Abstract: We report extensive laser-induced damage threshold measurements on dielectric materials at wavelengths of 1053 and 526 nm for pulse durations $\ensuremath{\tau}$ ranging from 140 fs to 1 ns. Qualitative differences in the morphology of damage and a departure from the diffusion-dominated ${\ensuremath{\tau}}^{\frac{1}{2}}$ scaling of the damage fluence indicate that damage occurs from ablation for $\ensuremath{\tau}l~10$ ps and from conventional melting, boiling, and fracture for $\ensuremath{\tau}g50$ ps. We find a decreasing threshold fluence associated with a gradual transition from the long-pulse, thermally dominated regime to an ablative regime dominated by collisional and multiphoton ionization, and plasma formation. A theoretical model based on electron production via multiphoton ionization, Joule heating, and collisional (avalanche) ionization is in quantitative agreement with the experimental results.

Abstract: We present a complete set of analytic fits to the non-relativistic photoionization cross sections for the ground states of atoms and ions of elements from H through Si, and S, Ar, Ca, and Fe. Near the ionization thresholds, the fits are based on the Opacity Project theoretical cross sections interpolated and smoothed over resonances. At higher energies, the fits reproduce calculated Hartree-Dirac-Slater photoionization cross sections.

Abstract: Laser-induced damage threshold measurements were performed on homogeneous and multilayer dielectrics and gold-coated optics at 1053 and 526 nm for pulse durations τ ranging from 140 fs to 1 ns. Gold coatings were found, both experimentally and theoretically, to be limited to 0.6 J/cm2 in the subpicosecond range for 1053-nm pulses. In dielectrics, we find qualitative differences in the morphology of damage and a departure from the diffusion-dominated τ1/2 scaling that indicate that damage results from plasma formation and ablation for τ ≤ 10 ps and from conventional heating and melting for τ > 50 ps. A theoretical model based on electron production by multiphoton ionization, joule heating, and collisional (avalanche) ionization is in quantitative agreement with both the pulse-width and the wavelength scaling of experimental results.

Abstract: We examine the interaction of intense, femtosecond laser radiation with the large (50{endash}200 A) clusters produced in pulsed gas jets. Both experiment and simulation show that the plasmas produced during these interactions exhibit electron temperatures far in excess of that predicted by above-threshold ionization theory for a low-density gas. Efficient heating of the clusters by the laser is followed by rapid expansion of the clusters and long-lived x-ray emission from hot, decaying, underdense plasma. {copyright} {ital 1996 The American Physical Society.}

Abstract: A theoretical model for electron‐impact ionization cross sections, which has been developed primarily for atoms and atomic ions, is applied to neutral molecules. The new model combines the binary‐encounter theory and the Bethe theory for electron‐impact ionization, and uses minimal theoretical data for the ground state of the target molecule, which are readily available from public‐domain molecular structure codes such as GAMESS. The theory is called the binary‐encounter Bethe (BEB) model, and does not, in principle, involve any adjustable parameters. Applications to 19 molecules, including H2, NO, CH2, C6H6, and SF6, are presented, demonstrating that the BEB model provides total ionization cross sections by electron impact from threshold to several keV with an average accuracy of 15% or better at the cross section peak, except for SiF3. The BEB model can be applied to stable molecules as well as to transient radicals.

Abstract: A highly tetrahedrally bonded, hydrogenated amorphous carbon (ta-C:H) has been deposited from an acetylene-fed plasma beam source. The plasma beam source is operated to provide a highly ionized, monoenergetic plasma beam of ${\mathrm{C}}_{2}$${\mathrm{H}}_{2}^{+}$ ions. The resulting films are characterized in terms of their ${\mathit{sp}}^{3}$ content, mass density, intrinsic stress, surface roughness, radial distribution function, C---H bonding, Raman spectra, optical gap, electrical conductivity, gap states, Youngs modulus, and hardness. The ${\mathit{sp}}^{3}$ content reaches a maximum value of 75% at an ion energy of 200 eV, or 92 eV per C ion. The density and stress also reach a maximum at this ion energy. The formation of ta-C:H is interpreted in terms of the subplantation of ${\mathrm{C}}^{+}$ ions, which produces a densified, ${\mathit{sp}}^{3}$ bonded network. The C-H vibration spectra suggest that C ${\mathit{sp}}^{2}$ sites form C=C alkene groups. The optical gap ${\mathit{E}}_{04}$ reaches a maximum of 2.85 eV and increases with the ${\mathit{sp}}^{3}$ fraction. The spin density due to defects is high and decreases with increasing ion energy. The Youngs modulus and hardness measured by microindenter reach maximum values of 290 and 61 GPa, consistent with the highly ${\mathit{sp}}^{3}$ bonding. The variation of hardness with ${\mathit{sp}}^{3}$ fraction is consistent with the constraint model of network rigidity. \textcopyright{} 1996 The American Physical Society.

