Showing papers on "Ionization published in 1989"

Above-threshold ionization in the long-wavelength limit

Abstract: In the long-wavelength limit, above-threshold ionization (ATI) is primarily the result of the interaction of a newly freed electron with the laser field. Classical physics requires that linearly and circularly polarized light produce very different ATI spectra. Measurements performed using both linearly and circularly polarized, picosecond, 10-\ensuremath{\mu}m pulses confirm these conclusions.

Triply-differential cross sections for ionisation of hydrogen atoms by electrons and positrons

Abstract: A derivation is given of the exact form of the three-body Coulomb wavefunction in the asymptotic region where the separation of all particles tends to infinity. Using a modification of the method of Pluvinage (1951), an approximate three-body scattering wavefunction is derived that satisfies this boundary condition. Triply-differential cross sections (TDCS) for electron impact ionisation of atomic hydrogen calculated with this scattering wavefunction, which contains no free parameters, show excellent agreement with measurements at impact energies greater than 150 eV. The corresponding TDCS for positron impact ionisation are also presented.

Tunneling ionization of noble gases in a high-intensity laser field.

Abstract: Studies of multiphoton ionization of noble gases have been carried out using 1-\ensuremath{\mu}m, 1-ps laser pulses with intensities up to the mid- ${10}^{16}$ W/${\mathrm{cm}}^{2}$. To our knowledge, this work represents the first study of the production of highly ionized noble-gas ions done exclusively in the tunneling regime. It is found in this regime that the ionization at a given intensity depends on both the ionization potential and the charge state of the species. The onset of ionization occurs when the sum of the Coulomb and laser electric potentials causes the electron to be unbound.

The photoevaporation of interstellar clouds. I: Radiation-driven implosion

Abstract: An approximate analytical solution is developed for the evolution of an interstellar neutral cloud exposed to the ionizing radiation of a newly formed star. The structure of a steady photoevaporation gas flow off a spherical ionization front is derived even for cases where the ionization front cannot be considered thin, and the downstream flow is not in ionization and thermal equilibrium. Under a wide range of conditions, the ionization front is approximately D-critical, and the ionized gas expands supersonically into the intercloud medium. The ionization front typically drives a strong shock into the initial cloud.

Collision‐induced dissociation of Fe+n (n=2–10) with Xe: Ionic and neutral iron binding energies

Abstract: Cross sections for collision‐induced dissociation (CID) of Fe+n with Xe, 2≤n≤10, are presented Experiments were performed on a newly constructed guided ion beam mass spectrometer, the design and capabilities of which are described in detail The single mechanism for dissociation of iron cluster ions is sequential loss of iron atoms with increasing collision energies There is no evidence for fission to molecular neutral products The cross section threshold energy dependences are analyzed to give the bond dissociation energies (BDEs), D0(Fe+n−1–Fe) Data analysis employs an empirical model that incorporates RRKM theory to account for inefficient dissociation on the time scale of the experiment Results show that Fe+6 has the strongest BDE, D0(Fe+5–Fe) =344±018 eV, while Fe+3 is the most weakly bound, D0(Fe+2–Fe) =164±015 eV Neutral cluster BDEs are derived from ionic binding energies and known ionization potentials Branching ratios and other cross section features are also discussed with respect to

The neutralization of organic cations

Abstract: Trois types de spectrometrie de masse sont etudies: la spectrometrie NIBS («Neutralized ion beam spectroscopy»), la spectrometrie NRMS («Neutralization-reionization mass spectrometry») et la spectrometrie de masse CIDI («Collision-induced dissociative ionization»)

Cosmic-ray-induced photodissociation and photoionization rates of interstellar molecules

Abstract: In the Prasad-Tarafdar mechanism, ultraviolet photons are created in the interior of dense interstellar clouds by the impact excitation of molecular hydrogen by secondary electrons generated by cosmic-ray ionization. Detailed calculations of the emission spectrum are described, and the resulting photodissociation and photoionization rates of a wide range of interstellar molecules are calculated. 84 refs.

Cold-plasma production for recombination extreme-ultraviolet lasers by optical-field-induced ionization

Abstract: For high-intensity, linearly polarized light, above-threshold ionization results from the time difference between the instant of ionization and the electric field maximum. Cold plasmas suitable for recombination extreme-ultraviolet (XUV) lasers can be produced if appropriate wavelength-ionizing radiation is used. Multiphoton ionization intensities are calculated for representative XUV laser schemes.

