Speaker
Ursula Keller
(ETH Zürich)
Description
Theoretical models often fail to describe the dynamics due
to numerical limitations,
and we need to develop better approximate models for the
ultrafast dynamics on an
atomic scale. Novel time-resolved attosecond streaking
techniques such as energy
streaking and the attoclock, and in addition more recently
interference techniques are
currently being applied in an attempt to answer a very
fundamental question in
quantum mechanics: how fast can light remove a bound
electron from an atom or a
solid?
We have used the attoclock technique [1] to gain more insight in strong laser field ionization, where a the strong laser field bends the binding potential to emit an electron by tunneling (tunnel ionization). Initially we have measured an instantaneous tunneling delay time in helium [1]. More recently we have confirmed no measurable tunneling delay time over a larger intensity regime with the Keldysh parameter well below unity for the first time (which is actually the real regime of tunneling) and extend our studies to argon with the same outcome with regards to the tunneling delay time [2].
Tunneling is described by three important parameters: tunneling rate, tunneling time and tunnel geometry. While the first two have been discussed widely in the last decades, we have recently determined the tunnel geometry for the first time [2]. The attoclock is a unique tool that directly reveals the tunnel geometry and therefore the more complex electron ion interaction. We show that the Coulomb correction alone is not sufficient and multi-electron effects can be important even for atoms such as argon. Possible systematic errors are eliminated with the attoclock by simply using clockwise and anticlockwise streaking fields. In collaboration with Prof. Lars B. Madsen’s group from Aarhus University we developed a modified semiclassical model that agrees well with our attoclock experiments [2]. The theory represents a "textbook" quality and represents everything a theory has to do: describes the experiment, relies on accurate assumptions, and new effects are clearly identified, stated and quantified. Even more so, the polarizabilities that enters the model are calculated and measured using other independent methods. Attosecond measurements have typically used attosecond streaking techniques for which the detailed electron ion interaction need to be understood on an attosecond time scale. If this interaction is not understood correctly, wrong conclusions could be drawn on possible time delays. To date attosecond streaking has been applied to atomic, molecular or solid target. Multielectron effects as pointed out here are even more important for more complex targets.
[1] P. Eckle, A. Pfeiffer, C. Cirelli, A. Staudte, R. Dörner, H. G. Muller, M. Büttiker, U. Keller, Attosecond ionization and tunneling delay time measurements Science, vol. 322, pp. 1525-1529, 2008
[2] A. N. Pfeiffer, C. Cirelli, M. Smolarski, D. Dimitrovski, M. Abu-samha, L. B. Madsen, U. Keller, Attoclock reveals geometry of laser-induced tunnel ionization arXiv:1103.4803v1 [physics.atom-ph] online 25. March 2011
We have used the attoclock technique [1] to gain more insight in strong laser field ionization, where a the strong laser field bends the binding potential to emit an electron by tunneling (tunnel ionization). Initially we have measured an instantaneous tunneling delay time in helium [1]. More recently we have confirmed no measurable tunneling delay time over a larger intensity regime with the Keldysh parameter well below unity for the first time (which is actually the real regime of tunneling) and extend our studies to argon with the same outcome with regards to the tunneling delay time [2].
Tunneling is described by three important parameters: tunneling rate, tunneling time and tunnel geometry. While the first two have been discussed widely in the last decades, we have recently determined the tunnel geometry for the first time [2]. The attoclock is a unique tool that directly reveals the tunnel geometry and therefore the more complex electron ion interaction. We show that the Coulomb correction alone is not sufficient and multi-electron effects can be important even for atoms such as argon. Possible systematic errors are eliminated with the attoclock by simply using clockwise and anticlockwise streaking fields. In collaboration with Prof. Lars B. Madsen’s group from Aarhus University we developed a modified semiclassical model that agrees well with our attoclock experiments [2]. The theory represents a "textbook" quality and represents everything a theory has to do: describes the experiment, relies on accurate assumptions, and new effects are clearly identified, stated and quantified. Even more so, the polarizabilities that enters the model are calculated and measured using other independent methods. Attosecond measurements have typically used attosecond streaking techniques for which the detailed electron ion interaction need to be understood on an attosecond time scale. If this interaction is not understood correctly, wrong conclusions could be drawn on possible time delays. To date attosecond streaking has been applied to atomic, molecular or solid target. Multielectron effects as pointed out here are even more important for more complex targets.
[1] P. Eckle, A. Pfeiffer, C. Cirelli, A. Staudte, R. Dörner, H. G. Muller, M. Büttiker, U. Keller, Attosecond ionization and tunneling delay time measurements Science, vol. 322, pp. 1525-1529, 2008
[2] A. N. Pfeiffer, C. Cirelli, M. Smolarski, D. Dimitrovski, M. Abu-samha, L. B. Madsen, U. Keller, Attoclock reveals geometry of laser-induced tunnel ionization arXiv:1103.4803v1 [physics.atom-ph] online 25. March 2011
