Tracking the Ionization Site in Neutral Molecules

Open Access

Tracking the Ionization Site in Neutral Molecules

L. Ortmann, A. AlShafey, A. Staudte, and A. S. Landsman

Phys. Rev. Lett. 127, 213201 – Published 16 November 2021

Abstract

When a diatomic molecule is exposed to intense light, the valence electron may tunnel from a higher potential (corresponding to an upfield atom) due to the suppressed internuclear barrier. This process is known as ionization enhancement and is a key mechanism in strong field ionization of molecules. Alternatively, the bound electron wave function can evolve adiabatically in the laser field, resulting in ionization from the downfield atom. Here, we introduce a method to quantify the relative contribution of these two processes. Applying this method to experimentally measured electron momenta distributions following strong field ionization of $N_{2}$ with infrared laser light, we find approximately a $2 ∶ 1$ ratio of electrons ionized from a downfield atom, relative to upfield. This suggests that the bound state wave function largely adapts adiabatically to the changing laser field, although the nonadiabatic process of ionization enhancement still contributes even in neutral molecules. Our method can be applied to any diatomic neutral molecule to better understand the evolution of the initially bound electron wave packet and hence the nature of the molecular ionization process.

Received 16 October 2020
Revised 19 September 2021
Accepted 15 October 2021

DOI:https://doi.org/10.1103/PhysRevLett.127.213201

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Open access publication funded by the Max Planck Society.

Published by the American Physical Society

Physics Subject Headings (PhySH)

Atomic & molecular processes in external fields Multiphoton or tunneling ionization & excitation Ultrafast phenomena Ultrashort pulses

Atomic, Molecular & Optical

Authors & Affiliations

L. Ortmann ^1,2,*, A. AlShafey², A. Staudte³, and A. S. Landsman^2,†

¹Max Planck Institute for the Physics of Complex Systems, Nöthnitzer Straße 38, D-01187 Dresden, Germany
²Department of Physics, The Ohio State University, Columbus, Ohio 43210, USA
³Joint Attosecond Science Lab of the National Research Council and the University of Ottawa, Ottawa, Ontario K1A 0R6, Canada

^*ortmann.1@osu.edu
^†landsman.7@osu.edu

Article Text

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Supplemental Material

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References

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Issue

Vol. 127, Iss. 21 — 19 November 2021

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Images

Figure 1
Illustration of a diatomic potential for alignment of the internuclear axis along the polarization axis. The ionization barrier for the upper well is considerably lowered by the influence of the lower well, which can lead to ionization enhancement. However, the force $F$ of the electric field can drive the electron toward the downfield well, which is expected to play a role if the electron position adapts adiabatically to the laser field.
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Figure 2
Top panel: experimental 2D photoelectron momentum distribution for the 45° case from [43]. The inset shows a sketch of the molecular orientation and the laser polarization direction. Bottom panel: $v_{z, mean}$ as a function of $v_{x}$ . The green line represents the values extracted from the experimental data (top panel) minus the orientation averaged experimental data (for details see Supplemental Material [44]). The ensemble of curves was obtained from QTMC simulations over a range of ionization ratios $q$ , which is specified by the color of the curve (see color bar). The theoretical curve that matches the experimental data best according to the average offset momentum $a$ (see Fig. 4) is shown as a bold line with black dashes. In both the theoretical and experimental evaluation leading to the displayed curves the 2D momentum distribution was restricted to $v_{x} \in [- 1, 1]$ and $v_{z} \in [- 1, 1]$ .
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Figure 3
(a) Field strength and vector potential in the pulse. Quarter cycles with decreasing absolute field strength are highlighted in green, whereas increasing absolute field strengths are marked in orange. (b),(d) Decreasing absolute field strength. Each panel shows two trajectories of electrons with an initial transverse momentum of the same absolute value but opposite sign. The two electrons go around the parent molecular ion and the fact that they are deflected differently results in an asymmetry of the momentum distribution at the detector. (c),(e) Increasing absolute field strength. The electrons have a positive final velocity again because the same sign of $A$ is chosen. The electrons go directly to the detector and are deflected less strongly by the asymmetric Coulomb potential. Note that these trajectories were calculated with an oversized molecule for illustrative purposes. For an accurate calculation with trajectories of the real system we refer to Supplemental Material [44].
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Figure 4
Average offset momentum $a$ according to Eq. (4) as a function of the ionization site ratio $q$ according to Eq. (2). The colors of the dots are chosen according to the color coding for $q$ in the color bar of Fig. 2. The error bars denote the uncertainty for $\pm 5 °$ deviations from 45° orientation. The experimental value is plotted as a larger green dot, with the horizontal and vertical green lines at the upper and lower error bar.
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