Detachment Of Ions Research Articles

Study on emission yields of negative- and positive-ion induced secondary electron from thin SiO 2 film

The emission yield of secondary electrons from thin SiO 2 films due to negative- and positive-ion bombardments was studied experimentally for the purpose of the investigation of the charging phenomenon of insulators during the negative-ion implantation. We used 120- and 360-nm-thick SiO 2 films on Si and low ion currents for obtaining the true yield for the insulator without charging effect. The yields (the current ratio of emitted electrons to incident ions) were measured as a function of the C − current density. Although the measured yield for each ion energy showed the value of unity at a high ion current density, it was saturated to the same value irrespective of the film thickness with a decrease in ion current density. It is considered that the charging effect was reduced by compensation due to the leakage current and that the true yield was obtained at a sufficiently low ion current density. As results from C − (C 2 −) and·C + ion bombardments, the true yield for negative ions was found to be larger (approximately by one unit) than that for positive ions due to the detachment of a loosely attached electron from the negative ion. The emission mechanism of secondary electrons for negative ions is considered to consist of kinetic emission related to ion momentum and emission related to electron detachment of negative ions.

Polyelectrolyte complexes from poly(N‐ethyl‐4‐vinylpyridinium bromide) and poly(sodium methacrylate). A stopped‐flow/light scattering investigation

AbstractTime‐resolved light scattering measurements performed in conjunction with the stopped‐flow method revealed information on the formation of polyelectrolyte complexes (PECs) of the weak polyelectrolyte (PE) couple poly(N‐ethyl‐4‐vinylpyridinium bromide) and poly(sodium methacrylate). The stability of the PECs depends, apart from other parameters, on the total ionic strength of the solution [I], the PE concentration and the concentration ratio (Φ) = [PVPE]/[PMA]. On the basis of Φ‐[I] diagrams, obtained for three PE combinations differing in average molar mass, the stability of PECs is characterized by the regimes A, B and C referring to conditions, where PECs are quite stable and water‐insoluble (A), PECs are not very stable and water‐soluble (B) and PECs are instable (not formed). The course of the curves separating regime A from regime B reflects the action of two competing effects regarding the stability of PECs, namely the increase in charge density and the increase in ion condensation with growing total ionic strength: Kinetic measurements revealed that, at low ionic strength right upon mixing, supramolecular complexes and later large aggregates are formed. The aggregates undergo rearrangements leading eventually to smaller complexes. At high ionic strength aggregates are formed rather slowly after mixing. This behavior is related to ion condensation. PECs can form only after the small ions occupying the charged sites at the PE chains are removed. Detachment of small ions from PE chains is a relatively slow process and is the rate‐determining step in PEC formation at moderately high ionic strength of the solution.

Coulomb trajectory effects on electron-impact detachment of negative ions and on mutual neutralization cross sections

Cross sections of electron-impact detachment of negative ions and of mutual neutralization between positive and negative ions are calculated using a model based on the semiclassical theory for ion - atom collisions but modified to include Coulomb trajectory effects. It is shown that the method allows us to obtain accurate results for collisions down to low energies. The Coulomb repulsion between the electron and the negative ion accounts for the rapid decrease of detachment cross sections at low energies, and the Coulomb attraction between positive and negative ions accounts for the rapid increase of neutralization cross sections at low velocities.

Two-photon detachment of negative ions via magnetic dipole transitions.

Two-photon detachment spectroscopy of the ${\mathrm{Ir}}^{\ensuremath{-}}$ $({5d}^{8}{6s}^{2}{}^{3}{F}_{4})$ and ${\mathrm{Pt}}^{\ensuremath{-}}$ $({5d}^{9}{6s}^{2}{}^{2}{D}_{5/2})$ ions is conducted using laser intensities $\ensuremath{\sim}{10}^{10}$ W/c${\mathrm{m}}^{2}$. Sharp resonances in the two-photon detachment yields, attributed to magnetic dipole transitions, are observed at photon energies of $7087.3\ifmmode\pm\else\textpm\fi{}0.4$ and $9740.9\ifmmode\pm\else\textpm\fi{}0.5{\mathrm{cm}}^{\ensuremath{-}1}$ for ${\mathrm{Ir}}^{\ensuremath{-}}$ and ${\mathrm{Pt}}^{\ensuremath{-}}$, respectively. Multiconfiguration Dirac-Fock calculations predict fine structure splittings of 6671 and $9535{\mathrm{cm}}^{\ensuremath{-}1}$ for ${\mathrm{Ir}}^{\ensuremath{-}}$ $(J\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}4\ensuremath{-}3)$ and ${\mathrm{Pt}}^{\ensuremath{-}}$ $(J\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}5/2\ensuremath{-}3/2)$, and A coefficients of 7 and $14{\mathrm{s}}^{\ensuremath{-}1}$, respectively, for the upper fine structure levels of the ions.

