**5. References**

344 Modern Metrology Concerns

With a step size roughly matching the XUV pulse duration the resulting spectrogram can look as in Fig. 5 and it is immediately evident that this so called "streaking spectroscopy" reveals the temporal evolution of the laser electric field (more precisely its vector potential) (E Goulielmakis et al. 2004) (Fiess et al. 2010). Beyond that, this sort of spectrograms resembles the signal of frequency resolved optical gating devices (FROG) known from laser science (Trebino & Kane 1993); (Mairesse & Quéré 2005); (Justin Gagnon & Vladislav S. Yakovlev 2009). Since their origin is a cross-correlation of two unknown pulses recorded with a known response function (here the photoionization) adaptive algorithms can extract all relevant parameters of **both** pulses involved (Michael Hofstetter et al. 2011). Such algorithms identify the temporal structure of the attosecond pulses and their temporal and spectral phase and thus fully characterize these light bursts that are the shortest signals that can be synthesized in the laboratory. It is an interesting feature of this streaking concept that the reconstruction actually explores the temporal structure of the released photoelectron wavepacket rather than of the light pulse and thus gives access to characteristics of the photo emission process that so far remained inaccessible to experimental physics. For photon energies that are sufficiently high to address two or more individual atomic orbitals and thus liberate electrons from different energetic states (as shown in Fig. 5) the streaking technique facilitates the investigation of subtle timing differences between them. The growing ability to shape the XUV emission according to the requirements of a particular experiment extends the technique to a wider spectral range. With specially tailored attosecond XUV pulses it was possible to discover that the emission of electrons from the neon 2s shell precedes the liberation of electrons originally bound in the 2p shell by about 20 attoseconds, the shortest time interval ever sampled directly

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

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

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?

(M Schultze et al. 2010).

regarding their physical origin.

complex and intertwined processes in real time.

**4. Conclusion** 


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