Formation of double-layer in the early stage of nanosecond laser ablation

Abstract

Relying on classical spectroscopy we investigate the early stage of ablation process and expansion of plasma induced on copper target by 532 nm harmonic of Nd:YAG laser. The experiment is done at reduced atmosphere pressure 2–50 Pa. Radially resolved spectroscopy is employed for expansion velocity estimation and monitoring fluorescence of surrounding gas. We show, relying on probe measurements, that emission of the fast electrons starts during the laser pulse duration. In less than 30 ns and on 1 mm scale ions reach velocity of ∼ 50 km/s or ∼ 800 eV energy. It suggests that electrons, during the laser action, have to gain more than 800 eV energy from the laser radiation to maintain the charge separation. We show that the expansion velocity of ions/atoms is proportional to their charge, Cu++ ions are faster than Cu+ ions and Cu atoms. These results are in consistence with double-layer model. The spectrum emitted from the cold argon region consists only of very narrow Ar I and Ar II spectral lines while the hot plasma plume, during the initial stage, radiates continuous spectrum in the investigated range 200–650 nm.