Showing papers on "Ionization published in 1987"

Two‐dimensional imaging of state‐selected photodissociation products detected by multiphoton ionization

Abstract: A new technique is presented that makes it possible, with a single laser pulse, to determine the three‐dimensional spatial distribution of state‐selected photoproducts. Initially, absorption of a photon from a laser beam causes fragmentation of a molecule. Multiphoton ionization is used to select the internal state of a desired fragment without perturbing its velocity. Following a short delay, the three‐dimensional spatial distribution caused by the fragment velocities is projected onto two dimensions by accelerating the state‐selected fragment ions into the surface of a channel plate particle multiplier. Electrons emerging from the multiplier are imaged onto a phosphorescent screen for analysis by a digital‐image processing device such as a two‐dimensional optical multichannel analyzer. The three‐dimensional spatial distribution is reconstructed by taking the Hankel transform of the Fourier transform of the projection. The technique is illustrated by recording the spatial distribution of methyl fragments produced in their vibrational ground state by the 266 nm photodissociation of CH3I. From this study it is determined that the fraction of CH3(v=0) formed in coincidence with I(2P1/2) is greater than 0.95, the rest being formed in coincidence with I(2P3/2) ground state.

Above-threshold ionization with subpicosecond laser pulses

Abstract: Above-threshold ionization (ATI) is investigated in xenon with pulses of light at 616 nm ranging from 15 to 0.4 psec. Significant energy shifts and broadenings of the ATI peaks are observed. For pulse widths less than 1 psec, the individual ATI peaks break up into a narrow fine structure, apparently due to resonance enhancements in the ionization process produced by ponderomotive shifts of states.

Height-integrated Pedersen and Hall conductivity patterns inferred from the TIROS-NOAA satellite data

Abstract: The series of polar-orbiting National Oceanic and Atmospheric Administration spacecraft TIROS, NOAA 6, and NOAA 7 have been monitoring the particle influx into the atmosphere since late 1978. This data base has been used to construct statistical global patterns of height-integrated Pedersen and Hall conductivities for a discrete set of auroral activity ranges. The observations of energy influx and “characteristic electron energy” have been binned in a 1° latitude and 2° magnetic local time grid and ordered by an auroral activity index. This index is an estimate of the energy deposited into a single hemisphere by incident particles, a parameter generated directly from the particle observations and, therefore, internally consistent with the statistical patterns that are constructed. An average electron spectrum is associated with each characteristic energy, which enables a height profile of ionization rate in the upper atmosphere to be determined. The use of a pressure coordinate system insures that the normalized ionization rate profiles are independent of atmospheric model parameters. To create the statistical pattern of height-integrated conductivities, however, vertical profiles of atmospheric temperature and composition are assumed, and the ion density enhancements are evaluated from a chemical balance between ion production and recombination based on an “effective” recombination coefficient. The data base can also provide the statistical pattern of particle heating rates and ionization rates over a three-dimensional grid suitable as input to more sophisticated ionospheric and neutral thermospheric codes.

Absolute cross sections for electron-impact ionization of the rare-gas atoms by the fast-neutral-beam method

Abstract: A new apparatus has been constructed for the measurement of absolute partial electron-impact-ionization cross sections of neutral atoms, molecules, and free radicals. A fast neutral beam is prepared by charge-transfer neutralization of a mass-selected ion beam and is ionized as it crosses an electron beam. From careful analysis of the apparatus, the absolute accuracy of measured cross sections is calculated to be \ifmmode\pm\else\textpm\fi{}12%. Combined with statistical errors, this gives about \ifmmode\pm\else\textpm\fi{}15% for the overall accuracy. Measurements of ionization cross sections for the four rare gases He, Ne, Ar, and Kr from 0 to 200 eV agree with the most reliable previous values within \ifmmode\pm\else\textpm\fi{}6%. The measured Xe ionization cross section is 12% greater than the previous best value. Ratios of double- and triple-ionization to single-ionization cross sections for Ar, Kr, and Xe confirm the recent measurements of Stephan, Helm, and M\"ark [J. Chem. Phys. 73, 3763 (1980)].