Multiphoton angular distributions with elliptically polarized light. II. Three- and four-photon detachment of halogen negative ions

For pt.I see ibid., vol.28, p.3845-59 (1995). The negative ions F-, Cl- and I- have been submitted to multiphoton excitation by elliptically polarized light from a Nd:YAG laser at lambda =1054 nm. Photoelectron angular distributions measured in the polarization plane with various ellipticities are fitted by the analytic formula that was established in the previous paper. Elliptic asymmetry parameters are obtained for three-photon detachment of I- and four-photon detachment of Cl- and F-. Ordinary 'linear' asymmetry parameters can be deduced from these elliptic asymmetry parameters, which makes it possible to compare the present results with those of previous measurements and calculations.

Above-threshold multiphoton detachment from the H- ion by 10.6- microm radiation: Angular distributions and partial widths.

We present a general procedure for accurate nonperturbative treatment of the angular distribution and partial widths for multiphoton above-threshold detachment (ATD) of atoms or negative ions in intense laser fields. The procedure consists of the following two steps: (1) The resonance wave function is determined by means of the non-Hermitian Floquet Hamiltonian method. The Floquet Hamiltonian is discretized by the complex scaling generalized pseudospectral method recently developed by Yao and Chu [Chem. Phys. Lett. 204, 381 (1993)]. No computation of potential matrix elements is required, and the kinetic-energy matrix elements can be evaluated analytically. (2) The angular distribution and partial rates are calculated, based on an exact differential expression, and a procedure is developed for the backrotation of the total complex resonance wave function to the real axis. The method is applied to the study of multiphoton ATD of ${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$ in strong fields at 10.6 \ensuremath{\mu}m. An accurate one-electron model potential [Laughlin and Chu, Phys. Rev. A 48, 4654 (1993)], which reproduces the known ${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$ binding energy and the low-energy e-H(1s) elastic scattering phase shifts, is employed. At this low frequency, the resonance wave functions can be obtained efficiently and rather accurately by means of a nonperturbative adiabatic approach recently developed by Telnov [J. Phys. B 24, 2967 (1991)]. This adiabatic theory is also valid in the limit of weak fields, and its validation is justified by its agreement with the exact perturbation calculations for the seven- and eight-photon detachment of ${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$. Detailed results for the angular distribution and partial widths for multiphoton ATD of ${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$ are presented for the moderately strong laser intensity regime (${10}^{10}$--${10}^{11}$ W/${\mathrm{cm}}^{2}$) at 10.6 \ensuremath{\mu}m.

Theory of electric corona including the role of plasma chemistry

Abstract

The potentialities of H − beam diagnostics by laser photodetachment electrons

The previously proposed method of diagnostics for negative ion beams based on electrons produced from single-photon detachment, is considered. The potentialities of such diagnostics by laser probing of a H − beam are theoretically investigated. Analytical expressions for the power density of a photon target, in which two-photon detachment of ions causes the deterioration of the accuracy in nonperturbative measurements of beam parameters, are obtained. The estimation for the H − beam is made at the Moscow Meson Factory Linac.

Longer wavelengths require lower intensity in multiphoton detachment of negative ions.

Negative chlorine ions, held in a Penning ion trap, are irradiated by intense (up to 2\ifmmode\times\else\texttimes\fi{}${10}^{16}$ W/${\mathrm{m}}^{2}$) Raman-shifted light of a Nd:YAlG laser with a wavelength of 1098 nm and a pulse duration of 30 ps. Electron-energy spectra are obtained showing up to 11-photon absorption. In the case of circularly polarized light suppression of the low-energy channels is observed. A saturation intensity of (8.5\ifmmode\pm\else\textpm\fi{}2.5)\ifmmode\times\else\texttimes\fi{}${10}^{15}$ W/${\mathrm{m}}^{2}$ is found for multiphoton detachment of ${\mathrm{Cl}}^{\mathrm{\ensuremath{-}}}$ by 1908-nm light. This result is counterintuitive because it is lower than the saturation intensity for multiphoton detachment at 1064 nm.

