Metal evaporation under powerful optical radiation

Abstract

The experimental results of our studies of metal evaporation under powerful optical radiation are presented. The theory of this phenomenon based on a liquid-vapor phase transition is developed. An approximate solution of the Clapeyron-Clausius equation is applied to the present problem. The method permits calculations of metal surface temperature versus incident light intensity I <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</inf> . When a certain critical intensity is exceeded ( <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I_{md} \sim 10^{7}-10^{8}</tex> W .cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> ) a new effect is shown to arise due to disappearance of the metallic properties of the target. The new effect is a "transparency wave" in whose front a liquid metal turns into a liquid dielectric. For <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I_{0} &gt; I_{md}</tex> it is the surface "transparent" (dielectric) layer that is evaporated. Its temperature is no longer raised and it remains below a critical value. This layer is separated from the metal by the transparency wave front, which propagates towards the interior of the metal. The transparency wave causes some other effects to arise, such as a sharp drop of the reflectivity from the metal surface, an essential change in the dependence of the observed evaporation front speed on I <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</inf> , and, finally, occurrence of a maximum on the curve of specific recoil impulse versus I <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</inf> . These other effects may be used to identify the transparency wave. The experimental results support some corollaries of our theoretical model. The vapor dynamics of metal evaporation under powerful millisecond optical radiation are investigated. Vapor heating near the target under laser light has been observed. The initial conditions of vapor motion are studied. From the gas-dynamic measurements the mass flow of the gas phase j <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</inf> is calculated. The dependence of j <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</inf> upon the incident light intensity is indicative of the fact that the metal surface has attained the temperature <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T = T_{md} &lt; T_{cr}</tex> , which corresponds to the liquid metal-liquid dielectric transition.