PROMPT ELECTRON-EMISSION AND COLLISIONAL IONIZATION OF AMBIENT GAS DURING PULSED-LASER ABLATION OF SILVER

A silver target kept under partial vacuum conditions was irradiated wi th focused nanosecond pulses at 1.06 mu m from a Nd:YAG laser. The ele ctron emission monitored with a Langmuir probe shows a clear twin-peak distribution. The first peak which is very sharp has only a small del ay and it indicates prompt electron emission with energy as much as 60 +/- 5 eV. Also the prompt electron emission shows a temporal profile with a width that is same as that for the laser pulse whereas the seco nd peak is broader, covers several microseconds, and represents the lo w-energy electrons (2+/-0.5 eV) associated with the laser-induced silv er plasma as revealed by time-of-flight measurements. It has been foun d that prompt electrons ejected from the target collisionally excite a nd ionize ambient gas molecules. Clearly resolved rotational structure is observed in the emission spectra of ambient nitrogen molecules. Co mbined with time-resolved spectroscopy, the prompt electrons can be us ed as excitation sources for various collisional excitation-relaxation experiments. The electron density corresponding to the first peak is estimated to be of the order of 10(17) cm(-3) and it is found that the density increases as a function of distance away from the target. Dep endence of probe current on laser intensity shows plasma shielding at high laser intensities.