Full-collision process

When facing a full collision process, the existence of such a partial electron transfer state is elusive and usually difficult to evidence experimentally. A major difficulty comes from its transient character. A convenient approach to stabilize the electron transfer between two encounters is to consider a complex formed with both of them. This was the case with the Au (H20) complex mentioned above. This is also the case with the neutral van der Waals complexes considered hereafter. In this case, the characterization of the charge-transfer state requires a careful experimental investigation. We review here how spectroscopy of these clusters can characterize partial electron transfers. 

In this section we will provide the formalism for an extension of the Born-Oppenheimer adiabatic approximation that deals with unstable autoioniz-ing molecules and takes into account the coupling between electronic and nuclear motion. The new formalism will be applied in two cases (i) full collision process of vibrational excitation of H2 molecule by electron impact [8], (ii) half collision process of interatomic Coulombic decay of electronically excited Ne cationic dimer [9,10]. 

If a particle A must know B s total information content before colliding, the collision process must be delayed until A has full access to that information. However, such a delay is consistent neither with classical nor quantum mechanics, Minsky instead suggests that the collision proceeds immediately, but with the particles both working with less than all the information that is classically required i.e, the incoming particles momenta are estimated. Outgoing momenta are determined via conventional classical rules, but, because of the estimation errors, each scattered particle leaves behind a receipt recording how much momentum was really taken away in the process. Receipts not only mark prospective event-locations at which future collisions might take place, but harbor information that can be used to estimate new real momenta. 

Takatsuka and Gordon (21a) have developed a "full collision" formulation of photodissociation which describes a multichannel process on the repulsive surface for both direct and indirect events. The scattering wavefunctions that are used to generate the T-matrix and the FC overlaps are not zeroth-order uncoupled functions, but solutions of the coupled-channel problem. 

The full collision of metastable Xe(6s, 3P2) atoms with BrCCl3 favors the formation of XeBr(B, C) rather than XeCl by a ratio of 2 1 (Setser and Qim 1991). This behavior is expected since in the ground state of BrCCl3 the electron is more likely to be located on the C Br antibonding orbital. In this process, the reaction starts on the covalent Xe -RX potential and evolves to the Xe+ RX potential, thus leading to XeX(B, C) products. 

Thus, the polarization aspects of such a wide class of photoprocesses, as discussed in the present section, namely photodissociation and photoionization, make it possible to obtain information both on the stereodynamics of the process and on the properties (for instance, symmetry types) of the states through which the transition takes place. It ought to be mentioned that photodissociation can be considered not only as a reaction of a photon with a molecule, but as a halfcollision , in which only the second stage of a collision is present, namely the departure of the products without their previous approach. In the following section we will dwell on the polarization of molecules in full collision, both reactive and non-reactive. 

Since the homogeneous width y of the Lamb-dip profile increases with pressure p, the maximum allowed deflection angle e in (8.1) also increases with p. A comparison of pressure-induced effects on the kernel and on the background profile of the Lamb dips and on the Doppler profile therefore yields more detailed information on the collision processes. Velocity-selective optical pumping allows the measurement of the shape of velocity-changing coUisional line kernels over the full thermal range of velocity changes [979]. 

Figure 22.5 shows a pictorial representation of the so-called two-vector correlation, both in photodissociation (half-collisions) and in atom-exchange reactions (full collisions). The important point to consider is that photodissociation is an anisotropic process in which the polarization of the electric field Sp of the photolysis laser defines a direction with respect to which the vector describing both products and parent molecule can be correlated. As a consequence, one can measure and analyse the correlation between the parent transition dipole moment fi and the recoil photofragment velocity vector, i.e. the v correlation. Thus, the angular distribution of the photofragments I 6) can be described in the form (Zare, 1972)... 

Ultrashort pulses are able to prepare localized wave packets which (for short times) move in the average classically. This motion clearly defines a reaction path for the case that the reaction starts right at the transition state and the molecular fragments evolve into the arrangement channels. In this sense the laser induced process is a half collision , since the first part of a full collision where the atomic and molecular species approach each other is missing. 

Fig. 18.49 Off-centre collision process at different time moments, (6, S, particle orientation) = (90°, 20 240,1), (a) Full penetration, B = 0.46. (b) Partial penetration and ejection, B = 0.2...

The preferable theoretical tools for the description of dynamical processes in systems of a few atoms are certainly quantum mechanical calculations. There is a large arsenal of powerful, well established methods for quantum mechanical computations of processes such as photoexcitation, photodissociation, inelastic scattering and reactive collisions for systems having, in the present state-of-the-art, up to three or four atoms, typically. " Both time-dependent and time-independent numerically exact algorithms are available for many of the processes, so in cases where potential surfaces of good accuracy are available, excellent quantitative agreement with experiment is generally obtained. In addition to the full quantum-mechanical methods, sophisticated semiclassical approximations have been developed that for many cases are essentially of near-quantitative accuracy and certainly at a level sufficient for the interpretation of most experiments.These methods also are com-... 

Recently, we have developed a full theoretical treatment of electron capture processes involving an ab initio molecular calculation of the potential energy curves and of the radial and rotational couplings followed, according to the collision energy range concerned, by a semi-classical [21-23] orquantal [24] collision treatment. 

Optimization of conditions was effected following a two-step process. In the first step, a full-scan spectrum was obtained for each of the steroids. Product-ion spectra at five different collision energies (0.5,1,1.5,2, and 2.5 V) were then obtained for the main precursor ions of the full-scan spectra. Through these studies, candidates were chosen for both precursor and product ions in the MS/MS mode. Once all transitions had been chosen for the MS/MS mode, the final tuning process was carried out... 

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