Abstract: Theory of Photoionization: VUV and Soft Xray Region M.Ya. Amusia. Manyelectron Effects in Photoionization M. Kutzner. Fundamental Aspects of Atomic Photoionization with High Brightness Light Sources A.F. Starace, S.T. Manson. Core Relaxation Effects in Molecular Photoionization J. Schirmer, et al. Partial Cross Sections and Angular Distributions U. Becker, D.A. Shirley. High Resolution Electron Spectrometry of Atoms M.O. Krause, C.D. Caldwell. Valence Ionization Processes in the VUV Region J. Berkowitz, et al. Photoionpair Formation J. Berkowitz. Electronic and Nuclear Relaxation of Core Excited Molecules I. Nenner, P. Morin. Resonances and Nearthreshold Processes G.C. King, K.H. Schartner. Resonant and Nonresonant Auger Recombination H. Aksela, et al. Laserbased UV and VUV Spectroscopy of Doubly-excited Atoms J.P. Connerade. Ion Yield Spectroscopy with Soft Xrays T. Hayaishi, P. Zimmermann. Coincidence Measurements on Ions and Electrons J.H.D. Eland, V. Schmidt. Spin Polarization in Photoionization U. Heinzmann, N.A. Cherepkov. Photoionization of Excited and Ionized Systems F.J. Wuilleumier, J.B. West. Photoionization of Oriented Systems and Circular Dichroism G. Schonhense, J. Hormes. Index.

Abstract: Valence ionization energies of a set closed‐shell molecules calculated in a partial third‐order (P3) quasiparticle approximation of the electron propagator have an average absolute error of 0.19 eV. Diagonal elements of the self‐energy matrix include all second‐order and some third‐order self‐energy diagrams. Because of its fifth power dependence on basis set size and its independence from electron repulsion integrals with four virtual indices, this method has considerable potential for large molecules. Formal and computational comparisons with other electron propagator techniques illustrate the advantages of the P3 procedure. Additional applications to benzene and borazine display the efficacy of the P3 propagator in assigning photoelectron spectra. In the borazine spectrum, 2E′ and 2A2′ final states are responsible for an observed feature at 14.76 eV. Another peak at 17.47 eV is assigned to a 2E′ final state.

Abstract: Molecular dynamics simulations were used to study the acid ionization of hydrochloric acid (HCl) at the basal plane surface of ice at 190 kelvin, as a model for the acid ionization process in Antarctic polar stratospheric clouds (PSCs). Initial conditions for HCl placement within the top bilayer of the ice lattice were selected on the basis of relevant dynamic equilibrium adsorption-desorption conditions. Free energy changes calculated for the first step in the stepwise acid ionization mechanism ranged from −5.8 to −6.7 kilocalories per mole for various likely initial conditions. These results indicate that acid ionization is thermodynamically favorable and that this process has important implications for ozone depletion mechanisms involving PSCs.

Abstract: Using ultrashort pulse multiphoton ionization, we launch a wave packet in the ${\mathrm{I}}_{2}^{\phantom{\rule{0ex}{0ex}}2+}$ state that dissociates into the fragments ${\mathrm{I}}^{2+}$ and I. We measure its multiphoton ionization probability as a function of its internuclear distance with an intense delayed probe pulse. If the probe pulse is polarized parallel to the internuclear axis, we observe a peak in the ionization yield at a critical distance of 5--6 \AA{}. No such enhancement is seen with perpendicular polarization in agreement with the model of enhanced ionization by electron localization.

