+ Taylor & Francis JOURNAL OF MODERN OPTICS, 2003, VOL. 50, NO. 314, 451-459 0 TaylwhFrancisGroup Strong-field photoionization of argon: a comparison between experiment and the SAE approximation R. WIEHLE, B. WITZEL, V. SCHYJA, H. HELM Department of Molecular and Optical Physics, Albert-Ludwigs- Universitat Freiburg, Germany and E. CORMIER CELIA, University of Bordeaux, F-33405 Talence, France (Received 15 March 2002) Abstract. We monitor the intensity dependence of photoionization of argon at 800 nm at pulse durations of 100 fs employing momentum-resolved photo- electron imaging spectroscopy. We compare our data with numerical solutions of the time-dependent Schrodinger equation (TDSE) using a model potential for argon and the single active electron approximation (SAE). We find quanti- tative agreement in all dominant features in energy and angle. The energy and angular distributions of the photoelectrons allow us to identify selective dominant ionization channels. 1. Introduction The study of photoionization in short intense laser pulses has been an active topic of research over the past 20 years. After the discovery of above-threshold ionization (ATI), fine structure within each AT1 order was discovered and attributed to transient resonances of ac-Stark shifted Rydberg states at specific intensities within the laser pulse [l-31. More recent work has concentrated on the high energy portion of the photoelectron spectrum, which displays plateau and jet features [4, 51. These features are in part attributed to elastic backscattering of a revisiting photoelectron from its parent ion. Little quantitative work has been reported on the intensity dependence of the photoelectron energy spectra at low electron energy, in particular a detailed comparison of current state-of-the-art theory and experiment is missing. It is the purpose of this contribution to give an account of a joint effort in analysing the complex angular and energy patterns which appear in strong-field ionization of the rare gases. The very significant dependence of the photoelectron energy and angular distributions on the peak laser intensity is shown to be quantitatively accounted for in the single active electron approximation. 2. Experiment Photoelectrons are recorded in an imaging spectrometer described previously [6]. The electrons formed in the tight focus of the laser beam are projected onto a Journd of Modern Optics ISSN 0950340 print/ISSN 1362-3044 online 0 2003 Taylor & Francis Ltd http://www.tandf.co.uk/journals DOI: 10.10X0/095003402101526X4 Downloaded by [Univ Bordeaux 1 Bat A33] at 03:16 12 April 2013