  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 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.
