Crossed Beam Technique Research Articles

Depth profiling of weakly absorbing samples by the crossed-beam photothermal deflection technique

Abstract The depth profiling of a weakly absorbing sample using the crossed-beam photothermal deflection technique is theoretically described in terms of imaging resolution and contrast. The results show that, for the transverse deflection amplitude image of an optical or thermal defect in weakly absorbing material, the image contrast and depth-profiling ability increase markedly with increasing intersection angle of the two beams. Therefore an effective way to improve the resolution and contrast of the image is to increase the intersection angle, and the best image quality is obtained at normally crossed-beam case. The crossed-beam photothermal depth-profiling technique provides a quantitative microanalysis tool for the detection of optical or thermal defects located at any depth of weakly absorbing materials.

Depth profiling of weakly absorbing samples by the crossed-beam photothermal deflection technique

The depth profiling of a weakly absorbing sample using the crossed-beam photothermal deflection technique is theoretically described in terms of imaging resolution and contrast. The results show that, for the transverse deflection amplitude image of an optical or thermal defect in weakly absorbing material, the image contrast and depth-profiling ability increase markedly with increasing intersection angle of the two beams. Therefore an effective way to improve the resolution and contrast of the image is to increase the intersection angle, and the best image quality is obtained at normally crossed-beam case. The crossed-beam photothermal depth-profiling technique provides a quantitative microanalysis tool for the detection of optical or thermal defects located at any depth of weakly absorbing materials.

A Triple-Crystal Diffractometer for High-Energy Synchrotron Radiation at the HASYLAB High-Field Wiggler Beamline BW5

The triple-crystal diffractometer installed at HASYLAB beamline BW5 with a high-field wiggler of critical energy 27 keV for DORIS III, operated at 4.5 GeV electron energy, is described. Samples can be mounted in large cryostats or furnaces normally used in neutron scattering experiments. The instrument has been successfully applied to measure structure factors S(Q) in liquids and amorphous materials, to collect full data sets of highly accurate structure factors for charge-density work, to measure the spin component of the ground-state magnetization in transition-metal and rare-earth compounds, to study the diffuse scattering from stacking faults and dislocation loops in Si single crystals, and for the investigation of various aspects of structural phase transitions: critical scattering in SrTiO3, oxygen order and stripe order in high-To materials. A crossed-beam technique allows for local studies of texture, internal strain and phase changes in the bulk of materials.

Dissociative electron attachment cross section to CHCl3 using a high resolution crossed beams technique

Using a crossed electron-molecule beam ion source in combination with a quadrupole mass spectrometer we have studied the electron energy dependence of the dissociative attachment process CHCl3+e→Cl− at electron energies from about 0 to 2 eV and in a target gas temperature range of about 300–430 K. The energy resolution and working conditions of this newly constructed crossed beams machine have been characterized using CCl4 as a test and calbrant gas. Utilizing the improved energy resolution of the present experimental setup (which allows measurements with FWHM energy spreads down to below 5 meV) it was possible to determine the accurate shape and magnitude of the cross section function in the low-energy range. This leads to the conclusion that between an electron energy of about 20 and 130 meV the reaction proceeds via deBroglie s-wave capture, whereas at higher energy (above about 0.4 eV) autodetachment plays a significant role. Moreover, the present measurements allow us to clarify previously reported differences in the absolute cross section, the number of peaks and in the energy position of these peaks. Finally, by analyzing the measured strong temperature dependence of the cross section close to zero electron energy the activation barrier for this dissociative attachment was determined to be 110±20 meV in good agreement with thermochemical data from swarm experiments performed under thermal equilibrium. Taking into account the present results it is also possible to discuss the mechanism for the existence of the second peak.

