4 states are excited by the attosecond pulse (3 p , 4 p , 5 p ) and obtain a measure of their relative strengths. Our ultimate goal is of course a complete characterization in both amplitude and phase. Determining the amplitudes and phases of a wave-like object by interference with a reference wave is a well-known approach. The experi- ment and the analysis we have presented demonstrates that we can probe an attosecond electron WP using a coherent reference WP that is in the continuum. With this proof of principle in hand, the method can be ex- tended to include the retrieval of a phase associated with each WP component from the interferogram. This phase retrieval, which will be addressed in a future theoretical work [24], yields the phase difference between the two paths into the continuum and includes a phase contribu- tion from both XUV and IR ionization steps. If it can be arranged that both ionization leading to the creation of the reference WP and ionization by the probe field do not add extra amplitude or phase variation or that these possible variations are well known then a complete phase characterization can be made. Conversely, experiments such as that presented here could be used to characterize the effect of the ionization by the probe field, or some structure in the continuum used for the reference WP. In conclusion we have demonstrated experimentally a new interferometric technique using a reference contin- uum WP and a delayed probe excitation. The method demonstrated can be used to probe the temporal evolu- tion of bound or quasi-bound electron WPs, e.g. created by shake up [7] with high spectral and temporal resolu- tion simultaneously, thus providing an increased preci- sion when doing attosecond experiments. This research was supported by the Marie Curie IEF (ATTOCO), RTN (XTRA), EST (MAXLAS) programs, the Integrated Initiative of Infrastructure LASERLAB- EUROPE, the Swedish and European Research Council and the National Science Foundation through grant num- ber PHY-0701372. IZ, SZ, TU and MFK are grateful for support by the DFG through the Emmy-Noether pro- gram and the Cluster of Excellence: Munich Center for Advanced Photonics. This work is part of the research program of the ”Stichting voor Fundamenteel Onderzoek der Materie” (FOM), which is financially supported by the ”Nederlandse Organisatie voor Wetenschappelijk On- derzoek” (NWO). KJS acknowledges support from the Ball Family Professorship. an expansion coefficient, and L$_{max}$ the maximum angu- lar momentum component of the ionized wave function, which corresponds to the maximum absorbed angular momentum. After extraction of the individual expansion coefficients, β$_{J}$ ( E, t ), we apply the same Fourier analysis that was already applied to the photoelectron spectrum measured along the laser polarization axis. Fig. 4c and 4e shows the extracted expansion coefficients β$_{1}$ ( E, t ) and β$_{2}$ ( E, t ) respectively, together with their Fourier trans- forms in Fig. 4d and 4f. Simple parity arguments dic- tate that interference between ionization processes that end in states of definite parity (either even or odd with respect to reflection along the polarization direction) will appear in the even J expansion coefficients, while inter- ference between ionization processes that end in mixed parity states (both even and odd) will appear in the odd J expansion coefficients. The direct-indirect interference process involves electrons that have absorbed different numbers of photons (one for the direct process, two for the indirect process, since the bound states involved can be ionized by absorption of one IR photon) resulting in a mixed parity state. Indeed, we see that the characteristic hyperbolic interference fringes appear clearly in β$_{1}$ ( E, t ) (Fig. 4c and d). In contrast, the process that produces quantum beats results in states of definite parity. Ac- cordingly, the quantum beat signal dominates β$_{2}$ ( E, t ) (Figs. 4e, f), thus confirming the interpretation presented above. Using the above analysis, we can determine which FIG. 4: (Color online). Analysis of experimental results. (a) Zoom of the low-energy region in Fig. 2. (b) Fourier analysis showing the WP components 3 p , 4 p , 5 p as well as a quantum beat 4 p -5 p . (c) β$_{1}$ ( E, t ) and (d) its Fourier analysis. (e) β$_{2}$ ( E, t ) and (f) its Fourier analysis. Energy, E (eV) (a) 0 1 2 (b) 3 p 4 p 5 p QB Energy, E (eV) (c) 0 1 2 (d) Delay, t (fs) Energy, E (eV) (e) 0 10 20 30 40 0 1 2 Fourier frequency, E ′ (eV) (f) 0 1 2 [1] P. M. Paul et al. , Science 292 , 1689 (2001). [2] M. Hentschel et al. , Nature 414 , 509 (2001). [3] G. Sansone et al. , Science 314 , 443 (2006). [4] E. Gustafsson et al. , Optics Letters 32 , 1353 (2007). [5] E. Goulielmakis et al. , Science 320 , 1614 (2008). [6] P. Johnsson et al. , Phys. Rev. Lett. 99 , 233011 (2007). [7] M. Uiberacker et al. , Nature 446 , 627 (2007).