Luminescence of F and F+ centers in magnesium oxide.

Abstract: Time-resolved spectra of luminescence from F and ${F}^{+}$ centers in MgO excited at 248 nm are reported over 8 decades of time and intensity, from 20 ns to 2 s. The decay time of F and ${F}^{+}$ luminescence has been measured at temperatures ranging from 90 to 573 K. The luminescence yield and spectra were studied as a function of excitation-power density at 248 nm over a span of 10 decades, from 1 mW/${\mathrm{cm}}^{2}$ to 38 MW/${\mathrm{cm}}^{2}$. Higher pump-power density favors F luminescence over ${F}^{+}$ luminescence in thermochemically reduced crystals, and the luminescence yield saturates above about 50 kW/${\mathrm{cm}}^{2}$ of pump intensity. Transient absorption spectra were measured in the microsecond-to-seconds time range. The luminescence data show that the decay kinetics of excited F and ${F}^{+}$ centers are dominated by ionization from the excited state and charge recapture from traps which include the ${H}^{\mathrm{\ensuremath{-}}}$ center in thermochemically reduced MgO. It is furthermore inferred that electrons are the charge carriers having primary influence on kinetics of both F and ${F}^{+}$ centers. This implies that the ${F}^{+}$ first-excited state lies close to the conduction-band edge. We confirm that excitation at 248 nm also causes release of valence holes, and propose a mechanism which may explain the hole release, the increase of the F-to-${F}^{+}$ intensity ratio with increasing pump intensity, and the saturation of luminescence with increasing pump intensity.

Methyl rotation, vibration, and alignment from a multiphoton ionization study of the 266 nm photodissociation of methyl iodide

Abstract: The photodissociation dynamics of CH3I and CD3I have been examined by using multiphoton ionization to probe the CH3, CD3, I(≡5 2P3/2) and I*(≡I 5 2P1/2) photoproducts. The parent compounds were cooled in a supersonic expansion, collimated into a molecular beam, and dissociated at 266 nm. For the CD3I dissociation, the ratio of CD3(v=0)/(v=2) was estimated to be about 1.1, with multiple determinations ranging from 0.47–2.1. The quantum number v here denotes the nascent excitation of the ν2 ‘‘umbrella’’ mode. Measurements of the CD3(v=1) and (v=3) vibronic bands indicated that the (v=1)/(v=3) ratio is greater than unity, with some measurements suggesting values as large as 10. A value for the CH3(v=0)/(v=2) ratio from dissociation of CH3I could not be estimated, although it was clearly larger than that for CD3. The CH3(v=0) and CD3(v=0) products from this dissociation are fit by 120±30 K and 105±30 K rotational distributions, respectively. The dissociation mechanism produces alignment in the molecular frame...

Hydrogen atom detection in the filament‐assisted diamond deposition environment

Abstract: The resonance‐enchanced multiphoton ionization (REMPI) technique was employed for detection of gas phase atomic hydrogen in the filament‐asisted diamond growth environment. The H atom REMPI signal varied significantly with the reactant CH4/H2 fraction as well as with the filament temperature. We interpret these observations as evidence for the surface role of atomic hydrogen in the diamond growth mechanism. The spatial resolution of the REMPI technique allowed us to confirm that hydrogen atom transport in the deposition region occurs by diffusion.

Electron and chemical kinetics in methane rf glow‐discharge deposition plasmas

Abstract: Experimental measurements and theoretical modeling of methane deposition plasmas have led to the identification of the most likely homogeneous and heterogeneous reaction paths leading to the deposition of amorphous carbon thin films. Experimental measurements of the voltage, current waveforms, mass flow rates, and pressure are used as inputs to the model. The magnitude and flow‐rate dependence of the discharge luminosity, film deposition rates, and downstream mass spectra are compared with the model predictions and used to identify the dominant reaction paths. The model uses Monte Carlo simulation of the electron kinetics to predict the electron impact dissociation and ionization rates. These rates provide input for a plug flow chemical kinetics model.

Spectroscopic determination of impurity influx from localized surfaces

Abstract: The flux of impurity atoms into plasmas from limiting surfaces is considered. It is shown how the flux of an impurity released from a surface can be derived from spectroscopic measurements along a line-of-sight directed at the surface. A theoretical atomic level population model is developed to obtain the 'ionization per emitted photon' quantities which link the spectroscopic measurement to the flux. Metastable states and finite density plasma effects are taken into account and observations at visible wavelengths are emphasized. Detailed studies and calculations are performed for C+1, C+2, O+1, O+2, Cr, Cr+1, Fe, Fe+1, Ni and Ni+1. Tabulations and graphs of relevant quantities are provided. The application of the theory to impurity influxes in the JET Tokamak is described.