Abstract: A new photoelectron spectrometer has recently been used to analyze the energy and spatial distribution of photoelectrons produced by multiphoton ionization of rare gases. It is based on the analysis of the image obtained by projecting the expanding electron cloud resulting from the ionization process onto a two‐dimensional position sensitive detector by means of a static electric field. In this article, we present the principle of this imaging spectrometer and the relevant equations of motion of the charged particle in this device, together with an inversion method that allows us to obtain the energy and angular distribution of the electrons. We present here the inversion procedure relevant to the case where the electrostatic energy acquired in the static field is large as compared to the initial kinetic energy of the charged particles. A more general procedure relevant to any regime will be described in a following article.

Abstract: We report a picosecond laser study of the transient absorption of hydrated electrons generated by the 3−5 eV multiphoton ionization of liquid water. The geminate kinetics indicate that eaq- is produced by at least three different mechanisms over this energy range. Power dependence of the signal amplitude shows a two-photon threshold for 4.0 eV excitation and a three-photon threshold absorption at 3.47 eV, consistent with two- or three-photon excitation of the A(1B1) lowest excited state. For (three-photon) excitation in the range 3.02−3.47 eV very little (≤15%) geminate recombination is observed while for the (two-photon) excitation at shorter wavelengths significant recombination (≥55%) is observed. In the region of 3.85−4.54 eV, photon-energy-independent kinetics indicate that eaq- is produced via two-photon excitation of the A state followed by an ionization process in which the electrons do not obtain any excess kinetic energy. For photon energies in the range of 4.75−5.05 eV, the escape fraction incr...

Abstract: We have developed a general theoretical model for the interaction of ionizing radiation with chromatin. Chromatin is modeled as a 30-nm-diameter solenoidal fiber comprised of 20 turns of nucleosomes, 6 nucleosomes per turn. Charged-particle tracks are modeled by partitioning the energy deposition between primary track core, resulting from glancing collisions with 100 eV or less per event, and delta rays due to knock-on collisions involving energy transfers >100 eV. A Monte Carlo simulation incorporates damages due to the following molecular mechanisms: (1) ionization of water molecules leading to the formation of OH, H, eaq, etc.; (2) OH attack on sugar molecules leading to strand breaks: (3) OH attack on bases; (4) direct ionization of the sugar molecules leading to strand breaks; (5) direct ionization of the bases. Our calculations predict significant clustering of damage both locally, over regions up to 40 bp and over regions extending to several kilobase pairs. A characteristic feature of the regional damage predicted by our model is the production of short fragments of DNA associated with multiple nearby strand breaks. The shapes of the spectra of DNA fragment lengths depend on the symmetries or approximate symmetries of the chromatin structure. Such fragments have subsequently been detected experimentally and are reported in an accompanying paper (B. Rydberg, Radiat, Res. 145, 200-209, 1996) after exposure to both high- and low-LET radiation. The overall measured yields agree well quantitatively with the theoretical predictions. Our theoretical results predict the existence of a strong peak at about 85 bp, which represents the revolution period about the nucleosome. Other peaks at multiples of about 1,000 bp correspond to the periodicity of the particular solenoid model of chromatin used in these calculations. Theoretical results in combination with experimental data on fragmentation spectra may help determine the consensus or average structure of the chromatin fibers in mammalian DNA.

Abstract: p‐ and n‐type doping of GaN have been realized by ion implantation of Ca and O, respectively. Rapid thermal annealing at 1100 °C or higher is required to achieve p‐type conduction in Ca or Ca+P implanted samples with an estimated ionization level of 169 meV and a corresponding activation efficiency of ∼100%. This is the first experimental report of an acceptor species in GaN, other than Mg, with an ionization energy level less than 180 meV. O‐implanted GaN displays an ionization level of ∼29 meV but with an activation efficiency of only 3.6% after a 1050 °C anneal that may result from insufficient vacancy generation for the lighter O ion or from the existence of a second, deeper O energy level. Neither Ca or O displayed measurable redistribution, based on secondary ion mass spectrometry measurements, even after a 1125 °C anneal.

