Ion Measurements Research Articles

Electron transport, ionization, and attachment coefficients in C2F4 and C2F4/Ar mixtures

Measurements of electron transport, effective ionization, and attachment coefficients are reported for C2F4. In addition, measurements of the electron drift velocity and the effective ionization coefficient as a function of the density-reduced electric field E/N are reported for mixtures of C2F4 with Ar. The measured effective ionization coefficients in C2F4/Ar mixtures indicates a contribution to these coefficients from Penning ionization in dilute C2F4/Ar mixtures. The rate constant for electron attachment to C2F4 as well as the product of the longitudinal electron diffusion coefficient and the gas number density NDL in mixtures of C2F4 with Ar as functions of E/N are also reported.

Investigation of Remote Antenna Assembly for Radio Communication with Reentry Vehicle

Consideration is given to the experimental investigation of the original method of providing radio communications with a reentry vehicle along the trajectory section when a vehicle is surrounded by plasma of the ionized shock layer. The method suggests placing of the antennas in special small containers. The containers are located at about zero angle of attack on a pylon ahead of the bow shock wave generated near a vehicle. Because of the small container bluntness low level of ionization near the antennas could be provided. But many problems connected with heating and thermal protection of the container and pylon arise in this case. Therefore along with the measurements of ionization near the container, heating and thermal protection of the container and pylon and their ine uenceon thevehicleaerodynamicswereinvestigated.Theexperiments wereperformed in fourhypersonicwind tunnels of different types in Mach-number range from 6.5 to 20.5. Various measurement methods were used. The investigation shows that the remote antenna assembly can provide uninterrupted radio communication with the reentry vehicle.

Generation of resonance-enhanced fifth harmonic in xenon with Bessel–Gauss laser beams

Generation of a tunable resonance-enhanced fifth harmonic in xenon under excitation by tightly focused Gaussian and Bessel–Gauss laser beams has been studied through multiphoton ionization measurements. It has been shown that the transformation of a Gaussian beam into a Bessel–Gauss one results in a red shift of the fifth-harmonic excitation profile toward the atomic resonance. The tuning curves and the relative efficiency of harmonic generation in Gaussian and Bessel–Gauss laser beams have been compared and discussed.

Uncertainty in Estimation of Wet Deposition of Sulphur

A model is described for predicting wet deposition ofsulphur in Britain from rainfall and site measurementsof ion concentration in precipitation. This modelincludes orographic enhancement of both rainfall andion concentration. The model output is comparedagainst available measurement data. Sensitivity anduncertainty analyses are used to predict the outputuncertainty. If the stated assumptions can be shown tobe correct, the wet deposition for Britain at the 5 kmscale is accurate to ±35% across the country. Theanalyses show a larger uncertainty in central Englandand a possible bias towards underestimation of wetdeposition, the latter being of importance incalculating critical load exceedances in remote areas.

Energy disposal in collisions of nitric oxide with molecular adlayers on transition metal single crystal surfaces: Rotational energy disposal

The dynamics of the scattering of nitric oxide (NO) from well-characterized and ordered layers of carbon monoxide (CO) and ethylidyne (CCH3) adsorbed on a Pt(111) substrate have been investigated through resonance-enhanced multiphoton ionization (REMPI) measurements of the rotational quantum state distributions of the scattered NO molecules for a range of incident NO translational energies between 10 and 50 kJ mol−1 from scattering substrates at 108 and 300 K. The measurements reveal that, where in principle the interaction potential resembles that of the NO–Ag(111) system more so than that of the NO–Pt(111) system, the scattering differs remarkably from that expected from a weakly bound system in that it occurs both into a trapping-desorption channel and a direct inelastic channel. Experimental data are presented that indicate both an incident translational energy dependence and substrate surface temperature dependence of the scattering, which is interpreted in terms of a hard-cube model.

