2010, Mauritsson, J., Remetter, T., Swoboda, M., Klünder, K., L'Huillier, A., Schafer, K.J., Ghafur, O., Kelkensberg, F., Siu, W., Johnsson, P., Vrakking, M.J.J., Znakovskaya, I., Uphues, T., Zherebtsov, S., Kling, M.F., Lépine, F., Benedetti, E., Ferrari, F., Sansone, G., Nisoli, M.

We present an interferometric pump-probe technique for the characterization of attosecond electron wave packets (WPs) that uses a free WP as a reference to measure a bound WP. We demonstrate our method by exciting helium atoms using an attosecond pulse (AP) 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 AP. By measuring the delay-dependent photoelectron spectrum we obtain an interferogram that contains both quantum beats as well as multipath interference. Analysis of the interferogram allows us to determine the bound WP components with a spectral resolution much better than the inverse of the AP duration. © 2010 The American Physical Society.