Characterization Of Femtosecond Laser-induced Plasma Under Low Pressure In Argon

Femtosecond laser is a quickly developed super ultrafast laser device in20century. The characteristics of ultra-short, high electric field and super wide frequency spectrum distinguish femtosecond laser pulses with long-pulsed laser and CW laser. Exotic phenomenon which called plasma can be produced, when femtosecond laser interacts with matter. The work of this paper mainly studied the characteristics of femtosecond laser induced plasma under low pressure in Argon.The LIBS apparatus was used to have spectroscopy diagnosis on laser-induced Ar plasma. With the features of non-contact testing, the emission spectroscopy is an important method of plasma diagnostics. The time-resolved spectra of certain atoms and ions under low atmosphere were obtained by spectrograph, and based on the spectral diagnostics, we studied the characteristics of laser-induced Ar plasma. The ambient pressure dependence of the electron temperature and electron density were also obtained by analysis of the spectrum. In summary, the content and the results of this paper are as follows:1. When the ambient pressure changed, the study on the emission mechanism of the evolution of laser induced Ar plasma emission spectra intensity was done. The present research tells us that the intensity of Ar ionic line is more intense than that of Ar atomic line under low ambient pressure, and Ar ionic lines almost are submerged in the background when the ambient pressure increased. The reason for these phenomena to happen is that with the increase of ambient pressure, the average photon numbers absorbed by an atom become smaller. We also explained the reason that the intensities of ArⅡ480.602nm increase at very beginning and then decrease as the time evolution.2. We studied the lifetime of laser-induced plasma emission line, which is quantified by the duration from the initiation of the spark to the time for the intensity of spectral line down to1/e of the peak. It is founded that all of the lifetimes increase firstly, then decrease as ambient pressure increases and the ArⅡ have shorter lives than Ar Ⅰ,which due to the thermal conductivity between plasma and surrounding atmosphere and recombination between electrons and ions. 3. Under the assumption of local thermodynamic equilibrium (LTE), Boltzman plots and the Stark broadening were employed to obtain the electron temperatures and electron densities, the results show the excitation temperatures are not strongly dependent on the ambient pressure and the electron densities decreases with increasing time more quickly at higher ambient pressure. Finally, the electron densities we obtained were consistent with the assumption of LTE used to calculate the plasma temperature.