Showing papers on "Ionization published in 1980"

Core-level binding-energy shifts for the metallic elements

Abstract: A general treatment of core-level binding-energy shifts in metals relative to the free atom is introduced and applied to all elemental metals in the Periodic Table. The crucial ingredients of the theoretical description are (a) the assumption of a fully screened final state in the metallic case and (b) the ($Z+1$) approximation for the screening valence charge distribution around the core-ionized site. This core-ionized site is, furthermore, treated as an impurity in an otherwise perfect metal. The combination of the complete screening picture and the ($Z+1$) approximation makes it possible to introduce a Born-Haber cycle which connects the initial state with the final state of the core-ionization process. From this cycle it becomes evident that the main contributions to the core-level shift are the cohesive energy difference between the ($Z+1$) and $Z$ metal and an appropriate ionization energy of the ($Z+1$) atom (usually the first ionization potential). The appearance of the ionization potential in the shift originates from the assumption of a charge-neutral final state, while the contribution from the cohesive energies essentially describes the change of bonding properties between the initial and final state of the site. The calculated shifts show very good agreement with available experimental values (at present, for 19 elements). For the other elements we have made an effort to combine experimental ionization potentials with theoretical calculations in order to obtain accurate estimates of some of the atomic-core-level binding energies. Such energies together with measured metallic binding energies give "pseudoexperimental" shifts for many elements. Our calculated core-level shifts agree exceedingly well also with these data. For some of the transition elements the core-level shift shows a deviating behavior in comparison with that of neighboring elements. This is shown to be due to a difference in the atomic ground-state configuration, such as, for example, ${d}^{5}s$ in chromium relative to the ${d}^{n}{s}^{2}$ configuration in vanadium and manganese. When the core-level shift is referred to, the ${d}^{n}{s}^{2}$ (or ${d}^{n+1}s$) atomic configuration for all the elements in a transition series, a quite regular behavior of the shift is found. However, some structure can still be observed originating from a change of screening within the $d$ band from a bonding to an antibonding type as one proceeds through the series. For elements beyond the coin metals the screening of a core hole is performed by $p$ electrons, which provide a less effective screening mechanism than the $d$ electrons for the transition metals. The coin metals are intermediate cases, partly due to a dominating $s$-electron screening and partly due to $d$-electron bonding in the initial state. The effect of the electron-density redistribution between the free atom and the solid on the core-level shift is particularly striking in the case of the rare-earth elements Pr-Sm and Tb-Tm. Here the remarkable situation is that a deep core electron is less bound in the atom than in the solid. Also for the actinides the electronic redistribution upon condensation gives rise to pronounced effects on the core-level shifts. Further, it is shown that the measured $6{p}_{\frac{3}{2}}$ binding energy in metallic uranium provides a clear demonstration of the occupation of the $5f$ level in this metal. The present treatment of the core-level shift for bulk metallic atoms can easily be generalized to surface atoms. From an empirical relation for the surface energy a simple expression for the shift of the surface core-level relative to the bulk can be derived. For the earlier transition metals, it is found that the core electrons are more bound at the surface than in the bulk, while for the heavier ones the opposite situation exists. This change of sign of the surface shift depends on the bonding-antibonding division of the $d$ band. To illustrate how the present approach can be applied to alloy systems, a treatment of core-level shifts for rare-gas atoms implanted in noble metals is undertaken.

Mechanism of pulsed surface flashover involving electron‐stimulated desorption

Abstract: A simple model is proposed to explain how a breakdown avalanche of secondary emission electrons can lead to surface flashover when an insulator in vacuum breaks down a few nanoseconds after high voltage is applied. The case of a plane insulator–vacuum interface perpendicular to parallel electrodes is considered. Positive surface charging is assumed to occur almost immediately upon application of the voltage, and the attendent secondary emission avalanche is assumed to be maintained at saturation throughout the prebreakdown time delay by field emission from the cathode electrode. Bombardment of the insulator by avalanche electrons desorbs a cloud of gas, which is partially ionized as it drifts through the swarm of electrons in the avalanche. The electric field at the cathode end of the insulator becomes enhanced as positive ions accumulate, which in turn increases the field emission and the rates of gas desorption and ionization. This and other regenerative processes rapidly lead to breakdown. Field enhanc...

