Influence of adsorbed water on the optical strength of infrared optics elements

Spectroscopic and mass-spectroscopic methods were used in an investigation of the composition of the substances ejected from the surfaces of polished transparent infrared materials acted upon by CO₂ laser pulses. The main components were found to be water, hydroxyl groups, and original material. A preliminary laser cleaning procedure reduced the amount of ejected water and hydroxyl groups, and also increased the surface breakdown threshold. The threshold of a surface free of an adsorbed water layer (for example a surface cleaved in vacuum) was equal to the bulk damage threshold of the material being investigated. The dynamics of the breakdown plasma formed near the surface was investigated and its temperature was measured. An analysis was made of possible ways of increasing the surface damage threshold of infrared optics elements by reducing the adsorptivity of the surface. The example of NaCl was used to demonstrate that chemical etching of a mechanically polished surface produced a short-term (5 min) increase in the breakdown threshold by a factor of 5–6 and a long-term (at least 3 months) increase by a factor of 2.5–3.