Time-dependent Hartree-Fock theory of multiphoton ionization: Helium

Abstract: The multiphoton ionization of helium has been determined for a number of laser wavelengths and intensities using the time-dependent Hartree-Fock model. Conclusions about the ionization dynamics for very-short-pulse, high-intensity lasers are discussed. The limits and characteristics of the time-dependent Hartree-Fock method for atomic physics processes are also evaluated.

Photoionization mass spectrometric studies of AsHn (n=1–3)

Abstract: A photoionization mass spectrometric study of SiH4 at T=150 K reveals the presence of SiH+4 with an adiabatic threshold at 11.00±0.02 eV. The implications for the structure of this Jahn–Teller split state are discussed. The appearance potentials of SiH+2 and SiH+3 are 11.54±0.01 eV and ≤12.086 eV, respectively. The reaction of F atoms with SiH4 generates SiH3 (X 2A1), SiH2 (X 1A1 and a 3B1), and SiH (X 2Π) in sufficient abundance for photoionization studies. The measured adiabatic ionization potentials (eV) are: SiH3, 8.01±0.02; SiH2 (X 1A1), 9.15±0.02 or 9.02±0.02; SiH2 (a 3B1), 8.244±0.025; SiH, 7.91±0.01. The singlet–triplet splitting in SiH2 is either 0.78±0.03 or 0.91±0.03 eV. The dissociation energy of SiH is 2.98±0.03 eV. A Rydberg series is observed, converging to SiH+ (a 3Π) at 10.21±0.01 eV. Heats of formation of the various neutral and ionic species are presented, as are the stepwise bond energies of SiH4.

Absolute Cross Sections for Molecular Photoabsorption, Partial Photoionization, and Ionic Photofragmentation Processes

Abstract: A compilation is provided of absolute total photoabsorption and partial‐channel photoionization cross sections for the valence shells of selected molecules, including diatomics (H2, N2, O2, CO, NO) and triatomics (CO2, N2O), simple hydrides (H2O, NH3, CH4), hydrogen halides (HF, HCl, HBr, HI), sulfur compounds (H2S, CS2, OCS, SO2, SF6), and chlorine compounds (Cl2, CCl4). The partial‐channel cross sections presented refer to production of the individual electronic states of molecular ions and also to production of parent and specific fragment ions, as functions of incident photon energy, typically from ∼20 to 100 eV. Total photoabsorption cross sections above the first ionization threshold are reported from conventional optical measurements obtained using line and continuum sources and from ‘‘equivalent‐photon’’ dipole (e,e) electron scattering experiments. Partial photoionization cross sections for production of electronic states of molecular ions are obtained from photoelectron spectroscopy and from dip...

