. The two-electron response in laser driven helium L.R. Moore, et al. The helium atom in strong and short laser pulses: multielectron H. Bachau, R. Hasbani. Dynamics of a two-electron system driven by an ultrashort and intense laser pulse B. Piraux, G. Lagmago-Kamta. Angular distributions for double ionization by an ultrashort, intense laser pulse: the case of Li- G. Lagmago-Kamta, A.F. Starace. Two-and-three electron atoms in strong laser fields P. Lambropoulos, et al. Part II: Ionization and Dissociation of Molecules. Tunnelling ionization and the Franck-Condon principle J.H. Posthumus, et al. Dissociative ionization of few-electron molecules in intense laser fields D. Dundas, et al. One-photon breakup of H+2 in a strong DC field Z. Mulyukov, R. Shakeshaft. Part III: Interaction of Clusters with Very Intense Femtosecond Laser Pulses. Nuclear fusion in gases of deuterium clusters and hot electron generation in droplet sprays under irradiation with an intense femtosecond laser T. Ditmire, et al. The absorption of energy by large atomic clusters from superintense laser pulses V.P. Krainov, M.B. Smirnov. Part IV: Production of Very Intense Femtosecond Laser Pulses. A semi-classical model for high-harmonic generation D.B. Milosevic. Beyond the simple maris model for high harmonic generation M. Brewezyk, K. Rzazewski. Anisotropy induced polarization effects in harmonic generation by an absorptive medium B. Borca, et al. About a new method of high harmonic amplification E.A. Nersesov, et al. X-ray generation via stimulated recombination of electrons and Bohr's correspondence principle A. Jaron, et al. Part V: Stabilization and Relativistic Effects in Super Strong Fields. Interference stabilization: LAMBDA- and V-schemes, dynamics of ionization, initial coherent population of Rydberg levels and quantum phase control of the ionization yield M.V. Fedorov, N.P. Poluektov. Computer experiments on atomic stabilization in a strong laser field A.M. Popov, et al. Quasistationary stabilization of the decay of a weakly-bound level and its breakdown in a strong laser field N.L. Manakov, et al. The strong field limit of atomic stabilization in ultrashort pulses M. Dorr, et al. 3-D numerical calculations of laser atom interactions -- subrelativistic and weakly relativistic regime A. Scrinzi, et al. Momentum space description of hydrogen atom interacting with a low frequency strong laser field A. de Bohan, et al. Relativistic effects in the atomic res

Electron‐impact dissociation of nitrogen

Abstract: The electron‐impact dissociation of N2 to form two nitrogen atoms is observed in a crossed beam experiment at electron energies between 18.5 and 148.5 eV. Detection of the correlated dissociation fragments with a time and position sensitive detector permits detection of both ground and excited state fragments, but excludes interference from dissociative ionization products. The observed translational energy releases in the N2 dissociation are consistent with predissociation to N(2D)+N(4S) fragments as the primary dissociation mechanism. Absolute cross sections for the electron impact dissociation are measured and compared with previous measurements. Recommended values of this cross section are given for electron‐impact energies between 10 and 200 eV.

Global Model of Plasma Chemistry in a High Density Oxygen Discharge

Abstract: the gas phase kinetics and plasma chemistry of high density oxygen discharges are studies. A self-consistent, spatially-averaged model is developed to determine positive ion, negative ion and electron densities, ground state and metastable free radical densities, and electron temperature as functions of gas pressure, microwave input power, and cylindrical source diameter and length. For an electron cyclotron resonance (ECR) discharge, the reduction in radial transport due to the confining magnetic field is also modeled. The kinetic scheme includes excitation, dissociation, and ionization of neutrals due to electron impact, electron attachment and detachment, and ion-ion neutralization. In addition, ion neutralization at the reactor walls is included. Model results show that for a low neutral pressure, high plasma density discharge, oxygen molecules are almost completely dissociated to form oxygen atoms, and the dominant positive ion is O+ rather than O2+. The metastable species are not important for the pressure range studies (0.5 - 100 mTorr), and the confining magnetic field significantly affects the plasma chemistry, the total positive ion density, and the electron temperature. Comparisons are made to experimental data, and qualitative agreement between experiment and model is observed.

