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Polarization effects in two-photon nonresonant ionization of argon with extreme-ultraviolet and infrared femtosecond pulses
O'Keeffe, P; Lopez, Rodrigo; Mauritsson, Johan; Johansson, Ann; Lhuillier, A; Veniard, V;Taieb, R; Maquet, A; Meyer, MPublished in:Physical Review A (Atomic, Molecular and Optical Physics)
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Citation for published version (APA):O'Keeffe, P., Lopez, R., Mauritsson, J., Johansson, A., L'Huillier, A., Veniard, V., ... Meyer, M. (2004).Polarization effects in two-photon nonresonant ionization of argon with extreme-ultraviolet and infraredfemtosecond pulses. Physical Review A (Atomic, Molecular and Optical Physics), 69(5). DOI:10.1103/PhysRevA.69.051401
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Polarization effects in two-photon nonresonant ionization of argon with extreme-ultravioletand infrared femtosecond pulses
P. OKeeffe,1 R. Lpez-Martens,2 J. Mauritsson,2 A. Johansson,2 A. LHuillier,2
V. Vniard,3 R. Taeb,3 A. Maquet,3 and M. Meyer1,41LURE, Centre Universitaire Paris-Sud, Btiment 209D, F-91898 Orsay Cedex, France
2Department of Physics, Lund Institute of Technology, P.O. Box 118, S-22100 Lund, Sweden3LCP-MR, Universit Pierre et Marie Curie, 11 Rue Pierre et Marie Curie, F-75231 Paris Cedex 05, France
4CEA/DRECAM/SPAM, CEN Saclay, F-91105 Gif-sur-Yvette, France(Received 18 June 2003; published 4 May 2004)
We report the results of experimental and theoretical investigations of the two-color, two-photon ionizationof Ar atoms, using femtosecond pulses of infrared laser radiation in combination with its extreme-ultravioletharmonics. It is shown that the intensities of the photoelectron lines resulting from the absorption of photonsfrom both fields strongly depend both on the respective phases of the fields and on atomic quantities such asthe asymmetry parameter. These phases, which are notoriously difficult to measure, can be estimated bychanging the polarization state of the laser radiation.
DOI: 10.1103/PhysRevA.69.051401 PACS number(s): 32.80.Rm, 32.80.Fb, 42.65.Ky
Photoionization measurements provide invaluable infor-mation on the structure of the electronic cloud of an atom ora molecule. The general features of photoionization, namely,the energy dependence, the angular distribution of the pho-toelectrons, and resonance effects, have been studied in greatdetail using mainly synchrotron radiation sources .More specific investigations have been carried out by meansof two-photon pump-probe experiments combining laser andsynchrotron radiation [4,5]. Novel high-order harmonic gen-eration (HHG) sources providing ultrashort extreme-ultraviolet (XUV) or x-ray pulses lasting a few femtoseconds or less [7,8] will provide a deeper insight into the elec-tronic relaxation processes and into the dynamics of photo-ionization . Today, femtosecond XUV sources such ashigh-order harmonic radiation or the recent TESLA free-electron laser  are in the process of being developed andcharacterized. The number of pump-probe studies with thesesources is therefore still rather limited and experiments oftenserve as a characterization tool for the sources themselves[7,8,11].
Electron spectra obtained from irradiation of a gas samplewith harmonic emission, reflecting the harmonic intensitydistribution, contain a series of lines separated by 2v. Thecombination of the harmonic pulse with part of the funda-mental infrared (IR) beam in the interaction chamber willgive rise to additional lines, when the intensity of the dress-ing IR beam is high enough s.1010 W/cm2d  and whenboth pulses overlap in time and space (see Fig. 1). In theintensity ranges considered here, these so-called sidebandsare the result of a two-photon ionization process, as the pres-ence of the IR beam leads to an additional absorption oremission of one IR photon simultaneously with the absorp-tion of the XUV photon. The intensity and shape of the side-bands depend strongly on the characteristics of the two fem-tosecond pulses  and can therefore be utilized tocharacterize the XUV pulses [11,14]. Since a given sidebandis made up of contributions from two consecutive harmonics,the sideband intensity will depend on the phase differencebetween the two harmonics as well as on the angular mo-
menta and the relative phases of the outgoing electrons[15,16]. The former property has been recently used to showthat harmonics generated in argon were phase locked, indi-cating that the light was emitted as a train of attosecondpulses .
