Emission spectra of microplasma generated by femtosecond laser pulses

Laser-induced breakdown spectra (LIBS) produced in ambient air and pure gases with tightly focused, micro Joule, femtosecond Ti:sapphire laser pulses have been studied experimentally. The breakdown region typically measures around 10 µm in diameter and 120 µm in length. Both line spectrum and blue-shifted continuum spectrum are recorded for air, N2, O2, Ar, Kr, and Xe when the optical intensity reaches 1014W/cm2, using 40 fs pulses. It is also found that the profile of the continuum varies only slightly for different gases. For a given gas the continuum profile changes along different positions of the narrow micro plasma track. In particular, the blue-shifted continuum is stronger at the initial position of the plasma track (closer to the focal lens), whereas the line spectrum is more dominant when light collection is made at the further end of the plasma track. Unlike the results reported by others for longer pulses, higher energy and also larger plasma zone, the majority of line spectrum we obtained is found around 800 nm, the central wavelength ofthe input Ti:sapphire pulses.