Low-ionization broad absorption lines in quasars

Abstract: The absorption line properties of six Mg II broad absorption line quasars (BALQs) are examined using a BALQ sample of Weymann et al. (1991). It is found that Al III absorption is always stronger than Al II absorption; C IV absorption tends to be very deep and broad; and low ionization absorption tends to be narrower and lie at the low velocity end of the C IV trough. The results indicate that the outflows in Mg II BALQs are not accelerating monotonically. It is shown that a hot cloud-confining outflow can be decelerated by adding thick absorbing clouds provided the ionization parameters of these clouds are high. It is argued that the MG II BALQ phenomenon might be a manifestation of a quasar's efforts to expel a thick shroud of gas and dust.

The weakest bond: Experimental observation of helium dimer

Abstract: Helium dimer ion was observed after electron impact ionization of a supersonic expansion of helium with translational temperature near 1 mK The dependence of the ion signal on source pressure, distance from the source, and electron kinetic energy was measured The signal was determined to arise from ionization of neutral helium dimer

Dissociative ionization of small molecules in intense laser fields

Abstract: With the development of high power, subpicosecond lasers, it has become possible to study the process of multielectron dissociative ionization of small molecules at intensities in excess of 1014 W cm-2. At such intensities the conventional multiphoton approach is too complex and the field ionization model becomes more attractive. This article reviews the experimental results obtained to date and indicates how certain aspects of the process, namely the mode of multiple ionization, the thresholds for multiple ionization, the charge symmetry of the fragment ion production and their peaked angular distributions, can be explained in terms of a field ionization, Coulomb explosion model. However, a more sophisticated approach is undoubtedly required to explain the lack of variation of fragment ion kinetic energies with laser risetime, the variation of kinetic energies with laser wavelength and the molecule specificity of the process.

Rotational line intensities in zero kinetic energy photoelectron spectroscopy (ZEKE-PES)

Abstract: Recent advances in the understanding of the factors which determine line intensities in zero kinetic energy photoelectron spectroscopy (ZEKE-PES) are reviewed. The relative importance of direct ionization and autoionization is assessed. Explicit consideration of the channel interactions which take place in the vicinity of molecular ionization thresholds leads to a general discussion of rotational line intensities in ZEKE-PES. A series of limiting cases is proposed to assist in the interpretation of experimental results. Finally a new dynamical interpretation of ZEKE experiments based on the trapping of the ZEKE electrons in non-penetrating high-l Rydberg orbitals by weak electric fields appears to give a satisfactory explanation of most experimental results obtained to date.

Sharp edges to neutral hydrogen disks in galaxies and the extragalactic radiation field

Abstract: It is shown that the very sharp truncation of the neutral hydrogen distribution seen in NGC 3198 (and probably M33) is well modeled as the result of ionization of the atomic gas by the extragalactic radiation field. Below a critical column density of about a few times 10 exp 19/sq cm the gas is dominantly ionized and undetectable in the 21-cm line. It is inferred from the photoionization models that the total disk gas distribution in NGC 3198 is actually fairly axisymmetric. The critical column density for ionization is not a strong function of galaxy mass or mass distribution; thus, all galaxies should show a cutoff at approximately the same column density. Specific models of 3198 suggest that the extragalactic ionizing photon flux is 5000-10,000 photons/sq cm s.