Abstract: Publisher Summary This chapter discusses the molecular applications of quantum defect theory. Quantum defect theory provides a unified description of discrete and continuous spectra in terms of the same parameters pertaining to the physics at short range. It accounts for the Rydberg structures as well as for their decay into the continuum. This decay is called preionization (or autoionization), or predissociation if the continuum is associated with nuclear rather than with electronic motion. It displays two molecular spectra exhibiting striking effects due to both preionization and predissociation. Both spectra result from photoexcitation of nitric oxide and they cover the same wavelength region. The upper spectrum is a recording of the ion current resulting after excitation just above the ionization threshold. The small peaks superimposed on the flat continuum are Rydberg structures whose presence reveals the occurrence of preionization. The lower trace is an absorption spectrum taken under comparable experimental resolution. The significant different appearance of the two spectra reveals that a second decay channel, corresponding to production of neutral atomic fragments, must also interfere. Moreover, the different appearance of the Rydberg peaks in the two spectra indicates that predissociation is strong and competes with preionization.

Abstract: A convenient computational approach for the calculation of the p Kas of ionizable groups in a protein is described. The method uses detailed models of the charges in both the neutral and ionized form of each ionizable group. A full derivation of the theoretical framework is presented, as are details of its implementation in the UHBD program. Application to four proteins whose crystal structures are known shows that the detailed charge model improves agreement with experimentally determined pKas when a low protein dielectric constant is assumed, relative to the results with a simpler single-site ionization model. It is also found that use of the detailed charge model increases the sensitivity of the computed pKas to the details of proton placement. © 1996 by John Wiley & Sons, Inc.

Abstract: High precision measurements of helium photoelectron energy and angular distributions for a broad intensity range reflect the change in the continuum dynamics that occurs as the ionization process evolves into the pure tunneling regime. Elastic rescattering of the laser-driven free electron from its parent ion core leaves a distinct signature on the spectra, providing a direct quantitative test of the various theories of strong field multiphoton ionization. We show that it takes a relatively complete semiclassical rescattering model to accurately reproduce the observed distributions. [S0031-9007(96)01922-9] PACS numbers: 32.80.Rm, 31.90.+ s, 32.80.Fb Strong field photoelectron spectra have attracted considerable attention over the past decade and a half, but a comprehensive understanding of the underlying dynamics which produce these spectra and related phenomena has solidified with the development of high repetitionrate, short pulse lasers which can span the entire intensity range of importance [1]. Keldysh [2] showed that at infrared and visible wavelengths the dynamics of strong field atomic ionization undergoes a change in character as the laser intensity increases. In weak fields electrons are promoted into the continuum by the simultaneous absorption of enough photons to increase their energy above the ionization potential. This is called multiphoton ionization (MPI). However, as the laser intensity increases, a completely separate mode of escape becomes possible. At large distances from the nucleus, the electrostatic attraction of the ion core can be overwhelmed by the laser’s instantaneous electric field, producing a barrier through which a valence electron can tunnel. In this regime a quasistatic tunneling picture becomes appropriate: the laser field varies so slowly compared to the response time of the electron that the ionization rate becomes simply the cycle average of the instantaneous dc tunneling rate. Tunneling becomes important when the ratio of the frequency of the applied field to the tunneling rate becomes smaller than unity. This ratio, known as the Keldysh or adiabaticity parameter, g, is given by p Ipys2Upd, where Ip is the binding energy of the electron and Up › Iy4v 2 is the ponderomotive energy in atomic units of a free electron in the laser field of frequency v and intensity I. The majority of experimental studies on neutral atoms exposed to intense, short‐pulse laser fields have been carried out in the MPI regime ( g$ 1). A few experiments [3 ‐ 6] have extended into the tunneling regime, but these measurements have been limited to observations of total ionization rates or electron energy distributions over a small dynamic range. In this Letter, we report upon the first systematic experimental investigation in the strong field tunneling limit. Our study, by virtue of the enhanced dynamic range accessible with kilohertz laser technology, follows the change in the electron spectrum and angular distributions as the ionization evolves from predominantly MPI to pure tunneling. We find that the electron distributions in the tunneling regime are very different from any previous reports obtained in the MPI or mixed regimes [7 ‐ 9]. We achieve a quantitative description of these spectra using a rescattering picture [9,10] which mimics the time evolution of a tunnel-ionized continuum wave packet in the combined fields of the laser and ion core. Since rescattering events are known to be important in other short-pulse, strong-field emission phenomena (e.g., harmonic generation), this investigation better defines the underlying dynamics of these processes. In the experiments presented here, a 150 fs, 1 kHz repetition rate, titanium sapphire laser operating at 0.78 mm was focused by fy4 optics into an ultrahigh vacuum chamber, producing a maximum intensity of 20 PW/cm 2 . The sample gas was 99.999% helium, which was further scrubbed to ,0.1 ppm for O2 ,H 2 ,H 2O, CO2, and hydrocarbon impurities. A 30 cm long time-of-flight photoelectron (PE) spectrometer provides energy and angular resolution of 0.05 eV and 65 mrad, respectively. Data collection used 1 ns binning of discriminated electron events operating at low event probability (#0.25/ shot), ensuring space charge free conditions. The spectrometer’s energy calibration was obtained by recording the high order (.40 photon absorption), long pulse abovethreshold ionization spectrum of xenon. The intensity was calibrated using measurements of the total ion yield, xenon short-pulse PES resonances, and spot size [11]. The uncertainty in the reported intensities is ,25%. The current PE spectra were recorded between 0.5 and 1.5 times the saturation intensity (Isat › 0.8 PW/cm 2 ) for neutral