The use of an electron beam ion trap in the study of highly charged ions

Abstract: The Electron Beam Ion Trap (EBIT) is a relatively new tool for the study of highly charged ions. Its development has led to a variety of new experimental opportunities; measurements have been performed with EBITs using techniques impossible with conventional ion sources or storage rings. In this paper, I will highlight the various experimental techniques we have developed and the results we have obtained using the EBIT and higher-energy Super-EBIT built at the Lawrence Livermore National Laboratory.

Collisional losses from a light-force atom trap

Abstract: We have studied the collisional loss rates for very cold cesium atoms held in a spontaneous-force optical trap. In contrast with previous work, we find that collisions involving excitation by the trapping light fields are the dominant loss mechanism. We also find that hyperfine-changing collisions between atoms in the ground state can be significant under some circumstances. PACS numbers: 32.80.Pj, 34.50.Rk Spontaneous-force light traps' have provided a way to obtain relatively deep static traps for neutral atoms. These allow one to produce samples containing large numbers of very cold atoms. In this paper we present an experimental study of the collisions which eject atoms from such a trap. These collisions are of considerable interest because the temperatures of the trapped atoms (10 K) are far lower than in usual atomic collision experiments. The theory of such low-energy collisions and their novel features have been discussed by several authors. Perhaps the most notable feature is that the collision times are very long, and the collision dynamics are dominated by long-range interactions and spontaneous emission. These collisions also have important implications with regard to potential uses of optically trapped atoms. For many applications the maximum density that can be obtained is a critical parameter, and these collisions limit the attainable density. There have been two experimental studies of collisions in optical traps. Gould et al. measured the cross section for associative ionization of sodium. However, there is no evidence that this process is significant in limiting trapped-atom densities, and for some atoms, including cesium, it is energetically forbidden. Prentiss et al. studied the collisional losses which limited the density of sodium atoms which were held in a spontaneous-force trap. Their surprising and unexplained results were a direct stimulus for our work. In particular, they observed no dependence of the loss rate on the intensity of the trapping light. This was quite surprising, because a ground and an excited atom interact at long range via the strong 1/r resonant dipole interaction, and a portion of the excited-state energy can be converted into sufficient kinetic energy to allow the atoms to escape from the trap. By comparison, two atoms in their ground states interact only through a much weaker short-range 1/r Van der Waals attraction, and even when such collisions occur, they may not produce significant kinetic energy to cause trap loss. This implies that the dominant collisional loss mechanism would involve the excited atomic states, and thus depend on the intensity of the light which causes such excitations. In this paper we present measurements which show that, in contrast with Ref. 9, the collisional loss rate has a marked dependence on the trap laser intensity. We will present strong circumstantial evidence that the dependence at very low intensities is due to hyperfinechanging collisions between ground-state atoms. We believe that the loss rates at higher intensities are associated with collisions involving excited states, and are the type discussed by Gallagher and Pritchard. As discussed in Ref. 7, these collisions are very different from normal ground-excited-state atomic collisions in which two initially distant atoms, A and A approach, collide, and separate in a time much less than the radiative lifetime of the excited state. In contrast, for these very-low-temperature collisions the absorption and emission of radiation in the midst of the collision drastically alter the motion. In particular, if the excitation takes place when the two atoms are far apart (R) 1000 A) they will reradiate before being pulled into the small-R region where energy transfer occurs. However, if they are sufficiently close when excited, they can be pulled close enough together for substantial potential energy to be transferred into kinetic energy before decaying. The two dominant transfer processes are excited-state fine-structure changes and radiative redistribution. In the first, A changes its fine-structure state in the collision and the pair acquire a fine-structureinterval worth of kinetic energy. The second process, radiative redistribution, refers to A Areemitting a -photon which, because of the A-A* attractive potential, has substantially less energy than that of the photon which was initially absorbed. This energy difference is transferred to the subsequent kinetic energy of the ground-state atoms. The trap loss rate depends on the probability of exciting such "close" A-A pairs, and this probability is determined by the frequency and intensity of the exciting radiation. Light which is tuned to the red of the atomic resonance frequency, vo, excites pairs which are closer together (and shifted in energy) and thus is more eN'ective at causing trap loss than light which is at vo. We tested this hypothesis by examining how the loss

Multiphoton processes in an intense laser field: Harmonic generation and total ionization rates for atomic hydrogen.