Kinetics of Nonequilibrium Low-Temperature Plasmas

Abstract: One Low-Temperature Plasma. General Information.- 1.1. Quasineutrality. Debye Screening.- 1.2. Ideal Plasma.- 1.3. Equilibrium Plasma.- 1.4. Local Thermodynamic Equilibrium. Elementary Processes.- 1.5. Features of the Transport Phenomena.- 1.6. Nonequilibrium Low-Temperature and High-Temperature Plasmas.- Two Elementary Processes in a Low-Temperature Plasma.- 2.1. Elastic Collisions.- 2.1.1. Coulomb Collisions.- 2.1.2. Elastic Scattering of Electrons by Atoms and Molecules.- 2.2. Inelastic Collisions of Electrons with Atoms, Ions, and Molecules.- 2.2.1. Excitation and Quenching by Electron Impact.- 2.2.2. Electron Impact Ionization and Ternary Recombination (Electron-Ion-Electron).- 2.2.3. Excitation of Vibrational and Rotational States of Molecules by Electron Impact.- 2.3. Inelastic Collisions with Heavy Particles.- 2.3.1. Excitation and Quenching.- 2.3.2. Ionization and Three-Particle Recombination.- 2.3.3. Associative Ionization and Dissociative Recombination.- 2.3.4. Processes Forming Molecular Ions.- 2.3.5. Charge Transfer.- 2.3.6. Formation of Negative Ions.- 2.4. Elementary Radiative Processes.- 2.4.1. Bound-Bound Transitions.- 2.4.2. Photoionization and Radiative Recombination.- 2.5. Average Energy Transferred to an Atom in Collisions.- 2.5.1. Energy Transfer in Elastic Collisions of Atoms with Electrons.- 2.5.2. Energy Transfer in Inelastic Collisions of Electrons with Atoms.- 2.5.3. Diffusion Coefficients for a Weakly Bound Electron in the Energy Space of the Atom.- Three Radiative Transport of Excitation.- 3.1. Basic Characteristics of Radiative Excitation Transport.- 3.2. Radiative Excitation Transport Equation.- 3.3. Effective-Lifetime Approximation.- 3.4. Radiative Excitation Transport in an Inhomogeneous Medium.- 3.5. Domain of Application of the Theory.- Four Criteria for the Onset of Nonequilibrium States.- 4.1. Criterion for Detachment of the Electron Temperature.- 4.2. Criterion for Equilibrium Ionization and for an Equilibrium Distribution of the Atoms over Levels.- 4.3. Criterion for the Breakdown of the Maxwellian Distribution.- Five Population Kinetics of Excited States.- 5.1. Qualitative Picture of the Population Distribution in a Nonequilibrium Plasma.- 5.2. System of Kinetic Balance Equations for the Populations of Excited States.- 5.3. Numerical Methods of Solving the System of Kinetic Equations for the Populations.- 5.4. Diffusion Approximation.- 5.5. Discrete Methods and the Modified Diffusion Approximation.- 5.5.1. Single-Quantum Approximation.- 5.5.2. Single-Quantum Approximation with Allowance for Radiative Transitions, the Predominant-Sink Model.- 5.5.3. Modified Diffusion Approximation.- 5.6. Comparison of the Populations Found Analytically with the Results of Computer Calculations and Experiments.- 5.7. Influence of Atom-Atom Collisions on the Population Distribution.- 5.8. Allowance for Sources of Excited Atoms in the System of Balance Equations.- 5.8.1. Processes of External Population of an Excited Level.- 5.8.2. Processes Which Destroy Excited States.- 5.9. Features of the Impact-Radiation Kinetics in a Rarefied Plasma.- 5.10. Some Applications of the Theory.- 5.10.1. Determining the Electron Temperature and Density from the Measured Populations of Excited States.- 5.10.2. Population Inversion in the Recombinational Decay of a Plasma.- Six Kinetics of Ionization and Recombination.- 6.1. Elementary Kinetics of Ionization and Recombination.- 6.1.1. Electron-Impact Ionization and Ternary Recombination.- 6.1.2. Ionization and Recombination in Collision with Heavy Particles.- 6.1.3. Penning Ionization.- 6.1.4. Associative Ionization and Dissociative Recombination.- 6.1.5. Radiative Recombination.- 6.1.6. Dielectronic Recombination.- 6.1.7. Recombination of Positive and Negative Ions.