Crossed Beam Technique Research Articles

Measurements of Cross Sections for Single and Double Ionization of Na+and K+by Electron Impact

Cross sections for single and double ionization of K + and Na + by electron impact have been measured as a function of electron energy from their respective threshold to approximately 1500 eV with the use of the electron-ion crossed-beam technique. The results for single ionization of Na + and K + are in good agreement with the previous crossed-beam data within the experimental uncertainties. Contributions from an autoionization and an L-shell direct ionization followed by autoionization (Auger effect) have been found in the results of single and double ionization cross sections for K + .

Total charge-transfer cross sections in collisions of Ca + ions with Mg and Sr atoms

The total charge-transfer cross sections in Ca + + Mg and Ca + + Sr collisions are investigated experimentally by the crossed-beam technique in the energy range 80–1000 eV.

Ionisation and charge transfer in collisions of H+and He2+with lithium

A crossed-beam coincidence technique has been used to determine cross sections for single and double ionisation of lithium by H+ and He2+ impact within the range 22-2100 keV amu-1. There have been no previous measurements and the results provide as assessment of theoretical predictions based on first Born, close-coupling and classical calculations. Estimates of the separate contributions to single ionisation from electrons in the L and K shells have been obtained from the measured cross sections. Cross sections for one-electron capture have also been determined together with cross sections for transfer ionisation.

Low-frequency density fluctuations in a tokamak plasma

CO2 laser scattering and correlation techniques are used to study low-frequency density fluctuations in plasmas in the Alcator C tokamak. Using crossed-beam correlation techniques, these fluctuations are found to have largest amplitudes near the plasma edge. In discharges with mean plasma densities n̄≳2×1014 cm−3, the normalized fluctuation amplitude is ñ/n≂0.5 and peaks at the limiter radius; however, at lower plasma densities, ñ/n peaks in the plasma interior at a normalized minor radius r/a≂0.75 (where a is the limiter radius). The radial and poloidal wave vectors of the fluctuations are comparable, with mean wavenumbers k̄∼20 cm−1. At any given k, the observed frequency spectrum is broad. For plasma densities n̄≲1.5×1014 cm−3, the poloidal group propagation velocity vθ of the fluctuations is measured to be approximately 2×105 cm/sec in the electron diamagnetic drift direction, while at plasma densities n̄≳2×1014 cm−3, vθ≂9×104 cm/sec in the ion diamagnetic drift direction. The nature of these fluctuations is also studied in discharges exhibiting a boundary-layer instability (i.e., ‘‘marfes’’).

An improved crossed-beams technique for the measurement of absolute cross sections for electron impact ionisations of ions and its application to Ar+ions

Based on the animated-beam method of Brouillard and co-workers (1983) an improved experimental technique has been developed to measure cross sections for electron impact ionisation of ions. In a crossed-beams arrangement the overlap of the electron and the ion beam is periodically varied by a mechanical displacement of the complete electron gun system. Simultaneously with the ionisation signal, the parent ion beam current and the velocity of the gun movement are registered. As a test of the new method cross sections for single ionisation of Ar+ ions are measured from threshold up to 850 eV electron energy. The results are in excellent agreement with the crossed-beams data of Woodruff et al.

Electron impact double ionisation of Ar+and Ar4+ions

Employing an improved crossed-beam technique described in the preceding paper, cross sections sigma q,q+2 for electron impact double ionisation of Arq+ ions (q=1 and q=4) were measured for electron energies from threshold to about 700 eV. The improved experimental accuracy allowed contributions of L-shell ionisation-autoionisation to sigma 1,3 to be identified for Ar+. With increasing q the relative importance of the L-shell contribution to sigma q,q+2 strongly increases. As a result the cross section sigma 4,6 for Ar4+ is by far dominated by the L-shell contribution at energies above 350 eV. Calculations on the basis of semiempirical and semiclassical theories are made to compare the relative strengths of direct double ionisation and inner-shell ionisation with subsequent electron emission.

Initial distribution of vibration of the OH radicals produced in the H + O 3 → OH(X 2Π 1/2,3/2) + O 2 reaction. Chemiluminescence by a crossed beam technique

The chemiluminescence in the H + O 3 → OH(X 2Π 1/2,3/2) + O 2 reaction was observed using a crossed beam technique. The initial population of the OH radicals in the vibrational state v (4 ⩽ v ⩽ 9) were found to be N(9) = 1.0, N(8) = 0.95 ± 0.1, N(7) = 0.9 ± 0.2, N(6) = 0.2 ± 0.1 and N(5) = N(4) = 0.2 ± 0.2. The total cross section of the formation of OH(4 ⩽ v ⩽ 9) was estimated to be of the order of 10 −2 nm 2. No chemiluminescence was observed in the reaction D + O 3 → OD(X 2Π 1/2,3/2) + O 2 under the present experimental conditions.

