Using Surface-Enhanced Raman Spectroscopy to Probe Surface-Localized Nonthermal Plasma Activation.

Low-temperature, nonthermal plasmas generate a complex environment even when operated in nonreactive gases. Plasma-produced species impinge on exposed surfaces, and their thermalization is highly localized at the surface. Here we present a Raman thermometry approach to quantifying the resulting degree of surface heating. A nanostructured silver substrate is used to enhance the Raman signal and make it easily distinguishable from the background radiation from the plasma. Phenyl phosphonic acid is used as a molecular probe. Even under moderate plasma power and density, we measure a significant degree of vibrational excitation for the phenyl group, corresponding to an increase in surface temperature of ∼80 °C at a plasma density of 2 × 10 10 cm -3 . This work confirms that surface-localized thermal effects can be quantified in low-temperature plasma processes. Their characterization is needed to improve our understanding of the plasma-induced activation of surface reactions, which is highly relevant for a broad range of plasma-driven processes.