Low-background laser plasma induced by Nd-YAG laser at low pressures

An experimental study has been carried out on the dynamical process taking place in the secondary plasma generated by a Q-sw Nd-YAG laser (26 mJ, 1,06 4 nm, 8 ns) on a variety of metal samples at a reduced pressure ol. 0.5 Tor r. It was found that light emission of plasma produced at such a low pressu re was ascribable to collisions ut. ultrafast atoms with surrounding gas mo lecules, instead of the shock wave excitation mechanism proposed earlier fo r plasma generated at pressures around a few Torr or higher. This plasma ex hibited excellent characteristics for spectrochemical applications because of its extremely low background emission. Using air as a surrounding gas, t he background emission at 0.5 Torr was reduced by 10-fold compared to that at 1 Torr. It was also demonstrated that replacing air with helium as a sur rounding gas further reduced the background emission by more than 10-fold. Temperature measurement performed by means of the Boltzmann two-line method showed that the average temperature of plasma generated below 1 Torr had t he tendency to increase with decreasing pressure, contrary to the trend obs erved at higher pressures. It was also found that the propagation length of the emission front varied with time at constant velocity, which is signifi cantly different from that predicted by the shock-wave model.