Influence of distance between sample surface and geometrical focal point on CN emission intensity from femtosecond laser-induced PMMA plasmas

Abstract

In this study, a femtosecond laser is used to ablate polymethyl methacrylate and produce plasma with CN molecules to investigate the influence of the distance between the sample surface and geometrical focal point on CN emission intensity in air. The laser wavelength is 800 nm with a pulse width of 50 fs, and the laser energy ranges from 0.1 mJ to 1.5 mJ. It is determined that the spectral characteristics of the CN molecule depend on the distance between the sample surface and geometrical focal point, as well as the energy of the femtosecond laser. At the same energy, the spectral intensity of the CN molecule first increases, and then decreases with an increase in the distance. The position with the strongest spectral emission is not the geometrical focal point of the focusing lens, but away from the geometrical focal point. As the laser energy increases, the position with the strongest spectral emission moves toward the focusing lens. In addition, the vibration temperature and full width at half maximum (FWHM) are also calculated by fitting CN spectral data for specific distances and energies. The vibration temperature and FWHM of CN at the position with strongest emission are the lowest, and the vibration temperature and FWHM at the left side (close to the lens) of the strongest emission position are higher than those at the right side (away from the lens) of the strongest emission position.