Peaking, forward-backward

Figure 19(a) shows the QM simulation of the differential cross-section (DCS) in the HF + D channel, over the same extended energy range as in Fig. 5. The agreement with experiment is seen to be qualitatively reasonable. The forward-backward peaking and direct reaction swathe observed in the experiment also occur in the QM calculation, although the relative magnitudes are not consistent. Thus fully quantitative agreement between QM calculations and experiment in all of the reaction attributes must await further refinements of the PES, and/or a more rigorous treatment of the open-shell character of the F(2P) atom.90...

In acetonitrile solution, the Cu Cu1 reduction of [CunL]2+ appears as a quasireversible process, giving rise to a forward-backward peak-system with AEp = 114 mV, at a scan rate of 0.1 V s l. 

We see that for each cnerg the total DCS has a roughly forward-backward symmetry which is a characteristic of a reaction with mi intermediate complex [2]. However the full symmetry is not obtained mid a slight preference for forward scattering is found. Then this insertion reaction is not purely statistical. An interesting point is related to the forward/backward peak ratio whicli is always larger than one for cill energies. 

An obvious difference between these two systems is that the former leads to a distribution which, while symmetric about the CM, is strongly forward-backward peaked (Fig. 21). On the other hand, the latter is more nearly isotropic (Fig. 22). This degree of forward-backward peaking is a direct indication of the angular momentum of the complex. Because of... 

The typical characteristic features of the forward/backward peak system for an uncompUcated reversible charge transfer are given by ... 

The aS3rrametric forward-backward peaking may be modelled by assuming that the angular distribution, 1(0), can be factored into a term with forward-backward symmetry and an "osculation" term ... 

The second row in Fig. 15 shows examples at the three selected photon energies of the C=0 15 difference spectra obtained for both enantiomers. After normalization by the mean spectrum the asymmetry factor F(54.7°) is plotted along the bottom row. After correction for the cos(54.7°) term arising from the specific experimental geometry the net forward-backward asymmetry, y, can be estimated to reach a peak 15% in the hv = 298.7-eV photoionization. 

Fig. 2a displays the ion time-of-flight (TOF) distribution obtained when (n) = 1.6 104 Xe clusters interacted with a Fourier Transform-limited 100 fs 800 nm, 1015 W cm 2 laser pulse. The TOF displays a number of peaks corresponding to ions up to Xe1,+. The peaks in the TOF are quite broad, and even display a double peak structure due to the fact that ions are emitted in forward-backward directions with respect to the detector. Both the charge state reached and the kinetic energy of the ions are signatures of collective effects in the cluster ionisation. For example, when only atoms were present in the atomic beam, the maximum charged state reached was 4+. 

In cyclic SWV (Fig. 7.25), the typical peak-shaped response is obtained in both the forward and reverse scans for any bulk concentrations of the ion. In contrast to the behavior at macro-interfaces, the forward and backward peaks are not symmetrical with respect to the potential axis, and the peak height of the reverse scan is dependent on the vertex potential. The position of the voltammograms is well described by the analytical solution (7.50) whenoswv < 0.1, with a difference between analytical and numerical results for the peak potentials of less than 8 mV at 7 298 K for typical SWV conditions sw = 10 — 50mV and AEs = 2 — 10mV. 

The influences of the pulse amplitude ( sw) and the potential step (A s) in SWV are shown in Fig. 7.26. Similarly to the case of macro-interfaces, the increase of sw gives rise to higher and broader peaks. Thus, for very large sw values (> 100 mV), a plateau rather than a peak is obtained, whereas for smaller pulse amplitudes (< 50 mV), the forward and backward peaks are well defined. 

When only one peak is obtained (A > —71.2mV), the determination of the formal potentials can be made from the peak current and peak potential. Indeed, the average of the forward and backward peak potentials corresponds to the value of the average formal potential, i.e.,... 

It is worth noting that Phg,a is determined by a single parameter, the dimensionless asymmetry factor (gx), that varies from isotropic (g = 0) to a narrow forward peak (gA = 1) or to a narrow backward peak (gA = -1) this parameter can be estimated employing the method described in Satuf et al. (2005). 

Pigure 9 shows an electron-ion covariance map obtained at the same time as Pig. 8b. Por convenience the time-averaged electron and ion TOP spectra are shown alongside the x and y axes. The electron TOP is virtually structureless, the slight forward-backward asymmetry and the peak at 292 ns being due to a small inhomogeneity in the extraction field. The forward electrons are unaffected and a kinetic energy scale is shown for both electrons and protons. In order to ensure that this lack of structure was not the result of poor instrumental resolution, an electron TOP spectrum of Xe was taken under identical conditions. A series of ATI peaks was observed in both the forward and backward direction, separated by about 2 eV. 

We also observe that the backward scattering produces primarily HF product in v=2, while the forward scattering peak corresponds primarily to HF products in v=3. This angular separation is virtually identical to what is predicted by the single-surface HSW calculations. 

---