Negative Fluorine Ions Research Articles

Fluorine and sodium depletion followed by refractive index modification imprinted on fluorophosphate glass surface by thermal poling

Thermal poling using microstructured electrodes on oxyfluoride glasses have been investigated and the impact on optical properties was evaluated. A micrometer refractive index modification based on the sodium and fluorine migration has been evidenced, through the creation of a layer with different chemical composition as compared to the original glass. Topological and optical properties modifications after poling treatment are discussed. For the first time, we described the migration of negative fluorine ion in the direction of the cathode from the glass network. Na and F migration led to a layer richer in oxides with a strong contrast of + 0.06 of the refractive index in the imprinted structures.

Atomic processes in bicircular fields

Various laser-induced and laser-assisted processes in atoms exposed to bicircular fields are investigated. The strong-field ionization process is exemplified by the direct and by the high-order above-threshold detachment of negative fluorine ions both for corotating and counterrotating bicircular fields. The symmetries of the electron distributions in the momentum plane are analyzed in detail. The results for laser-assisted electron-ion recombination and for high-order harmonic generation are presented for the bicircular field with counterrotating components.

Landau–Zener model for electron loss of low-energy negative fluorine ions to surface cations during grazing scattering on a LiF(001) surface**Project supported by the National Natural Science Foundation of China (Grant Nos. 11175075, 11405078, 11474140, 11404152, and 11305083).

There is no available theoretical description of electron transfer from negative projectiles at a velocity below 0.1 a.u. during grazing scattering on insulating surfaces. In this low-velocity range, electron-capture and electron-loss processes coexist. For electron capture, the Demkov model has been successfully used to explain the velocity dependence of the negative-ion fraction formed from fast atoms during grazing scattering on insulating surfaces. For electron loss, we consider that an electron may be transferred from the formed ionic diabatic quasi-molecular state to the formed covalent diabatic quasi-molecular state by the crossing of the potential curves of negative projectiles approaching the surface cations, which can be described by the Landau–Zener two-energy-level crossing model. Combining these two models, we obtain good agreement between the experimental and calculated data for the F−–LiF(001) collision system, which is briefly discussed.

Anomalous Positive Ion Formation in Negative Ion Collisions with HOPG

Charge transfer on graphite, a typical substrate and one of the fusion first wall materials, is of great importance in the plasma–wall interaction, thin-film growth, and surface catalysis. We present an experimental study of 8.5–22.5 keV energy carbon, oxygen, and fluorine negative ions scattering from a highly oriented pyrolytic graphite (HOPG) surface at a scattering angle of 8°. It is found that the positive ion fraction decreases monotonically with the increase of both incident velocity and angle. In particular, these dependences are very different from those presented in previous studies. A molecular dynamics simulation reveals that, around the critical condition for planar surface channeling, a number of projectiles may penetrate into the subsurface and become energetic atoms when they emerge from the surface. Hence, an exponential scaling related to the penetration probability has been proposed to well describe the velocity and angle dependences of positive ion fractions.

Closed-orbit theory for photodetachment in a time-dependent electric field

The standard closed-orbit theory is extended for the photodetachment of negative ions in a time-dependent electric field. The time-dependent photodetachment rate is specifically studied in the presence of a single-cycle terahertz pulse, based on exact quantum simulations and semiclassical analysis. We find that the photodetachment rate is unaffected by a weak terahertz field, but oscillates complicatedly when the terahertz pulse gets strong enough. Three types of closed classical orbits are identified for the photoelectron motion in a strong single-cycle terahertz pulse, and their connections with the oscillatory photodetachment rate are established quantitatively by generalizing the standard closed-orbit theory to a time-dependent form. By comparing the negative hydrogen and fluorine ions, both the in-phase and antiphase oscillations can be observed, depending on a simple geometry of the contributed closed classical orbits. On account of its generality, the presented theory provides an intuitive understanding from a time-dependent viewpoint for the photodetachment dynamics driven by an external electric field oscillating at low frequency.