Cross-section and rate formulas for electron-impact ionization, excitation, deexcitation, and total depopulation of excited atoms

Abstract: For electron-induced ionization, excitation, and de-excitation, mainly from excited atomic states, a detailed analysis is presented of the dependence of the cross sections and rate coefficients on electron energy and temperature, and on atomic parameters. A wide energy range is covered, including sudden as well as adiabatic collisions. By combining the available experimental and theoretical information, a set of simple analytical formulas is constructed for the cross sections and rate coefficients of the processes mentioned, for the total depopulation, and for three-body recombination. The formulas account for large deviations from classical and semiclassical scaling, as found for excitation. They agree with experimental data and with the theories in their respective ranges of validity, but have a wider range of validity than the separate theories. The simple analytical form further facilitates the application in plasma modeling.

Use of one‐electron theory for the interpretation of near edge structure in K‐shell x‐ray absorption spectra of transition metal complexes

Abstract: We report the results of multiple scattered wave SCF X‐alpha calculations of the one‐electron cross section for K‐shell photoabsorption in the molecular complexes MoO4−−, CrO4−−, and MoS4−−. We show that the method can successfully account for energy separations and relative cross sections of spectral features both below and above the K‐shell ionization threshold. Furthermore, we show: (a) that the first fairly intense peak on the low energy side of the rising edge for molybdate and chromate is due to a dipole allowed transition to a bound antibonding state of mainly nd character on the metal ion; this transition is possible because of the mixing with the ligand p orbitals having the proper T2 symmetry induced by the tetrahedral molecular potential; (b) the shoulder on the rising absorption edge can be explained by the beginning of the steplike continuum absorption when convolved with a Lorentzian function of frequency to imitate lifetime and monochromator broadening: (c) the main absorption peak is due t...

K, L, and M shell generalized oscillator strengths and ionization cross sections for fast electron collisions

Abstract: Generalized oscillator strengths and ionization cross sections by fast electron impact are calculated for K, L, and M shells. A nonrelativistic Hartree–Slater program is used to generate the initial states as well as the continuum wave functions. Core edge shapes and their dependence on momentum transfer are computed within this atomic model up to some tens or hundreds of eV above threshold. Some comparisons are made with experimental measurements and though details near threshold are not predicted, the gross shape of the spectrum is in quite good agreement with these data. While we confirm that the hydrogenic model is a reasonable approximation for K edges, we expect our computations to be useful in obtaining more accurate total cross sections for L and M shell excitation.

Stark widths of doubly- and triply-ionized atom lines☆

Abstract: In this paper, we report modifications of well known semiempirical and semiclassical approximation furmulas for Stark line-width calculations. Comparisons with experiments for doubly ionized atoms yield, as an average ratio of measured to calculated widths 1.06 ± 0.31 for a modified semiempirical formula and 0.96±0.24 for a modified semiclassical formula. For triply ionized atoms these ratios are 0.91±0.42 and 1.08±0.41, respectively. Comparison with other theoretical calculations have also been made.

Mass spectrometric determination of partial electron impact ionization cross sections of He, Ne, Ar and Kr from threshold up to 180 eV

Abstract: A new approach to mass spectrometric measurement and analysis has been used to study the electron impact ionization of the rare gases in the low electron energy regime (<180 eV). Critical considerations are given to the problems of ion extraction from the ion source (Nier‐type) and the transmission of the extracted ions through the mass spectrometer system (double focusing sector field Varian MAT CH5). Accurate relative partial cross section functions were obtained for production of He+ and He2+ from He; Ne+, Ne2+, and Ne3+ from Ne; Ar+, Ar2+, Ar3+ from Ar, and Kr+, Kr2+, Kr3+ and Kr4+ from Kr. In addition, the cross section ratios for the partial ionization cross sections of each individual gas were determined. These ratios were used to sum up the relative partial cross section functions for each gas, hence obtaining relative total ionization cross section functions by weighting each partial cross section function with the respective charge number. The shape of the relative total ionization cross section...

