Supersonic beam experiments, dissociative

N2 dissociation on Fe crystal planes seems to be an example also of the presence of mixed adsorption channels, which has led to some confusion over the detailed nature of the potential energy surface for this system. Ertl et al. (1982) have claimed that there is a zero net barrier on Fe(lll), with adsorption dominated by precursor kinetics, whereas highly activated adsorption is measured in supersonic beam experiments (Rettner and Stein, 1987). This probably relates to the different regimes of measurement, Ertl using low gas temperatures, while Rettner and Stein varied the gas energy. 

Figure 7.2. The uptake of carbon by dissociative adsorption of methane on Ni(lll) follows f rst-order kinetics. The experiment involved dosing the surface with a supersonic beam of molecular methane at the indicated...

Alford J M, Weiss F D, Laaksonen R T and Smalley R E 1986 Dissociative chemisorption of H2 on niobium cluster ions. A supersonic cluster beam FT-ICR experiment J. Rhys. Chem. 90 4480... 

One issue that is particularly interesting for activated dissociation is the importance of translational vs. vibrational activation since this relates to the topology of the barrier location on the PES (see Section 2.3.1). In analyzing experiments, it has been traditional to define the vibrational efficacy rjv as in eq. (2.5). This analysis, however, assumes that A(v) is the same for all v and this may not be universally true. In this case, describing vibrational efficacy is more complicated. Very recently, experiments for CH4 dissociation on transition metals even combine supersonic nozzle molecular beams with laser state preparation techniques to probe the reactivity of specifically prepared vibration rotation states [115-118] (see Section 4.3.1.3). 

Figure 7 HREEL spectra recorded after O2 adsorption at different Ex on Ag(41 0) at T = 105 K. The supersonic molecular beam hits the surface at 0 = 31°, i.e. normally to the step heights, in all experiments. Only dissociative adsorption is observed at the lowest E as witnessed by the vibration at 40 meV. An additional adatom species (hoy = 32 meV) forms at slightly larger Ex and admolecules are stable for Ex above 0.30 eV.

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... 

The infrared photodissociation of weakly bound complexes has attracted much interest in recent years. In these experiments a vibrational mode of one molecular component is excited by an infrared photon. If the photon energy is larger than the binding energy of the complex, the complex predissociates. Typically, the clusters are prepared in a supersonic expansion and the dissociation is measured by monitoring the depletion of the molecular beam as a function of the laser frequency. The... 

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