Crossed molecular beam technique

REACTION DYNAMICS OF CARBON-CENTERED RADICALS IN EXTREME ENVIRONMENTS STUDIED BY THE CROSSED MOLECULAR BEAM TECHNIQUE... 

Which experimental approach can best reveal the chemical dynamics of carbon-centered radicals Recall that since the macroscopic alteration of combustion flames, atmospheres of planets and their moons, as well as of the interstellar medium consists of multiple elementary reactions that are a series of bimolecular encounters, a detailed understanding of the mechanisms involved at the most fundamental microscopic level is crucial. These are experiments under single collision conditions, in which particles of one supersonic beam are made to collide only with particles of a second beam. The crossed molecular beam technique represents the most versatile approach in the elucidation of the energetics and... 

Crossed molecular beam machines count among the experimental arrangements which allowed a significant breakthrough in reaction dynamics. A recent review by Casavecchia et al. shows how improvements in the crossed molecular beam technique made possible recent progresses in the understanding of gas-phase reaction dynamics [14]. 

The reactions of vdW molecules and clusters can be divided into intra- and intercluster processes, and further into neutral and ionic cluster reactions. The latter were recently reviewed by Mark and Castleman. Therefore the scope of this contribution will be limited to neutral species only. We distinguish between intra- and intercluster reactions. In intracluster processes reactions are induced inside a cluster, usually by light. Examples of such reactions are the reaction of excited mercury atoms with various molecules attached to them, reactions that follow photodissociation in the cluster, and charge transfers inside a large cluster. In intercluster reactions the cross molecular beam technique is usually applied in order to monitor scattered products and their internal energy. The intercluster reactions may be divided into three major categories recombination processes, vdW exchange reactions, and reactions of clusters with metal atoms. 

In this chapter, we have shown how the recent advances in the crossed molecular beam technique allow us to study complex polyatomic reactions of relevance in astrochemistry. The focus was on the CN radical reactions with simple alkynes, but the same approach has been also applied to the study of other CN radical reactions with unsaturated small organic molecules, such as ethylene, benzene, and allene, which are of relevance in astrochemistry as well [77,81,84]. 

Kaiser R.L, Balucani N. The formation of nitriles in hydrocarbon-rich atmospheres of planets and their satellites Laboratory investigations by the crossed molecular beam technique. Acc. Chem. Res. 2001, 34. 699-706. 

Experiments using the crossed molecular beams technique are yielding increasingly detailed information concerning the outcome of reactive and non-reactive molecular collisions.1,2 The theoretical description of such experiments may be broken up into three steps as follows ... 

The continuous nature of the emission is associated with the variable quantities of energy that are taken up by the translational motion of the molecules of stannic and stannous chloride which move off in opposite directions. Such a conclusion is obviously verifiable by the use of crossed molecular beam techniques (see Section 3). 

Interferometric measurements of i.r. emissions from vibrationally excited HF molecules have yielded evidence for reaction (1). Crossed molecular beam techniques were used to study the reaction between F2 and 12 above a threshold... 

Using the H-atom Rydberg tagging time-of-flight crossed molecular beam technique, Dr. Ren studied the reaction resonance and non-adiabatic effects at a full quantum resolved level in the F + H2 system. Through state-to-state resolved experiments, he provided the first conclusive evidence of reaction resonances in the F %,2) + H2 -> HF + H reaction. The dramatic difference between the dynamics for the F( P3/2) + H2(j = 0,1) reactions represents a textbook example of the role of reactant rotational level in resonance phenomena in this benchmark system. Dr. Ren has also carried out a very high-resolution experimental study on the dynamics of the isotope substituted reaction, F( P3/2) -I- HD -> HF -I- D, with spectroscopic accuracy (a few cm ). These findings provided a very clear physical picture for reaction resonances in this benchmark system, which has eluded us for more than 30 years. 

The nature of chemical reactions is the breaking of old bonds and the formation of new bonds. Molecular reaction dynamics is the study on how the old bonds are broken and how the new bonds are formed. In the past few decades, molecular reaction dynamics is an important field of physical chemistry and chemical physics. Its main task is to study elementary chemical reaction processes on the atomic scale and femtosecond (even attosecond) time scale. The in-depth study of this field offers important knowledge to atmospheric chemistry, interstellar chemistry, as well as combustion chemistry, and deepens our understanding of the essential nature of chemical reactions in nature. In the 1980s, the improvement of the crossed molecular beam technique [1] and femtosecond chemistry [2] enabled the molecular reaction dynamics to have a rapid development, and gradually mature. In the last ten years, the molecular reaction dynamics had a lot of progress and made a series of achievements, due to the many new technologies, the development, and innovation of new experimental methods, and advances in theory and computation [3]. 

Molecular Beam and Crossed Molecular Beam Techniques... 

Casavecchia P, Capozza G, Segoloni E et al (2005) Dynamics of the 0( P) -h C2H4 reaction Identification of five primary product channels (vinoxy, acetyl, methyl, methylene, and ketene) and branching ratios by the crossed molecular beam technique with soft electron ionization. J Phys Chem A 109 3527-3530... 

Using HRTOF crossed molecular beam technique, we studied the F( P) + D2 DF + D reaction systematically. The differential cross sections and the relative integral cross sections in this reaction were provided. We measured the F( P3/2)/ F ( Pi/2) r tio in the F atom beam and thus got the F( P3/2)/F ( Pi/2) reactivity ratio, which is in perfect agreement with the theoretical results. In Sect. 4.1 of this chapter, we review non-adiabatic efifects in the F -b H2 system the measurement of the F( P3/2)/F ( Pi/2) ratio in the F atom beam is described in Sects. 4.2, 4.3 presents the crossed molecular beam experimental results and discussion summary is given in the last section. 

B. Cross Section Measurements In recent years crossed molecular beam techniques have been employed by Lee and co-workers ( ) to study extensively Ihe reactions of fluorine atoms with several different olefins. Very high cross-sections were observed, consistent with the rapid thermal rate constants measured above. In these ejqieriments, multichannel decomposition is observed (i. e., loss of H or CH3 after F addition to the ff -bond). Since the C-F bond is stronger dian any other single bond with a carbon atom, C-F bond scission teck to the original reactants does not compete with the exothermic processes involving breakage of C-C and C-H bonds. A tabulation of AH values for addition reactions of thermal F to various olefins is included (Table II) ( ). Some hi ly endothermic pathways are listed for the purposes of comparison. The ei eri-mental measurements of D/S in this table confirm that the only radicals undergoing appreciable C-C bond rupture with carbene formation after thermal F addition are those that form CF2. The extraordinary stability of the CF2 radical makes C-C bond rupture exothermic for these radicals. 

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