Crossed molecular beam method experiments

In this chapter, the information available from crossed beam experiments on the reactions of CN radicals with simple unsaturated hydrocarbons leading to the formation of unsaturated nitriles will be reviewed after a brief introduction on the crossed molecular beam method. The results presented here are part of a systematic study which has been undertaken to characterize the reactions of CN radicals with small unsaturated organic molecules at the microscopic level [60,75-84]. 

Another popular method for studying combustion species is via crossed molecular beams. In this technique, the reactant molecules of interest are propelled as beams toward an intersection where their molecular collisions bring about reactions. For a more complete discussion of crossed molecular beam experiments, see reference 59. 

In this chapter we have discussed the successful implementation in our laboratory, for the first time, of the soft (i.e. low energy) electron-impact ionization method for product detection in crossed molecular beams reactive scattering experiments with mass spectrometric detection. Analogous to the approach of soft photoionization by tunable VUV synchrotron radiation,... 

One of the first approaches to study the microscopic kinetics i.e. state-to-state cross sections and reaction probabilities of a chemical reaction was the crossed molecular beam experiments. The principle of the method consists of intersecting two beams of the reactant molecules in a well-defined scattering volume and catching the product molecules in a suitable detector (Fig. 9.33). 

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... 

In light of previous experimental and theoretical work on the F f H2 reaction, it can be seen why an experisient of this complexity is necessary in order to observe dynamic resonances in this reaction. The energetics for this reaction and its isotopic variants are displayed in Figure 1. Chemical laser (11) and infrared chemiluminescence (12) studies have shown that the HF product vibrational distribution is hi ly inverted, with most of the population in v=2 and v°°3. A previous crossed molecular beam study of the F + D2 reaction showed predominantly back-scattered DF product (13). These observations were combined with the temperature dependence of the rate constants from an early kinetics experiment (14) in the derivation of the semiempirical Muckerman 5 (M5) potential energy surface (15) using classical trajectory methods. Although an ab initio surface has been calculated (16), H5 has been the most widely used surface for the F H2 reaction over the last several years. 

A number of these halogen reactions have been studied in crossed molecular beam experiments, and the availability of direct measurements of overall rate constants from the discharge-flow method should be valuable in understanding the dynamics of the colhsion processes involved. Beam studies of Cl -1- Bra, > the most fully investigated of the above reactions, confirm a high integral cross section for this reaction approaching the bimolecular collision frequency. It does not appear possible to reconcile earlier, indirect, much lower values for the rate constants of Cl - - Bra, Cl -f- BrCl, Cl -b ICl, Cl - - CINO, based on analysis of photochemical reactions, with the direct measurements from resonance fluorescence. - ... 

We highlight some recent work from our laboratory on reactions of atoms and radicals with simple molecules by the crossed molecular beam scattering method with mass-spectrometric detection. Emphasis is on three-atom (Cl + H2) and four-atom (OH + H2 and OH + CO) systems for which the interplay between experiment and theory is the strongest and the most detailed. Reactive differential cross sections are presented and compared with the results of quasiclassical and quantum mechanical scattering calculations on ab initio potential energy surfaces in an effort to assess the status of theory versus experiment. 

New UCLA experiments [62] represent a test of this concept, and demonstrate by a purely physical method the degree of orientation of an "oriented molecule beam", hitherto inferred from crossed molecular beam reactive asymmetry experiments. (The only related observations are those of Ref. 63 in which a partially state-selected and oriented beam of CH3I was subjected to UV photoionization and the anisotropy of the photoelectron angular distribution measured.)... 

Meth. LC QB OS ED MW MB EPR DR method of measurement for the values in this line of the table level crossing spectroscopy quantum beat spectroscopy optical spectroscopy electric deflection method microwave spectroscopy molecular beam resonance experiment-electric or magnetic resonance electron paramagnetic resonance double resonance experiments (MODR, RF/DR)... 

For reaction (5.18) it has been shown from a crossed molecular beam experiment that the main process is CH2OH + H, and the process of CH3O + H is not important (Lin et al. 1998). Also for the physical deactivation pathway (5.20), Wine and Ravishankara (1982) and Takahashi et al. (1996) reported that the ratio is less than a few%, while the recent high precision experiment of Vranckx et al. (2008b) by using the chemiluminescence method showed that it is 0.2 0.3 % and that it is negligible as an atmospheric reaction. 

That there are differences in reactivity for different orientations of reactants in collisions has long been a basic idea in chemistry. The actual effects of orientation on reactivity were first explicitly measured in crossed molecular beam experiments in 1966, and since then there have been continuous efforts to explore these effects in various systems by both experimental and theoretical methods. An example of the use of classical trajectories to investigate the influence of orientation effects in reactions is a recent study of the reaction ... 

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