Distribution Of Neutral Atoms Research Articles

Experimental Study of Space Charge Structure and Expansion Dynamics of Laser Ablation Plasma

Using the method of spectrally selective photorecording in a nanosecond temporal resolution, the distribution of neutral atoms and single- and double-charged gallium ions in a plasma plume is investigated under the conditions of laser ablation of a gallium-indium target and so is the expansion dynamics of both components and the plume as a whole. The expansion velocity of a neutral atomic component of the plume is found. The expansion velocity of the glow boundary and the shift velocity of the glow intensity maximum are found to be close to 8.5∙103 and 6∙103 m/s, respectively. The kinetic energy range, corresponding to these values for the gallium atoms (13–26 eV) is quite consistent with the position of local maximum measured earlier from the energy distribution of single-charged gallium ions.

Neutral argon density profile determination by comparison of spectroscopic measurements and a collisional-radiative model (invited)

Neutral atoms play important roles in non-fully-ionized plasmas. In helicon sources, neutral pumping and neutral damping of waves are poorly understood. Measurement of the spatial distribution of neutral atoms is possible with spectroscopic diagnostics such as laser-induced fluorescence (LIF) and passive emission spectroscopy. However, these measurements typically apply to an excited neutral atom state, rather than the entire neutral population. With a collisional-radiative (CR) model employing Langmuir probe measured electron parameters in argon helicon source plasmas, we have reproduced LIF and emission spectroscopy measured radial profiles for three excited neutral states. The CR model indicates a neutral depletion on axis of at least 60%. Simple calculations based on measured edge neutral pressures and peak plasma densities significantly underestimate the degree of ionization in the core of the helicon plasma.

Measurement of the velocity distribution of neutral atoms using laser-induced fluorescence

Laser-induced fluorescence has been used to measure the Doppler-shifted emission spectra in the atomic beam of gadolinium. The velocity distribution of gadolinium atoms that was deduced from these spectra shows good agreement with a Maxwellian distribution. Based on this good agreement, temperatures of the evaporating source have also been estimated. A new experimental method has been devised in our measurements, which can also be used for other multiline systems similar to gadolinium. This method allows one to effectively eliminate the distorting interference present in Doppler profile measurements for the multiline systems mentioned above.

Two-dimensional calculation of spatial distribution of neutral atoms for a planar inductively coupled oxygen discharge

Two-dimensional self-consistent fluid simulation of inductively coupled oxygen plasma is presented. The model equations include continuity equations for charged species and neutral oxygen atom, the Poisson equation, and the electron energy balance equation. The drift–diffusion approximation is employed. From the Maxwell equations, the induced electric field and absorbed power are calculated. The two-dimensional spatial distributions of charged species densities, charged species flux, density and flux of atomic oxygen, electric potential, and electron temperature are calculated. The effect of the gas pressure on the plasma uniformity is investigated. As the pressure increases, the spatial distributions of charged species and neutral atom have their peak values in the toroidal region where a large amount of the input power is absorbed by the plasma.

CHARACTERIZATION OFAl+SECONDARY ION EMISSION PRODUCED BYNe+ANDAr+BOMBARDMENT OF ALUMINIUM SURFACE

This paper reports the characterization of the velocity (energy) dependencies of the Al+secondary ion emission produced by 0.5 keV and 5 keV Ne+and Ar+bombardment of polycrystalline pure aluminium. The distributions of secondary Al+ions over their kinetic energy were measured for emission energies of 1–1000 eV without applying electric fields to force the ions into the mass–energy analyzer. To extract the ionization probability, the measured energy distributions of emitted ions were normalized with respect to reference energy distributions of neutral atoms. The reference distributions were obtained by original numerical simulations, as well as analytically, through a sophisticated normalization of the Thompson distribution. It was shown that for both extraction methods, the logarithmic plots of the normalized secondary ion fraction versus the normal component of the reciprocal ion velocity (the reciprocal or inverse velocity plots) are nonmonotonic, with two peaks and two linear portions situated at a low emission energy (Ek=5–25 eV ) and at a high emission energy (Ek=80–280 eV ). The linear portions were fit by exponential dependency P+∝ exp (-v0/vn) with two different values of the characteristic velocity v0. For the low emission energy, the value v01~(3.3±0.2)×106cm / s was independent of the mass and energy of the projectiles. However, for the high emission energy, the characteristic velocity depended on the projectile's mass, M, namely v02~(5.3±0.3)×106cm / s for Ne+and v02~(8.1±0.3)×106cm / s for Ar+; the ratio v02( Ne+)/v02( Ar+) is close to the value [Formula: see text]. This indicates that ballistic mechanisms might contribute to affect the high-energy part of the reciprocal velocity plots along with nonballistic ionization processes, which are generally believed to be the only significant factor for the plots.

