Chemical imaging with Laser Ablation – Spark Discharge – Optical Emission Spectroscopy (LA-SD-OES) and Laser-Induced Breakdown Spectroscopy (LIBS). (March 2020) - Grünberger, Stefan
Highlights: Laser-triggered localized spark excitation and optical spectroscopy (LA-SD-OES). Chemical imaging by LA-SD-OES and laser-induced breakdown spectroscopy (LIBS). Differences in optical emission spectra measured by LA-SD-OES and LIBS. Hyperspectral image contrast in LA-SD-OES and LIBS discussed. Physical processes involved in laser-triggered spark formation analyzed. Abstract: Chemical imaging by Laser ablation – spark discharge – optical emission spectroscopy (LA-SD-OES) is presented. A low-energy laser pulse triggers an electric spark between a metal electrode tip and a sample and the excited plasma is analyzed by OES. Without a laser pulse the spark is suppressed as the applied electric field strength is below the self-breakdown threshold in air. The laser-triggered discharge is spatially confined to a plasma channel between tip and ablated spot on the sample. This localization of the electric spark enables for element imaging with high spatial resolution not feasible with spark-OES. Human fingerprints, mesoscopic layered samples, and micro-patterned polymer layers are spectrochemically imaged. The image contrast defined as signal to background ratio of the analyte emission line intensity is higher for LA-SD-OES than for Laser-induced breakdown spectroscopy (LIBS) at low laser energy. We also measure the lag time between laser pulse and spark depending on the applied electrode voltage and the distance between tip and sample and discuss the results in terms of avalanche ionization in air. The optical spectra measured by LA-SD-OES and LIBS in the same experimental setup are similar. Higher ionized emission lines are detected with LA-SD-OES only.