Analysis of minor elements in steel and chemical imaging of micro-patterned polymer by laser ablation-spark discharge-optical emission spectroscopy and laser-induced breakdown spectroscopy

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 mu J 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.