Abstract: While the physical chemistry of gas-phase ions has its roots in traditional mass spectrometry, the growth of the field during the past two decades can be attributed to the development and application of new experimental and theoretical techniques. Among these are guided ion beams for investigating ion−molecule reactions at very low translational energies, ion chromatography in which an ion’s electronic state or geometrical structure can be determined and selected, pulsed field ionization/zero kinetic energy photoelectron spectroscopy with ±0.1 meV resolution for determining ionization energies and ion vibrational frequencies, and multiphoton ionization and photoelectron−photoion coincidence methods for state selecting ions in uni- and bimolecular ionic reactions. The results from all of these studies have been greatly enhanced by concomitant advances in ab initio molecular orbital methods which provide heats of formation, structures, and vibrational frequencies of stable ion structures as well as transiti...

Abstract: Our major objective in this paper is to describe a new method we have developed for the analysis of trace gases at partial pressures down to the ppb level in atmospheric air, with special emphasis on the detection and quantification of trace gases on human breath. It involves the use of our selected ion flow tube (Sift) technique which we previously developed and used extensively for the study of gas phase ionic reactions occurring in ionized media such as the terrestrial atmosphere and interstellar gas clouds. Before discussing this analytical technique we describe the results of our very recent Sift and flowing afterglow (FA) studies of the reactions of the H3O+ and OH− ions, of their hydrates H3O+(H2O)1,2,3 and OH− (H2O)1,2, and of NO+ and O2 +, with several hydrocarbons and oxygen-bearing organic molecules, studies that are very relevant to our trace gas analytical studies. Then follows a detailed discussion of the application of our Sift technique to trace gas analysis, after which we presen...

Abstract: The angle-resolved N 1s photoelectron spectrum of has been measured with high resolution in the threshold region An analysis of the vibrational fine structure yields a vibrational energy of and a lifetime broadening of for the core-ionized molecule As in the case of C 1s ionization of CO, the vibrational fine structure changes considerably with photon energy in the region of both the shape resonance and the double excitations Both the vibrationally resolved partial photoionization cross sections and asymmetry parameters have been determined in this photon energy range The calculated cross sections in the literature are in reasonable agreement with experiment In addition, we have measured the cross sections and asymmetry parameters of the most intense direct and conjugate shake-up satellites The behaviour of the satellites is found to differ significantly from that of the corresponding C 1s satellites of CO

Abstract: Intense electromagnetic pulses (EMPs) released by lightning discharges produce bright optical emissions at 80–95 km altitudes emitted in a thin (∼30 km) cylindrical shell expanding to radial distances of up to >150 km, lasting for ∼400 µs, and appearing in limb-view as a thin layer with ∼400 km lateral extent.

