Types of pairwise collisions

Both heavy (atoms, ions, molecules) and light (electrons, photons) particles can be involved in collisions. Polyatomic molecules have internal degrees of freedom (vibrational and rotational motion of atoms) and, in this sense, they have an internal structure. 

Let particle A exists in the quantum state i and has the velocity and particle B is in the quantum state j and has the velocity v . The processes of three types can occur at collisions of particles A and B 

Process 1 corresponds to elastic collisions in which the quantum states of particles A (i) and B (j) remain unchanged and only the vectors of the velocities of their motion change. Process 2 corresponds to inelastic collisions when the quantum states of particles A and B change along with the velocities. These processes include various forms of exchange of the energy of vibrational, rotational, and electronic motions. Process 3 corresponds to collisions accompanied by the redistribution of atoms to form new particles C and D. In this book, we consider mainly collisions of the last type, which are named chemical collisions or chemical reactions. 

Such physical magnitudes as the collision cross section and rate constant of the collision are quantitative characteristics of the collision process. Describing processes in experiments on scattering (molecular beams), researchers usually use the notion of collision cross section, whereas for collision processes in the bulk the notion rate constant of the process is used. First, let us introduce the notion collision cross section . 

The scheme of the experiment with molecular beams is presented in Fig. 2.1. Particles A and B with the velocities a and V can collide in the intraaction zone (hatched r ion in figure). The interaction products fly apart at differrait angles and arc detected by a detector, which can be replaced around the interaction zone. 

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