Crossed beam apparatus

Turner et al. (23) have measured the cross-section for this reaction with a cross-beam apparatus using ion energies down to 4 e.v. These results are given in Table I. The agreement with the present results is gratifying in view of the uncertainties discussed above. 

Recent studies with a crossed-beam apparatus not only show that the products shown above are the correct ones, but that both the linear and cyclic isomers, each of which is a detected interstellar molecule, are formed.47 Crossed-beam studies also show that other reactions between C atoms and unsaturated hydrocarbons proceed to form similar products 48... 

Fig. 3.15. Schematic illustration of the crossed-beam apparatus developed by Hyder et al. (1986) for the measurement of positron elastic differential scattering cross sections. Reprinted from Physical Review Letters 57, Hyder et al, Positron differential elastic scattering cross section measurements for argon, 2252-2255, copyright 1986 by the American Physical Society.

In 1969 it was still possible to consider separately the results of experiments carried out with crossed molecular beams and spectroscopic measurements of the product states of bulb reactions. Not only had there been few attempts to fuse together these two techniques, but also the lists of reactions which, up to that time, had been studied by the two methods were almost mutually exclusive. One result of progress in the 1970s is that this clear distinction has now been removed. The operation of crossed-beam apparatus... 

Threshold energy for formation of HD + CDjBr Crossed-beam study of K + HF as a function of rotational state and translational energy Photoreactions leading to the production of laser snow studied in crossed-beam apparatus Study of CsH formation... 

Rutherford and Vroom studied A1+ collisions with O2 and N2 at ion energies ranging from 1 to 5000 eV in a crossed beam apparatus involving a modulated neutral beam. ° The aluminum ions were produced by surface ionization of AICI3 vapor on a hot tungsten filament and product ions were detected mass spectrometrically. In the AI+ + O2 collision system, the 02" " formation cross section was found to be at the detection limit of 0.01 A at 1 keV ion energy ( 115 km/s), after which it rose to values above 0.1 A at 5 keV. N2" formation in A1+ -f N2 collisions was only measurable above 1.5 keV ( 150 km/s). The dissociative charge transfer processes ... 

Process 30 has been studied in the gas phase using a crossed-beam apparatus [301], The reaction product has not been clearly identified as being BaF or BaF2, but its measured angular and velocity distributions reveal that the reaction proceeds via the formation of a long-lived BaSFe intermediate, which is believed to be the electron-transfer complex Ba -SFg. Hence the reaction can be tentatively in-... 

Fig. 8. Crossed beam apparatus of Herman et al. for the study of ion—molecule reactions. A, Ion source B, focusing and decelerating lenses C, molecular beam source D, collision region E, chopper wheel F, energy analyser G, mass spectrometer H, electron multiplier I, scattering chamber J, magnet. (From ref. 103.)...

Experimental Details. A view of the essential parts of the crossed-beam apparatus using short-lived radioisotope labeling and detection is shown in Figure 9 (23). An HAt beam is produced in the beam source (A) from the... 

Figure 9. View of the essential parts of the crossed beam apparatus using short-lived radioactive labeling and detection (23) Ay radioactive beam source By scrubber-furnace C, LN -cooled collimator D, shut-off plug Ey nozzle beam furnace and cryopump F, gate valve G, hodoscope H, LN -coohd beam trap 7, calibrated beam monitor /, silicon surface barrier detectors K, halogen crossed beam L, radioactive beam M, rotary feed-through used to close the source stopcock.

The first such crossed-beam apparatus was described by Turner et The machine was designed to measure angular distributions and differential cross sections, but not energy spectra. Since it has not been extensively used, it will not be described further here. 

A crossed-beam apparatus (EVA) used by the authors of this chapter includes the following features A mass-selected primary ion beam suf-... 

Fig. 4. Crossed-beam apparatus with velocity and angular resolution (EVA). ...

Although cross sections can be measured in crossed-beam apparatus, the density of target molecules is more difficult to establish than if a gas chamber is used. While this is a disadvantage of the technique, an apparatus such as EVA may readily be adapted to incorporate a gas chamber when precise cross-section measurements are needed. 

A common problem in crossed-beam apparatus is that reaction with the neutral gas may take place not only at the beam intersection, but elsewhere along the track of the ion beam. This difficulty is readily avoided by pulsing the neutral beam using a shutter of some kind. Products resulting from the beam crossing are thus similarly pulsed and may be distinguished from the dc background by a phase-sensitive amplifier. 

