Collision effects Penning ionization collisions

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. 

In order to record excitation spectra, the radical ions must first be thermalized to the electronic ground state, which happens automatically if they are created in condensed phase (e.g. in noble-gas matrices, see below). In the gas-phase experiments where ionization is effected by collision with excited argon atoms (Penning ionization), the unexcited argon atoms serve as a heat bath which may even be cooled to 77 K if desired. After thermalization, excitation spectra may be obtained by laser-induced fluorescence. 

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. 

PENNING EFFECT. An increase in the effective ionization rate of a gas due to the presence of a small number of foreign metastable atoms. For instance, a neon atom has a metastable level at 16.6 volts and if there are a few neon atoms in a gas of argon which has an ionization potential of 15.7 volts, a collision between the neon metastable atom with an argon atom may lead to ionization of the argon. Thus, the energy which is stored in the metastable atom can be used to increase the ionization rate. Other gases where this effect is used are helium, with a metastable level at 19.8 volts, and mercury, with an ionization level at 10.4 volts. 

---