Capture collisions

Dugan and Magee (1967) and Dugan et al. (1968, 1969) have made extensive numerical calculations on the trajectories of ion-molecule collisions and defined capture collisions for polar molecules. Their major findings may be summarized as follows ... 

The phenomenon of the capture collision can best be understood by consideration of the effective radial potential of a collision, For a particular value of translational angular momentum L, this potential is... 

Capture Collisions for Rotating Molecules. Molecules will generally be rotating before a collision. The rotational motion of a dipolar molecule will be hindered by the field of the ion at separations such that the relation ... 

The cross section for capture collisions obtained by setting the right-hand side of (17) equal to is... 

A summary of the properties of rotating molecules is given in Table 112,14 jg noted that both and can be positive or negative. Thus the effective polarizability of a collision can be reduced to zero for certain states of rotation of a polar molecule for such states, capture collisions do not occur. 

The Langevin trajectories for ion-molecule collisions (described in the Introduction) have a very simple property they either lead to a capture collision or they do not. It is a simple all-or-nothing situation. The treatment leading to the analytical maximum cross section for a dipolar molecule assumed that these collisions have the same property it is also for an all-or-nothing situation. The calculated ion-dipolar molecule collisions do not have this property. There is no all-or-nothing answer to the question of capture in an ion-permanent-dipole collision for a fixed initial impact parameter. For... 

A reasonable definition of the capture collision cross section is... 

Fig. 7. Variation of ion velocity and polar molecule rotational energy during N0.2 -I- HCI single reflection capture collision.

Fig. 9. Variations of polar angle 0 for translational motion of Ar+ relative to CO molecule during multiple reflection capture collision.

Fig. 11. Variations of dipole moment vector and ion-dipole orientation angle during CHjCN-CHgCN+ multiple reflection capture collision with several turning points.

Fig. 13. Variations of azimuthal angle (p for translation motion of NO2+ relative to HCl molecule during multiple reflection capture collision.

Fig. 14. Variation of ion projections tracing translational motion in Ar+ + CO capture collision with multiple reflections.

Fig. 15. Variation of azimuthal angle

Figure 16 is a superposition of movie frames of a CHjCN capture collision involving only one reflection. It provides a correlated history of ion-dipole interaction. The results are not presented stroboscopically since a variable step size is used. The post reflection hindering of the CH3CN rotor at 5-15 A is demonstrated in the movie. It has been suggested that preferential orientation of the negative end of the dipole toward the positive ion will favor a specific chemical reaction. ... 

A more complete treatment of the classical dynamics of a system containing an ion and a rotating polar molecule involves the numerical solution of the equation of motion (trajectory calculation) by Dugan et al. [64—68]. In their treatment, a capture collision is defined by an ion trajectory that penetrates to within a certain value of r. Their results also show that the locking in of the dipole is not likely to occur because of the conservation of angular momentum. 

T. Aberg, A. Blomberg, J. Tiilkki and O. Goscinski Maximum Entropy Theory of Recoil-Charge Distributions in Election Capture Collisions Phys. Rev. Letters 52, 1207 (1984). 

J. V. Dugan, Jr. and J. L. Magee, Capture collisions between ions and polar molecules, J. Chem. Phys. 47, 3103-3112 (1967). 

From the conditions that a SV r) dr=0 and V r)=E r), at r=r, a capture collision cross section and collision rate constant are deduced by... 

Let us compute the transmittance of this slab on the basis of the following two physical models (1) Neutrons are not scattered in the slab but move straight through, unless they experience a capture collision, in which case they are removed from the beam. (2) Neutrons enter a diffusion process the instant they strike the slab. 

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