Beam sources dissociative

The results for Pt(5 3 3) (Figs. 11 and 26) are consistent with the increased sticking probability observed for a Maxwellian source of H2 on Pt(9 9 7) over Pt(l 1 1) [81]. It also provides direct evidence for an additional channel to dissociative adsorption through step sites which was invoked to explain the enhanced rate of H2 + D2 exchange reaction at Pt(3 3 2) over that observed on Pt(l 1 1) surfaces investigated using a Maxwellian beam source [82]. 

The generation of reactive species by a fast pre-reaction is a well established process in kinetics, particularly in flow systems, although this technique is now being used in molecular beam sources [see (f), below]. As atoms are commonly generated by thermal or microwave dissociation (see above), reactions are mainly used to produce radicals. However, some atoms may be produced by reaction as a more convenient alternative to direct vaporisation or dissociation. Scandium and yttrium atoms have been generated [25] by the reaction... 

The substitution reactionsof fluorine atoms with unsaturated hydrocarbons have been investigated in a comprehensive range of experiments.153-159. A velocity selected beam of F atoms, formed by thermal dissociation of F2 in a nickel oven, is intersected by a hydrocarbon beam from a supersonic nozzle beam source. Angular153 and velocity154 distributions of C2H3F product were measured for the displacement of a hydrogen atom from ethylene... 

Much has been written about the high reactivity of steps at metal surfaces and there are several examples in chemisorption. For example, the 110 plane of W has a low sticking probability ( 3 x 10 3) for nitrogen chemisorption at 300 K, but on the stepped 320 plane, with 110 terraces, s0 = 0.70. On the other hand, a singular, step-free W 100 surface is also very reactive (s0 = 0.59) and it has been suggested that the reactivity of stepped planes such as 320 is related to the structural similarity of the steps to the 100 plane itself [47]. A further dramatic illustration of the dependence of s0 on the presence of steps is demonstrated by the work of Salmeron et al. [353] on the stepped Pt surface. Using a molecular beam source, they demonstrated that the reactivity for dissociative chemisorption of H2 (as indicated by the formation of HD from a mixture of H2 and D2) was 7 times higher when the beam was directed at the steps than when it was directed over the steps, as shown in Fig. 20. 

Thus, electron impact can excite any dissociative state of a molecule and reduce it to fragments, and this is a key mechanism by which radicals and molecular fragments are produced in situations ranging from planetary atmospheres to molecular beam sources from experiments in molecular collisions. Another important process is electron-impact ionization, because this is the mechanism by which ions are created in any gas discharge system, and because it can produce any ionic state of the target. 

Thennal dissociation is not suitable for the generation of beams of oxygen atoms, and RF [18] and microwave [19] discharges have been employed in this case. The first excited electronic state, 0( D), has a different spin multiplicity than the ground 0( P) state and is electronically metastable. The collision dynamics of this very reactive state have also been studied in crossed-beam reactions with a RF discharge source which has been... 

Formation of normal ions in an ion source. In this example, some initially formed ions (m,+) dissociate (fragment) to give smaller ions (mj ) and a neutral particle (iio). Unchanged ions (m +) and the fragment ions (mj" ) are drawn out of the source as beams moving with velocities v, V2, respectively. 

The use of inadiation or electron bombardment offers an alternative approach to molecular dissociation to the use of elevated temperamres, and offers a number of practical advantages. Intensive sources of radiation in the visible and near-visible are produced by flash photolysis, in which a bank of electrical capacitors is discharged tlrrough an inert gas such as ktypton to produce up to 10 joule for a period of about 10 " s, or by the use of high power laser beams (Eastham, 1986 (loc.cit.)). A more sustainable source of radiation is obtained from electrical discharge devices usually incorporating... 

Merged-beam measurements 23-26 have consistently shown that the measured recombination cross section depends on conditions in the ion source. The authors have ascribed the effect to differing vibrational distributions. In one of the later measurements,16 the Hj vibrational state was inferred from the threshold energy for electron-ion dissociative excitation,... 

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