Dissociative excitation of potassium in collisions of electrons with KI molecules

Abstract

An experimental study of dissociative collisions of monokinetic electrons with simple molecules is of interest for developing a theory of the structure of molecules and electron-molecular interactions. At the same time, the information obtained on the cross sections of dissociative excitation can be used as reference values for solving problems of theoretical and applied plasmachemistry, laser technology, etc. Among simple molecules (with not more than five atoms) ones of halogenides of alkali metals are of particular interest. A characteristic feature of these molecules is the presence of a comparatively strong bond in the electron ground state with a very small binding energy in electron excited states: calculations performed for some of these molecules give binding energy values in electron excited states equal to about 0.0t eV [1, 2 I. Therefore, inelastic collisions of electrons with molecules of halogenides of alkali metals lead first of all to their dissociation with the possible resulting formation of excited atoms. Investigation of the dissociative excitation of molecules of the class considered began only in recent years using the method of extended crossing beams. Up to now inelastic collisions of monoenergetic electrons with the KCI I3 l, LiCI 14 l, and LiF 151 molecules have been investigated. In the present work the method of extended crossing beams, whose use for thc study of electron-molecular collisions is discussed in detail in [3, 4 I, is used for measuring the dissociative excitation cross section of potassium in collisions of monoenergetic electrons with potassium iodide molecules. A beam of KI molecules was produced by vaporizing potassium iodide from a tantalum crucible and subsequently limiting it to transverse dimensions of 26 x 200 mm by water-cooled diaphragms. The crucible was heated by an electron beam with an electron energy of 10 keV. To obtain more uniform heating, the beam was defocused to a spot diameter of 30-40 mm. At the temperature of the upper surface of the crucible 850 K the concentration of molecules in the region of the crossing of beams at a distance of 280 mm from the cruicible was 2.3.1011 cm -3. Since at such a concentration the mean free path of the molecules attains several dozens of meters, collisions of molecules in the beam play a negligibly small role. Moreover, reabsorption of atomic lines, including resonance ones, is also negligibly small, since atoms in the molecular beam have a concentration of only 107 cm -3 or less. A ribbon beam of monoenergetic electrons with transverse dimensions of 13x200 mm and a drift space of 30 mm was formed by a special low-voltage gun, the design and characteristics of which are discussed in detail in [6, 7 ]. The current density in the electronic beam did not exceed 0.7 mA/cm 2 in the entire range of electron energies 0-100 eV. The width of the energy distribution of electrons constituted 0.9 eV for 90Yo of the electrons at an energy of 100 eV and 1.0 eV at 20 eV. To whom correspondence should be addressed.