- 6.2. Basic Kinetic Equations for Ionization and Recombination and the Results of Their Numerical Solution.- 6.2.1. Definition of the Ionization and Recombination Coefficients.- 6.2.2. Results of Numerical Determination of the Ionization and Recombination Coefficients.- 6.3. Coefficients of Impact-Radiative Recombination in the Diffusion and Modified Diffusion Approximations.- 6.3.1. The Diffusion Approach in the Kinetics of Recombination and Ionization.- 6.3.2. Coefficients of Impact-Radiative Ionization and Recombination in the Modified Diffusion Approximation.- 6.3.3. Comparison with Experimental Data and Numerical Results.- 6.3.4. Influence of Collisions with Heavy Structureless Particles on the Ionization and Recombination Coefficients.- 6.3.5. Impact-Dissociative Recombination and Impact-Associative Ionization.- 6.4. Electron Density under Steady Nonequilibrium Conditions.- 6.4.1. Equation for the Nonequilibrium Electron Density.- 6.4.2. Discussion of the Experimental Data and Comparison with Theoretical Results.- Seven Electron Energy Distribution and Energy Balance.- 7.1. Kinetic Equation and the Electron Energy Balance.- 7.1.1. Weakly Ionized Plasma.- 7.1.2. Highly Ionized Plasma.- 7.2. Inelastic Collisions. Their Influence on the Electron Energy Balance, Excitation Rate, and Ionization.- 7.2.1. Inelastic Collision Integral. Energy Loss in Inelastic Collisions.- 7.2.2. The Distribution Function and the Rate of Excitation and Ionization in a Highly Ionized Plasma.- 7.2.3. Rate of Excitation and Ionization and the Distribution Function in a Weakly Ionized Plasma.- 7.2.4. Distribution Function in the Presence of a Source of Fast Electrons.- 7.3. Self-Consistent Distributions for Electrons over Energies and Atoms over Excited States.- 7.3.1. Self-Consistent Distributions of Atoms over Levels and Electrons over Energies.- 7.3.2. Influence of a Non-Maxwellian Distribution on the Coefficient of Impact-Radiative Ionization.- 7.4. Electron Energy Distribution in a Strong Electric Field.- 7.4.1. Electron Energy Distribution in a Weakly Ionized Plasma.- 7.4.2. Townsend Ionization Coefficient.- Eight Transient Nonequilibrium Plasmas.- 8.1. Criteria of Quasisteadiness.- 8.1.1. Quasisteadiness in the Excited States.- 8.1.2. Quasisteadiness of the Electron Temperature.- 8.2. Ionizational Relaxation.- 8.2.1. Ionization Growth during Optical Heating and behind Shock Fronts.- 8.2.2. Recombinational Decay of a Plasma.- 8.2.3. Recombination of Electrons in an Expanding Plasma.- 8.3. Radiation from Transient Plasmas.- 8.4. Relaxation of the Distribution Function.- 8.5. Instabilities of a Nonequilibrium Plasma in an External Electric Field.- 8.5.1. Ionizational Instability of a Highly Nonequilibrium Plasma.- 8.5.2. Overheating Instability of the Electron Gas.- Nine Some Topics in the Kinetics of Molecular Plasmas.- 9.1. Electron Energy Balance.- 9.2. Electron Energy Distribution Function.- 9.3. Distribution of Molecules over Vibrational Levels.- 9.3.1. Quasiequilibrium. The Structure of the Vibrational Distribution.- 9.3.2. Steady-State Distribution in the Case of Strong Excitation.- 9.3.3. Vibrational Energy Balance.- 9.4. Electron-Ion Recombination in Molecular Gases.- 9.4.1. Recombination Accompanied by the Excitation of Molecular Rotations.- 9.4.2. Recombination Accompanied by Excitation of Molecular Vibrations.- 9.4.3. Limits of Applicability. Comparison with Experiment.- 9.5. Some Topics in the Kinetics of Atomic-Molecular Plasmas.- Appendixes.- Appendix 1. Values of the Level-Population Coefficients of Atomic Hydrogen.- Appendix 2. Differential and Finite-Difference Fokker-Planck Equations.- Appendix 3. Energy Level Schemes of Atoms.- Appendix 4. Sample Calculation of the Populations of Excited Atoms in the Modified Diffusion Approximation.- References.