Ultraviolet photoelectron studies of the ground-state electronic structure and gas-phase tautomerism of purine and adenine

Abstract: Ultraviolet photoelectron spectroscopy has been employed to study electronic structure and tautomerism in adenine and purine and in various methyl-substituted derivatives of these molecules. The assignment of the photoelectron spectra has been aided by results from HAM^ molecular orbital calculations which are empirically parameterized to provide orbital ionization potentials. The calculations predict that the lone-pair orbitals in purine and adenine are similar in spacial distribution and have similar ionization potentials. This conclusion is supported by the spectroscopic data which indicate that in purine the three highest occupied lone-pair orbitals have ionization potentials of -9.6, - 10.6, and - 11.7 eV. In adenine these orbitals have ionization potentials of -9.6, - 10.5, and 11.39 eV. The planar exocyclic amino group in adenine causes the A structure of this molecule to be significantly different than that in purine. In adenine the ionization potentials of the five highest occupied A orbitals are 8.48, -9.6, -10.5, 12.10, and 13.21 eV. In purine the ionization potentials of the four highest occupied a orbitals are -9.6, -10.2, -11.9, and 13.10 eV. The band arising from the fifth highest occupied A orbital in purine has an ionization potential greater than 13.5 eV and is not resolved. A comparison of the photoelectron spectra of purine, adenine, and their 7- and 9-methyl-substituted derivatives indicates that the spectra of purine and adenine are much more similar to the spectra of their 9-methyl derivatives than to the spectra of their 7-methyl derivatives. This observation supports the conclusion that in an isolated environment the N(9)H tautomers of both purine and adenine are more stable than the N(7)H tautomers.

Transport theory for kinetic emission of secondary electrons from solids

Abstract: Kinetic secondary electron emission from a solid target resulting from incidence of keV electrons or keV and MeV ions is treated theoretically on the basis of ionization cascade theory. The energy and angular distribution and the yield of secondary electrons are calculated for a random target. These quantities are determined from the solutions to a system of Boltzmann transport equations. Input quantities are the cross sections for collisions between the involved particles and the surface barrier of the target. A general power cross section has been utilized in the analytical procedure. It is shown that liberated electrons of low energy move isotropically inside the target in the limit of high primary energy as compared to the instantaneous energy of the liberated electrons. The connection between the spatial distribution of kinetic energy of the liberated electrons and the secondary electron current from a solid is derived. To find the former, existing computations for ion slowing down and experimental and theoretical ones for electron bombardment can be utilized. The energy and angular distribution of the secondary electrons and the secondary electron yield are both expressed as products of the deposited energy at the surface of the target and a factor which depends only on the properties of the escaping secondary electrons. Corrections for energy transport away from the surface by energetic recoil electrons are partly included. Also the contribution from recoiling target atoms at heavy-ion bombardment in the keV region is largely taken into account. The predicted energy and angular distribution agree with absolute spectra for incident electrons, whereas the agreement with absolute spectra for incident protons is less satisfactory. Extrapolation of the energy distribution down to the vacuum level gives a spectrum which shows good agreement with experimental data. The electron- and proton-induced yields from aluminum are evaluated on the basis of existing low-energy-electron stopping-power data. The agreement with existing experimental data is good. Also, experimental yields from electrons, protons, and noble gas ions incident on copper agree within the accuracy of the treatment.

Visible and UV multiphoton ionization and fragmentation of polyatomic molecules

Abstract: It is now known that tunable dye laser light leads to resonance enhanced photoionization of molecules in a mass spectrometer via multiphoton absorption. In some cases this process also leads to ion fragmentation. Time delayed tandem laser pulses of different wavelengths are employed to study the mechanism of this multiphoton ionization and fragmentation process in benzene. The experimental results strongly indicate that excitation does not proceed up an autoionization ladder of the neutral molecule, rather the multiphoton process is shown to be due to the climbing of two independent ladders, one up to the ionization potential in the neutral molecule, the second in the new species of molecular parent ions produced. Hence multiphoton ionization could also form a new basis of ion spectroscopy.

Ionization and electron capture to the continuum in the H + -hydrogen-atom collision

Abstract: The equivalence of the two-body interactions between the final particles of the ion-atom ionization process is considered in the $T$-matrix formalism. This defines a final wave function which is product of Coulomb waves and describes simultaneously the capture to the continuum and the direct ionization. The relation of that approach with the wave equation for the three-particle system and the asymptotic behavior are discussed. The doubly differential cross section for the ionization of hydrogen atoms by proton impact is calculated, in first order, and compared with experimental values.

