Paper SS2-TuA8
Mechanism of Negative Ion Formation during the Laser Ablation of Alkali Halides¹

Tuesday, October 16, 2007, 4:00 pm, Room 611

Pulsed excimer laser radiation produces a variety of surface and near surface defects in the alkali halides that contribute to particle emission. Quadrupole mass spectrometry during pulsed 248-nm laser irradiation of cleaved LiF, NaCl, KCl, and KBr shows intense emissions of positive alkali ions at fluences well below the threshold for visible plume formation. We argue that these ions are emitted directly from surface defect sites when nearby electron traps are photoionized. In this work, we describe somewhat weaker emissions of negative alkali ions with kinetic energies similar to that of the positive ions. Measurements of induced charge on a metal sheet mounted behind a thin sample show that the surface develops a positive charge during the laser pulse. Thus the direct emission of negative ions at these low fluences is not expected. In previous work, we have shown that the cloud of positive alkali ions is accompanied by enough electrons to compensate much of the positive charge. These electrons are electrostatically confined to the ion distribution to produce a mixed charge cloud. Although these electrons are available for electron attachment processes, detailed measurements on KCl show no Cl^– at our level of sensitivity, despite the presence of the neutral Cl, with its high electron affinity. We attribute the lack of Cl^- to the absence of spatial overlap between neutral Cl and the charge cloud. Since the detected neutral K and Cl have similar velocity distributions, negative ion formation by electron attachment to neutral K desorbed directly from the surface is similarly difficult. These considerations suggest that K^- is formed by double electron attachment to K⁺. Measurements of the total ion and electron emissions imply that the electron and ion densities immediately after the laser pulse are sufficient to account for the production of the observed negative alkali ions by collisional electron-ion and electron-neutral recombination. Although electron attachment is frequently observed among the products of ablation at high fluences, it is remarkable that the conditions required for double electron attachment are attained at the low fluences probed in this work. Further study of the behavior of charged emissions in this well-studied system is required to clarify how these conditions develop.

¹This work was supported by the US Department of Energy under Grant DE-FG02-04ER-15618.