Molecular beam studies

Ref. 205). The two mechanisms may sometimes be distinguished on the basis of the expected rate law (see Section XVni-8) one or the other may be ruled out if unreasonable adsorption entropies are implied (see Ref. 206). Molecular beam studies, which can determine the residence time of an adsorbed species, have permitted an experimental decision as to which type of mechanism applies (Langmuir-Hinshelwood in the case of CO + O2 on Pt(lll)—note Problem XVIII-26) [207,208]. 

Lee Y T 1987 Molecular beam studies of elementary chemical processes Science 236 793-8... 

Campbell E B and Hertel I V 1992 Molecular beam studies of fullerenes Carbon 8 1157-65... 

These can be determined experimentally to very high accuracy from the Stark effect and molecular beam studies. The experimental accuracy is far beyond the capabilities of ab initio studies. At the other extreme, the original route to these quantities was through studies of the dielectric polarization of species in solution, and there is currently interest in collision-induced dipole moments. In either case, the quantities deduced depend critically on the model used to interpret the experiment. 

Steady-state molecular beam studies of the reaction of methylacetylene on reduced Ti02 (001) surfaces were undertaken to determine whether this reaction could be performed catalytically under UHV conditions. A representative experiment is presented in Figure 1. Prior to each experiment, the surface was sputtered and annealed to a temperature between 400 K and 550 K surfaces prepared in this manner have the highest fraction of Ti(+2) sites (ca. 30% of all surface cations) of any surface we have been able to create by initial sputtering [3]. Thus these are the surfaces most active for cyclotrimerization in TPD experiments [1]. Steady-state production of trimethylbenzene (as indicated by the m/e 105 signal detected by the mass spectrometer) was characterized by behavior typical of more traditional catalysts a jump in activity upon initial exposure of the crystal to the molecular beam, followed by a decay to a lower, constant level of activity over a longer time scale. Experiments of up to 6 hours in duration showed... 

Campbell CT, Ertl G, Kuipers H, Segner J. 1981. A molecular beam study of the adsorption and desorption of oxygen from a Pt(lll) surface. Surf Sci 107 220-236. 

Zaera, F. (2002) Infrared and molecular beam studies of chemical reactions on solid surfaces , Int. Rev. Phys. Chem., 21, 433. 

Birks et al. reported chemiluminescence from the A3n0+ and B3 states of IF in the reaction of F2 with I2 and suggested that the reaction kinetics were consistent with a four-center reaction forming the products IF + IF [74], In a series of molecular beam studies, it was shown that the reaction actually forms a collision complex that decomposes to form two sets of products, IF + IF and I2F + F [75-77] ... 

It is now possible to design the experiments using molecular beams and laser techniques such that the initial vibrational, rotational, translational or electronic states of the reagent are selected or final states of products are specified. In contrast to the measurement of overall rate constants in a bulk kinetics experiment, state-to-state differential and integral cross sections can be measured for different initial states of reactants and final states of products in these sophisticated experiments. Molecular beam studies have become more common, lasers have been used to excite the reagent molecules and it has become possible to detect the product molecules by laser-induced fluorescence . These experimental studies have put forward a dramatic change in experimental study of chemical reactions at the molecular level and has culminated in what is now called state-to-state chemistry. 

The primary photochemical reaction for nitromethane in the gas phase is well supported by experiments to be the dissociation of the C—N bond (equation 98). The picosecond laser-induced fluorescence technique has shown that the ground state NO2 radical is formed in <5 ps with a quantum yield of 0.7 in 264-nm photolysis of nitromethane at low pressure120. The quantum yield of NO2 varies little with wavelength, but the small yields of the excited state NO2 radical increase significantly at 238 nm. In a crossed laser-molecular beam study of nitromethane, it was found that excitation of nitromethane at 266 nm did not yield dissociation products under collision-free conditions121. 

Based on the studies of alkane dissociation at high pressures, two conclusions were reached regarding the applicability of molecular beam studies for predicting high pressure behavior . First, the effects of vibr-... 

The exchange between the gas-phase and chemisorbed states of small molecules plays a vital role in such technologically important fields as heterogeneous catalysis and corrosion. The dynamics involved in these processes, however, are not currently well understood. Molecular-beam studies combined with classical trajectory calculations have proven to be a successful tool for understanding the underlying features of atomic-scale motion in the gas phase. The extension of these techniques to surfaces has also helped in elucidating the details of gas-surface reactions. 

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