Ionization and Mass Spectrometry of Decaborane for Shallow Implantation of Boron into Silicon

Future generations of Si electronic devices will need very shallow p-n junctions, in the tens of nanometer range. Implantation of B to form p‐type junctions of such low depth requires very low energies, below 1 keV, where the ion beam formation and transport are hindered by space‐charge effects. Shallow implantation also can be achieved using higher energy beams of ionized large molecules, such as decaborane , since the atoms are implanted with only a fraction of the beam energy. Measurements of electron impact ionization and breakup of decaborane in the electron energy range, 25–260 eV, and temperatures up to 350°C are reported here. Ions containing 10 B atoms were found to be the dominant component in all measured mass spectra. In another set of experiments, the beams of the cluster ions were generated in an electron impact ionization source, mass analyzed, transported through a 2.5 m long ion beam line, and implanted into Si. No significant breakup of the ions and no neutral beam component were found. Beams of ions with ten B atoms were formed more easily and are more robust than initially thought. The results confirm the potential of decaborane cluster ions for low energy implantation of boron. © 2000 The Electrochemical Society. All rights reserved.

Complete measurements of two-photon ionization of atomic rubidium using elliptically polarized light

We report in detail our complete characterization of atomic photoionization via a single measurement, namely, that of the two-photon photoelectron angular distribution using elliptically polarized light. Through these measurements we are able to determine the atomic parameters necessary to unambiguously describe two-photon ionization of atomic rubidium. These parameters include relative cross sections for photoionization into various outgoing channels as well as the phase difference between the continuum S and D partial waves. We present a derivation of the angular distribution with arbitrarily polarized light in terms of these cross sections and phases, demonstrate the advantages of using elliptically polarized light, discuss details of the experiment, and report the results of our measurements in this paper. While measured phase differences are in excellent agreement with expected values over the entire range of photoelectron kinetic energy of our measurements from 0 to 0.5 eV, ratios of cross sections for photoionization into the two D channels are anomalous, suggesting unexpected fine structure effects in these continua.

L-shell ionization measurement of tungsten by electron impact

L-shell partial production cross sections of Lα−, Lβ−, Lγ− rays by electron impact were measured by observing the counts of X-ray from impacted thin tungsten target. Total production cross sections and mean ionization cross sections were deduced from these measured results. The electron beam energy range was from 11 to 36 keV. Tungsten was sputtered onto a carbon backing to reduce bremsstrahlung of the backing. The effect of electrons reflected by the backing has been corrected. Comparison with two theoretical calculations has performed. The experimental results agree rather well with the theoretical predications.

Ionization measurements in optically pumped discharges

The kinetics of ionization and electron removal in opticallypumped non-equilibrium plasmas sustained by a CO laser are studied usingnon-self-sustained dc and RF electric discharges. Experiments in opticallypumped CO/Ar/N2 mixtures doped with O2 and NOdemonstrated that associative ionization of CO produces free electrons at arate up to S = 1015 cm-3 s-1. The ionization ratecoefficient, inferred from the CO vibrational population measurements, iskion = (1.1-1.8)×10-13 cm3 s-1. It isshown that excited NO and possibly O2 molecules also contribute tothe vibrationally stimulated ionization process. In a CO/Ar plasma,applying a dc bias to the cell electrodes resulted in the rapidaccumulation of a deposit on the negative electrode due to a large clusterion current. The average mass of an ion in this plasma, estimated bymeasuring the mass of the deposit, is m≅250 amu, which isconsistent with the mass spectrometer analysis of the deposit. The depositdid not accumulate when small amounts of O2 and NO were added tothe CO/Ar plasma, which presumably indicates the destruction of the clusterions.It is demonstrated that adding small amounts of O2 to theoptically pumped CO/Ar plasmas significantly increases the electrondensity, from ne = (4-7)×109 cm-3 tone = (1-2)×1011 cm-3. This effect occurs at anearly constant (within 50%) electron production rate S, indicating asubstantial reduction in the overall electron removal rate. This reductioncan be qualitatively interpreted as the destruction of rapidly recombiningcluster ions in the presence of the O2 additive, and theirreplacement by monomer ions with a slower recombination rate. Furtherstudies of the ion composition in optically pumped plasmas are suggested.

