**4. Conclusion**

Attosecond pulses in general allow taking snapshots of the fastest processes known in the microcosm. The permitted to resolve processes, so far only predicted, in real time and facilitated the discovery of ultra-fast mechanisms that are not yet fully understood regarding their physical origin.

The quantum mechanical tunnel effect belongs to the first category. As an implementation of *attosecond tunneling spectroscopy* a first experiment demonstrated how adding attosecond tools to ion mass spectrometry can shine –time-resolved- light on very fundamental processes. According to the explanation of Leonid Keldysh the probability for tunnel ionization is high at the maxima of the laser electric field und thus leads to a stepwise evolution of the number of ionized atoms exposed to intense light (Mishima et al. 2002) at the same time the time structure of subsequent Auger decay cascades in Xenon and Krypton have been resolved (Uiberacker et al. 2007). So far only known from (indirect) line width measurements, attosecond pulses turned out to be the only tool that allows unraveling complex and intertwined processes in real time.

The investigation of transport effects of electrons in a solid is an example for the second category(A. L. Cavalieri, Müller, et al. 2007). A timing difference of electrons reaching the surface after photoionization was discovered that depends on the initial state of the released electron. It was found that electrons originating from the 4f band of a tungsten crystal take about 50 attoseconds longer to reach the surface compared to those starting in the conduction band. Is that solely a transport effect or signature of the crystal band structure?

Recently attosecond science was successfully combined with *transient absorption spectroscopy*. This experimental approach, that revolutionized quantum chemistry after the advent of pulsed laser sources, was used to track the oscillations of a coherent superposition in the valence shell of atoms in real time (Eleftherios Goulielmakis et al. 2010).
