X-ray and extreme ultraviolet emission induced by variable laser pulse-width irradiation of Ar and Kr clusters and droplets

Abstract

Summary form only given. We examine X-ray (>1.5 keV) and EUV (/spl lambda/ = 2-44 nm) emissions from laser heated supersonic argon and krypton gas jets at room temperature and at cryogenic temperature, using laser pulses of constant energy (50 mJ) and variable pulse width in the range 100 fs to 10 ns. The jet is operated in two regimes: cluster generation (uncooled Ar and Kr, and cooled Ar), where the cluster radius is much smaller than a laser wavelength (<100 nm), and a droplet formation regime (cooled Kr), where the droplet radii are comparable to or larger than the laser wavelength (>1 /spl mu/m). We find that for both cooled and uncooled Ar (clusters), the EUV peak emission of the highest observed ion stages occurs for pulses in the range 100-300 fs. For uncooled Kr (clusters), the emission peaks at less than /spl sim/ 1 ps while for cooled Kr (droplets) a broad, dominant peak appears in the range /spl sim/ 50-300 ps. We also measured X-ray emission above 1.5 keV for the same conditions and find that peak emission occurs at /spl sim/ 300 fs for both cooled and uncooled Ar and Kr. We find that the results for both clusters and droplets can be understood in terms of two natural laser plasma time scales. The first is a longer time scale during which the plasma from an individual cluster or droplet remains above critical density. The second, shorter, time scale relates to the finite time for establishing an optimal critical density layer on the periphery of the expanding cluster or droplet plasma in order to maximize resonance absorption. This suggests that the EUV peak at /spl sim/ 100 ps represents maximization of the bulk thermal heating, while the X-ray peak at /spl sim/ 300 fs represents maximum hot electron generation, which would be consistent with optimal resonant absorption.