Coupled cluster Green's function method: Working equations and applications

Abstract: Detailed working equations are derived for the ionization part of the single-particle Green's function within the coupled cluster Green's function (CCGF) framework. The CCGF method is applied to the calculation of vertical ionization potentials (IPs) of a number of small molecules, notably, HF, N2, CO, F2, CS, C2H4, H2O, and H2CO. The results for the outer-valence IPs, with an average error of 0.12 eV, compare favorably to third-order equation-of-motion calculations within the same basis set (average error 0.28 eV) and outer-valence GF (OVGF) values taken from the literature (average error 0.17 eV). Ground-state properties that derive from the CCGF are compared to expectation values obtained in the related normal coupled cluster methods (NCCM) approximation from a formal point of view. Correlation energies obtained in CCGF are compared to CCSD results for the above series of molecules and, in addition, the so-called true correlation energy density as obtained from the CCGF is compared to the result from an accurate MR–CI calculation for a highly correlated system: the HF molecule at large internuclear separation. © 1993 John Wiley & Sons, Inc.

Chemically selective laser ion-source for the CERN-ISOLDE on-line mass separator facility

Abstract: Radioactive atoms produced in proton-induced nuclear reactions and released from thick targets have been ionized resonantly by laser radiation in a hot tube connected to the target container. Pulsed tuneable lasers with a repetition rate as high as 10 kHz have been applied for stepwise resonant excitation and photoionization in the last step. In this way the efficiency and selectivity of the target and ion source system which serves as an injector to the on-line isotope separators at CERN-ISOLDE could be improved. In a series of off-line and on-line studies the ionization of Sn ( E i = 7.3 eV), Tm ( E i = 6.2 eV), Yb ( E i = 6.2 eV) and Li ( E i = 5.4 eV) was investigated. An ionization efficiency of up to 15% was obtained for Yb. The ratio of the laser-ionized and surface-ionized ion currents was measured as a function of temperature for different ionization cavity materials (W, Ta, Nb and TaC). It was shown that this ratio, i.e. the selectivity, rises for Tm from 10 to 10000 with falling temperature and is strongly dependent on the material. Since the lasers are pulsed the ion beam becomes bunched with a pulse width of about 10–50 μs. This width is strongly dependent on the potential drop along the tube (caused by the electric current used for heating the tube) and on the alignment of the laser beams with respect to the tube axis. The selectivity could be further improved by a factor of 10 using gated detection of the bunched ion beam.

Matrix‐assisted laser desorption/ionization mass spectrometry with additives to 2,5‐dihydroxybenzoic acid

Abstract: Selected benzoic acid derivatives and related substances were used as additives to 2,5-dihydroxybenzoic acid (2,5 DHB) and the performance of the mixtures in matrix-assisted laser desorption/ionization mass spectrometry was investigated. Using benzoic acid derivatives substituted at position 2 and /or 5 or related substances as a co-matrix in the 1-10% range with 2,5 DHB results in improved ion yields and signal-to-noise ratio of analyte molecules, especially for the high-mass range. The enhanced performance is prominent for 2-hydroxy-5 methoxybenzoic acid and exists for both proteins and oligosaccharides. It is suggested that the improvement is caused by a disorder in the 2,5 DHB crystal lattice allowing «softer» desorption. Charge transfer from matrix ions to additive molecules at the expense of analyte ionization gives a simple explanation for the deteriorating effects of some tested additives

A two‐dimensional fluid model for an argon rf discharge

Abstract: A fluid model for an argon rf discharge in a cylindrical discharge chamber is presented. The model contains the particle balances for electrons and ions and the electron energy balance. A nonzero autobias voltage is obtained by imposing the condition that the time‐averaged current toward the powered and grounded electrode is zero. Particle densities and ionization profiles peak strongly in front of the smaller, powered electrode. There electric fields are stronger and the electron current density is higher, resulting in more ohmic heating and therefore higher ionization rates. The radial uniformity of the plasma in front of the powered electrode gives a homogeneous ion flux toward this electrode. The asymmetric character of the profiles of the cylindrical geometry is in clear contrast with the essentially one‐dimensional infinite parallel‐plate geometry, which is fully symmetric with respect to the center of the discharge and has a zero dc autobias voltage. A comparison with results of a one‐dimensional model shows that the average ion density, the average ion flux, and the average ionization rate in the cylindrical reactor are comparable to those in a parallel‐plate reactor. The numerical treatment of the time evolution of the transport equations and Poisson’s equation needs an implicit method to avoid numerical instabilities. The resulting system of discretized equations is solved by a multigrid technique. The spatial discretization uses the Sharfetter–Gummel scheme.

