Interaction of wide band gap single crystals with 248nm excimer laser radiation. XII. The emission of negative atomic ions from alkali halides

Abstract

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Interaction of vacuum ultraviolet excimer laser radiation with fused silica. III. Negative ion formation

“…Negative ion emission from the surface is not expected due to the positive surface charge that builds up during the laser pulse. 4 The resulting electric fields hinder the escape of negative ions. ͑Insulating surfaces typically emit more electrons than positive ions during the laser pulse.…”

“…2 In a previous study of alkali halides exposed to pulsed 248 nm radiation, we showed that negative ions are formed by electron attachment to previously emitted particles. 4 Here, the electrons that are available for attachment processes are confined to a localized cloud of positive ions ͑principally O + and Si + ͒ and negative charge ͑principally electrons͒, where the electrons are electrostatically coupled to the positive ions. 4,[9][10][11] We propose that the negative ions observed when fused silica is exposed to 157 nm radiation are formed when electrons traveling with a similar Ϯ charge cloud attach to neutral atoms and molecules from the surface.…”

“…4 Here, the electrons that are available for attachment processes are confined to a localized cloud of positive ions ͑principally O + and Si + ͒ and negative charge ͑principally electrons͒, where the electrons are electrostatically coupled to the positive ions. 4,[9][10][11] We propose that the negative ions observed when fused silica is exposed to 157 nm radiation are formed when electrons traveling with a similar Ϯ charge cloud attach to neutral atoms and molecules from the surface. In this case, the spatial overlap between this Ϯ charge cloud and the neutral particles is a critical issue 4 addressed below.…”

“…4 Because of its high intensity, we focus our discussion on O − production. During electron irradiation, most surfaces charge negative and negative ions ͑including O − ͒ are often detected.…”

“…4 In the alkali halides, the overlap between the charge cloud and neutral particles emitted from the surface is negligible. Given sufficient electron densities in the charge cloud, however, negative ions can be formed by a two-step process, where electron-ion attachment produces neutral particles which are then converted to negative ions by a second attachment event.…”