Authors: J T Dickinson
Publish Date: 2008/05/28
Volume: 92, Issue: 4, Pages: 1025-1030
Abstract
Below laser fluences where a plasma is formed the socalled plasma or plume formation threshold a number of fundamental phenomena can occur where particles such as atomic and molecular ions atoms and molecular neutrals and electrons can be emitted An understanding of such processes is necessary to develop predictive models for material removal from laser irradiated surfaces—at the foundation of laser etching machining and pulsed laser deposition We have reported on a number of the mechanisms for such emission processes Here due to space limitations we present a summary of our studies on the formation of negative alkali ions from single crystal KCl during exposure to pulsed 248nm radiation at fluences well below the threshold for plasma formation Despite the high electron affinities of the corresponding halogen atoms negative halogen ions were not detected Significantly the positive and negative alkali ion distributions overlap strongly in time and space consistent with K formation by the sequential attachment of two electrons to K+ Negative alkali ions are also observed under comparable conditions from LiF NaCl and KBr In each material the strong overlap between the positive and negative alkali ion distributions and the lack of detected negative halogen ions suggest that negative ion formation involves a similar mechanism
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