Spatial confinement effect on CN emission from nanosecond laser-induced PMMA plasma in air

摘要

In this paper, a PMMA target was ablated to study spatial confinement effect on CN emission of laser-induced breakdown spectroscopy in air. Four cylindrical cavities with same depth and different diameters (4, 6, 8, and 10 mm) were used to confine the nanosecond laser-ablated PMMA plasma plume. Laser pulse energies were 40, 50, and 60 mJ. The influence of cavity diameter on emission band and vibration temperature of CN molecule was demonstrated, the results showed an obvious increase in the emission and vibration temperature of CN molecule by using the cylindrical cavity. When the cavity diameter was smaller at a fixed laser energy, the emission enhancement of CN molecule appeared earlier, and the CN molecule had stronger emission and vibration temperature. For a fixed diameter cavity, when the laser energy was higher, the CN molecule presented stronger emission and higher vibration temperature, and the emission enhancement also appeared earlier. The enhanced effect on the spatial confinement mainly comes from shock wave produced by the laser-induced PMMA plasma. The spatial confinement cavity can reflect the shock wave, and the shock wave compresses the plasma plume, resulting in an improvement in the temperature and density of the plasma. Therefore, the optical emission of CN molecule increases as the shock wave interacts with the PMMA plasma.