Laser-pulsed plasma chemistry: Laser-initiated plasma oxidation of niobium

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Abstract
We report the first observation of the chemical modification of a solid surface exposed to an ambient gas plasma initiated by the interaction of laser radiation with the same surface. A new technique, which we designate laser‐pulsed plasma chemistry (LPPC), is proposed for activating heterogeneous chemical reactions at solid surfaces in a gaseous ambient by means of a plasma initiated by laser radiation. Results for niobium metal in one atmosphere oxygen demonstrate single‐pulse, self‐limiting oxide growth induced by a pulsed CO2 laser. X‐ray photoelectron spectroscopy (XPS or ESCA) was used to monitor surface chemical composition changes and thickness control of thin (1 to 5 nm) reaction product layers. The dependence of single‐pulse oxide growth upon laser fluence is observed to be monotonic for oxide thicknesses up to 5 nm. Composition of the oxide Nb2O5−δ, formed by such an optically driven plasma, is similar to that formed by low‐temperature oxidation processes such as rf plasma oxidation; however, the valence defect δ of the LPPC oxide is a least two to five times lower. Interdiffusion at the oxide/metal interface becomes important at higher irradiances and is activated by direct optical coupling with the solid or by plasma‐mediated thermal coupling. Under ultrahigh vacuum, CO2 laser irradiances greater than 0.9 J cm−2 per pulse thin the surface oxide.

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