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. 

Penning ionization collisions provide an efficient means of selective excitation and their importance in metal vapour... 

Penning ionization collisions of the type represented by equation (11.23) were proposed as the main excitation mechanisms for certain laser transitions discovered in the spectra of zinc and cadmium ions which are listed in Table 11.2. 

Fig.11.7. Energy-level diagram for Cd showing known laser transitions pumped either by Penning ionization collisions with metastable helium atoms, He(2 S,),...

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. 

In this chapter, firstly, a very brief survey is given of recent advances in such studies as classified according to the detection technique of transient species in pulse radiolysis. Secondly, examples are chosen from our recent investigations, with special emphasis on the important contributions of pulse radiolysis methods to gas-phase collision dynamics one is electron attachment, the other is Penning ionization and related processes. The detection techniques and corresponding reaction processes, together with major references, are given below ... 

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... 

Besides Penning ionization, electron tunneling is also registered in other autoionization phenomena in atomic collisions. For example, if the ionization energy of the A atom exceeds the sum of the first two ionization energies of the B atom, then a collision between the A+ ion and the B atom may involve an autoionization process [24]... 

Figure 25. Associative Penning ionization total ionization ratio as function of collision energy for Ne(3P2 0)-Kr. Results obtained by three different experimental methods are compared.

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)...

Spontaneous Ionization in Slow Collisions III. PENNING IONIZATION—COMPLICATIONS... 

Figure 28. Electron spectrum for collision system He -Kr at various collision energies. Broad distribution at low electron energies is a result of Penning ionization, and narrow peaks arise from atomic autoionization of krypton following excitation transfer from He to Kr.77...

Fig. 14. Geometry of a single cell of a penning gauge. Space charge of the trapped, circulating electrons equalizes the axis potential with that of the cathode. Thus, the electric field is radial. Electron density is at a maximum a short distance from the anode. Electrons progress radially toward the anode only as they lose kinetic energy, mainly through inelastic (ionizing) collisions with molecules (40).

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