Abstract: Plasma-immersion ion implantation (PIII) is an emerging technology for the surface engineering of semiconductors, metals, and dielectrics. It is inherently a batch-processable technique that lends itself to the implantation of large numbers of parts simultaneously. It thus offers the possibility of introducing ion implantation into manufacturing processes that have not traditionally been feasible using conventional implantation.In PIII the part to be treated is placed in a vacuum chamber in which is generated a plasma containing the ions of the species to be implanted. The plasma based implantation system does not use the extraction and acceleration methods of conventional mass-analyzing implanters. Instead the sample is (usually) repetitively pulsed at high negative voltages (in the 2–300 kV range) to implant the surface with a flux of energetic plasma ions as shown in Figure 1. When the negative bias is applied to a conducting object immersed in a plasma, electrons are repelled from the surrounding region toward the walls of the vacuum chamber, which is usually held at ground potential. Almost all the applied voltage difference occurs across this region, which is generally known as a sheath or cathode fall region. Ions are accelerated across the sheath, producing an ion flux to the entire exposed surface of the work-piece. Because the plasma surrounds the sample and because the ions are accelerated normal to the sample surfaces, implantation occurs over all surfaces, thereby eliminating the need for elaborate target manipulation or masking systems commonly required for beam line implanters. Ions implanted in the work-piece must be replaced by an incoming flow of ions at the sheath boundary, or the sheath will continue to expand into the surrounding plasma.Plasma densities are kept relatively low, usually between 108 and 1011 ions per cm3. Ions must be replenished near the workpiece by either diffusion or ionization since the workpiece (in effect) behaves like an ion pump. Gaseous discharges with thermionic, radio-frequency, or microwave ionization sources have been successfully used.Surface-enhanced materials are obtained through PIII by producing chemical and microstructural changes that lead to altered electrical properties (e.g., semiconductor applications), and low-friction and superhard surfaces that are wear- and corrosion-resistant. When PIII is limited to gaseous implant species, these unique surface properties are obtained primarily through the formation of nitrides, oxides, and carbides. When applied to semiconductor applications PIII can be used to form amorphous and electrically doped layers. Plasma-immersion ion implantation can also be combined with plasma-deposition techniques to produce coatings such as diamondlike carbon (DLC) having enhanced properties. This latter variation of PIII can be operated in a high ionenergy regime so as to do ion mixing and to form highly adherent films, and in an ion-beam-assisted-deposition (IBAD)-like ion-energy regime to produce good film morphology and structure.

Abstract: We present a method for calculation of differential ionization cross sections from theories that use the close-coupling expansion for the total wave function. It is shown how, from a single such calculation, elastic, excitation, and ionization cross sections may be extracted using solely the T-matrix elements arising from solution of the coupled equations. To demonstrate the applicability of this formalism, the convergent close-coupling theory is systematically applied at incident energies of 150–600 eV to the calculation of e-He ionization. Comparison with available measurements is generally very good.

Abstract: Multiphoton ionization of helium at high laser intensities has been investigated by direct numerical integration of the full time-dependent two-electron Schrodinger equation on a Cray T3D. At field intensities above , two-photon ionization occurs in a few field periods, and double ionization and high harmonic generation are prominent. We present calculations of double ionization yields, investigate the sensitivity of double ionization to the electron - electron electrostatic repulsion, and present evidence of unexpected non-exponential behaviour in ionization decay rates.

Abstract: He atoms have been ionized by a combination of two pulses the fundamental (800 nm, 150 fs) from a Ti:sapphire laser, and its 21st harmonic (38 nm). The resulting above-threshold ionization spectra comprise peaks that scale as the cross-correlation function, which can be mapped out by variation of the time delay between the two pulses. Using this technique, we have carried out the first measurement, to our knowledge, of the duration of high-order harmonic pulses with subpicosecond resolution. The result indicates a duration of 40 fs. This duration is much shorter than the duration of the fundamental pulse, in agreement with existing model calculations.

Abstract: Three models of the charge collection process in small dosimetric ionization chambers exposed to pulsed radiation are discussed. All three models allow for the presence of a free-electron component in the charge transfer, incorporating this into the model in slightly different ways, and the resulting collection efficiency formulae are compared over the range of variables normally met within clinical dosimetry. Measurements of the free-electron fraction for plane-parallel ionization chambers and for a Baldwin - Farmer 0.6 chamber are presented. The proportion of free electrons at the normal operating voltage is often high in small chambers but it is obvious that this can only lead to an increase in collection efficiency if the f-value calculated for purely ionic conduction allows for some improvement. Thus, a 50% free-electron fraction in a chamber which collects ions with efficiency f = 0.9950 at low pulse doses will increase this efficiency to only 0.9982. The same chamber, at the same operating voltage, and therefore the same free-electron fraction, if exposed to larger pulse doses, yielding an efficiency of 0.9531 as calculated for ions alone, would have a true efficiency of 0.9830 - a large change.