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
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.
PACS: 32.70.Cs – Oscillator strengths, lifetimes, transition moments / 32.80.Fb – Photoionization of atoms and ions / 42.65.Ky – Harmonic generation, frequency conversion
© EDP Sciences, Società Italiana di Fisica, Springer-Verlag, 2003