A coordinated flowing afterglow and crossed beam study of electron attachment to CCl 3Br

Dissociative electron attachment to CCl 3Br has been studied using a flowing afterglow/Langmuir probe (FALP) and a crossed beam technique. In the FALP experiment the overall attachment rate coefficients and the branching ratio into the Cl − and the Br − product channels, R = Cl − ( Cl − + Br − ), were measured as a function of the gas temperature, T g, in the range of 300–540K and the electron temperatures, T e, from T g to 4000K indicating that R approached the statistical value of 0.75 at the highest T g. At T g = 540K both Cl 2 −1 and ClBr − molecular ions were observed at about the 2% level. An apparent activation energy of 55 meV for the overall attachment reaction was derived using a model developed previously to describe the dependence of dissociative electron attachment rates on T g and T e. The crossed beam experiment provided relative attachment cross-sections for the production of Cl − and Br − as a function of electron energy, E, from near zero up to ∼2 eV at several T g within the range 311–423 K. The absolute cross-sections at T g = 311K were obtained from the FALP value using a calibration procedure. At low E the overall attachment cross-section varies as E −1 in accordance with s-wave capture theory. In accordance with the FALP data R increases from 0.2 at low E and the lowest T g to the statistical value of 0.75 at high E and high T g. A peak observed in the cross-section at an E of about 0.7 eV is tentatively attributed to p-wave electron attachment. The rapid decrease of this peak cross-section with T g is ascribed to autodetachment.

Collision-induced dissociation dynamics of the enolic acetone cation: evidence for non-ergodic behavior

Angle- and energy-resolved collision-induced dissociation (CID) of the enolic acetone cation to an acetyl cation plus methyl radical has been studied using the crossed-beam technique. The results, presented in the form of center-of-mass velocity contour plots, suggest that a single dynamical mechanism is followed in the CID of enolic acetone cation as the collision energy is increased from 3.4 eV to 41 eV (CM). When interpreted according to the usual two-step model for CID, our data show that energy deposition from translational to internal modes is about 2.6 ± 0.5 eV and is independent of collision energy. We also find that angular scattering is characterized by a constant reduced scattering angle, τ = E cm θ cm. The only decomposition path induced by collision with Ar and He in this energy range is the reaction producing acetyl cation and methyl radical. The simplest interpretation for this relatively large energy deposition and moderate reduced scattering angle parameter is that collisional activation proceeds mainly by electronic excitation, followed by rapid evolution of products onto the ground electronic state hypersurface of the keto ion. However, a low energy tail observed in the fragment ion kinetic energy distributions indicates that a significant fraction of collisionally activated ions survive for 2 to 10 μs before undergoing dissociation. This delayed CID is postulated to follow the same mechanism responsible for the unimolecular metastable dissociation of enolic acetone cations.

Reactive scattering of crossed supersonic beams: Cl+ CH I I Cl+ CH I 2 2 2

The reaction Cl + CH2I→ICl + CH2I has been studied at a relative collision energy of 53·2kJ mol-1 using the crossed-beam technique. Angular and translational energy distributions of reactive scattered ICl products have been measured. The angular distribution is consistent with reaction occurring through a short-lived complex with lifetime about 0·8 of the rotational period. The kinetic energy distributions measured for this system show strong coupling to scattering angles. The forward- and backward-scattered products have higher translational energy than those that are sideways scattered. A phase-space model is developed which explains the experimental energy distributions well.

Ionisation and fragmentation of fullerene ions by electron impact

Absolute cross-sections for electron impact induced fragmentation, fragmentation-single-ionisation and fragmentation-double-ionisation were measured for the fullerene ions C+0 and C+58. For the fullerene ion C2+60, fragmentation-single-ionisation was measured with a loss of n C2 molecules (n = 1, 2, 3). The cross-sections were determined from the respective threshold up to an electron energy of 1 keV using the animated crossed-beams technique.

Observation of Interference in Charge Exchange Scattering inHe2++He+Collisions

We report the first observation of interference in charge exchange collisions between two ions. Employing the crossed-beams technique in conjunction with signal recovery methods, angular differential cross sections have been measured for charge transfer in He{sup 2+}+He {sup +} collisions at barycentric energies between 0.5 and 10.2keV. The oscillatory structure observed is in agreement with quantum calculations and can be interpreted in terms of interference between scattering into {ital gerade} and {ital ungerade} molecular states, which arise due to the identity of the nuclear charges. {copyright} {ital 1997} {ital The American Physical Society}

Measurement of absolute differential cross sections for the vibrational excitation of molecular nitrogen by electron impact in the2Πgshape resonance region