Molecular multiphoton ionization spectroscopy

Abstract: Multiphoton ionization (MPI) spectroscopy is rapidly evolving into a valuable adjunct to VUV spectroscopy for the study of the highly excited states of molecules. The technique is described, and several examples are given of its applications. Studies of the 3pz Rydberg states of the methylbenzenes and of the 1B2u state of benzene show that state assignments can be made and new multiphoton structure seen that is not available in one-photon spectra. MPI of benzene in a supersonic jet demonstrates dramatic resolution improvement and shows that natural linewidth information can be gained for large molecules. Collisional effects in MPI spectra are briefly discussed with reference to Xe and to laser enhanced collisional broadening of nitric oxide in the supersonic beam.

Wavelengths and Transition Probabilities for Atoms and Atomic Ions. Part I. Wavelengths. Part II. Transition Probabilities

Abstract: : Wavelengths for about 47,000 spectral lines of atoms and atomic ions, as well as transition probabilities A for about 5000 lines, are tabulated. The data were selected in such a way as to include the prominent lines over a wide spectral region. Part I contains wavelengths of lines of neutral through quadruply ionized atoms in the range 40 to 40,000 A. This information is presented in two different ways: (1)separate line lists grouped according to chemical element and further subdivided according to stage of ionization; and (2) a general table of wavelengths ordered numerically, with relative intensity, chemical element, and stage of ionization indicated for each line. Part II contains transition probability data for atoms in various stages of ionization, with emphasis on the neutral and singly ionized species. This table is arranged according to the chemical element and is further subdivided according to stage of ionization. Estimates of the accuracies of the A-values are provided. Wavelengths, energy levels, and statistical weights serve to identify the lines and to provide useful data for plasma spectroscopy applications. (Author)

Relativistic Charged Particle Identification by Energy Loss

Abstract: A detailed review, both theoretical and experimental, is given of the problems and progress involved in the identification of relativistic particles by ionization measurements in a medium. Argon is used as one example of such a medium. (AIP)

Multiphoton ionization of azulene and naphthalene

Abstract: Molecular beams of azulene and naphthalene are irradiated by the output of a tunable pulsed laser and the charged species produced by multiphoton ionization (MPI) are analyzed using a quadrupole mass spectrometer. A study of the ion fragmentation pattern as a function of laser power and laser wavelength permits these two C10H8 isomers to be readily distinguished. Compared to electron impact (EI) ionization, the MPI process can produce more gentle ionization at low laser powers and more extensive fragmentation at high laser powers. The former permits the study of azulene and naphthalene dimers from which their heats of dissociation are estimated. In the case of azulene, two‐color excitation yields a mass spectrum that differs from the mass spectrum obtained by excitation with either color, alone. The ionization efficiency of MPI is very high, exceeding that of EI during the time the pulsed laser is on. A qualitative description involving autoionizing states is presented to account for this efficiency and t...

Post-Ionization of Field-Evaporated Ions

Abstract: It is shown that the final observed charge state of a field-evaporated ion can be explained by the occurrence of post-ionization whereby an ion being accelerated away from a metal surface in a strong electric field loses one or more electrons by tunneling into the substrate. Calculations using an approximate analytic formula derived from a simple model potential predict the probability of post-ionization for many different elements.

A thermalized ion explosion model for high energy sputtering and track registration

Abstract: A velocity spectrum of neutral sputtered particles as well as a low resolution mass spectrum of sputtered molecular ions was measured for 4.74 MeV F-19(+2) incident of UF4. The velocity spectrum is dramatically different from spectra taken with low energy (keV) bombarding ions, and is shown to be consistent with a hot plasma of atoms in thermal equilibrium inside the target. A thermalized ion explosion model is proposed for high energy sputtering which is expected to describe track formation in dielectric materials. The model is shown to be consistent with the observed total sputtering yield and the dependence of the yield on the primary ionization rate of the incident ion.

