Single collision conditions

Spectroscopic detemiination of the HE rotational distribution is another story. In both the chemical laser and infrared chemiluminescence experiments, rotational relaxation due to collisions is faster or at least comparable to the time scale of the measurements, so that accurate detemiination of the nascent rotational distribution was not feasible. However, Nesbitt [40, 41] has recently carried out direct infrared absorption experiments on the HE product under single-collision conditions, thereby obtaining a fiill vibration-rotation distribution for the nascent products. 

The only previous estimate of the branching ratio under truly single-collision conditions is that carried out in one of these CMB studies Schmoltner et al.65 used a beam of lsO in order to be able to detect, in... 

The spectroscopy methods such as LIF and REMPI are utilized not only to detect the free radicals as discussed above, but also to directly measure the internal state distributions of the photoproducts in the photodissociation of free radicals. In this approach, the photochemistry is carried out in the free radical beam under single-collision conditions with well-defined... 

The precursor ion is mass-selected with Q and subjected to single-collision conditions, collision gases Ar or Xe, in the AC only quadrupole Q2. The CID-pro-duced ions are detected with Q3 (not shown in Figure 9). For additional details see experimental parts of papers.63-64... 

Under ideal circumstances, reactions under investigation are carried out under what is called "single-collision" conditions. These typically require low pressures, so that a product molecule bom in a particular stale is not modified by subsequent collisions before the time of interrogation Has expired. Very short laser pulses usually are used, thus making the time between excitation and interrogation less than 10 second. However, at this speed, higher pressures can be used so that collisions occurring between formation of a product and its detection and identification can he minimized or essentially eliminated. 

Single-collision conditions, that is, there is one and just one collision in the reaction zone defined as the zone where the beams cross, and no collisions prior to or after this collision. 

Counterparts under Gas Phase, Single-Collision Conditions... 

The molecular beam deflection method is shown schematically in Figure 3-17 (Buck et al. 1985). It is based on momentum transfer between clusters entrained in a molecular beam and rare gas atoms which are the constituents of a second molecular beam at 90° to the cluster beam. Collisions between the rare gas atoms and the clusters under single-collision conditions deflect a small percentage of the clusters from their original path. The maximum deflection angle depends on the mass of the cluster. For example, binary clusters may be deflected into a broad range of angles with a well defined upper limit set by the momentum conservation... 

Figure 3-17. Schematic of the molecular beam deflection method. Under single-collision conditions, collisions between clusters and He atoms deflect the clusters to angles whose maxima are determined by momentum conservation.

Visible chemiluminescence has been observed from Ba + N20 under single collision conditions [338, 342], although the nature of the excited state of BaO is uncertain. Similarities between the observed spectra from Ba + NzO and Ba + 03 were noted [338]. Higher pressure studies show emission characteristic of BaO (A S), but it is believed that this... 

Cross sections corresponding to single collision conditions should be analyzed. As noted above, this requires that cross sections be measured at several... 

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... 

FIGURE 11.10 Products formed in the reactions of ethynyl and cyano radicals with unsaturated hydrocarbon molecules under single collision conditions. The reaction of ethynyl radicals with benzene is currently being studied in our laboratory. 

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