Caloric curves of small fragmenting clusters.

The kinetic energy release distribution of neutral atoms emitted from photoexcited clusters Sr(+)(n) with n=4-15, has been obtained by time-of-flight velocity dispersion. The deduced temperature is plotted as a function of the excitation energy. For small sizes n<7 a general increase is observed. For cluster sizes larger than n=9, the deduced caloric curves first increase, and then show evidence of a plateau regime as excitation energy increases. This limiting temperature in neutral atom ejection is consistent with a bound cluster-vapor phase transition in a microcanonical system.

Estimates of differential sputtering yields for deposition applications

Angular and angle-resolved energy distributions of neutral atoms sputtered from elemental targets under ion bombardment are investigated by numerical and analytic modeling. Special emphasis is made on sub-keV ion bombardment, which is typical for sputter deposition sources. Scaling laws that define the distribution anisotropies are formulated. An approximate semiempirical description is suggested for anisotropic energy distributions.

The Ion Temperature Distribution Measurement by Abel-Inverted Doppler Broadening Method

Doppler Broadening methods have been intensively used in measuring the temperature of neutral atoms and ions in plasma diagnostics. However, since only the line-integrated emission can be measured in the experiments, the local temperature can not be found directly. To solve this problem, we first measured the spatial distribution of each spectrum by Abel inversion and then obtained the Doppler broadening at each radial position by analytically combining the spectra. The emissions are collected through five optical fibers which are located at intervals of 48 mm and inserted into the slit of a spectrometer. The dispersed output from the spectrometer is measured with a CCD camera. Since the abscissa of a CCD frame represents the spectra and the ordinate represents the spatial distribution, the Abel-inverted Doppler broadening is easily measured. By using this method, we measured the temperature distribution of neutral atoms and ions in the Hanbit device.

Energy distributions of neutral species ejected from well-characterized surfaces measured by means of multiphoton resonance ionization spectroscopy

The energy distributions of neutral atoms ejected from the polycrystalline Cu target, the Si(111)-7×7 surface, and the Si(111)-“5×5”-Cu surface by 5 keV Ar+ ion bombardment have been measured with very high efficiency by means of the multi-photon resonance ionization spectroscopy, in order to obtain the surface binding energies. The energy distributions for Cu from polycrystalline Cu target, Si from the Si(111)-7×7 surface, and Cu from the Si(111)-“5×5”-Cu surface have been found to have a peak at energies of around 3.0, 5.0 and 1.5 eV, and the function shapes of high energy tails to be proportional to E−1.9, E−1.2 and E−1.3, respectively. Based on the linear collision cascade theory, the surface binding energies are determined to be 5.7, 6.0 and 2.0 eV, and the power factor m in the power law approximation to the Thomas–Fermi potential are determined to be 0.1, 0.4 and 0.3 for Cu from the Cu polycrystalline, Si from the Si(111)-7×7 surface, and Cu from the Si(111)-“5×5”-Cu surface, respectively. In conclusion, the results indicate that the energy distributions of ejected particles are well characterized by the linear collision cascade theory developed by Sigmund.

