Eur. Phys. J. D 22, 3-11 (2003)
https://doi.org/10.1140/epjd/e2002-00213-1

Two-color pump-probe experiments in helium using high-order harmonics

¹ Department of Physics, Lund Institute of Technology, P.O. Box 118, 221 00 Lund, Sweden
² Institute of Physics and Astronomy, University of Aarhus, 8000 Aarhus C, Denmark
³ LURE, Centre Universitaire Paris-Sud, Bâtiment 209D, 91898 Orsay Cedex, France and CEA/DRECAM/SPAM, CEN Saclay, 91105 Gif-sur-Yvette, France

Corresponding author: ^a anne.lhuillier@fysik.lth.se

Received: 3 June 2002
Revised: 14 August 2002
Published online: 22 October 2002

Abstract

A pump-probe technique has been applied for measuring the lifetimes and absolute photoionization cross-sections of excited He states. The 1s2p ¹P and 1s3p ¹P states of He are excited by using the 13th and the 14th harmonic, respectively, of a tunable 70 ps dye laser generated in a Kr gas jet. The states are ionized after a varying time delay, by absorption of probe photons with energies between 1.6 and 4.5 eV. Lifetimes of  ns and  ns are determined with a precision of about 15%. A significant enhancement of the number of ions present in the lifetime curves at zero time delay for pressures above  mbar is attributed to direct two-photon ionization of He in combination with AC Stark broadening of the excited state and absorption of the XUV light in the medium. Absolute photoionization cross-sections from the He 1s2p ¹P and He 1s3p ¹P states in the threshold region are determined by measuring the saturation of the ionization process with a precision of %. In addition, the variation of the relative orientation between the polarization vectors of the pump and probe beams enables the determination of partial photoionization cross-sections.