An ion source using laser ablation

This article describes the measurement of the fractional ionization in an ablated plume produced by excimer laser, and the application of the plume as an ion source. Laser ablation of iron, aluminum, and tantalum with a KrF excimer laser was performed in a vacuum chamber. The ablated plume was caught by a cup type electrode used to measure the amount of charged particles as current wave forms. Applying electrostatic potential larger than 40 V, the current wave forms were separated into electropositive and electronegative peaks. Each peak indicates ions and electrons, respectively. The amount of ions, 6×1013 ions/pulse, was estimated by integrating the electropositive peak. The ionization degree of 1.7% was also estimated by a series of analyses. For an aluminum target, the ion current reached 2 A at the peak and 1 mA on average at 60 kJ/m2, 500 mJ, and 50 Hz.

Test of a small prototype of the KLOE drift chamber in magnetic field

We report on a beam test of a small prototype of the KLOE drift chamber in magnetic field. The chamber was operated with a 90% He–10% iC 4H 10 gas mixture. Drift space–time relations were studied. The average spatial resolution is about 110 μm in the absence of magnetic field and 120 μm in a 0.6 T magnetic field. Measurements of the primary ionization and drift velocity are reported. The measurement of d E/d x in a He-based gas mixture is also discussed.

Molecular dynamics study of benzene–benzene and benzene–potassium ion interactions using polarizable potential models

We have constructed a polarizable potential model for benzene using molecular dynamics techniques. The atomic site polarizabilities for carbon and hydrogen were taken from the recent work of Applequist [J. Phys. Chem. 97, 6016 (1993)], which reproduced the experimental molecular polarizability of the benzene molecule very accurately. Our model describes well the available experimental data such as the structure and thermodynamic properties of liquid benzene and the equilibrium properties of the liquid/vapor interface of benzene. The lowest minimum-energy structure of the benzene dimer predicted by our model has a T-shape with a potential energy of −2.5 kcal/mol. This value agrees with the experimentally obtained value (−2.4±0.4 kcal/mol), which was determined from a high-precision ionization measurement. The cyclic minimum-energy structures are found for both the benzene trimer and tetramer clusters. The computed density profile shows that the interface is not sharp at a microscopic level and has a thickness about 5 Å at 300 K. The calculated surface tension is 25±2 dyn/cm, which is in excellent agreement with the experimentally obtained value of 28 dyn/cm. The results of our model also compare well with the corresponding results for benzene obtained by Jorgensen and Severance [J. Am. Chem. Soc. 112, 4768 (1990)], who used nonpolarizable potential parameters. We also report the details of our study of K+(C6H6)n=1–6 clusters. We found that the polarization effects were quite significant in these systems.

Determination of cross sections and continuum phases of rubidium through complete measurements of atomic multiphoton ionization

We present a complete experimental determination of the atomic parameters necessary to unambiguously characterize two-photon ionization of atomic rubidium, i.e., the relative photoionization cross sections and the continuum wave function phase difference. We obtain these complete results through a single measurement, that of the two-photon photoelectron angular distribution using elliptically polarized light. While measured phase differences are in excellent agreement with expected values, the ratios of photoionization cross sections of the two D channels are not.

Ionization measurement at very low temperature for nuclear and electron recoils discrimination by ionization-heat simultaneous measurement for dark matter research

To achieve a high level of discrimination between nuclear recoils and electron recoils for dark matter research, we realized and studied ionization-heat detectors working at very low temperature (10–20mK). To understand the mechanisms underlying the ionization measurement at this temperature range, we made systematic studies of detectors performances (time stability, energy resolution, etc.) in X- and γ-rays detection for Ge and Si detectors. Results are presented and discussed. We found that the time stability is governed by the progressive space charge build-up due to impurities ionization by far-infrared radiation. Moreover if the energy resolution of HPGe-pin detectors is limited by the noise, it appears to be intrinsically limited by carrier trapping on neutral impurities in Si detectors.

Multi‐color photometric measurements of ionization and energies in sprites

Recent time‐resolved multi‐color photometric data obtained on one class of lightning‐related transient upper‐atmospheric electromagnetic events called sprites have confirmed an impulsive ionization emission during the sprite initiation. Data have also been obtained on some sprites which do not exhibit observable tendrils and which exhibit ionization emission that, if present, is below our detection limit. This suggests that some sprite events exhibit strong ionization while others do not. These results indicate that conditions causing sprite optical emissions are highly variable.