Continuum modeling of argon radio frequency glow discharges

Abstract: A continuum model has been developed which successfully describes the concentration, movement, and energetics of charged particles within a rf discharge. This model includes continuity equations for all charged particles, Poisson’s equation to determine the local electric fields, and an electron energy balance to determine the ionization and energy‐loss rates. All input parameters (diffusivity, mobility, ionization, and energy‐loss rate) were defined using reported values determined in dc field experiments.

Multiphoton ionization of hydrogen: A time-dependent theory

Abstract: Results of calculations using a novel numerical method for intense-field, low-order multiphoton ionization of hydrogen are reported and compared to previously published values. The technique involves the explicit numerical solution of the time-dependent Schr\"odinger equation using a finite-difference representation of the electronic wave function.

Dissociation energies of the benzene dimer and dimer cation

Abstract: The dissociation energy of the benzene dimer was measured indirectly via a precision measurement of its ionization potential, using tunable synchrotron radiation and an argon-benzene nozzle beam optimized for (C6H6)2. This ionization potential, 8.690 +/- 0.023 eV (1427 +/- 4A), was combined with other data from the literature to obtain D0((C6H6)2) = 2.4 +/- 0.4 kcal mol and D0((C6H6)2 ) = 15.3 +/- 0.9 kcal mol . The corresponding heats of formation are H/sub f/ ((C6H6)2) = 30.4 +/- 0.4 kcal mol and H/sub f/ 0((C6H6)2 ) = 230.8 +/- 0.9 kcal mol . The threshold portion of the photoionization efficiency function of (C6H6)2 shows no sign of autoionizing Rydberg structure, so direct ionization is the process that determines the threshold function shape. This shape also indicates that the geometry of (C6H6)2 is significantly different from that of (C6H6)2 , which is thought to have a sandwich structure, but is still sufficiently similar to permit the threshold to be distinctly observed.

Size dependence of the gradual transition to metallic properties in isolated mercury clusters

Abstract: Single-photon ionization thresholds are reported for mercury clusters with up to 70 atoms prepared in a supersonic beam. The size dependence of the ionization potentials provides the first direct experimental evidence that a heretofore undetected size-dependent gradual transition from van der Waals--type to metallic properties occurs for mercury clusters in the size range between 20 to approximately 70 atoms.

Correlation of Particle-Induced Displacement Damage in Silicon

Abstract: Correlation is made between the effects of displacement damage caused in several types of silicon bipolar transistors by protons, deuterons, helium ions, and by 1 MeV equivalent neutrons. These measurements are compared to calculations of the nonionizing energy deposition in silicon as a function of particle type and energy. Measurements were made of displacement damage factors for 2N2222A and 2N2907A switching transistors, and for 2N3055, 2N6678, and 2N6547 power transistors, as a function of collector current using 3.7 - 175 MeV protons, 4.3 - 37 MeV deuterons, and 16.8 - 65 MeV helium ions. Long term ionization effects on the value of the displacement damage factors were taken into account. In calculating the energy dependence of the nonionizing energy deposition, Rutherford, nuclear elastic, and nuclear inelastic interactions, and Lindhard energy partition were considered. The main conclusions of the work are as follows: 1) The ratio of the displacement damage factors for a given charged particle to the 1 MeV equivalent neutron damage factor, as a function of energy, falls on a common curve which is independent of collector current. 2) Deuterons of a given energy are about twice as damaging as protons and helium ions are about eighteen times as damaging as protons.

Effect of closed classical orbits on quantum spectra: Ionization of atoms in a magnetic field.

Abstract: Measurements of the absorption spectrum near the ionization threshold for an atom in a strong magnetic field showed that the spectrum is a superposition of many oscillatory terms (``quasi-Landau oscillations''). We have developed a quantitative theory which shows that each classical closed electron orbit which begins and ends near the nucleus contributes an oscillatory term to the average oscillator strength. The theory gives new understanding of the behavior under combined Coulomb and Lorentz forces, and it elucidates the roles of isolated closed orbits in chaotic systems. The first results of this theory are shown to be in good agreement with experimental results.

