Final Report Summary - ATTOCLOCK (Clocking fundamental attosecond electron dynamics)
While the experiments seem to agree that quantum tunneling does not happen instantaneously, there is no consensus yet on the theoretical side and both statements “cannot be measured because time is not an operator” and “just follow the peak of the wavepacket” do not resolve this issue. Following the peak of a wavepacket, for example, can be tricky and often misleading – not only in tunnel ionzation but also in single-photon ionization.
Refined attoclock measurements revealed a real (not instantaneous) tunneling delay time over a large intensity regime in He, using two independent experimental apparatus. Only two theoretical predictions are compatible within our experimental error: the Larmor time, and the probability distribution of tunneling times constructed using a Feynman Path Integral (FPI) formulation. The latter better matches the observed qualitative change in tunneling time over a wide intensity range, and predicts a broad tunneling time distribution with a long tail. The implication of such a probability distribution of tunneling times, as opposed to a distinct tunneling time, would imply that one must account for a significant, though bounded and measurable, uncertainty as to when the hole dynamics begin to evolve [Optica, vol. 1, No. 5, pp. 343-349, 2014].
This can be understood in very simply terms: In contrast to a light pulse, an electron wavepacket disperses even in vacuum. Since the propagation of the peak of the wavepacket is defined by the group delay, almost any group delay can be measured during propagation in combination with an appropriate energy-dependent transmission filter [Phys. Rev. Lett., vol. 115, 133001, 2015]. In fact we could show that strong-field ionization in the dipole approximation (i.e. tunnel ionization) is much faster than the group delay of the electron wavepacket (i.e. Wigner delay).
This is in contrast to the simplest case in photoemission time delays, when the electron is promoted into a flat (non-resonant) continuum by direct laser-assisted photoionization. In this case the measured delay after absorbing a single XUV photon is related to the group delay of the departing electron wave packet induced by the ionic potential and laser field, respectively. This delay is also referred to as the Wigner delay.
We believe that our angle and spectrally resolved results [Nature Comm. 9, 955, 2018], presented both for non-resonant and resonant case, represent the most complete study of the atomic time delay evolution across an autoionizing state, and thus allow direct access to the role of electron correlation in the auto ionization process. Our results clearly demonstrate that not only the phase of the photoelectron wave packet is significantly distorted in the presence of resonances, but that this distortion depends on the electron emission angle. This prevents one from interpreting the Wigner delay as photoemission time delay! This is very analogous to the tunneling delay time which is not correctly explained by the Wigner time – in fact is much faster.
Furthermore molecules are even more challenging. For example with lighter atoms involved - as in most organic and biological molecules containing hydrogen - the electron and nuclear motions significantly affect each other even in the attosecond regime as shown with a detailed study of the electronic-nuclear coupling in dissociative ionization of H2 [Nature Phys., accepted]. In addition for example we showed that orientation- and energy-resolved measurements supply an experimental observable, the stereo Wigner time delay, which provides direct information on the localization of the excited electron wave packet within the asymmetric CO potential at the moment of ionization [Science, in review].
Refined attoclock measurements revealed a real (not instantaneous) tunneling delay time over a large intensity regime in He, using two independent experimental apparatus. Only two theoretical predictions are compatible within our experimental error: the Larmor time, and the probability distribution of tunneling times constructed using a Feynman Path Integral (FPI) formulation. The latter better matches the observed qualitative change in tunneling time over a wide intensity range, and predicts a broad tunneling time distribution with a long tail. The implication of such a probability distribution of tunneling times, as opposed to a distinct tunneling time, would imply that one must account for a significant, though bounded and measurable, uncertainty as to when the hole dynamics begin to evolve [Optica, vol. 1, No. 5, pp. 343-349, 2014].
This can be understood in very simply terms: In contrast to a light pulse, an electron wavepacket disperses even in vacuum. Since the propagation of the peak of the wavepacket is defined by the group delay, almost any group delay can be measured during propagation in combination with an appropriate energy-dependent transmission filter [Phys. Rev. Lett., vol. 115, 133001, 2015]. In fact we could show that strong-field ionization in the dipole approximation (i.e. tunnel ionization) is much faster than the group delay of the electron wavepacket (i.e. Wigner delay).
This is in contrast to the simplest case in photoemission time delays, when the electron is promoted into a flat (non-resonant) continuum by direct laser-assisted photoionization. In this case the measured delay after absorbing a single XUV photon is related to the group delay of the departing electron wave packet induced by the ionic potential and laser field, respectively. This delay is also referred to as the Wigner delay.
We believe that our angle and spectrally resolved results [Nature Comm. 9, 955, 2018], presented both for non-resonant and resonant case, represent the most complete study of the atomic time delay evolution across an autoionizing state, and thus allow direct access to the role of electron correlation in the auto ionization process. Our results clearly demonstrate that not only the phase of the photoelectron wave packet is significantly distorted in the presence of resonances, but that this distortion depends on the electron emission angle. This prevents one from interpreting the Wigner delay as photoemission time delay! This is very analogous to the tunneling delay time which is not correctly explained by the Wigner time – in fact is much faster.
Furthermore molecules are even more challenging. For example with lighter atoms involved - as in most organic and biological molecules containing hydrogen - the electron and nuclear motions significantly affect each other even in the attosecond regime as shown with a detailed study of the electronic-nuclear coupling in dissociative ionization of H2 [Nature Phys., accepted]. In addition for example we showed that orientation- and energy-resolved measurements supply an experimental observable, the stereo Wigner time delay, which provides direct information on the localization of the excited electron wave packet within the asymmetric CO potential at the moment of ionization [Science, in review].