Crossed Beam Technique Research Articles

Absolute Measurements of Emission Cross-Sections for Dissociative Excitations of H2O by Electron-Impact with Crossed-Beam Apparatus and High Sensitive Quartz Spring Balance

Absolute measurements of the emission cross-sections for the A - X band-spectrum of OH and the H β line in the dissociative excitation processes of H 2 O by electron-impact have been carried out by means of the crossed-beam technique, with the aid of a high sensitive spring balance made of fused quartz. The spring has a tiny pan, which is surrounded by a specially designed trap of liquid nitrogen. The spring words well for direct intensity measurements of the water molecular beam, because the condensation coefficient of H 2 O on ice is large enough at lower temperatures. The following results have been obtained; (4.2±2.1)×10 -18 cm 2 for the H β transition, and (7.1±3.5)×10 -18 cm 2 for the OH ( A , 2 Σ + - X , \(^{2}\varPi\)) transition, at 300 eV respectively.

Excitation of the 2s and 2p states of hydrogen in fast collisions between hydrogen atoms

A modulated crossed-beam technique has been used to determine cross sections for the processes H(1s)+H(1s) to H(2p or 2s)+H( Sigma ) within the energy range 4-26 keV. Results have been based on measurements of the Lyman- alpha radiation emitted from the beam intersection region both in the absence and in the presence of an electric field large enough to produce complete quenching of the metastable 2s state. A Doppler-shift technique was used to distinguish between the emissions from both the target and projectile atoms. In general, the measured cross sections are larger than the values predicted theoretically. As part of a check on the experimental procedure measurements have also been carried out using Ar and H2 targets and good agreement is found between the results and previous experimental data.

Formation of H(2p) and H(2s) atoms in collisions of 2-26 keV protons with hydrogen atoms

A modulated crossed-beam technique has been used to determine cross sections for the four reactions: H++H(1s) to H(2s or 2p)+H+ (electron capture) and H++H(1s) to H++H(2s or 2p) (excitation) within the energy range 2.3-26 keV. Results have been based on measurements, using a Doppler-shift identification technique, of the Lyman alpha radiation emitted from the beam intersection region both in the absence and in the presence of an electric field. While cross sections for excitation of the 2s state have not been reported previously, cross sections for capture into the 2s state agree well with those measured by Bayfield (1969a) using a furnace target technique. The H(2p) formation cross sections are also shown to be in reasonable accord with values based on the crossed-beam measurements of Stebbings et al. (1965).

Absolute electron impact ionization cross sections of Li in the energy range from 100 to 2000 eV

The absolute electron impact ionization cross sections for lithium in the energy range between 100 and 2000 eV were measured by the nonmodulated crossed beam technique. The neutral lithium beam is produced by a Knudsen cell and crossed at right angles with the electron beam. The ions formed are collected on a plate and their intensity determined with a dc amplifier. The neutral beam is condensed on a cold trap cooled with liquid nitrogen, this temperature being much lower than that required to obtain total condensation. The amount of metal deposited is measured by the isotopic dilution technique and by atomic absorption, and the density of the atoms in the neutral beam is calculated. The total absolute ionization cross sections can then be determined. All possible errors have been carefully analyzed and their magnitudes estimated. The absolute ionization cross section for Li at an energy of 500 eV is QLi=0.358 × 10−16 cm2. This value is half of that obtained by McFarland and Kinney. The partial ionization cross sections for the singly and doubly charged ions, is determined with a mass spectrometer attached to this apparatus. For the singly charged ions, the variation of the cross section with the energy of the ionizing electrons is in agreement with the optically allowed transition law Q = A log BE/E. From the variation of Q with E, the squared matrix element of the transition moment |Mi|2 is determined. New calculations of the ionization cross section of Li were performed in the framework of the Born-Bethe approximation as modified by Gaudin and Botter to take into account collisions with large momentum variation of the incident electron. Hartree-Fock type wavefunctions for the ground state atom (tabulated by Clementi) were used. The calculated values are in good agreement with our experimental results and with the former theoretical results calculated by various methods. This work also indicates that the experimental results of McFarland and Kinney for Li are too large.