High resolution zero kinetic energy photoelectron spectroscopy of benzene and determination of the ionization potential

Abstract: We report the direct high‐resolution determination of the ionization potential of benzene and the observation of rotational structure in the vibronic ground state of the benzene ion. We employ a new technique of two‐color high‐resolution zero kinetic energy photoelectron spectroscopy (ZEKE‐PES) via selection of an intermediate resonant (neutral) state. The ZEKE‐PES method allows for the investigation of rotationally resolved photoionization dynamics and the direct high‐precision determination of ionization potentials, the precision being derived directly from the laser wavelength calibration. For benzene we determine an IP of 74 555.0±0.4 cm−1. We compare this method to ionization potential measurements from ion yield measurements under various conditions.

Electron impact ionization in the vicinity of comets

Abstract: The solar wind interacts very strongly with the extensive cometary coma, and the various interaction processes are initiated by the ionization of cometary neutrals. The main ionization mechanism far outside the cometary bow shock is photoionization by solar extreme ultraviolet radiation.Electron distributions measured in the vicinity of comets Halley and Giacobini-Zinner by instruments on the VEGA and ICE spacecraft, respectively, are used to calculate electron impact ionization frequencies. Ionization by electrons is of comparable importance to photoionization in the magnetosheaths of Comets Halley and Giacobini-Zinner. The ionization frequency in the inner part of the cometary plasma region of comet Halley is several times greater than the photoionization value. Tables of ionization frequencies as functions of electron temperature are presented for H2O, CO2, CO, O, N2, and H.

Multiple ionization of He, Ne, and Ar by fast protons and antiprotons.

Abstract: Single and multiple ionization of He, Ne, and Ar has been studied experimentally by impact of fast protons and antiprotons. The single-ionization cross sections obtained with protons and antiprotons are found to be the same. The double-ionization cross sections obtained with antiprotons, however, are much larger than those obtained with protons at equal velocity. This difference is found for all three gases but the effect is largest for He and Ne, where the difference is about a factor of 2 at 1 MeV/amu. The difference is discussed in terms of interference between two collision mechanisms which both result in double-electron escape. Experimental information on the magnitude of the interference term is obtained by inclusion of double-ionization data, partly obtained in this work, for fast electron and \ensuremath{\alpha}-particle impact. For triple ionization of Ne, we also find that antiprotons yield much larger cross sections than protons do. Identical cross sections, however, are found for triple ionization of Ar with protons and antiprotons. This is believed to be due to the fact that triple ionization of Ar is mainly a consequence of a single vacancy produced in an inner shell followed by electronic rearrangement. This observation supports the interpretation that the observed charge effect is due to an interference effect in the outermost shell.

Absolute electron-impact-ionization cross-section measurements of the halogen atoms

Abstract: Electron-impact-ionization cross sections have been measured for the halogen atoms F, Cl, Br, and I from threshold to 200 eV. The absolute accuracy is \ifmmode\pm\else\textpm\fi{}14%, except for F which is \ifmmode\pm\else\textpm\fi{}20%. The halogen cross sections have shapes similar to those of the neighboring rare gases but are greater in magnitude. Simple semiclassical formulas of electron-impact ionization are able to accurately predict the experimentally determined halogen to rare gas cross-section ratios.

s and d Rydberg series of NO probed by double resonance multiphoton ionization

Abstract: The s and d Rydberg states of NO with v = 0 have been observed for 6 leqslant; n* leqslant; 40 by double resonance multiphoton ionization which was three-photon resonant with the 3pπC 2∏ (v = 0) Rydberg intermediate state. Strong parity selection is shown to occur in the intermediate level despite the fact that the Λ-doublets are not resolved. The OODR spectra are thereby considerably simplified. A unified description of the rotational-electronic structure of the s and d Rydberg states in terms of quantum defect theory is presented, extending from the low n* s, d supercomplexes up beyond the ionization threshold. Electronic radial dipole transition moments are calculated and included in the MQDT analysis, giving evidence for a Cooper minimum in the region of the supercomplex n* = 5. For 25 leqslant; n* leqslant; 40 fringes are observed in the OODR spectrum and are interpreted in terms of rotational l-uncoupling, electronic s ∼d mixing and energy dependent electronic dipole transition moments. The fringe f...

