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# Attosecond electron spectroscopy using a novel interferometric pump-probe technique 

J. Mauritsson, 1 T. Remetter, 1 M. Swoboda, 1 K. Kl¨ under, 1 A. L’Huillier, 1 K. J. Schafer, 2 O. Ghafur, 3 

F. Kelkensberg, 3 W. Siu, 3 P. Johnsson, 3 M. J. J. Vrakking, 3, 4 I. Znakovskaya, 5 T. Uphues, 5 

S. Zherebtsov, 5 M. F. Kling, 5 F. L´ epine, 6 E. Benedetti, 7 F. Ferrari, 7 G. Sansone, 7 and M. Nisoli 7 

$^{1}$Department of Physics, Lund Institute of Technology, P. O. Box 118, SE-221 00 Lund, Sweden 

$^{2}$Department of Physics and Astronomy, Louisiana State University, Baton Rouge, Louisiana 70803-4001, USA 

$^{3}$FOM-Institute AMOLF, Science park 113, 1098 XG Amsterdam, The Netherlands 

$^{4}$Max-Born-Institut f¨ ur Nichtlineare Optik und Kurzzeitspektroskopie (MBI), Max-Born-Strae 2 A, 12489 Berlin, Germany 

$^{5}$4 Max-Planck-Institut f¨ ur Quantenoptik, Hans-Kopfermann-Strasse 1, D-85748 Garching, Germany 

$^{6}$Universit Lyon 1; CNRS; LASIM, UMR 5579, 43 bvd. du 11 novembre 1918, F-69622 Villeurbanne, France 

$^{7}$Politecnico di Milano, Department of Physics Istituto di Fotonica e Nanotecnologie, CNR-IFN Piazza L. da Vinci 32, 20133 Milano, Italy 

We present an interferometric pump-probe technique for the characterization of attosecond elec- tron wave packets (WPs) that uses a free WP as a reference to measure a bound WP. We demon- strate our method by exciting helium atoms using an attosecond pulse with a bandwidth centered near the ionization threshold, thus creating both a bound and a free WP simultaneously. After a variable delay, the bound WP is ionized by a few-cycle infrared laser precisely synchronized to the original attosecond pulse. By measuring the delay-dependent photoelectron spectrum we obtain an interferogram that contains both quantum beats as well as multi-path interference. Analysis of the interferogram allows us to determine the bound WP components with a spectral resolution much better than the inverse of the attosecond pulse duration. 

Attosecond science [1, 2] promises to achieve tempo- ral resolution comparable to the duration ( τ ) of the light pulses, e.g. , of the order of 100 as [3, 4] or even below [5]. An important issue, however, is whether this is possi- ble only to the detriment of spectral resolution, thus considerably limiting the scientific interest of such light sources. When attosecond pulses interact with atoms or molecules, they create broad electron wave packets (WPs), partly in the continuum, but often including also a number of bound states excited by direct absorption [6] and/or by shake-up processes [7]. A spectral resolution given by the Fourier limit, i.e. , of the order of 1 /τ , pre- vents any detailed analysis of such complex WPs. 

Because of the low intensity of currently available at- tosecond pulses, most of the techniques used to char- acterize attosecond electron WPs have used as a probe an infrared (IR) pulse which is synchronized to the ex- treme ultraviolet (XUV) attosecond pulse. The required XUV-IR synchronization is inherent to the attosecond pulse generation process and several methodologies, such as chronoscopy [7], streaking [8, 9], stroboscopy [10] and interferometry [11] have been demonstrated. All of these techniques require that the attosecond and IR pulses overlap temporally. The IR field is therefore not only probing the electron WP after it has been created but it also perturbs the formation. In addition, the spectral resolution of these techniques is Fourier limited. 

Several techniques could be considered to achieve a spectral resolution higher than the inverse of the pulse duration. Although trains of pulses, i.e. frequency combs have the potential to achieve extremely high spectral res- olution (see for example [12]), pairs of pulses, as in tra- 

PACS numbers: 32.80.Rm, 32.80.Qk, 42.65.Ky 

ditional Ramsey spectroscopy, might be easier to imple- ment for time-resolved measurements [13]. In this paper we present a novel interferometric technique, using a sin- gle attosecond pulse and a delayed IR pulse, that resolves both issues previously discussed: The spectral resolution is much better than the Fourier limit of the exciting pulse and the delayed probe pulse does not perturb the excita- tion process (see Fig. 1). 

In our method, an attosecond pulse is used to excite a 

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