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arXiv:1611.09352 · Journal of Physics B Atomic Molecular and Optical Physics

Comment on ‘Time delays in molecular photoionization’

Denitsa Baykusheva, Hans Jakob Wörner

Published Mar 15, 2017• 6 citations
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Abstract

Abstract In a recent article by Hockett et al (2016 J. Phys. B: At. Mol. Opt. Phys. 49 095602), time delays arising in the context of molecular single-photon ionization are investigated from a theoretical point of view. We argue that one of the central equations given in this article is incorrect and present a reformulation that is consistent with the established treatment of angle-dependent scattering delays (Eisenbud 1948 PhD Thesis Princeton University; Wigner 1955 Phys. Rev. 98 145–7; Smith 1960 Phys. Rev. 118 349–6; Nussenzveig 1972 Phys. Rev. D 6 1534–42).

DOI
10.1088/1361-6455/aa62b5

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The age of attosecond physics was ushered in by the invention of methods for probing phenomena on a time scale less than femtoseconds [F. Krausz, M. Ivanov, Rev. Mod. Phys. 81, 163 (2009)]. In photoemission, the Eisenbud-Wigner-Smith (EWS) time delay [R. Pazourek, S. Nagele, J. Burgdorfer, Rev. Mod. Phys. 87, 765 (2015)] is the interval between photon annihilation and emission of the electron, usually measured in attoseconds ($10^{-18}$ seconds), and varies with the kinetic energy and the angle of emission of the electron wave packet. We present a new interferometric method of measurement, using two phase-locked Extreme Ultraviolet pulses of frequency $\omega$ and $2\omega$, from a Free-Electron Laser. Phase differences between one- and two-photon ionization channels are extracted, and used to derive the relative EWS delay for one- and two-photon ionization of neon as a function of emission angle. Agreement with theory is excellent. Competing techniques require the use of an IR field, which necessitates corrections, but our method is correction-free. This new quantum clock is a powerful tool for investigating ultrafast processes in atoms and molecules.

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Journal of Physics Conference Series•Jul 1, 2017•1 citation
Abstract In a recent article by Hockett et al (2016 J. Phys. B: At. Mol. Opt. Phys. 49 095602), time delays arising in the context of molecular single-photon ionization are investigated from a theoretical point of view. We argue that one of the central equations given in this article is incorrect and present a reformulation that is consistent with the established treatment of angle-dependent scattering delays (Eisenbud 1948 PhD Thesis Princeton University; Wigner 1955 Phys. Rev. 98 145–7; Smith 1960 Phys. Rev. 118 349–6; Nussenzveig 1972 Phys. Rev. D 6 1534–42).
This article addresses topics regarding time measurements performed on quantum systems. The motivation is linked to the advent of 'attophysics' which makes feasible to follow the motion of electrons in atoms and molecules, with time resolution at the attosecond (1 as = 10 −18 s) level, i.e. at the natural scale for electronic processes in these systems. In this context, attosecond 'time-delays' have been recently measured in experiments on photoionization and the question arises if such advances could cast a new light on the still active discussion on the status of the time variable in quantum mechanics. One issue still debatable is how to decide whether one can define a quantum time operator with eigenvalues associated to measurable 'time-delays', or time is a parameter, as it is implicit in the Newtonian classical mechanics. One objective of this paper is to investigate if the recent attophysics-based measurements could shed light on this parameter-operator conundrum. To this end, we present here the main features of the theory background, followed by an analysis of the experimental schemes that have been used to evidence attosecond 'time-delays' in photoionization. Our conclusion is that these results reinforce the view that time is a parameter which cannot be defined without reference to classical mechanics.
The reported observation of two-photon ionization of He by 13eV13\phantom{\rule{0.3em}{0ex}}\mathrm{eV}13eV photons of the TESLA Facility in T. Laarmann, A. R. B. de Castro, P. G\"urtler, W. Laasch, J. Schulz, H. Wabnitz, and T. M\"oller, Phys. Rev. A72, 023409 (2005) is ambiguous, as it is not supported by the experimental data presented. We discuss the lack of evidence for the claimed two-photon ionization.
The age of attosecond physics was ushered in by the invention of methods for probing phenomena on a time scale less than femtoseconds [F. Krausz, M. Ivanov, Rev. Mod. Phys. 81, 163 (2009)]. In photoemission, the Eisenbud-Wigner-Smith (EWS) time delay [R. Pazourek, S. Nagele, J. Burgdorfer, Rev. Mod. Phys. 87, 765 (2015)] is the interval between photon annihilation and emission of the electron, usually measured in attoseconds (10−1810^{-18}10−18 seconds), and varies with the kinetic energy and the angle of emission of the electron wave packet. We present a new interferometric method of measurement, using two phase-locked Extreme Ultraviolet pulses of frequency ω\omegaω and 2ω2\omega2ω, from a Free-Electron Laser. Phase differences between one- and two-photon ionization channels are extracted, and used to derive the relative EWS delay for one- and two-photon ionization of neon as a function of emission angle. Agreement with theory is excellent. Competing techniques require the use of an IR field, which necessitates corrections, but our method is correction-free. This new quantum clock is a powerful tool for investigating ultrafast processes in atoms and molecules.