Characteristics of the secondary plasma induced by focusing a 10-mJ XeCl laser pulse at low pressures

A XeCl excimer laser (20 ns, 10 mJ) was focused on a copper target at a relatively high power density (10(10) W/cm(2)) in a surrounding gas of air in the pressure range from 1 Torr to about 10 Torr. Plasma characteristics were examined in detail with the use of a time-resolved spatial distribution technique on the emission lines Cu(I) 5218 Angstrom, Cu(I) 5105 Angstrom, and Cu(II) 4674 Angstrom. The results show that the majority of atoms are ablated in the form of neutral atoms, and that the temperature of the plasma increases with time in the initial explosion stage. These facts suggest that the secondary plasma is excited by a shock wave and the recombination emission process is not predominant. It was also demonstrated that the plasma produced at 1 Torr is due to another excitation mechanism; namely, the collision of ultrafast atoms with surrounding gas molecules. This plasma is also suitable for analytical spectrochemical application.