Study of the reactions H2+ + He→HeH++H and HeH+ + H→H2++He using crossed beam techniques

The proton transfer reactions H2+ + He→HeH++H and HeH+ + H→H2++He have been studied in the laboratory energy range 1–25 eV. The first process, which is endothermic for ground state reactants, has been studied over the indicated energy range with different degrees of vibrational excitation present in the H2+ particles. Below 9 eV, the reaction is found to depend strongly on vibrational energy while above this energy the reaction cross sections appear to depend mainly on the kinetic energy of the reactants. The exothermic reverse reaction exhibits a 1 / ν dependence in the low-energy region indicating an ion-dipole mechanism. Methods of measuring the degree of dissociation obtained in forming the hydrogen atom beam are discussed.

Computation of the sound field of a jet from characteristic turbulence parameters measured by crossed beam techniques

The sound field radiating from a jet is strongly dependent upon the turbulence in the jet. To describe the sound sources in a hot jet, a method has been developed, based upon the measurement of infra-red radiation of the jet, and a hybrid processing of the measured signal, which enables the computation of characteristic properties of the turbulence at various points inside the jet (convection speed, integral length scale, life time and intensity of turbulence).

Electron impact differential and integral cross sections for excitation of the n = 2 states of helium at 29.2 eV, 39.2 eV and 48.2 eV

A high resolution electron impact spectrometer using electrostatic monochromators and the cross-beam technique is described. Considerations related to its constraction and use in measuring cross sections are discussed. This instrament was used to obtain absolute values for differential and integral excitation cross sections of the n = 2 manifold states of helium at 29.2 eV, 39.2 eV and 48.2 eV. The results are presented and compared with currently available theoretical calculations. (auth)

Electron impact ionization of ions trapped in a hollow electron beam

Introduction of a molecular beam into a hollow cylindrical electron beam generated by a toroidal cathode produces positive ions which oscillate radially for periods of the order of a second: they are ionized by bombardment with an axial beam of electrons of variable energy; ions of different charge states are extracted axially through an orifice into a quadrupole mass-spectrometer and channeltron detector. In this way cross section functions for ionization of multiply charged ions are obtained for electron energies between threshold and 500 eV. The cross sections, not being absolute, are normalized against data obtained by crossed-beam techniques. The normalization factors which are governed by the ion trapping time, are independent of the energy of the axial beam but may depend on the residual gas pressure. They are used with +or-30% confidence for multiply charged ion cross section functions for which only the singly charged ion crossed beam data are available.

Angular Distribution of Electrons Elastically Scattered fromN2

The angular distribution of electrons elastically scattered from ${\mathrm{N}}_{2}$ as a function of energy has been measured utilizing a crossed-beam technique. Measurements have been made of monoenergetic electrons with energies between 5 and 90 eV scattered from a collimated beam of ${\mathrm{N}}_{2}$ at angles from - 114\ifmmode^\circ\else\textdegree\fi{} to + 160\ifmmode^\circ\else\textdegree\fi{}. The wide angular range of the measurements has enabled the determination of the total elastic-scattering and momentum-transfer cross sections. The measurements are relative and have been normalized to Fisk's theoretical calculation at 5 eV. The results generally agree well with other published measurements and with theory.

Measurement of Low Energy Scattering Cross Sections for the He–N2 System

Total elastic scattering cross sections for the He–N2 system were measured for relative velocities from 1.68× 104 to 1× 105 cm/sec utilizing the crossed beam technique. An orbiting resonance below the j=0 to 2 transition was observed. Two collimated, aerodynamically intensified molecular beams intersecting at 10° were used. Each beam had a speed distribution comparable to that obtained from mechanical velocity selectors. The scattering center density was determined from a calibration of the detector utilizing a thermal beam source, carefully considering the geometry of the system. Attenuation of the scattering signal due to the background gas was determined without recourse to pressure-gauge readings. One beam was modulated and integrating techniques were used to recover the signal. The time-of-flight technique was used to determine the initial conditions in each beam. The measurements were compared with quantum mechanical calculations performed at the University of Bonn using a LJ (12,6) potential. Generally, the measurements were larger than these calculations, but one resonance was partially resolved.

