While femtosecond Laser Induced Breakdown Spectroscopy has been studied in the macro-scale (i.e. ablation crater sizes of tens to hundreds of micrometers), the spectral emission mechanisms at the micron/sub-micron scale remain largely unknown, mainly because of the challenges associated with spectral emission acquisition from the limited amounts of ablated mass at these small lengthscales. In this work we study the limits of detection, the laser-induced plasma properties and spectral emission efficiency of Cu at the micron/submicron scale. Although the corresponding number electron densities and temperatures are similar to those reported for macroscale laser ablation, our findings suggest less efficient luminous spectral emission per ablated volume as we scale down in laser energy and crater sizes. These results provide a first insight into fs laser-induced plasma properties at the micron/sub-micron scale regime.

Recently, laser ablated molecular isotopic spectrometry (LAMIS) has expanded its capability to explore molecules formation mechanism in laser-induced plasma in addition to isotope analysis. LAMIS is a powerful tool for tracking the origination of atoms that is involved in formation of investigated molecules by labeling atoms with their isotopic substitution. The evolutionary formation pathways of organic molecules, especially of C₂ dimers and CN radicals, were frequently reported. However, very little is known about the formation pathways for metallic radicals and heterodimers in laser ablated plasma. This research focuses on elucidating the formation pathways of AlO radicals in femtosecond laser ablated plasma from ¹⁸O-labeled Al₂O₃ pellet. Plasmas expanding with strong forward bias in the direction normal to the sample surface were generated in the wake of a weakly ionized channel created by a femtosecond laser. The formation mechanism of AlO and influence of air were investigated with multiple plasma diagnostic methods such as monochromatic fast gating imaging, spatiotemporal resolved optical emission spectroscopy, and LAMIS. An advanced LAMIS fitting procedure was used to deduce the spatiotemporal distributions of Al¹⁸O and Al¹⁶O number densities and also their ratios. We found that the Al¹⁶O/Al¹⁸O number density ratio is higher for plasma portion closer to the sample surface, which suggests that chemical reactions between the plasma plume and ambient air are more intense at the tail of the plasma. The results also reveals that direct association of free Al and O atoms is the main mechanism for the formation of AlO at the early stage of the plasma. To the contrast, chemical reactions between plasma materials and ambient oxygen molecules and the isotope exchange effect are the dominant mechanisms of the formation of AlO and evolution of Al¹⁶O/Al¹⁸O number density ratio at the late stage of the plasma.