Far-Ultraviolet Excited Laser-Enhanced Ionization Spectrometry of As, Se, Cu, and Sb in Air/Acetylene and Ar/O2/Acetylene Flames

Laser-enhanced ionization (LEI) measurements of As, Se, Sb, and Cu in air/acetylene and Ar/O2/acetylene flames are reported. The use of far-UV excitation wavelengths to perform one- and two-step LEI excitation from the ground states of As, Se, and Sb is reported for the first time. Significant ionization background signals are observed for far-UV excitations in the air/acetylene flame, but are reduced by substituting an (80:20) Ar/O2 mixture for air as the flame oxidant. The sensitivities of the individual LEI techniques are also increased by the Ar/O2 gas substitution. The limits of detection for As, Se, Sb, and Cu are 30, 0.5, 9 and 2 ng/mL, respectively, and compare favorably with previous LEI reports. Due to the poor atomization characteristics of As and Se, the utility of hydride generation (HG) sample introduction with LEI detection has been explored. Although high sensitivity has been observed for the HG-LEI approach, limits of detection were not evaluated due to nonlinear responses of the signals with respect to analyte concentration.

Trace detection of NO and NO 2 by photoionization and laser induced fluorescence techniques

The analytical capabilities of photoionization spectroscopy (PIS) and laser induced fluorescence (LIF) techniques have been evaluated to determine the relative merits of each technique for measurements of NO and NO 2 at the ppb and sub-ppb level. For both techniques, excitation is accomplished via the A–X (1,0) band of NO near 215 nm. For the ionization measurements, a (1 + 1) PIS scheme is utilized where NO molecules are photoionized by a second laser operating at 355 nm. Nitrogen dioxide (NO 2) is determined by a photofragmentation approach where NO is photofragmented from the parent molecule and then detected by PIS. Limits of detection by the PIS approaches are 0.05 ppb for NO in air and 6 ppb for NO 2 in air. For LIF, fluorescence emission is measured via the A–X (0,1) band at 237 nm. Limits of detection by LIF are 0.2 ppb and 30 ppb for NO in N 2 and air, respectively.

Antiprotonic atom formation and spectroscopy-ASACUSA experiment at CERN-AD

This talk describes the experiments on atomic spectroscopy and atomic collisions as proposed by the ASACUSA collaboration for the forthcoming AD facility at CERN. They consist of high-precision spectroscopy of antiprotonic atoms, the study of antiprotonic atom formation processes, and stopping power and ionization measurements in low-pressure gases.

Ionization of high Rydberg states of calcium by electric fields

Measurements of electric field ionization of high Rydberg states populated by two-photon excitation are reported. We use a novel technique, in which the extraneous ionization background due to other causes is suppressed, resulting in higher accuracy and sensitivity than otherwise achievable. The method is applied to a series in the calcium spectrum which exhibits an interesting anomaly around n = 63 in previous experiments by laser spectroscopy using thermionic diode detection. In the present experiments, collisions are highly improbable, and it is demonstrated that electric field ionization then occurs without any detectable anomaly around n = 63. The implications of this for the interpretation of the effect are discussed.

Evaluation of effective source position for local linear accelerator and inserts

All types of treatment planning systems need some input measured beam data. Such data differ in type and number depending on the model of electron beam algorithm used inside. In addition to the number of percentage depth dose (%DD) and cross beam profiles, the effective source surface distance parameter SSD eff was also recommended to be measured and transferred to the planning system. Output measurements were carried out using 0.6cc cylindrical Farmer type ion chamber placed in water phantom at depth of dose maximum, in Radiat. Phys. Dep., Malmö Hospital, Lund University, Sweden. Results were collected for all available energies combined with both applicators and inserts on a Varian Clinac 2100C. SSD eff was determined from the ionization measurements using equation given by Khan et al. (1991). Comparing the results with those obtained by Roback et al. (1995) on a similar Varian Clinac 2100C, it was found that the deviation of SSD eff was 7.5% for combination of energies, field sizes, and inserts except at 6 MeV. The variations of SSD eff than the nominal SSD reflect the importance of corrections against air gap present in irregular clinical situations. SSD eff should be measured during commissioning of both accelerator or/and local treatment-planning computer. Inserts used in measurements should be from the same material and thickness as that are used in clinical work.