Ionization, collisional

Collisional ionization can play an important role in plasmas, flames and atmospheric and interstellar physics and chemistry. Models of these phenomena depend critically on the accurate detennination of absolute cross sections and rate coefficients. The rate coefficient is the quantity closest to what an experiment actually measures and can be regarded as the cross section averaged over the collision velocity distribution. 

The two electrons emerging from the collision are again speeded until each produces another electron by collisional ionization of another atom of argon. The process continues so the first incident electron becomes two, the two become four, and so on. This cascade increases the number of electrons and ions in the gas to form a plasma within a few milliseconds. 

The intra-cluster gas in clusters of galaxies is generally hotter and in collisional ionization equilibrium, and the continuum is dominated by bremsstrahlung, making the interpretation of at least the hydrogen-like and helium-like K-shell emission lines relatively straightforward, but they are comparatively weak and an accurate determination of the temperature(s) is critical. 

The interaction of even simple diatomic molecules with strong laser fields is considerably more complicated than the interaction with atoms. In atoms, nearly all of the observed phenomena can be explained with a simple three-step model [1], at least in the tunneling regime (1) The laser field releases the least bound electron through tunneling ionization (2) the free electron evolves in the laser field and (3) under certain conditions, the electron can return to the vicinity of the ion core, and either collisionally ionize a second electron [2], scatter off the core and gain additional kinetic energy [3], or recombine with the core and produce a harmonic photon [4]. 

Lastly, we mention one more excitation mechanism that has been observed in molecules. It is well-established that following strong field ionization in atoms and molecules, under certain conditions, the ionized electron can be driven back to the ion core where it can recombine to produce high-harmonic radiation, induce further ionization, or experience inelastic scattering. However, there is also the possibility of collisional excitation. Such excitation was observed in [43] in N2 and O2. In both molecules, one electron is tunnel ionized by the strong laser field. When the electron rescatters with the ion core, it can collisionally ionize and excite the molecular ion, creating either N + or Ol+ in an excited state. When the double ion dissociates, its initial state can... 

Chatham, H. Hils, D. Robertson, R. Gallagher, A. Total and Partial Electron Collisional Ionization Cross Sections for Methane, Ethane, Silane, and Di silane. J. Chem. Phys. 1984, 81, 1770-1777. 

Figure 2. Simplified picture of atom-atom collisional ionization with crossing distance r. Heavy solid lines represent trajectories of neutral systems. At the first crossing (r= rj some fraction (1 - PJ of trajectories make adiabatic transitions and are represented by dashed lines (ion pairs). Those making diabatic transitions remain neutral and continue their flight relatively unaffected. Each of these trajectories then encounters r = r<- again, and again each trajectory can make an adiabatic or diabatic transition, resulting in ion pairs or neutrals depending on the trajectory. The ultimate production of ions requires one transition to be diabatic and one to be adiabatic, in either order. The inner circle represents the repulsive core.

The most drastic effect on the losses of the thermal energy is due to dissociation of molecular hydrogen. According to Fox and Wood (1985) as much as a half of the thermal energy behind the shock front is absorbed due to dissociation of Hg molecules. At the same time photodissociation of Hg molecules in the precursor causes retardation of the collisional ionization in the relaxation zone, whereas the precursor structure is very sensitive to the radiation flowing from the wake (Gillet and Lafon 1983 1984). So, the self-consistent model of the radiative shock is urgently needed. 

We fitted the combined data with model spectra based on the atomic data compiled by Raymond and Smith (1977). The model spectra employed here are both for collisional ionization equilibrium (CIE) and non-equilibrium ionization (NEI) models with cosmic abundances (Allen, 1977). Single Te spectrum for both models can not fit the data. Two components of different Te models can reproduce the data well for both models. The physical parameters obtained with CIE models are self inconsistent because the ionization parameter r ( the electron density n X the elapsed time t the after shock heating ) is about 1011cm 3sec which is too short by an order of magnitude for the CIE condition to be reached. 

Translational-energy Distribution Measurements on Product Ions from Collisional Ionization Mb... 

In a related type of experiment, collisional ionization (or so-called charge stripping) of high-kinetic-energy ions is employed. The major reaction studied is of the general form... 

The depopulation cross sections of the Rb nd states of 25 < n < 40 are 1000 A2, which is the same as the cross section of the Rb ns state if the ns —> (n - 3)1,1 > 3 contribution is subtracted. For the Rb nd states the calculated contribution of the scattering of the nd state to nl S 3 and (n—1)1 s 3 states with no change in the rotational state of the CO is <100 A2, so 90% of the cross section is due to the inelastic transitions leading to rotational excitation. Presumably it is because the resonant transfer accounts for 90% of the observed cross section that the structure in the cross section is more visible in the nd cross sections than in the ns cross sections. For both the ns and nd states minimal collisional ionization is observed and calculated in this n range, principally because there are too few CO molecules with energetic enough A/ = -1 rotational transitions. For example, only CO 7 > 18 states can ionize an n = 42 Rydberg state by a A7 = -1 transition, and only 3% of the rotational population distribution is composed of 7 > 18 states. 

Fig. 11.15 Principal quantum number dependence of the experimental rate constants for the total depopulation of the Xe nf states by NH3 ( ) and for collisional ionization (O). Also shown is the calculated ionization rate constants of Rundel (R) (ref. 69), Latimer (L) (ref. 10), and Matsuzawa (M) (ref. 70) (from ref. 64).

Fig. 11.18 Arrival time spectra of the products of the collisional ionization of Xe 26f high Rydberg atoms by CH3I, and C6F6. As shown, collisions with CH3 lead only to r. Collisions with C F14 lead to both C7F14 and e, as shown by the large signal at early times due to electrons. C6F6 leads to the production of a long lived autodetaching state of QF6 which produces a nearly continuous electron signal at early times (from ref. 79).

The logical extension of the state changing M and An collisions is collisional ionization, for Na Rydberg atoms and Ar+ the process... 

Conclusions (1) Hollander s "low" cross section for collisional ionization of Na is sufficient to model the opto-galvanic signal magnitudes as a function of excitation energy. Abnormally high cross sections are not required. 

Collisional Ionization of Sodium Atoms Excited by One- and Two-Photon Absorption in a Hydrogen-Oxygen-Argon Flame... 

In conclusion, we have presented three independent experimental results which can be explained on the basis of equilibrated collisional ionization using a laser-saturated level and electrical neutrality of the probed volume. 

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