CROSSED-BEAM CORRELATION TECHNIQUE FOR TURBULENT-FLAMES MEASUREMENTS

Emission-absorption spectroscopy methods for flame diagnostics have mostly been limited to axisymmetrical laminar flames. The development of tunable lasers with extremely narrow bandwidth has made possible the determination of time-averaged mean values of temperature and species concentration in axisymmetrical turbulent flames. In this work, a new method is presented to allow the measurement of both time-averaged mean values and fluctuation intensities of temperature and species concentration in turbulent flames. With this method, turbulent flames -irrespective of their geometries- may be investigated. The possibilities of these measurements using the crossed-beam correlation technique are studied and presented. The correlation between the fluctuation values of received radiation density of two laser beams crossed at a certain point in the turbulent flame gives information about the mean and fluctuation components of the spectral absorption coefficient at that point. Measurement of the absorption coefficient leads to determination of temperature and species concentration. The fluctuation intensities are measured using crossed laser beams having the same wavelength, whereas time-averaged mean values are determined when the wavelengths of the laser beams are different.

Single and double ionisation of helium by H+, He2+and Li3+ions

A crossed-beam technique incorporating time-of-flight spectroscopy together with electron-ion and ion-ion coincidence counting of the collision products has been used to study the ionisation of He by H+, He2+ and Li3+ ions. Cross sections for single and double ionisation have been determined with high precision at energies within the range 50-2380 keV amu-1. Cross sections for single ionisation have been compared with the predictions of the first Born approximation, while cross sections for double ionisation have been considered in terms of the 'two-step' and 'shake-off' mechanisms. The authors have also measured the corresponding cross sections for one- and two-electron capture and for the transfer-double-ionisation process; the latter is shown to be the main He2+ production mechanism at the lower impact energies.

A measurement of the cross section for ionisation of helium by electron impact using a fast crossed beam technique

The cross section for single ionisation of ground state helium atoms by electron impact has been measured over the energy range from threshold to 750 eV using a crossed electron-fast atom beam technique. The absolute value of the measured cross section is estimated to be accurate to within about +or-6% at a 90% confidence level. Ionisation contributions from excited atoms are negligible. These data are in excellent agreement with those of Rapp and Englander-Golden (1965) who applied the electron beam-static-gas target technique. The mean of these two cross sections is recommended as the best data currently available for ionisation of helium. The experimental procedure is fully described and a detailed assessment is made of the experimental errors.

Single ionisation of multiply charged xenon ions by electron impact

Single-ionisation cross sections sigma q,q+1 for electrons incident on Xeq+ ions (q=1, 2, 3, 4) have been measured for electron energies up to 700 eV by employing a crossed beams technique. The results for Xe3+ ions agree with previous experimental data of Gregory et al. (1983). The obtained cross sections increasingly exceed distorted wave with exchange (DWX) calculations of Younger (1980) for direct ionisation by factors of up to about five when the charge state of the parent ions is increased from q=1 to q=4. The cross sections sigma 3,4 and sigma 4,5 show an unexpected enhancement at electron energies around 100 eV. These features are attributed to indirect ionisation processes, which probably also cause another unusual result: for the first time the authors found a case where ionisation of an atomic ion becomes easier when its charge state is increased, namely, for electron energies above 250 eV sigma 4,5 is greater than sigma 3,4.

Total charge-transfer cross sections in collisions of Mg + ions with Sr and Ba atoms

The total charge-transfer cross sections in Mg + + Sr and Mg + + Ba collisions are investigated experimentally by the crossed-beam technique over the energy range of 50–1000 eV. Regular oscillations on the both cross sections are observed.

Crossed-beam coincidence studies of ionisation and electron capture in collisions of multiply charged ions with hydrogen atoms

The authors present further measurements of cross sections for ionisation of atomic hydrogen using a crossed-beam coincidence technique previously developed in this laboratory. Cross sections for ionisation by 400-9756 keV Ar(3-9)+ and 1000-4770 keV C(2-6)+ and O(2-6)+ ions have been determined and considered in terms of cross section scaling predictions. The authors have adapted the experimental technique to permit studies of one-electron capture by multiply charged ions; previous measurements have relied entirely on the furnace target method. The effectiveness of this alternative approach has been demonstrated by measurements of the cross section for electron capture by 140-4000 keV Ar(3-6)+ ions. The cross sections for the combined processes of charge transfer and ionisation have been considered in terms of classical scaling rules.

Ionisation of atomic hydrogen by 4.8 MeV C6+ions

A crossed-beam coincidence counting technique incorporating time-of-flight spectroscopy, previously developed in the authors' laboratory, has been used to determine the cross section for ionisation of H(1s) atoms by C6+ ions at 400 keV amu-1. The result is compared with previous measurements for bare nuclei projectiles and with recent theoretical predictions.