Collision‐induced dissociation of aluminum cluster ions: Fragmentation patterns, bond energies, and structures for Al+2–Al+7

Abstract: Collision‐induced dissociation (CID) of cooled, mass selected aluminum cluster ions (Al+2–7) by xenon, has been studied over an energy range of 0–10 eV (center of mass). These experiments were carried out in a new apparatus which is described in detail. From the product branching ratios and cross section magnitudes we derive qualitative structural information about the cluster ions. The fragmentation thresholds are analyzed to yield dissociation energies, approximate ionization potentials, and further structural information about the cluster ions and their neutral counterparts. Cluster stabilities range from 0.85±0.40 eV for Al+4 to 2.25±0.70 eV for Al+7. The results provide a stringent test for recent calculations on Al2–6.

G0.18-0.04: Interaction of Thermal and Nonthermal Radio Structures in the Arc Near the Galactic Center

Abstract: Using a high-resolution radio-continuum image made at a frequency of 4.8 GHz, the radio source G0.18-0.04 located at the intersection of the galactic center Arc with the galactic plane is discussed. In this direction, the radio Arc is composed of a set of linear, nonthermal filamentary structures superimposed on a narrow, sickle-shaped, thermal feature which crosses them. The radio image suggests a physical interaction between the thermal and nonthermal structures. It is argued that the magnetic field is dynamically important and that its strength is estimated to be very large (0.001 G). Furthermore, it is suggested that the ionization of G0.18-0.04 is caused by collisional ionization rather than ultraviolet photons from massive stars. The discussion presents possible ways in which some of the energy of the relativistc particles or the magnetic field in the nonthermal filaments might be extracted to ionize ambient gas in the galactic plane.

Detectors for particle radiation

Abstract: 1. Physics foundations 2. Measurement of ionization 3. Measurement of position 4. Measurement of time 5. Particle identification 6. Measurement of energy 7. Measurement of momentum 8. Applications of detector systems Index.

Photoionization dynamics and abundance patterns in laser vaporized tin and lead clusters

Abstract: Tin and lead clusters are produced by laser vaporization in a pulsed nozzle source and studied with laser photoionization mass spectroscopy. ‘‘Magic numbers’’ are observed in both cluster size distributions under a variety of laser wavelength and power conditions which can be understood in terms of ionization thresholds, relative ionization cross sections, and multiphoton‐induced fragmentation. After investigation of the photoionization dynamics, relative abundances of different sized clusters are estimated. Abundance patterns of tin and lead clusters are compared to those reported previously for other group IV elements (C, Si, Ge) to investigate the role of periodicity in cluster growth and bonding properties. Especially abundant 10‐atom cluster species are observed for both tin and lead, as has been observed previously for both silicon and germanium. Other features not observed for silicon and germanium, such as abundance patterns characteristic of atom closepacking geometries, are observed to a limited...

Interaction of metastable noble-gas atoms with transition-metal surfaces: Resonance ionization and Auger neutralization.

Abstract: Electron emission due to deexcitation of metastable noble-gas atoms occurs at transition-metal surfaces which are either clean or covered by atomic adsorbates via resonance ionization of the metastable atom and subsequent Auger neutralization. The resulting electron-energy distributions contain information on the local surface density of states. For analysis, the electron-energy distribution is approximated by a self-convolution function. The validity of this approximation is discussed in detail. The deconvolution of the experimental data yields an effective transition density function reflecting the main features of the electronic surface density of states. The deconvolution technique is described in the Appendix. Furthermore, results are presented on the angular distribution of electrons emitted by Auger neutralization as well as on the lowering of the ionization potential of meta- stable and ground-state noble-gas atoms at surfaces which provides information on the distance range where resonance ionization, Auger neutralization, and Auger deexcitation occur.