Time resolved transient absorption probing of a laser plasma

Abstract

The described experimental setup allows the kinetic analysis of a pulsed-laser generated gas-plasma by transient visible absorption spectroscopy at a time resolution of 10 ns and a spectral resolution of 0.3 cm-I. Absorption from the first-4s level manifold towards the 5p level manifold of the argon atom is reported in the spectral range around 420 nm. An example of time dependence is given for the 4s[3/2]~ level population. Since the advent of lasers their ability to generate plasmas has been recognized and widely used and the physical properties of such plasmas extensively studied [I]. Quite a number of those works dealt with rare gas laser plasmas but very few have been devoted to spectroscopical properties of these plasmas. However during the last two decades dense rare gases have attracted considerable attention due to their potentiality as laser media [2]. In this respect a detailed knowledge of plasma kinetics is of great importance. The present report describes a method allowing the kinetic study of a gas plasma by transient visible absorption spectroscopy at high spectral and temporal resolution. The experimental setup is sketched in figure I. The pulsed-laser generated plasma, recently used as a vacuum ultraviolet source [3], is created by focusing the 1064 nm radiation of a pulsed 585-20 Quantel Nd : YAG laser (YAG 1, 350 mJ) at the center of a 6-way gas cell by means of the spherical lens Ll of focal length 20 mm. The stainless steel gas cell, mounted with metallic Helicoflex gaskets from Cefilac, is evacuated with a turbopurnp and refilled before each run with a gas at a pressure typically in the atmospheric range (10~ Pa). This is a very clean technique as the medium is excited far from the cell walls. The probe beam is obtained by irradiating a laser dye cell by part of the frequency tripled radiation at 355 nm of a 2062 JOURNAL DE PHYSIQUE III M 12