Resonance enhanced two-photon ionization studies in a supersonic molecular beam: Bromobenzene and iodobenzene

Abstract: The 1B2(ππ*)←1A1 absorptions of two monosubstituted halobenzenes have been investigated using resonance enhanced two‐photon ionization in a pulsed supersonic molecular beam. Detection of the photoions was accomplished by means of a time‐of‐flight mass spectrometer. The 1B2←1A1 system of bromobenzene has been observed with good sensitivity using this technique, even though the total decay rate of the 1B2 state is greater than 1×1011 sec−1. No ion signal was observed when the same transition was probed in iodobenzene, allowing us to place a lower limit on its decay rate of 4×1013 sec−1.

Electron-impact ionization cross sections for highly ionized hydrogen- and lithiumlike atoms

Abstract: Electron-impact ionization cross sections for highly ionized atoms in the hydrogen and lithium isoelectronic sequences have been computed in several variants of the Coulomb-Born and distorted-wave approximations Electron exchange in the transition matrix element and Coulomb distortion of the partial waves were found to be important The results are compared to recent crossed-beam experimental data and to other theoretical predictions

Pressure ionization in laser-fusion target simulation

Abstract: Accurate simulation of high density target implosion requires material properties (ionization, pressure, energy, opacity and transport coefficients) at densities where bound electrons are significantly perturbed by neighboring atoms. In modern laser-fusion simulation codes, this data is supplied by tables and/or calculated from a Stromgren model for ionization equilibrium. Improvements have been made in the Stromgren average-atom model which aim at assuring thermodynamic consistency and obtaining better agreement with more elaborate calculations. Arbitrary degeneracy is allowed for the free electrons. Consistent Coulomb contributions to pressure and continuum lowering are obtained. A new pressure ionization scheme merges bound electrons into the continuum as a smooth function of density and the corresponding contribution to pressure is calculated. Results are shown for aluminum.

Laser induced multiphoton ionization mass spectrum of benzene

Abstract: Gas phase benzene is multiphoton ionized under low pressure, collision‐free conditions in the source region of a time of flight mass spectrometer using UV excimer laser radiation. All of the ions generated are identifiable and derive from C6H6. The differences between KrF and ArF laser induced mass spectra and total ionization yields are interpreted on the basis of known fragment ion appearance potentials, and a rate equations model which describes the ionization process. The observed mass spectra exhibit certain characteristics not found in conventional electron impact spectra. These idiosyncracies may enhance its potential as a mass spectrometric ionization source.

Radio‐frequency mass selective excitation and resonant ejection of ions in a three‐dimensional quadrupole ion trap

Abstract: The three−dimensional quadrupole ion store (QUISTOR) has been employed as a reactor for the study of ion/molecule reactions using external mass analysis, and as an ion trap for the study of stored ion dynamics using either external mass or in situ resonant absorption. Excitation of the axial frequency component of ion motion is accomplished by application of small (∠1 V) RF potentials to an end−cap of the QUISTOR. The frequencies at which ion resonance is observed differ from those predicted by theory and are dependent upon trapped ion density. Thus, the frequency shift may be related to both the space charge within the QUISTOR and perturbation of the stability diagram. the simultaneous application of in situ resonance absorption and external mass analysis has been utilized in three separate studies. (1) In ionized argon, Ar+ has been ejected by resonance absorption and the rate constant for the subsequent partial charge reaction of Ar2+ with argon has been determined to be 4.8±1.6×10−12 cm3 molecule−1s−1. (2) In ionized 2−propanol at ∠0.1 Pa and 4 ms after the ionizing pulse, we have found the ambient space charge experienced simultaneously by nine ion species to be sensibly constant. (3) The coupling of precursor to product ions in ion/molecule reactions of the 2−propanol system has been investigated: this particular application closely resembles the double resonance technique in ion cyclotron resonance mass spectrometry.

Current collection and ionic recombination in small cylindrical ionization chambers exposed to pulsed radiation

Abstract: The recombination correction for a small cylindrical ionization chamber exposed to pulsed radiation is reexamined in the light of new experimental evidence. A simple two-voltage testing technique is described and demonstrated. In general the existing theory for an air-filled chamber (Boag 1950) is found to be accurate over the range of charge densities examined (0.013 p/pulse to 3.8 p/pulse) where p=3.33 X 10(-4)C. m-3 (1 esu cm-3).