Beam-crossing configuration to control plasma position, improve spatial resolution, and enhance emissions in single-pulse, laser-induced breakdown spectroscopy in gases.

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We report a relatively simple configuration of laser-induced breakdown spectroscopy (LIBS) that is suitable for gas flow diagnostics with increased spatial resolution, signal intensity, and stability. In this optical configuration, two laser beams are generated by splitting a single laser beam, and then they are focused and crossed orthogonally at the detection volume from two different optical paths. Different from dual-pulse LIBS, this LIBS configuration uses only one laser source, and thus is of relatively low cost. Several advantages were found for this simple beam-crossing LIBS when it was demonstrated in air in the present work, particularly on signal enhancement and stabilization, confining plasma volume, and controlling plasma position. Both of the latter two advantages are relevant to spatial resolution improvement of LIBS in gases, which has rarely been discussed in previous reports. An enhancement factor of 2 was found for atomic hydrogen, nitrogen, and oxygen emissions with respect to conventional LIBS. Another advantage is that the position of breakdown can be precisely controlled through adjustment of the propagation of the two beams, also resulting in smaller plasma volume and stable emission intensity. Furthermore, the technique is moderately tolerant to dust particles neutrally present in the environment, avoiding the spark occurring at a position out of the detection volume. Beyond LIBS, the new configuration has other potential applications, e.g., laser-induced ignition, which is also briefly discussed.