Energy efficiency analysis of ArF excimer laser system

The reliable functioning and continual optimizing of ArF excimer laser system is of importance when it comes to productization into the market from a laboratory test machine. The analysis of dynamic characteristics of the system is vital to understanding its operating mechanism and optimizing the design theoretically. In this article, one-dimensional fluid model is used to analyze the excimer laser discharge mechanism, and the content ratio of fluorine gas, argon gas, and neon gas, which constitute a gas mixture, is studied in a simulated ArF excimer laser system. Particles are treated as a fluid, which significantly reduces the computing cost in fluid model, and therefore is suitable for high-pressure situation. Four equations are included in one-dimensional fluid model, i.e., Boltzmann equation that describes electron energy distribution, ion continue equation that illustrates ion number density, Poisson's equation that shows the distribution of electric field, and photon rate equation that demonstrates laser outputting process. By combining these four equations, high pressure plasma discharge process and particles stimulated radiation process are studied, and calculation continues from one time step to another until the end of discharging process. The result of the calculation presents energy transfer process from three aspects:energy deposition efficiency, ArF* formation, and laser outputting. In the energy deposition process, the energy deposition efficiency is sensitive to the change of fluorine gas ratio while the variation of the content ratio of other two gases has a less influence on this process. In addition, there exists an optimal fluorine gas ratio that causes the highest energy deposition efficiency. In the ArF* formation process, the reaction between excited argon ions and fluorine gas is the main channel that generates ArF*. The proper increasing of fluorine gas ratio helps form ArF*. In the laser outputting process, photon loss is mainly because of the reaction between fluorine negative ions and photons. Therefore superfluous fluorine gas in the mixture leads to less photons, which eventually results in low energy efficiency of laser. By summarizing the three aspects of energy transfer process, the fluorine gas ratio in the gas mixture plays a significant role in determining the energy efficiency of ArF excimer laser system. This theory is verified by experiments, showing that the deviation of the optimized fluorine gas ratio severely reduces energy efficiency. This conclusion can guide us in optimizing the design and steady reliable function of ArF excimer laser system.

Temporal interferences driven by a single-cycle terahertz pulse in the photodetachment dynamics of negative ions

We present theory and calculations of a real-time-domain interferometry for the photodetachment dynamics of negative ions in the presence of a single-cycle terahertz pulse. The photoelectron can follow two or more classical trajectories to arrive at a detector simultaneously allowing the electron waves to interfere quantum mechanically. Both the in-phase and antiphase oscillations can be observed in the photoelectron interferences from negative hydrogen and fluorine ions depending on the pulse strength and the observing angle. Especially, a temporal-caustic bifurcation is observed when the detection angle is not in line with the pulse polarization direction. Similar interferences and bifurcations are also expected in the angle-resolved energy spectrum, as a result of its approximate equivalence with the time-dependent electron flux at large distances.

Nonadiabatic dynamics in energetic negative fluorine ions scattering from a Si(100) surface.

The dependence of the negative-ion fractions on incident energy and angle is reported for 8.5-22.5 keV F(-) ions scattered from a Si(100) surface at a fixed scattering angle of 38°. The negative-ion fraction increases monotonically with incident velocity for specular scattering. In particular, the variation of the fraction with incident angle is bell shaped for a given incident energy. We interpret this variation using the incident-velocity effect at short distances where the yield of negative ions depends on the number of initial neutrals. It strongly indicates that at short distances, a dynamical equilibrium population is never achieved. This nonadiabatic feature is supported by simple calculations using modified rate equations.

Evidence for the incoming-velocity effect on negative fluorine ions scattering from a highly-oriented-pyrolytic-graphite surface

Negative-ion fractions depending on incident energies and angles have been measured for keV-energy fluorine negative ions scattering on a graphite surface at a scattering angle of 38\ifmmode^\circ\else\textdegree\fi{}. For specular scattering, the fraction increases monotonously with increasing incident energy. A specific feature is the nonmonotonic angular dependence of the fraction with the variation of incident angles. It strongly indicates the interaction-time-dependent electron transfer process. The incident-velocity effect has been taken into account to analyze the experimental results, in which an exponential scaling is found.

High-order above-threshold ionisation of atoms and negative ions: channel-closing effects and the low-frequency approximation

The so-called low-frequency approximation (LFA) is an improved version of the strong-field approximation. The LFA describes the rescattering step of high-order above-threshold ionisation by utilising the exact field-free scattering amplitude, which has to be calculated separately. We apply the LFA to high-order above-threshold detachment of the fluorine negative ion. By comparing the so-obtained angle- and energy-resolved spectra with the exact results obtained as solutions of the time-dependent Schrödinger equation we show that the LFA is a more adequate approximation than the so-called improved strong-field approximation used previously. The LFA is also superior to the recently introduced quantitative rescattering (QRS) model. We show this by applying the LFA to the analysis of the intensity-dependent enhancements in above-threshold ionisation spectra of argon atoms. These enhancements can be explained as channel-closing-induced effects. In particular, we have observed and explained the behaviour of these channel-closing enhancements for higher values of the electron emission angle.