Studies on energy distributions of species ejected from solid surfaces by means of multiphoton resonance ionization spectroscopy

This paper reviews the energy distributions of neutral atoms ejected from a polycrystalline Cu target, the Si(1 1 1)-7×7 surface, the Si(1 1 1)-5×5-Cu surface and positive Cu ions ejected from the Si(1 1 1)-“5×5”–Cu surface by 5 keV Ar + ion bombardment. These energy distributions have been measured with very high efficiency by means of multi-photon resonance ionization spectroscopy, in order to obtain the surface binding energies. The energy distributions for Cu from the polycrystalline Cu target, Si from the Si(1 1 1)-7×7 surface, and Cu from the Si(1 1 1)-“5×5”–Cu surface have been found to peak at energies of around 3.0, 5.0 and 1.5 eV, and the function shapes of high-energy tails have been found to be proportional to E −1.9, E −1.2 and E −1.3, respectively. Based on the linear collision cascade theory, the surface binding energies are determined to be 5.7, 6.0 and 2.0 eV. The obtained values of the surface binding energy and the energy distributions are discussed. The ionization probability of Cu atoms ejected from the Si(1 1 1)-“5×5”–Cu surface has been found from the ratio of positive and neutral energy distributions to be proportional to the exponential function of the inverse of velocity. The velocity dependence is also discussed in terms of bond breaking model.

Energy spectra of sputtered species under sub-keV ion bombardment: experiments and computer simulations

Energy distributions of neutral atoms sputtered from elemental Cu and two binary alloys (Cu 0.53W 0.47 and Ni 0.92W 0.08) under steady-state conditions were investigated both by experiments and by computer simulations. Ar + ions with impact energies ranging from 65 to 1030 eV were used as bombarding species. The experimental data were recorded with a secondary-neutral mass spectrometer that combines electron-gas post-ionization with a high-transmission double focusing mass spectrometer. Due to the instrument's high detection sensitivity, the energy spectra could be determined over a large intensity range, covering up to five orders of magnitude. A comparable (or even) larger emission-energy range could be monitored in the computer simulations which were done with the dynamic TRIDYN code. The computed energy spectra are in qualitative agreement with the corresponding experimental distributions. Not surprisingly, the data from both approaches indicate that the inverse-square emission-energy dependence of the yield predicted by analytical sputtering theory is generally not valid for the low (near-threshold) impact energies used in this work and for emission energies approaching the maximum possible values. Rather, the energy spectra exhibit, over a considerable energy range, an exponentially decreasing yield with increasing emission energy. The large range of emission energies detected provided the opportunity to derive estimates of the maximum emission energy and its dependence on the primary-ion energy E for the various atomic species. These values amount to ∼0.55 E for Cu and Ni atoms and to ∼0.35 E for W atoms sputtered from the alloys and to ∼0.6 E for Cu atoms sputtered from the pure Cu specimen; they are compared to simple estimates of the maximum emission energies in low-energy sputtering based on collision sequences leading to the ejection events that involve only a small number of atoms.

Angle and energy distributions of neutral atoms sputtered from Ni3Al(100)

Angle and energy distributions of neutral atoms sputtered from Ni3Al(100)

Spatial distributions of neutral atoms in the near-target plasma: Theory and experiment

Spatial distributions of neutral atoms in the near-target plasma: Theory and experiment

Spatial distributions of neutral atoms in the near-target plasma: Theory and experiment

In this paper analytical expressions are given for the distributions of neutral impurity atoms in the plasma near the surface, which acts as the impurity source. The effects of the angular and energy distribution of the sputtered atoms, of the experiment geometry (plasma and surface size) and the plasma parameters are taken into account. Using the analytical expressions and experimentally measured photon intensities of the neutral line emission, either the flux of eroded particles or the excitation rate coefficients can be determined experimentally, by knowing one of them. In the linear plasma generator PSI-1 such an experiment with Li, for which the excitation rate coefficients are available, has been performed. The measured flux of eroded Li atoms has been compared with erosion calculations taking into account both thermal sublimation and physical sputtering.

Subthermal kinetic energy distributions of neutral atoms photodesorbed from Na cluster surfaces

Two-photon laser techniques are employed to obtain high-resolution kinetic energy distributions of Na atoms which desorb from large, surface-bound Na clusters upon pulsed laser-excitation of surface plasmons. The measured translational energy distribution of the atoms is clearly non-Maxwellian with a cos 2θ angular dependence and most probable kinetic energies below that expected of atoms desorbing from a surface at thermal equilibrium. The experimental data is interpreted by a model which takes energy transfer between the desorbing Na atoms and the vibrating Na cluster surface into account.

