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Laser ablation - spark discharge - optical emission spectroscopy (LA-SD-OES) and Laser-induced breakdown spectroscopy (LIBS) are applied for the analysis of minor elements Manganese (Mn) and Chromium (Cr) in steel and for the chemical imaging of micro-patterned photoresist layers. In LA-SD-OES a weak laser pulse (typical energy 50 μJ to 5 mJ) triggers an electric discharge between a high voltage electrode and the laser ablated spot on the sample surface. Intense emission lines of neutral and singly-ionized atoms are observed with both spectroscopic techniques. However, lines of doubly-ionized atoms are detected with LA-SD-OES only. The line intensities of C III and Al III increase with decreasing laser pulse energy, surprisingly. The appearance of intense doubly-ionized lines indicates that the mechanisms of plasma excitation are different in LA-SD-OES compared to LIBS and that the combined plasma is not in local thermodynamic equilibrium at low laser energy. The calibration curves of Mn and Cr in industrial steel reveal steeper slope (higher sensitivity) for the Mn II and Cr II lines measured with LA-SD-OES and for the Mn I and Cr I lines measured with LIBS. Chemical imaging of the photoresist layer with LA-SD-OES at the atomic lines of C I and C III and the molecular band of CN reveals all details of the sample. Similar images are obtained for C I and CN using LIBS, however, no image contrast is obtained at the C III line. The very low pulse energy in LA-SD-OES enables soft sampling and may be beneficial also for the analysis and imaging of sensitive material. Unlabelled Image • Laser-triggered localized spark excitation and optical spectroscopy (LA-SD-OES) • Enhanced emission lines of singly and doubly ionized atoms with LA-SD-OES • Higher sensitivity for minor elements Mn and Cr in steel obtained with ionic lines • Chemical imaging of micro-patterned polymer using a C III emission line [ABSTRACT FROM AUTHOR]

• 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. 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. [ABSTRACT FROM AUTHOR]