4.1 Simple Collision Parameters (1)

1. Prof. Reinisch, EEAS 85.483/511

2. 4.1 Simple Collision Parameters (1) There are many different types of collisions taking place in a gas. They can be grouped into two classes, elastic and inelastic. Elastic Collisions: the particles conserve their masses, and the kinetic energy and momentum is conserved. Inelastic Collisions: kinetic energy can be transformed into rotational or vibrational energy, or excitation and ionization. Prof. Reinisch, EEAS 85.483/511

3. 4.1 Simple Collision Parameters (2) Collision Time and Frequency: Assume a molecule with radius r0 moves with velocity v through a cloud of electrons (F. 4.1). In the time t it sweeps out a cylindrical volume V = svt that was previously filled with nV electrons. Here s = p r02 is the collisional cross section of the molecule. If n is the number of electrons per unit volume, i.e., the electron number density there will be nV = nsvt collisions in the time t. The mean time t per collision is then t = t/ (nsvt) =1/nsv. The inverse is called the collision frequency c (Greek symbol, not velocity v): c  1/t = nsv. Mean Free Path Length: lmfp  vt = 1/sn Generally, the electrons have thermal (random) velocities and the relative velocities must be considered. Prof. Reinisch, EEAS 85.483/511

4. 4.2 Binary Elastic Collisions (1) The collision process between particles of species s and t is controlled by their relative velocities and the inter-particle force. We want to find the differential cross section sst(gst,q) required to calculate the Boltzmann collision integral (3.9). Here is the magnitude of the relative velocity, and q is the scattering angle. If the two colliding particles have comparable masses ms and mt, it is advantageous to perform the calculations in the center-of-mass system defined in equations (4.6) to (4.13). Using the laws of conservation of momentum and energy, it is easy to show that gst= gst‘ (gst before, gst‘after collision) . The direction of the relative velocity vector g changes changes at the collision, see Fig.4.3. We illustrate the collision process for the simple case of a Coulomb collision between an ion and and electron. Since the ion mass is so much larger than the electron mass it barely changes its velocity in response to the collision, i.e., the center-of-mass (CM) system is essentially anchored in the ion. Prof. Reinisch, EEAS 85.483/511

5. 4.2 Binary Elastic Collisions (2)Coulomb Collision The geometry of the electron-ion collision is shown in Fig. 4.4. The ion is at rest in the ion frame of reference. ‘Far away’ (before the collision) the electron has the momentum mev0. A line through the center of the ion parallel to v0 has the distance b0 from the electron when the electron is still far away. This distance is called the impact parameter. The Coulomb force is a so-called central force, i.e., it acts along the line connecting the two charges. The form of the Coulomb law suggests the use of polar coordinates r,f in the plane through the two particles (Fig. 4.4). Prof. Reinisch, EEAS 85.483/511

6. 4.2 Binary Elastic Collisions (3) Coulomb Collision (kinetic + potential energy)after = (kinetic + potential energy)before Prof. Reinisch, EEAS 85.483/511

7. 4.2 Binary Elastic Collisions (4) Coulomb Collision Prof. Reinisch, EEAS 85.483/511

8. 4.2 Binary Elastic Collisions (5)Coulomb Collision Prof. Reinisch, EEAS 85.483/511

9. 4.2 Binary Elastic Collisions (6)Coulomb Collision Prof. Reinisch, EEAS 85.483/511

10. Prof. Reinisch, EEAS 85.483/511