Angular, energy, and population distributions of neutral atoms desorbed by keV ion beam bombardment of Ni{001}

Multi-photon resonance ionization, time-of-flight mass spectrometry and imaging techniques have been employed to measure the polar-angle, kinetic energy, and population distributions of Ni atoms desorbed from 5 keV Ar ion bombarded Ni{001}. The measured angle- and energy-resolved intensity maps of the sputtering yield provide a set of data that can be used to examine the detailed interactions between the particles of the solid surface during the sputtering event. The results show a considerable degree of anisotropy associated with both the ejection angle as well as the crystallographic direction. In order to have an understanding of the interactions of the desorbed particles with the surface, molecular dynamics simulations of the ion-induced sputtering event are performed. The agreement between experimental and computer simulation results is excellent. Measurements performed on excited states of sputtered Ni show that the valence electron shell structure is an important factor in determining the angle-integrated kinetic energy distribution while the magnitude of the excitation energy is of secondary importance. Population distribution among different electronic states is obtained through two sets of measurements performed on different instruments. Both measurements employ the same resonant ionization schemes and laser fluences. The results show that the a 3D 3 and a 3D 2 states are more heavily populated than is predicted by a Boltzmann-type distribution.

Laser-Induced Breakdown of Metal Vapor

The interaction of laser radiation in the intensity range G = 106 ÷ 1010 W/cm2 and at the wavelength λ = 1.06 μm with copper vapor is simulated, using a collision-radiation model describing the kinetics of nonequilibrium ionization and recombination. It is shown that the interaction of laser radiation with vapor can proceed in two qualitatively different ways, namely, prebreakdown and optical breakdown regimes. If the radiation intensity is insufficient to induce avalanche ionization, the system transfers to a stationary state characterized by one temperature, the equilibrium between the radiation and the vapor being absent. For higher Values of laser intensity, optical breakdown starts, i.e., the nonequilibrium transition state from a partially ionized vapor to a fully ionized plasma where Coulomb collisions are predominant. Simulations confirm experimental results that optical breakdown refers to a distinct threshold in the radiation intensity. In the macroscopic description of breakdown, the threshold intensity values depend on the ionization potential and the electronic state distribution of neutral atoms as well as on the initial temperature of the evaporated material and the laser pulse width.

Boltzmann Equation Analyses for the Relaxation of Velocity Distribution of Neutrals with the Transient FTI Method

The relaxation process of the velocity distribution of neutral atoms injected into dilute atomic gases with Maxwellian and non-Maxwellian velocity distributions has been analysed by numerically solving the Boltzmann equation using the transient FTI method. Here, only isotropic scattering in the center of mass frame and the collision characteristics of constant collision frequency and of constant collision cross section are assumed. Numerical analysis for the relaxation of the velocity distribution of the entire gas with a non-Maxwell initial distribution was also carried out by linearizing the Boltzmann equation sequentially in time. The results obtained are very simple and suggestive. The mechanism driving the neutrals toward the Maxwell distribution is discussed.

Energy distributions of neutral atoms sputtered by very low energy heavy ions

Polycrystalline Cu was sputtered by normally incident, very low energy Ar + ions ( E 0 = 40–1000 eV). The kinetic energy ( E) distributions of the neutral Cu atoms sputtered normally from the Cu surface were measured, using secondary neutral mass spectrometry. For values of E 0 above approximately 600 eV, the observed energy distributions agreed closely with the Thompson-Sigmund theory. For values of E 0 less than about 600 eV the distributions fell off faster than predicted by the Thompson-Sigmund theory, and the peak value of the distribution shifted to somewhat lower energies. Both these effects were exaggerated as E 0 was further lowered. The average kinetic energy of the sputtered neutral Cu atoms increased with increasing E 0. The rate of this increase was less at higher values of E 0.

Kilo-electron-volt Ar–ion-induced neutral atom desorption from Rh{331}: Relation of angular distributions to surface structure

The surface structure of single crystal metals in reflected in the angular distributions of neutral atoms desorbed during keV Ar ion bombardment. Results are presented here of a study of desorption from a stepped surface, Rh{331}. Recently, classical dynamics simulations of the desorption process were shown to give excellent agreement with ejection data from Rh{111}, and the same model potential has been used to simulate desorption from Rh{331}. The agreement with experiment remains excellent despite the different coordination of the surface atoms on Rh{111} and Rh{331}. The azimuthal ejection distributions are strongly affected by the steps.