On the unimolecular fragmentation of C60+ fullerene ions: The comparison of measured and calculated breakdown patterns

Abstract: The stability of singly charged C60+ fullerene ions, produced by electron impact ionization of C60, has been studied as a function of the electron energy and the time elapsed from ionization in a Nier‐type ion source/double‐focusing, sector‐field mass spectrometer system. A huge kinetic shift of more than 34 eV (dependent on the observation time) was observed for the dissociation process C60+→C58++C2. The ionization efficiency curves for C58+, C56+, and C54+ fragment ions have been recorded with an energy resolution of approximately 0.5 eV. This allowed us to construct a time‐resolved breakdown graph of the decaying C60+ fullerene ion. Two different methods, i.e., the finite heat bath model of Klots and the Rice–Ramsperger–Kassel–Marcus (RRKM) expression, have been used to calculate the decay rates and the breakdown graph of the C60+ ion, and the results of the calculation have been compared with the experimentally obtained breakdown graph. The best fit leads to a dissociation energy (C58+−C2) of 7.1±0.4 ...

Electron‐impact dissociation of oxygen

Abstract: The electron‐impact dissociation of O2 to form two oxygen atoms is observed in a crossed beam experiment at electron energies between 13.5 and 198.5 eV. Detection of the correlated dissociation fragments with a time and position sensitive detector permits detection of both ground and excited state fragments, but excludes interference from dissociative ionization products. The observed translational energy releases in the O2 dissociation are consistent with production of O(1D)+O(3P) fragments following electron impact excitation to the B 3Σu−, B’ 3Σu−, and 2 3Πu states, and production of O(3P)+O(3P) fragments from excitation to the (unresolved) c 1Σu−, A’ 3Δu, and A 3Σu+ states. Absolute cross sections for the electron impact dissociation of O2 are measured.

Atoms in ultra-intense laser fields

Abstract: The interaction of an atom with an ultra-intense radiation field is characterized by the involvement of many photons in absorptions and emissions. Of course the word 'intense' has to be compared with some atomic reference: if the induced transition coupling between bound states exceeds inherent widths, then the dressed atom Rabi oscillations which dominate the atomic evolution are typical intense field effects, even though laser intensities may actually be quite modest. When laser intensities exceed IOi3 Wcm-', and infrared frequencies are employed, tben free electrons are dressed by interaction energies which exceed the photon energies. Non-perturbative continuum processes such as above- threshold ionization then occur, combined with the emission of very high order harmonics ofthe pump laser frequency. At higher laser intensities, the optical electric field can exceed the Coulombic binding electric field and allow aver-the-barrier ionization. which defines a new regime of high intensity physics. In this region (or at higher intensities) the atomic electron charge cloud oscillates in the laser field with large amplitude excursions from the nucleus, during which time it is unable to absorb further photons. This stabilization regime is predicted to persist until the electron dressing energy approaches the restmass energy when wholly unexplored regions remain 10 be investigated. In this topical review, we examine theorelied models of atoms dressed by intense fields. We review the breakdown of lowest-order perturbation theory and those 'essential states' methods adopted to include Rabi frequencies, Stark shifts, induced widths and continuum dressing. Newer methods more suitable for super-svong fields are described, such as Floquet and Valkov methods and the direct numerical integration ofthe Schr6dinger equation. Such methods are used to provide completely non-perturbative strong field descriptions of atomic dynamics. We conclude with a brief examination of the relativistic effects expected to be important when new high intensity ultrashort pulse lasers currently under development are employed in strong field physics.