Tunnelling is one of the key features of quantum mechanics. A related debate, ongoing since the inception of quantum theory, is about the value, meaning and interpretation of 'tunnelling time' 1-5 . Simply put, the question is whether a tunnelling quantum particle spends a finite and measurable time under a potential barrier. Until recently the debate was purely theoretical, with the process considered to be instantaneous for all practical purposes. This changed with the development of ultrafast lasers and attosecond metrology 6 , which gave physicists experimental access to the attosecond (1 as = 10 -18 s) domain. It is at this time scale
We have measured fully differential cross sections for photo double ionization (PDI) of helium 450 eV above the threshold. We have found an extremely asymmetric energy sharing between the photoelectrons and an angular asymmetry parameter β ≃ 2 and β ≃ 0 for the fast and slow electrons, respectively. The electron angular distributions show a dominance of the shake-off for 2 eV electrons and clear evidence of an inelastic electron-electron scattering at an electron energy of 30 eV. The data are in excellent agreement with CCC calculations.
Time delays of electrons emitted from an isotropic initial state and leaving behind an isotropic ion are assumed to be angle-independent. Using an interferometric method involving XUV attosecond pulse trains and an IR probe field in combination with a detection scheme, which allows for full 3D momentum resolution, we show that measured time delays between electrons liberated from the 1s 2 spherically symmetric ground state of helium depend on the emission direction of the electrons relative to the linear polarization axis of the ionizing XUV light. Such time-delay anisotropy, for which we measure values as large as 60 attoseconds, is caused by the interplay between final quantum states with different symmetry and arises naturally whenever the photoionization process involves the exchange of more than one photon in the field of the parent-ion. With the support of accurate theoretical models, the angular dependence of the time delay is attributed to small phase differences that are induced in the laser-driven continuum transitions to the final states. Since most measurement techniques tracing attosecond electron dynamics involve the exchange of at least two photons, this is a general, significant, and initially unexpected effect that must be taken into account in all such photoionization measurements.
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