Post-Threshold Energy Dependence of the Cross Section for Endoergic Processes: Vibrational Excitation and Reactive Scattering

The essential features of the translational energy dependence or excitation function σ(Etr) for two types of endoergic collisional processes are deduced on the basis of information on the inverse, exoergic processes. Microreversibility is conveniently exploited via the symmetric yield function, Y(E), which is uniquely determined at a given total energy, E. In the case of the vibrational excitation of diatomic molecules by atomic or molecular impact (T → V), use is made of the abundant data on the temperature dependence of the V → T relaxation time τ. The usual semiempirical two-parameter representation of the relaxation process is transformed to yield an explicit functionality for the translational energy dependence of the inelastic excitation cross section σ0 → 1 (Etr), in the adiabatic region. For diatomics whose relaxation is slow (e.g., slow enough to be studied in a shock tube) the vibrational excitation cross section σ0 → 1 is exponentially small in the post-threshold region. These systems would be unsuitable for study by the crossed beam technique; the latter thus complements rather than replaces the more traditional relaxation methods. Also considered are endoergic chemical reactions whose inverse processes involve negligible activation barriers (e.g., ion—molecule reactions, reactive atom—molecule reactions, etc.). In all cases the dominant post-threshold energy dependence of the reactive cross section σR(Etr) for the endoergic process is fixed via the microreversibility constraint, so experimental data in this region do not provide as much information as is available from studies of the exoergic process. The deviation from a simple Arrhenius cross section functionality is determined by the long-range final product interaction.

Ionization of potassium by 20-100 keV protons

A modulated crossed beam technique has been used to investigate the ionization of potassium by 20-100 keV protons. Absolute cross sections for electron production and cross sections for K+ and K2+ formation have been determined. The results are compared with recent calculations based on the classical impulse approximation and with experimental data on the ionization of potassium by electron impact.

Ionization Cross Sections of Ca, Sr and Ba by Electron Impact

Total ionization cross sections of Ca, Sr and Ba by electron impact have been measured by the crossed beam technique. Intensity of neutral atomic beam is determined by the deposition method. Sample atoms deposited onto a beam collector are quantitatively analyzed by atomic absorption spectrophotometry. The absolute cross sections are determined to be 6.2±0.2 A 2 at 13.6 eV for Ca and 7.2±0.3 A 2 at 12.6 eV for Sr. The absolute cross section of Ba could not be determined because of a poor sensitivity of spectrophotometer for Ba. Possible systematic errors are estimated to be between +8% and -19% for both of Ca and Sr. Relative cross sections of the three elements are obtained for electron energy from 0 eV to 1000 eV. Each curve for the total ionization cross section as a function of electron energy has two maxima in low energy region, one of which is considered to be due to autoionization. The results are compared with some experimental results and theoretical calculations by other workers.

"Polarization-Free" Vacuum-Ultraviolet Excitation of Helium by Electrons

"Polarization-free" excitation cross sections for the $1^{1}S\ensuremath{-}n^{1}P$ transitions of He have been measured from threshold to 2 keV using a crossed-beam technique. Results have been made absolute using the accurately known oscillator strengths. Maxima in the cross sections for the $n^{1}P$ levels occurred around 100 eV and were found to be 106\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}19}$ ${\mathrm{cm}}^{2}$ ($2^{1}P$), 31\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}19}$ ${\mathrm{cm}}^{2}$ ($3^{1}P$), and 13\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}19}$ ${\mathrm{cm}}^{2}$ ($4^{1}P$).

Measurements of Excited-State Population Ratios of Atomic Hydrogen Produced by Charge-Exchange Neutralization of Energetic Proton Beams

Excited-state population ratios for atomic hydrogen have been measured for energetic hydrogen ions neutralized by charge exchange with a number of different metallic vapors and permanent gases. Electric field ionization techniques were used to determine excitation ratios. Detection of the energetic ions and neutrals was accomplished using partially depleted surface-barrier detectors. Counting, using phase-detection techniques in which an add-subtract scaler was phased to the signal, provided an improved signal-to-noise ratio. Two methods were used to provide desired targets. Metallic vapor and permanent-gas thin targets of the order of ${10}^{\ensuremath{-}5}$ mTorr cm were produced as chopped neutral beams, using conventional crossed-beam techniques. Gas cells were used to extend these measurements with permanent gases to targets up to seven orders of magnitude thicker. In the latter case, the ion beam was interrupted electrically. Targets investigated were magnesium, potassium, barium, hydrogen, nitrogen, thallium, Freon, water vapor, ammonia, and perfluorodimethylcyclohexane (${\mathrm{C}}_{8}$${\mathrm{F}}_{16}$).