Excess-photon detachment of negative chlorine ions with 100 fs laser pulses

Multiphoton detachment spectra of Cl-, showing excess-photon absorption, have been obtained using short (100 fs) and intense (up to 5*1016 W m-2) optical pulses with a wavelength of 620 nm. In the short-pulse regime the shifts in the electron spectrum can be related directly to shifts of the continuum states. In this way, it is shown experimentally that excess-photon detachment can occur in the absence of channel closure.

Multiphoton detachment of negative ions in an intense radiation field.

We have applied a momentum-space method to the solution of the three-dimensional time-dependent Schr\odinger equation for electron detachment from a model negative ion, in particular ${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$. We have studied such phenomena as excess-photon detachment and threshold effects. For selected cases, we obtain excellent agreement with the Floquet calculations. We also present results in regimes of strong interest to experimental efforts.

Experimental observation of excess-photon detachment of negative ions.

Negative chlorine ions are stored in a Penning trap and are exposed to short (35 ps) and intense (up to 3\ifmmode\times\else\texttimes\fi{}${10}^{12}$ W/${\mathrm{cm}}^{2}$) optical pulses with a wavelength of 1064 nm. The detached electrons are detected in energy-resolved measurements. Absorption is observed of up to two photons in excess of what is required on the basis of the electron affinity.

One- and two-photon detachment of negative hydrogen ions: a hyperspherical adiabatic approach

The authors report results on one- and two-photon detachment cross sections of negative hydrogen ions from the 1Se ground state in lowest-order perturbation theory using an adiabatic hyperspherical approximation.

Positive Corona Processes in Electronegative Gaseous Dielectrics

Events occurring in a positive point/plane gap Subjected to quasi-step function voltages have been monitored by both terminal and optical measurements. Evidence is presented on the importance of the critical al initiation volume in the vicinity of the anode in determining both the time lag and the luminous extent of the corona. Changes in the corona characteristics are explained on the basis of spatial modifications of the ionization zone resulting from space charge distortion. An approximate theoretical basis for the results is explored in terms of the detachment of negative ions and illustrated with reference to seeding of the discharge with a low ionization potential additive.

A Modek for the Production of Initial Electrons by Detachment of SF-6-IONS

Experiments with positive pulses in a sphere-to-plane gap give evidence of the fact that initial electrons are provided by collisional detachment of SF6 ions. The position-dependent density of negative ions prior to a pulse application is calculated numerically for different geometries. The results are then included in a quantitative model for the electron production rate which is based on physical assumptions about the detachment coefficient. From a comparison with the measurement, ?/n is obtained as a function of E/Ecr.

Photodetachment, photodissociation, and photochemistry of surface molecules of icy solids containing NH3 and pure H2O ices

Photodetachment and photodissociation of surface molecules of pure and mixed ices containing NH3 and H2O at 90∼130 K have been investigated by using a mass spectrometer situated in a ultrahigh vacuum apparatus. NH3 molecules were excited to the lowest singlet state by an ArF laser at 193 nm (hν=6.4 eV). The detachment of NH3 molecules from pure NH3 solids was observed at a low laser fluence of 0.1 mJ/cm2. Two photons of 248 nm laser (2hν=10 eV) also detached H2O molecules from pure H2O ices. Mixed solids of NH3 and H2O released both molecules from the surface upon irradiation with 193 nm laser. The translational energies of the detached molecules were of the order of the intermolecular interactions of the constituent molecules (0.03∼0.2 eV) and the energy distributions were not changed by the increase of laser fluence. A detachment mechanism explaining all of the observations is proposed. The force acting on the detachment is ascribed to ‘‘electronic exchange repulsion between an excited molecule and a ground state molecule.’’ At a higher laser fluence (≳500 mJ/cm2), the detachment of ions was observed at m/e=17 and 18 with comparable intensities in a mixed solid of NH3 and H2O. Translational energy distributions of the ions peaked at ∼1 eV for both species. Dissociation of atomic hydrogen from the surface molecules was observed in the solid containing NH3. A fraction of the atomic hydrogen posseses translation energy (Et=17 kcal/mol=0.74 eV) which is similar to that observed in the photolysis of free NH3 molecles in a molecular beam. A very low energy component (Et=0.6 kcal/mol=0.026 eV) appeared and exhibited an intensity change depending upon the deposition and annealing conditions. Molecular hydrogen was also released with a translational temperature very close to the surface temperature. The photoproducts N2H4, N2H2, and NH2OH were detected in the mixed solid system of NH3 and H2O. Photochemical bimolecular condensation is proposed as a mechanism of the reaction forming a hydrogen molecule and a condensed molecule. The present study affords an experimental foundation of the photoejection model of physically adsorbed molecules on interstellar grains presented by Watson and Salpeter (Ref. 2).