By means of a crossed-beam technique, we have conducted measurements of absolute differential cross sections for the vibrational excitation of the electronic ground state of ${\mathrm{N}}_{2}$ by electron impact. The ${}^{2}{\ensuremath{\Pi}}_{g}$ shape resonance region was treated, with the impact energies being those of the resonance's first four elastic scattering peaks---approximately 1.9, 2.1, 2.4, and 2.6 eV. The scattering angles covered were from 12$\ifmmode^\circ\else\textdegree\fi{}$ through 156$\ifmmode^\circ\else\textdegree\fi{}$, in 12$\ifmmode^\circ\else\textdegree\fi{}$ increments. Absolute integrated-excitation cross sections were computed from the differential cross sections. Our results show the expected strong $D$-wave character, and are compared with the recent results of others, both experimental and theoretical.

Electron capture and multiple ionization in collisions of fast and ions with gallium atoms

A crossed-beam technique incorporating time-of-flight analysis and coincidence counting of the collision products has been used to study electron capture, and pure ionization of ground-state gallium atoms in collisions with and ions. Cross sections leading to the formation of up to six-fold ionized gallium in electron capture and up to four-fold ionized gallium through pure ionization have been determined within the energy range 38 - 1440 keV . Pure ionization rather than electron capture is found to provide the main contribution to formation over most of the energy range considered. A notable feature of the electron capture data is the dominance of the transfer ionization contribution which is believed to be indicative of the important role of Auger ionization processes. Calculations based on an independent electron model of ionization (which have provided a good description of our previous measurements in Fe and Cu) are shown to fit our measured pure ionization cross sections in Ga very satisfactorily. In the case of electron capture, when the model is modified to take some account of Auger ionization, formation is described reasonably for impact but with only limited success for impact.

Total cross sections for electron capture in slow Arq+/N2 (q = 6, 8) collisions

Total charge transfer cross sections σ6, 6-i (i = 1-4) and σ8, 8-i (i = 1-6) have been measured in the energy range of 0.3-3.8 · q keV. Two different methods based on the crossed beam technique were used to determine the cross sections. One method is based on a retarding field analysis of the ion beam flux after the collision. The other method uses a coincidence technique in order to determine both, the dissociative part of the reaction and the total cross section. In the observed energy range all measured total cross sections have no significant energy dependence and both methods give the same results. The mean values of the cross sections are σ6, 5 = (32 ± 7) · 10−16 cm2, σ6, 4 = (8.4 ± 1.6) · 10−16 cm2, σ6, 3 = (8.4 ± 1.3) · 10−16 cm2 and σ6, 2 = (1.3 ± 0.2) · 10−16 cm2. In the system Ar8+/N2 the cross sections for the capture of i = 1, 2,..., 5 electrons amount to σ8, 8-i = (132, 38, 17, 13, 3.8) · 10−16 cm2, respectively.

Electron impact ionization of the hydrogen-like ions: B4+, C5+, N6+ and O7+

Absolute electron impact ionization cross sections of hydrogen-like ions B4+, C5+, N6+ and O7+ have been measured at electron energies from below threshold up to about 6 keV using the crossed-beams technique. The cross sections are in good agreement with several theories and are compared with other available cross section data. The scaling behaviour of the cross sections along the hydrogen isoelectronic sequence is discussed.

Account: Impulsive excitation in collision-induced dissociation reactions of polyatomic cations

Our understanding of the mechanisms of energy transfer and subsequent dissociations in the collision of ions with a neutral [collision-induced dissociation, (CID)] has been significantly advanced by recent dynamics studies using the crossed-beam tandem mass spectrometry technique. In this account two classifications of impulsive excitation, both involving significant angular deflection of product ions from the direction of travel of the parent ion, are described in microscopic detail. Recent experimental examples of these classes of impulsive mechanisms are described, along with their dynamics “signatures”. Connections are drawn between these models, beam experiments and conventional studies of CID reactions by means of mass-analyzed ion kinetic energy spectroscopy (MIKES).

