A spectral study on the plume induced by femtosecond laser ablation of lithium niobate in vacuum

The plasma plume induced by femtosecond laser ablation of lithium niobate in vacuum is studied by spectroscopy. The spectra of the plasma plume consist of a continuous spectrum and four apparent spectral lines. The spectral intensity varies with the distance from sample's surface. The peak of continuous spectral intensity is at a distance of z = 0.4 mm, while there are two peaks for spectral lines, which are at z = 0.4 mm and z = 0.8 mm respectively. After deconvoluted by a Gaussian function with a FWHM of 0.5 nm, the spectral lines are well fitted by the Voigt function. The electron density Ne and the electron temperature Te are respectively derived by using Stark-broadening method and Boltzmann plot method. It is surprising that the peaks of Ne and Te of plasma plume are at z = 0.4 mm and z = 0.8 mm respectively, which is consistent with the intensity peaks of spectra. A two-flows model is introduced as a possible explanation for the formation mechanism of the plasma plume. The electron cloud with a high speed, which is ejected firstly, pulls the nonequilibrium ion plume to accelerate and encounters the ion plume at z = 0.8 mm, forming the plasma plume with the highest temperature. The dynamics of nonequilibrium plume near the surface is deduced to be responsible for the surprising phenomenon.