Energy loss of keV fluorine ions scattered off a missing-row reconstructed Au(110) surface under grazing incidence

A joint experimental and theoretical study of energy loss is presented for 1-to-4-keV fluorine negative ions in grazing scattering on a missing-row reconstructed Au(110) surface. Measurements of energy losses for various azimuthal orientations of the crystal have been performed by means of a time-of-flight method with a pulsed beam. The dependence of the fraction of surviving negative ions on azimuthal angles, was determined. Our energy-loss data are discussed in light of trajectory and stopping-power calculations, where the explicit inclusion of the nonuniform electron density at the surface provides good agreement with the experimental data. The simulation allows us to delineate various trajectory classes that correspond to different contributions in the energy-loss spectra for various azimuthal orientations of the surface.

Radial Density Profiles of Fluorine Negative Ions and Electrons around a Magnetized String Plasma in CF4 Gas

Radial density profiles of electrons and fluorine negative ions around a string-type CF4 plasma across a magnetic field are measured using a wire-type Langmuir probe. Modified Bohm theory is applied to evaluate negative-ion density, and it is confirmed that negative ions are well confined between two end plates. Parameter dependences of the density profiles are also measured. Simple density models for electrons and negative ions are proposed, and their radial density profiles are obtained by solving the equations of continuity. The experimental and theoretical results are in good agreement for electrons, whereas there are some discrepancies for negative ions. An unexpected small recombination rate coefficient of negative ions and radial electric field might be the reason for this discrepancy.

Velocity-gauge theory for the treatment of strong-field photodetachment

A recent experimental study of the photodetachment of negative fluorine ions by a strong, circularly polarized laser field has been used as a test case for comparison of different gauges in a strong-field theory. Earlier studies concluded that the length gauge was the more suitable theory, but it was necessary to view the experimental peak intensity as a fitting parameter, and this had to be altered to 145% of the measured peak intensity. That amount of variation is not possible. There were two independent experimental measurements of the peak intensity: one direct and the other through a measurement of the momentum distribution of the photodetached electron. These two measurements give the same result for the peak intensity, and so the 45% increase required to have a fit to the length gauge is not an acceptable theoretical explanation for the experimental results. In contrast, the velocity gauge calculation provides a good match to the spectrum peak with no alteration necessary for the measured peak intensity. The theoretical spectrum is somewhat narrower than the measured spectrum, but that is consistent with the fact that the intensity parameters are not quite large enough to ensure detailed accuracy of the strong-field approximation.

Photodetachment in a strong laser field: An experimental test of Keldysh-like theories

We investigate the photodetachment process in the negative fluorine ion in a strong linearly polarized laser field. Angle-resolved momentum distributions of photoelectrons are measured with the use of an imaging technique for a wide range of laser frequencies and peak intensities. The nonmonotonic structure recorded in photoelectron spectra is interpreted in terms of the quantum interference effect predicted by a Keldysh-like theory. In particular, the dependence of the interference term on the laser parameters is used to explain the origin of the observed spectral features. Our results unambiguously show that the length gauge is the proper one to use in the frame of the strong-field approximation.

Radial Diffusion of Negative Ions Produced by Magnetized String-Type CF4 Plasma

Fluorine negative ions (F-) produced by a column-type magnetized plasma with CF4 gas diffuse radially and accumulate around the plasma column. To explain the features of this accumulation, the radial profiles of the negative-ion density n- as functions of discharge parameters are measured using a hot-wire-type Langmuir probe. The modified Bohm theory is applied to evaluate n- as n-/ne>2 and the negative ions are well confined between two end plates. n- around the plasma column decreases with increasing magnetic field from 0.01 to 0.15 T. With increasing pressure from 0.25 to 1.1 Pa, n- around the plasma column gradually decreases.

