Pressure dependence of emission intensity in femtosecond laser-induced breakdown spectroscopy

Femtosecond laser produced plasma emission has been characterized as a function of pressure for applications in laser induced breakdown spectroscopy (LIBS). Experiments were performed with a Ti: sapphire laser system ( 130 fs, 800 nm), from atmospheric pressures down to 10(-3) Torr and at pulse energies on the order of 1 - 50 muJ, (0.1 - 5 J cm(-2)). Characteristic emission lines from Al, Mg, Si and Cu elements exhibited significant enhancement in signal intensity at a few Torr background air pressure as compared to atmospheric air pressure. Spatially and temporally resolved emission measurements indicate enhancement due to a longer lifetime of the plasma expanding to a larger size at lower background pressures. Further reduction in pressure down to 10(-3) Torr resulted in a decrease in signal intensity, as a result of a reduction of collisional excitation of the emission lines which occurs when the plasma plume expands into the ambient atmosphere. It has been also observed that signal enhancement at low pressure is very much dependent on the measurement delay time and on the transition being observed. With a delay time of 200 ns the integrated intensity of the neutral Al I lines at 396 nm exhibited 67 times enhancement in signal intensity at 4 Torr of pressure with respect to atmospheric pressure, whereas signal enhancement is only 4 times when no measurement delay time was used. The best signal to noise ratio of 850 was observed at 4 Torr pressure for an 85 ns delay time. Measurements of crater size showed no pressure dependent changes in the ablated mass, indicating that little plasma shielding occurs due to the short pulse duration of the femtosecond laser pulses.