Characterization of Femtosecond Laser-Induced Plasma under Low Pressure in Argon^*

An experiment of femtosecond laser-induced breakdown in argon with a pressure below normal atmospheric pressure is performed. The breakdown spectrum is mainly due to the electronic relaxation of excited Ar atoms and Ar ions. The lifetimes and characteristics of the Ar plasma are extensively studied by the time-integrated and time-resolved optical emission spectroscopy technique, which is also discussed. Under the assumption of local thermodynamic equilibrium (LTE), the plasma temperature is calculated. Moreover, the electron density is accessed from the Stark broadening of the ionized argon lines. Finally, the validity of applications of LTE is also discussed.