Abstract: We report results of large-scale nonperturbative (Floquet) calculations of rates for harmonic generation and total ionization of H(1{ital s}) by fields whose intensity ranges from about 10{sup 12} to 3{times}10{sup 13} W/cm{sup 2} and whose wavelength ranges from 265 to 1064 nm. At long wavelengths and moderate to high intensities, perturbation theory yields estimates of total ionization rates that are orders of magnitude too large, and estimates of harmonic-generation rates that exhibit a qualitatively incorrect behavior with respect to order. We have studied the influence of resonances on the ionization rates, and we illustrate the resonance enhancement of the ionization yield for a realistic pulse. Finally, we address several formal aspects of Floquet theory, including the convergence of the induced dipole moment, gauge invariance, and the normalization of the wave function.

Penetration of energetic neutral beams into fusion plasmas

Abstract: The efficiency of neutral beam heating and current drive depends crucially on the deposition of the energy and momentum of the beam in the plasma. This deposition is determined by the atomic processes involved in the stopping (or effective ionization) of the neutral beam atoms. These processes have been studied in detail for the energy range from 10 keV/u to 10 MeV/u. The processes considered include both the ground state and the excited state of the beam atoms, thus allowing for the multistep ionization of the beam in collisions with the plasma constituents and impurities. The effective beam stopping cross-section has been calculated for a wide variety of beam and plasma parameters. The atomic database necessary for these calculations has been documented using the best data available at present. The stopping cross-section data are also given in terms of a convenient analytic fit, which can be used either in computer calculations or for simpler analytic estimates of neutral beam penetration.

Nonlinear light scattering accompanying multiphoton ionization

Abstract: We confirm in computer experiments the presence of unexpected plateau and cutoff features in high-order harmonic generation accompanying multiphoton ionization. We show the connection between these features and above-threshold ionization.

Measurements of Swarm Parameters and Derived Electron Collision Cross Sections in Methane

Abstract: A pulsed drift tube has been used to measure the electron drift velocity in methane over the range of E/N from 10 to 1000 Td. In addition, measurements of the positive ion mobility and ionization coefficient have been made over the range of E/N from 80 to 1000 Td. Within the experimental sensitivity, no evidence of attachment has been observed in this range. A set of electron collision cross sections has been assembled and used in Monte Carlo simulations to predict values of swarm parameters. The cross‐section set includes a momentum transfer cross section which is based primarily on the present and previous drift velocity measurements, cross sections for vibrational excitation and ionization based on published experimental cross‐section measurements, and a cross section for dissociation into neutral products obtained by subtracting a measured dissociative ionization cross section from a measured total dissociation cross section. Isotropic scattering is assumed for all types of collisions in the Monte Car...

Coronaspray nebulization and ionization of liquid samples for ion mobility spectrometry.

Abstract: Ion mobility spectrometry after electrospray nebulization and ionization was investigated as a method for the detection of components dissolved in liquids. While electrosprary operating conditions proved promising, greater sensitivity was achieved when the electric potential applied to the sample introduction needle was increased above breakdown potential and a corona discharge was established. Passing the liquid through the corona discharge established a "coronaspray" that efficiently nebulized and ionized the solvent and analytes. In this initial investigation of coronaspray ion mobility spectrometry (CIMS), ion current as a function of potential, temperature, and liquid flow rate was studied; several IMS spectra were obtained; and a continuous monitoring mode of operation was demonstrated. The results from this study indicated that CIMS has potential as a versatile and sensitive detection method for a variety of analytical procedures involving liquid flowing streams such as flow injection analysis, liquid chromatography, capillary zone electrophoresis, and field flow fractionation.

Proton transfer from 1-naphthol to water: Small clusters to the bulk

Abstract: Excited‐state proton transfer from 1‐naphthol to water was studied as a function of solvent system size, from supersonically cooled neutral clusters, 1‐naphthol⋅(H2O)n, n=1–50, to bulk ice and water. Occurrence or nonoccurrence of proton transfer was detected and studied using cluster‐size‐specific laser‐spectroscopic techniques: resonant two‐photon ionization (R2PI) and laser‐induced fluorescence emission. Depending on cluster size or solution phase, three qualitatively different types of excited‐state behavior were observed: (1) For small clusters, n≤7, both the R2PI and fluorescence spectra of the clusters were similar in nature to the spectra of bare 1‐naphthol; (2) the medium‐size clusters (n=8–20) show incremental spectral shifts which indicate successive stages of molecular solvation, and the spectra approach that of 1‐naphthol in bulk ice at n≊20; (3) the fluorescence spectra for large clusters, n≥20, show increasing emission intensity below 25 000 cm−1, characteristic of the emission of the excit...

