Collision cross-sections Penning ionization

Ionization in Collisions of Heavy Particles. Estimate the adiabatic Massey parameter and ionization probability and cross section for ionization of an Ar atom at rest in collision with an Ar+ ion having kinetic energy twice that of the ionization potential. Compare the ionization probability in this case with the one for the Penning ionization. 

The effective cross section s for ionizing collisions depends on the type of gas. According to (2.25), the discharge current i is a function of the number particle density ng, as in a Penning gauge, and it can be used as a measure of the pressure in the range from 10 to 10 mbar. At lower pressures the measurements are not reproducible due to interferences from field emission effects. 

Figure 12 Calculated Penning ionization cross sections for collisions (--------) He(2 P)-Ar, (-----),...

Secondly, the cross section reflects the nature of the dependence of the partial widths on the intermolecular distance R. As expected from Eq. (20), is small for large R and increases gradually with decrease in R. On the other hand, Fgx in its empirical form (Eq. (17)) is extremely small at large distances and increases sharply near the repulsive wall. Therefore, the de-excitation probability due to Penning ionization is determined by the dipole-dipole part of the decay width in collisions with large impact parameter, whereas the probability for Penning ionization is already almost unity by the single contribution... 

Figure 19. Deviation of sum of partial cross sections from total ionization cross section, normalized to total ionization cross section, is plotted against the collision energy. All cross sections are calculated on basis of description of Penning process by local complex potential. Deviation is measure of inconsistency of their desorption for case of system He(2 S )-H.

Figure 26. Calculated cross section for formation of quasibound diatomics HeAr+ by Penning ionization of Ar by He(23S) at 42-meV collision energy, as function of time elapsed between ionization and observation (reproduced from Hickman and Morgner 22)...

The three non-adiabatic effects most easily studied by beam techniques are (1) collisional excitation followed by the emission of radiation for the upper state. (2) collisional ionization to A+ + B-, (3) Penning ionization. Collision induced fluorescence is discussed by V. Kempter in Article 9 the technique is a relatively sensitive one and cross sections down to 10-21 cm2 can be detected, though this still falls short of the sensitivity of bulb spectroscopic methods. The technique is also powerful in that separate cross sections for excitation of members of a multiplet can be obtained although, as always with a beam technique, absolute values are difficult to obtain. 

Formation of long-lived Ar-atoms was studied with the same experimental setup between threshold and about 16 eV collision energy.90 The long-lived atoms were detected by Penning ionization of acetylene. The cross section for production of metastables is practically constant over the range studied from 11-5 to 16-3 eV. The absolute magnitude is 3-6 ( 2-4). 10-20 cm2. 

If the electronic excitation energy of a metastable atom A exceeds the ionization potential of another atom B, their collision can lead to an act of ionization, the so-called Penning ionization. The Penning ionization usually proceeds through the intermediate formation of an unstable excited quasi molecule in the state of auto-ionization cross sections of the process can be very high. Cross sections for the Peniung ionization of N2, CO2, Xe, and Ar by metastable helium atoms He(2 S) with an excitation energy of 19.8 eV reach gas-kinetic... 

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