Measurements of absolute total cross sections for charge transfer and electron detachment of halide ions on chlorine

Absolute total cross section measurements are reported for electron detachment and for reactive scattering between the halide ions (Cl−, Br−, and I−) and chlorine gas Cl2. The charge transfer and dissociative charge transfer cross sections are found to be very large at their maxima, which occur at a relative collision energy of about 13 eV. The electron detachment cross section for I−+Cl2 is anomalously low. Some energy loss spectra are reported for I−+Cl2. They exhibit substantial inelastic scattering which is consistent with the calculated potentials of Cl2.

Role of photodetachment in initiation of electric discharges in SF6 and O2

The role of photodetachment in the initiation of electron avalanches near a positive point electrode was investigated for a discharge gap in which the negative ion flux was controlled by UV irradiation of the cathode. For irradiation of the anode region with light beams up to 500 mW, photodetachment was found to make a negligible contribution to avalanche initiation in SF6 and O2 at pressures from 50 to 500 kPA (∼0.5 to 5 atm). The conditions under which the role of photodetachment might be observed are discussed, and it is shown that for the conditions considered here the expected dominant electron release mechanism in the gap is through collisional detachment of stable negative ions. Previously reported enhancements in avalanche rates resulting from irradiation of a positive point can be explained as arising from increases in negative ion densities due to attachment of photoelectrons ejected by scattered radiation.

Mechanisms for Inception of DC and 60-Hz AC Corona in SF6

Using a pulse counting technique, inceptions of positive and negative point-plane corona in SF6 under dc and 60-Hz ac conditions were measured. Effects of gas pressure, UV-radiation, and point electrode size on differences between ac and dc, and between positive and negative inceptions were investigated. Inceptions were also calculated using the streamer criterion. Agreement was obtained with measured negative inceptions for both ac and dc conditions, but not with positive inceptions. The growth in the active-electron initiation volume with applied voltage was calculated and used to explain the observed polarity effect. The magnitude of the polarity effect is predictably reduced, either by irradiating the gap or by increasing the diameter of the point electrode. The difference between ac and dc positive inceptions is attributed to the enhancement of avalanche-initiating electron production by the residual ion space charge from negative corona in the previous half-cycle. Without radiation, it is hypothesized that electron initiation for positive corona occurs mainly via field-enhanced collisional detachment of negative ions, the probability of which depends on the degree of field nonuniformity in the gap. For negative corona, initiation is assumed to occur via field emission from the point electrode. The presence of radiation enhances initiation in the positive case by photodetachment of negative ions, and in the negative case by photoelectric emission.

Forced diffusion model for unipolar injection in low conductivity media

A unipolar charge injection model based on the Poisson equation and the one-dimensional forced diffusion equation is presented. Its application to low dielectric constant fluids provides a description of conductivity in such media. The first integral leads to the Ricatti equation which describes the electric field distribution. In the presence of image forces the boundaries of this equation must be displaced from the physical electrodes. The two boundary conditions of this system are stated in terms of injection and discharge conditions. Employment of the image potential gives rise to an expression for an injection law (relation between field and current) and its applicability is discussed along with the analytical solutions that are obtained for field profile, charge density, current and voltage. Detachment of ions bound to the electrode by image forces, whose description enters the injection law, is investigated for finite current flow in the system. The role of dielectric constant of the electrode on the shape of current—volgate curves is discussed.