In this paper, we investigate experimentally and theoreti-cally the influence of the state of polarization of the IR probepulse (linear polarization with an angle relative to that of theXUV field and the degree of ellipticity) on the formation ofthe sidebands resulting from interfering amplitudes involvingtwo harmonics. In the case of photoionization with linearlypolarized photons, the ionization signal is independent of theharmonic phases and the variation of the angle between thetwo polarization vectors of the XUV and IR field allows usto determine the asymmetry parameter for the one-photonionization process. On the other hand, the sideband intensityis very sensitive to the relative phase of two consecutiveharmonics, when the ellipticity of the IR photon is changed.It is demonstrated that the controlled variation of the polar-ization states of the light can provide detailed information onthe photoionization process, especially on the asymmetry pa-rameter b describing the angular distribution of the electronsproduced by absorbing the XUV photon, by measuring theangle integrated ionization signal.
The experimental conditions used for the present studyhave been described in detail elsewhere . Briefly, XUVphotons are produced by HHG in an Ar gas cell from anintense s1 mJ,50 fsd IR laser s810 nmd running at 1 kHzrepetition rate (see Fig. 2). The generated harmonics are fo-cused into the acceptance volume of a magnetic bottle elec-tron spectrometer (MBES) installed in an experimentalchamber filled with a static pressure of about 104 mbar Argas. One-photon ionization of the Ar 3p shell (binding en-ergy about 15.8 eV) by the harmonics leads to a series ofequidistant lines in the photoelectron spectrum correspond-ing to the photoionization process by the 11th s,17 eVd upto the 25th harmonic s,38 eVd. The observation of higherharmonics is limited by the reflectivity of the focusing goldmirror used in the experimental vacuum chamber. In the in-
PHYSICAL REVIEW A 69, 051401(R) (2004)
1050-2947/2004/69(5)/051401(4)/$22.50 2004 The American Physical Society69 051401-1
teraction region, the XUV beam intersects the IR probe beams0.6 mJd at an angle of about 8. The spatial overlap betweenboth beams and especially the size of the IR beam can becontrolled by mechanical adjustment of a convex lens sf=50 cmd. The temporal overlap is changed through an opti-cal delay line introduced in the path of the beam used forHHG. In order to manipulate the polarization of the IR(probe) beam, a half-wave plate is installed just in front ofthe focusing lens, enabling us to change the relative angle Qbetween the polarization vector of the IR and the harmonicbeam. Alternatively, a quarter-wave plate allows us tovary the degree of ellipticity of the IR beam from linear tocircular.
A typical photoelectron spectrum is shown in Fig. 1(b)giving the photoelectron intensity as a function of electronkinetic energy (horizontal axis) and time delay between thetwo pulses (vertical axis). Figure 1(c) shows the spectrumobtained when both pulses are best temporally overlapped.The dominant structures in Fig. 1(b) are the vertical lines,which are not much affected by the presence of the IR pulsesand which correspond to direct photoionization of Ar 3pelectrons by the XUV photons. The spin-orbit splitting ofabout 0.18 eV between the Ar+3p5 2P1/2 and
2P3/2 compo-nents  is not resolved in the present experiment. Thesidebands show up for temporal delays Dt between about40 and +40 fs. The harmonic pulse durations, extractedfrom the combined knowledge of the energy-integrated side-band intensity profile and the independently measured 50 fsduration of the IR pulse, are found to be
19th harmonics, can be neglected for the present experiment.The transition amplitude for the sideband denoted SB2N is
given by (atomic units are used, unless otherwise stated):
T 2Nsn,l,m,kWd = T2N1s+d sn,l,m,kWdeisf2N1+fLd
+ T 2N+1sd sn,l,m,kWdeisf2N+1fLd, s1d
where sn , l ,md defines the initial state of the atom and kW isthe wave vector of the outgoing electron. fL, f2N1, andf2N+1 are the absolute phases of the fields of the IR laser andof the neighboring harmonics, respectively. T s+d sT sdd cor-responds to the absorption semissiond of one laser photon.The fact that