Temporal and interference fringe analysis of TEM_01* laser modes

The properties of TEM01* doughnut modes have been examined by frequency analysis and by two-beam interference. Interpretation in terms of an evolving helix, made up to two orthogonal TEM01 modes of different frequency, is supported by computer simulation of fringe patterns. These patterns are shown to correspond closely with photographic recordings; the implications of the phenomena for the crossed-beam technique in laser velocimetry are outlined. Finally, the possibility of developing a beam of pure helical cophasal surface is discussed and its interference patterns analyzed.

Measured electron-impact ionization of Be-like ions:B+,C2+,N3+, andO4+

Electron-impact-ionization cross sections have been measured from threshold to about 20 times threshold for Be-like ions B/sup +/, C/sup 2 +/, N/sup 3 +/, and O/sup 4 +/ with the use of the crossed-beams technique. The ion beams consist of mixtures of ions in the 2s/sup 2/ /sup 1/S ground state and the 2s2p /sup 3/P/sub 0,1,2/ metastable states. For B/sup +/ and C/sup 2 +/ the metastable fractions could be changed, allowing estimates of ground-state cross sections, while for N/sup 3 +/ and O/sup 4 +/ the metastables appear to dominate the beams and the fractions could not be changed. The measured cross sections are compared with a variety of predictions. Distorted-wave calculations compare most favorably, but discrepancies up to 40% are found. The metastable-state--ground-state mixture complicates the comparisons for these beam experiments and is probably an issue for all environments where light (Z< or =15) Be-like ions occur.

Collisional angular-momentum mixing of Na Rydberg states with Xe

The collisional angular-momentum mixing of the $\mathrm{Na}\mathrm{nd}$ Rydberg states into states of higher angular momentum with Xe as the collisional partner has been studied in zero field and in the presence of a small electric field. The collisional $l$-mixing cross section in zero field has been measured for $9\ensuremath{\le}n\ensuremath{\le}23$ using a vapor-cell-fluorescence method and a crossed-beam technique with a selective-field-ionization (SFI) scheme as the state detector. Using the $|{m}_{l}|$ selectivity of the SFI, we have also investigated the final-$|{m}_{l}|$-state distribution of the collisionally populated states of $l&gt;2$. Our measurements suggest that the collisionally populated states have predominantly low $|{m}_{l}|$ values ($|{m}_{l}|\ensuremath{\le}2$). An application of a low electric field (\ensuremath{\sim} 50 V/cm) substantially reduces the collisional population of the low-$|{m}_{l}|$ states, thus reducing the total cross section as well.

Ionisation of H2by fast protons and multiply charged ions of He, Li, C, N and O

A crossed-beam coincidence technique, previously developed in this laboratory and used in measurements of the ionisation of H by protons and multiply charged ions, has now been applied to studies of non-dissociative and dissociative ionisation of H2. Cross sections sigma ni for nondissociative ionisation and cross sections sigma di for the sum of the dissociative ionisation channels have been separately determined for 38-1500 keV amu-1 H+, 31-550 keV amu-1 He2+, 30-388 keV amu-1 Liq+ with 1<or=q<or=3, 16-193 keV amu-1 Cq+ with 2<or=q<or=4, 14-166 keV amu-1, Nq+ with 2<or=q<or=5 and 12-145 keV amu-1 Oq+ with 2<or=q<or=5. Measured cross sections are dependent on and increase with q. Peak values of sigma ni and sigma di shift to higher velocities as q increases. Peak values of sigma i= sigma ni+ sigma di contain contributions from sigma di ranging from about 10% for H+ to about 30% for N5+ and O5+ impact. In all cases the contribution of sigma di to sigma i becomes dominant at low energies. In a few selected cases a fast-ion coincidence technique is used to show that the main contribution to these large values of sigma di arises from the transfer ionisation process. The ratios R= sigma i(H)/ sigma i(H2) of the previously measured cross sections in H to the present values in H2 are obtained. At the high-velocity limits, R is decreasing in all cases and has values ranging from about 0.45 for 1500 keV amu-1 H+ to about 0.61 for 145 keV amu-1 O5+.

Charge-transfer in collisions of Mg + ions with Ca atoms

The total charge-transfer cross section in Mg +Ca collisions is investigated experimentally by the crossed-beam technique over the energy range of 70–1000 eV.

Chemical reactions of atoms with molecules in colliding beams

Abstract In investigations into the nature of chemical reaction the emphasis has changed from experiments which determine bulk properties to those at the microscopic level. The role of the crossed beam approach in this fundamental study is discussed. The methods available for the production and detection of atomic and molecular beams, including analysis or selection of both the translational energy and internal state distributions, are considered. Selected experiments are used to highlight the development of the crossed beam techniques and to illustrate the type and quality of the information that can be deduced about reaction dynamics. This leads to the formulation of reaction models. The achievements of theoretical methods and their ability to predict reaction behaviour are reviewed. Finally, current developments in experimental techniques using laser technology are presented.