Figure 21,3 Schematic view of a differential crossed-beam apparatus. The two beams collide at 90° and the product detector rotates within the plane of the beams...

Figure 21,9 Universal crossed-beam apparatus with a rotatable electron-impact mass spectrometer, following the design of Lee et al. (1969). Adapted from Casavecchia, Rep. Prog. Phys., 2000, 63 355, with permission of lOP...

An example of the latter has been the study of the reactions Cl + RH HCl + R (R = aliphatic radical), carried out in the crossed-beam apparatus schematically shown in Figure 23.12 (Ahmed etal., 2000). 

Figure 23.12 Crossed-beam apparatus for investigating the dynamics of elementary reactions using imaging techniques. Adapted from Ahmed et at, Phys. Chem. Chem. Phys., 2000, 2 861, with permission of the PCCP Owner Societies...

Figure 24.1 shows a schematic layout of a crossed-beam apparatus in which the Na (FCH3) (n = 1 to 5) clusters were produced by the pick-up technique. For this, a hot effusive beam of Na atoms is crossed with a pulsed, cold supersonic beam of FCH3. 

In the crossed-beam geometry, the particles encounter each other only once. Alternatively, the decelerated beams can be loaded into a molecular synchrotron, located at the intersection point of the two molecular beams. In a synchrotron containing 20 counterpropagating packets, a packet that completed 100 round trips would have undergone 4000 encounters. We are currently constructing a molecular synchrotron as a collider for neutral polar molecules, which is designed to be combined with the crossed-beam apparatus. 

The details of the crossed-beam apparatus used in our experiment can be found in many earlier publications [17,18]. Briefly, the alkali dimer source consisted of a resistively heated molybdenum oven and nozzle assembly, with the temperatures of the nozzle and the oven being controlled independently by different heating elements. Sodium vapour carried by an inert gas, which was either He or Ne, expanded out of the 0.2 mm diameter nozzle to form a supersonic beam of Na/Na2/inert gas mixture. The Na2 concentration was about 5% molar fraction of the total sodium in the beam when He was used as carrier gas. The beam quality dropped severely when we seeded Na2 in Ne so the dimer intensity became much weaker. No substantial amount of trimers or larger clusters was detected under our experimental conditions. The Na2 beam was crossed at 90 by a neat oxygen supersonic beam in the main collision chamber under single collision conditions. The O2 source nozzle was heated to 473 K to prevent cluster formation. Both sources were doubly differentially pumped. The beams were skimmed and collimated to 2 FWHM in the collision chamber. Under these conditions, the collision energies for the reaction could be varied from 8 kcal/mol to 23 kcal/mol. 

Vestal ML, Blakley CR, Ryan PW, Futrell JH. New crossed-beam apparatus for the study of ion-molecule collision processes. Rev Sei Instrum. 1976 47 15-26. 

New data also exist on the reaction Li + HF(v=l j) -> LiF + H [8]. This reaction is of particular interest as ab initio surfaces exist [18,19,20] for which j-dependent cross sections were calculated [20,21]. The measurements were carried out in a crossed beam apparatus similar to the one shown in Fig. 1 except for the use of a pulsed chemical HF-laser and a pulsed HF-nozzle source. The results are displayed in Fig. 3f. As = 0.14 eV is only sli dy above dueshold the signal S(j) is essentially proportional to a(v=l,j). However, at = 0.32 eV, contributions to the signal arising fiom the v=0 reaction (c.f. Section 2) are certainly not negligible. Thus the observed decrease of S(j) might be caused by a(v=0,j) rising steeper with j than o(v=l,j) (for more details see Ref. 8). 

The laser-induced ffuorescence method was used to advantage to measure the BaCl distribution from Ba + CCI4. The experiments were done in a crossed-beam apparatus, although no angular measurements were made. This appears to be one of the more complete analysis of the fluorescence spectrum, although the same rotational temperature was still assigned to all V levels and no correlation between Ey and E was noted. The authors converted from BaCl number density to flux in assigning the distributions, which has not been done for the other entries in Table 2.11. The BaCl vibrational distribution extends from v = 25-58 with a sharp maximum at... 

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