The Low-Energy, Heavy-Particle Collisions–A Close-Coupling Treatment*

Abstract: Publisher Summary This chapter focuses on quantitatively reliable close-coupling treatments of slow and heavy-particle atomic collisions. The chapter explains the dynamics of such collisions and identifies remaining problems with the theory. It suggests possible extensions of ion–atom collision models to the study of ion–molecule and ion–surface collisions. Atomic and molecular collision physics is a particularly important and singularly challenging field. Such collisions are ubiquitous in nature. Atomic and molecular collisions are also very much at the heart of a large number of applications in such areas as magnetically or intertially confined fusion plasmas, laser systems, partially ionized gases and plasmas, chemical systems, surface interactions, channeling and energy loss in solids, energy and ionization balance in the earth's atmosphere, and astrophysical studies on a broad front.

Fluid hydrogen at high density: The plasma phase transition.

Abstract: We apply a new model equation of state, based on realistic interparticle potentials and a self-consistent treatment of the internal levels, to fluid hydrogen at high density. This model shows a strong connection between molecular dissociation and pressure ionization. We consider the possibility of a first-order plasma phase transition for which we give both the evolution in temperature and the critical point.

Mass spectroscopic investigation of the CH3 radicals in a methane rf discharge

Abstract: Neutral CH3 radicals in a capacitively coupled rf discharge in methane have been detected with a quadrupole mass spectrometer utilizing a threshold ionization technique. The absolute density of CH3 radicals was measured at pressures from 0.5 to 20 mTorr, together with the ionic composition of the methane plasma. The principal ionic species were CH+5 and C2H+5 , except in the low‐pressure region, suggesting the importance of ion‐molecule reactions in the plasma. The lifetime of CH3 radicals in the afterglow of pulsed rf discharges was measured and explained in terms of the recombination reaction CH3 +CH3 →C2 H6.

Computer program (SEQPEP) to aid in the interpretation of high-energy collision tandem mass spectra of peptides.

Abstract: An algorithm (SEQPEP) that aids in the interpretation of high-energy (greater than 1 kV) collision-induced dissociation mass spectra of peptide ions generated by fast atom bombardment (FAB) ionization is described. The only required input is a list of product ion masses and relative abundances generated by the mass spectrometer data system, the mass of the precursor [M + H]+ ion, and the mass of any C-terminal modification, if present (e.g., amide). Possible N-terminal modifications and amino acid compositions are not required as input. In the output, sequences are ranked according to the fraction of total product ion current that can be accounted for as either sequence-specific or non-sequence-specific fragment ions. These are listed by ion type. One of the major advantages of this program over algorithms described earlier is the incorporation of ion types more recently discovered. Also, this program is much faster, requiring less than 5 min of central processing unit time for an input of as many as 100 product ions. The results obtained from 50 peptides, including some generated when sequencing a protein of previously unknown structure, are discussed.

Nonadiabatic transitions in atomic collisions

Abstract: The adiabatic approximation is used in the physics of atomic collisions to calculate the parameters of inelastic (nonadiabatic) transitions between electronic states of colliding atoms (excitation, charge transfer, ionization) when, on the one hand, the motion of nuclei can be treated classically and, on the other, their relative velocities are low. The problem then reduces to solution of the secular Schrodinger equation for electrons with a Hamiltonian dependent on the internuclear distance and varying slowly with time. The review presents an asymptotic theory of nonadiabatic transitions between bound states as well as from bound states to a continuous spectrum without any limitations on the nature of the electron Hamiltonian of the kind encountered in exactly soluble models, except that a low value of the relative velocity of the nuclei is assumed. In addition to a general theory, the review deals with the various mechanisms of one-electron nonadiabatic transitions in the specific case of the simplest three-particle quasimolecular system (two nuclei and an electron). The concluding section deals with some modifications of the adiabatic approximation necessary for matching to the physical boundary conditions and calculation of the isotopic effects.