Collision energy dependence

Indeed, several identifiable resonance fingerprints in experimental observables were found.26-31 Concurrent theoretical simulations and analyses not only confirmed the experimental conjectures, but also provided deeper insights into the nature of this resonance state. For the integral cross-sections, a distinct step for Ec < 1 kcal/mol was observed in the reactive excitation function (i.e. the translation energy dependence of the reactive cross-section) for the HF+D product channel, whereas it is totally absent for the other DF+H product channel.26 Anomalous collision energy dependence of the HF vibration branching was also observed.28 For Ec < 1 kcal/mol more than 90% of the HF products are populated in the v = 2 state. However, as the energetic threshold for the formation of HF( / = 3) from... 

Fig. 13. The collision energy dependence of the Mg+ + 02- MgC" " + O reaction cross section. Figure printed with permission from Elsevier Science Publishers. ...

Fig. 18. Collision energy dependence of the near threshold ionization and excitation cross sections in K + O2 collisions. The K+ and e cross sections are those of Moutinho et The excitation cross sections are those of Kempter et al. (higher dotted curve) and Lacman and Herschbach (dot-dashed curve).The latter were reported as relative cross sections, while the former provided an estimated absolute cross section.

The following picture emerges from their illuminating work [131], The initial electron transfer forms the anion O2 with the bond length of the neutral molecule, shorter than that of the anion. Hence the newly formed O2 is compressed and starts to oscillate with a period of 3 x 10 s. At the chosen collision energy, depending on the impact parameter, the collision time varies between 0.5 and 3 times the vibration period. As a result, there is a family of impact parameters where OJ is back in position to return the captured electron on the exit channel of the collision, but there is another family also, for which the electron return is impossible. This leads to oscillations in the angular distribution of the Cs atoms, which can be related to the vibrational period of O. ... 

The collision fraction law fails in competitive q>eriments in which the reactive collision energy probability density distribution is strongly dependent on sample composition. Such behavior arises when the excitation functions for nonthermal reactions exhibit dissimilar collision energy dependences (7,9,10,19-26). Despite diis caveat, the simple collision fraction rule exhibits good utility for many types of competitive systems (9,23). 

Fig. 3.4 Collision energy-dependent DCS for the forward-scattering HF(v = 2) product of the reaction F -h H2(/ = 0). From [24], reprinted with permission from AAAS...

In order to look for more strong experimental evidences in the F -F H2(/ = 1) reaction resonances, we have measured the collision energy dependent forwardscattering signal (see Fig. 3.17) of the HF(v = 2) product from the F -F H2(/ = 1) reaction in the collision energy range of 0.13-0.31 kcal/mol. The main peak of the distribution is located at the colhsion energy of 0.18 kcal/mol. Full quantum... 

Collision Energy Dependence of the Backward-Scattering Signal... 

Fig. 3.29 Collision energy-dependent DCS for the backward-scattering HF(v = 2) products summed over at / = 0-3. The solid circles are the experimental data, and the solid lines are the calculated theoretical results based on the XXZ-PES and the new CCSD(T)-PES. Reprinted from [34]. 2008, National Academy of Sciences, USA...

The new PES can be a good description for the F -b HD, but is it accurate for the F -b H2 reaction The collision energy dependence for the forward-scattering HF(v = 2) product obtained on the new PES agrees with the experimental results very well, with a clear narrow peak predicted at 0.52 kcal/mol (see Fig. 3.34). The degree of the agreement on the DCSs between experiment and theory is also remarkable. 

Using collision energy dependence of backward scattering signal, collision energy dependence of F( P3/2) -I- — 0) reaction relative integral cross section... 

Fig. 4.7 Collision energy-dependent DCS for the backward scattering DF(v = 3) products summed over / = 0-7...

Fig. 4.8 a Collision energy dependence of the overall integral reactive cross sections, summed over product vibrational and rotational levels, for the F/F + T>2(j = 0) reactions for 0.25 < Ec < 1.2 kcal/ mol b the ratio of the cross sections shown in panel (a). The solid curves are the results of our theoretical calculations, whereas the points indicate the experimental results. The error bars indicate the range of measurement errors in the experiment. From [25], reprinted with permission from AAAS... 

Cheng, X.H., Wu, Z.C., Fenselau, C. (1993) Collision Energy-dependence of Proton-bound Dimer Dissociation - Entropy Effects, Proton Affinities, and Intramolecular Hydrogen-bonding in Protonated Peptides. J. Am. Chem. Soc. 115 4844-4848. 

At energies above the onset of the -wave regime, cross-sections will be dominated by contributions from nonzero angular momentum partial waves. If the interaction potential includes an attractive part, the effective potentials for nonzero angular momentum partial waves may possess centrifugal barriers that introduce shape resonances in the collision energy dependence of the cross-section. This is illustrated in Figure 3.4 for the vibrational relaxation of CO(v = 1, y = 0) in collisions with " He atoms. 

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