Time-resolved measurements of the E-to-H mode transition in electronegative pulse-modulated inductively coupled plasmas

Time-resolved measurements of electronegative pulse-modulated inductively coupled plasmas (ICPs) were carried out using various measurement techniques. In order to explain the experimentally observed results, it is proposed that the structure of an Ar∕CF4 plasma during an afterglow dynamically changes, passing through three stages when the period of the afterglow is long enough: (1) The first stage is the initial afterglow where the electron temperature suddenly decreases due to inelastic collision with CF4. Electron density decreases and the density of fluorine negative ions increases by electron attachment, but the sheath potential still exists and the negative ions are confined in the bulk plasma region. Since charge neutrality should be maintained, the density of positive ions is almost constant during this stage. (2) The second stage is the intermediate afterglow where the plasma consists mainly of negative and positive ions but sheath potentials remain, reducing the negative ion flux from the plasma. The sheath potential gradually disappears and the densities of all the charged particles (electrons and positive and negative ions) decrease because of increased loss to the wall surface. (3) The third stage is where the sheath structure has disappeared completely, and the plasma consists of mainly positive and negative ions and losses are dominated by ambipolar diffusion. The presence of these stages during the afterglow is very important in understanding the behavior of pulsed ICPs with E-to-H mode transitions. The state of an afterglow plasma at the moment power is reapplied determines the discharge characteristics of the pulsed ICPs (e.g., the appearance of an E mode, the duration of the E mode, stability of the plasma).

Experimental Study of Photodetachment in a Strong Laser Field of Circular Polarization

Negative fluorine ions are exposed to a circularly polarized infrared laser pulse with a peak intensity on the order of 2.6 x 10(13) W/cm(2). A fundamental difference, as compared to the case of linearly polarized field, is found in the absence of any structure in the photoelectron spectrum that can be associated with the quantum interference effect. This observation is in accord with our recent predictions [S. Beiser, Phys. Rev. A 70, 011402 (2004)10.1103/Phys. Rev. A.70. 011402]. The experiment reveals that the length gauge is appropriate for the description of the field interaction in the frame of the strong field approximation.

Cross-section set and chemistry model for the simulation of c-C4F8 plasma discharges

Great interest exists in c-C4F8 (octafluorocyclobutane or perfluorocyclobutane) etching plasma discharges due to their selectivity and potential for decreasing global warming gas emissions. In order to allow computational exploration of the discharge physics, a numerical model for a c-C4F8 discharge has been constructed. A set of cross sections has been assembled for electron collisions with c-C4F8 based on a combination of ab initio calculations, beam measurements, and swarm (i.e., electron transport coefficient) analysis. In addition, a chemical reaction set has been proposed and an axisymmetric numerical model has been used to test the cross section and chemical reaction set against experiments. Results show that measured trends are reproduced and absolute values are well represented. A mechanism is suggested for negative atomic fluorine ion (F−) behavior with respect to power.

Charging and trapping of macroparticles in near-electrode regions of fluorocarbon plasmas with negative ions

Charging and trapping of macroparticles in the near-electrode region of fluorocarbon etching plasmas with negative ions is considered. The equilibrium charge and forces on particles are computed as a function of the local position in the plasma presheath and sheath. The ionic composition of the plasma corresponds to the etching experiments in 2.45 GHz surface-wave sustained and 13.56 MHz inductively coupled C4F8+Ar plasmas. It is shown that despite negligible negative ion currents collected by the particles, the negative fluorine ions affect the charging and trapping of particulates through modification of the sheath/presheath structure.

Internal structure and ER properties in ER suspensions of disperse system under dc electric field

The interrelationship between the internal structure and the rheological properties in electrorheological (ER) suspensions subjected to a dc applied field was investigated using two ER suspensions [herein termed ERF(I) and ERF(II)]. There was a remarkable difference in the internal structure between ERF(I) and ERF(II), and the ER response in ERF(II) which contained the negative fluorine ions was larger than that in ERF(I) lacking these ions. These are related to an ionic interfacial activity of dispersed particles involving the polarization of a diffuse double sheath. The effects of the voltage polarity and the gap spacing on the rheological properties in two ER suspensions were also examined. Differences in the test results were attributed to the internal structure in the suspensions. It was also shown that further promotion of the ER response in ERF(II) is obtained by installation of a metallic mesh on the electrode surface. © 2000 Scripta Technica, Electr Eng Jpn, 132(4): 9–18, 2000