An optical study of turbulence

This paper describes theoretical and experimental work carried out at the Cavendish Laboratory of the University of Cambridge. The main object of the work was to develop a new technique for measuring the structure of fluid turbulence.A parallel beam of light is passed through the turbulent region, containing refractive index fluctuations, and analyzed on exit by gratings of periodic transmissivity. Two forms of analysis yield (a) the spatial power spectrum of the refractive index fluctuations in the turbulence, and (b) the velocity distribution within the beam aperture. The method does not disturb the fluid physically, does not depend on the existence of a mean flow velocity, and works well in liquids.One of the limitations of this single-beam method is that it produces information averaged along the path length of the beam in the turbulence, and to overcome this a cross-beam technique, using two beams intersecting at right-angles, has been developed in theory. This method gives the spatial power spectrum of the refractive index fluctuations, as does the single beam method, but the results are characteristic only of the volume of intersection of the beams.The paper first discusses the theory of the single-beam and crossed-beam techniques, and then experimental results obtained with the single-beam method.The turbulent region investigated was a rectangular tank of water, heated from below and cooled from above, producing convective turbulence of high Rayleigh number (4·1 × 108), a system difficult to analyze by conventional methods of measurement, such as the hot-wire anemometer.Spectral density functions (power spectra) of refractive index, and hence in this case temperature fluctuations, have been measured, as have velocity distributions. Statistical analysis of the results also gives useful information about the Eulerian time scale of the turbulent field.

Absolute Measurement of Total Ionization Cross Section of Mg by Electron Impact

Total ionization cross section of Mg by electron impact has been measured absolutely by the crossed beam technique. Intensity of neutral Mg beam is determined by the deposition method. Mg atoms deposited onto a beam collector are quantitatively analyzed by atomic absorption spectrophotometry. The absolute cross sections at 22.4 eV, 106 eV, 492 eV and 1000 eV are determined to be 9.1, 3.9, 1.7 and 1.0×10 -16 cm 2 , respectively. Systematic errors are estimated to be within -15% and +8%. Relative cross section is obtained as a function of electron energy from the threshold to 1130 eV. The results are compared with some experimental results and theoretical calculations by other workers.

Relative Electron Impact Ionization Cross Sections by a Double Crossed-Beam Technique

Accurate values of electron impact ionization cross sections of high-temperature species are of importance in mass spectrometric investigations as well as a variety of other fields. However, the present experimental techniques are subject to many inherent difficulties or are limited in applicability and have yielded relatively few reliable values. Theoretical calculations are not sufficiently accurate. An experimental technique for accurate measurement of relative cross sections is described. The measurement depends only on one experimentally determined quantity, as well as the mass and temperature of the gaseous species, and is applicable to a wide variety of systems. The technique utilizes a mass spectrometer that is fitted with a Knudsen cell and employs electron impact ionization. A second, modulated electron beam intercepts the molecular beam, causing a slight modulation in the intensity of the molecular beam that is reflected in the output signal of the mass spectrometer. The relative attenuations of the various component species of the molecular beam are proportional to the relative total ionization probabilities of these species. The relative cross sections for Ca, Tl, Ag, and Pb have been measured and are compared with literature values obtained by other techniques. Preliminary application has also been made to the molecular species W2O6 and W3O9.

Inelastic electron scattering from organic compounds

AbstractIn calculations of possible superconductivity in a system, knowledge of certain matrix elements determines whether or not bound electron pairs can exist and thus lead to superconductivity. These matrix elements involve ground and excited state wave functions. We are presently attempting to experimentally determine such matrix elements in certain organic compounds by measuring differential cross sections for inelastic electron scattering. Such cross sections involve the square of the matrix elements of interest. To do this we are using a crossed beam technique with an electrostatic electron spectrometer for the production of the incident electron beam and analysis of the scattered beam. The apparatus is described, as well as the present status of the experiment.

Measurements of Wind Speeds with an Optical Crossbeam System

An optical crossbeam technique has been developed for the remote sensing of atmospheric winds. The feasibility tests of a complete system are reported. The initial crossbeam system was found to measure atmospheric winds within ±20 percent of those measured with cup anemometers on a meteorological tower. The system consists of two telescopes (light beams) which collect light along narrow beams. The collected light is measured by photodiodes. The light is found to fluctuate in intensity due mainly to scattering by aerosols. The light source is available skylight only. The fluctuations in light are related to the convective wind velocity by employing space-time correlation techniques.