Threshold energy collisional ionization

Using a modulated photon beam and a detector to measure only ionization produced at the modulation frequency, Lee and Mahan were able to show from the phase shift involved that both (48) and (49) occur. The associative ionization reaction (48) was found in cesium to occur for absorbed photons whose energy was within 0.70 eV of the atomic ionization potential, that is, for the states of cesium in which the excited electron is in the 8P, 9P, lOP,. .. levels. However, reaction (49), which we call collisional electron release, is observed only for cesium atoms in the 12P, 13P, 14P,... levels, and thus has a threshold energy that lies only within about 0.2 eV from the atomic ionization potential. Similar results were observed for rubidium and potassium, namely that collisional electron release occurs in addition to associative ionization, but does so only for states of the atoms lying much closer to the ionization continum than is the case for the latter process. 

Figure 9.11 (Left) Potential energy curves for the covalent (Nal) and ionic (Na l ) forms of sodium iodide. AE is the nominal (i.e., asymptotic) energy gap between the two states, i.e., the difference between the ionization potential of Na and the electron affinity of I. Thus, from A o (eV) S 14.35// x (A) (Eq. (3.18)), / x = 7 A. It is known that for this system the actual electronic energy gap as discussed below is the much smaller value of = 0.025 eV. Note that the curves shown are "diabatic" ones. The electronic state is frozen and is not allowed to adjust to the position of the nuclei. The Born-Oppenheimer or adiabatic states do not cross. (Right) Cross-section for collisional ionization, in reduced units, logarithmic scale vs. the velocity in reduced units. The velocity Vm is defined in Eq. (9.24). The thresholds are indicated. Points experimental. Adapted from Moutinho etal., Physica, 53, 471 (1971) and Baede (1975).

In an ensuing study, Lacman and Herschbach studied collisional excitation and ionization of K with improved neutral beam intensities, permitting studies at beam energies as low as 2 eV. The study involved a number of diatomic molecules. Both ionization and excitation thresholds close to... 

Compton studied the formation of both positive and negative ions by electron ionization and electron attachment in UFs (Compton, 1977). The cross section for the electron ionization of UFe from the threshold electron energy, 14 eV, to 1 keV exhibited a maximum of 18 at 160 eV. Reactions of UFs with electrons and alkali metal atoms to produce uranium fluoride anions confirmed the high electron affinity of UFe and demonstrated that D(UF5 —F) and D(UF5-F ) are similar, to within 1 eV, but are both 2 eV greater than D(UF5-F). That the U—F bonds are stronger in UFe" than in UFs reflects the large electron affinity of UFe. Streit and Newton reported that exothermic electron transfer from halide anions and SFe to UFe occurs close to the collisional rate (Streit and Newton, 1980), a result that renders particularly intriguing the nonoccurrence of thermoneutral Eq. 8 due to an unanticipated barrier. 

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