Attosecond Electron Spectroscopy Using a Novel Interferometric Pump-Probe Technique

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.

Figure 1

Principle of attosecond electron interferometry. A broadband, AP with a spectrum centered on the ionization threshold of helium is used to coherently excite an electron WP consisting of a superposition of bound and continuum $p$ states (region I). The created WP evolves freely during a certain delay (II). The bound part of the WP is finally ionized by a few-cycle IR pulse (III), which is locked in phase to the AP and interferences with the previously created free WP are observed in the photoelectron spectra. In addition, several excited states can be excited leading to quantum beats in the ionization signal.

Figure 2

Experimental photoelectron spectra in He as a function of delay. The electron distribution is recorded using a VMIS and the spectra are obtained by selecting a small collection angle along the polarization direction. The interference fringes are clearly visible at low energies.

Figure 3

(a) Calculated photoelectron spectra in He as a function of delay between the AP and the IR pulse. Interference fringes are clearly seen where the AP precedes the IR probe. (b) Fourier transform of the photoelectron spectrum allowing the identification of the states that form the bound WP. The beat signals from the $2p$, $3p$ and $4p$ states can be seen as vertical lines while the direct-indirect interference gives rise to contributions at an angle of 45°. A line out of the transform at 4 eV is presented in (c).

Figure 4

Analysis of experimental results. (a) Zoom of the low-energy region in Fig. 2. (b) Fourier analysis showing the WP components $3p$, $4p$, $5p$ as well as a quantum beat $4p\mathrm{\text{−}}5p$. (c) ${\beta }_1(E,t)$ and (d) its Fourier analysis. (e) ${\beta }_2(E,t)$ and (f) its Fourier analysis.