Electron capture in collisions of He2+ ions with atomic and molecular oxygen

A crossed beam technique incorporating time-of-flight analysis and coincidence counting of the collision products has been used to measure cross sections for O+, O2+ and O2+ formation by electron capture in collisions of 5-67keV amu−1 He2+ ions with ground state O atoms and O2 molecules for the first time.

Dissociative electron attachment to using a high-resolution crossed-beams technique

Using a crossed electron - molecule beam ion source in combination with a quadrupole mass spectrometer we have studied the electron energy dependence of the dissociative attachment process (at electron energies from about 0 - 2 eV and with an energy resolution of about 60 meV) in a target gas range of about 300 - 420 K. Utilizing the improved energy resolution of the present experimental set-up it was possible to determine the accurate shape and magnitude of the cross section function in this low-energy range. This leads to the conclusion that close to 0 eV energy (below about 0.15 eV) the reaction proceeds via s-wave capture, whereas at higher energy (above 1 eV) autodetachment significantly determines the shape of the cross section. Moreover, the present measurements allow us to clarify previously reported differences in the absolute cross section, in the number of peaks and in the energy positions of these peaks. Finally, by analysing the measured strong temperature dependence of the cross section peak at about zero electron energy the activation barrier for this dissociative attachment was determined to be in good agreement with thermochemical data deduced from swarm experiments.

Absolute cross sections for electron-impact single ionization of Neq+ (q=2,4-6) ions.

Absolute total cross sections for electron-impact single ionization of ${\mathrm{Ne}}^{\mathit{q}+}$ (q=2,4--6) ions have been measured using a crossed-beams technique from below the ground-state ionization threshold to 800 eV with typical total uncertainties near the peak of the cross sections ranging from 9% for ${\mathrm{Ne}}^{2+}$ to 13% for ${\mathrm{Ne}}^{6+}$. Details of the apparatus and experimental procedures used in this study are presented along with a discussion of the experimental uncertainties. The measured cross sections for all four ions are dominated by direct ionization and are in excellent agreement with the Lotz semiempirical formula. Ionization rate coefficients and fitting parameters calculated from the measured cross sections are also reported. \textcopyright{} 1996 The American Physical Society.

Electron capture in collisions of 5 - 67 keV ions with oxygen atoms

A crossed beam technique incorporating time-of-flight analysis and coincidence counting of the collision products has been used to study and formation in both one- and two-electron capture by 5.3 - 66.7 keV ions in collisions with ground-state oxygen atoms. In the case of one-electron capture, cross sections for simple charge transfer, which are known to be dominated by the product channel at low energies, are found to attain a large peak value of about at about 20 keV . The low-energy trend of these cross sections is in satisfactory accord with the single value measured previously at 2 keV in this laboratory using a different technique. There have been no other experimental or theoretical studies of the - O collision system. Production of ions, mainly by transfer ionization involving one-electron capture, is surprisingly effective with the production ratio increasing from 0.23 to 0.64 over the energy range considered.

Electron-impact single ionization of low-charged molybdenum ions.

The electron-impact single ionization of low-charged Mo ions is studied both experimentally and theoretically. The absolute cross sections for ${\mathrm{Mo}}^{\mathit{q}+}$ ions in charge states q=1--8 are measured using a crossed-beam technique. Distorted-wave calculations are made including both direct ionization and excitation-autoionization contributions. For ions in charge states q=6--8, the measured cross sections are in good agreement with calculations involving only the lowest-energy metastable configuration. For ions in charge states q=3--8, the good agreement between experiment and theory confirms the dominant role played by excitation-autoionization contributions to the total ionization cross section. \textcopyright{} 1996 The American Physical Society.

Dissociative electron attachment to ozone using a high-resolution crossed beams technique

The production of O − and O 2 − by dissociative attachment to ozone has been studied as a function of electron energy (from about 0 up to about 9 eV) in a recently constructed high-resolution crossed beams apparatus combined with a quadrupole mass spectrometer. After absolute calibration of the ozone gas density and taking into account the different detection efficiency absolute attachment cross sections for both fragment anions could be deduced. The corresponding peak values are 2.8∗x10 −17 cm 2 at an electron energy of 1.33 eV and 1.83∗x10 −17 cm 2 at an electron energy of 1.05 eV for the O − and O 2 − production, respectively.