Heteronuclear Limit of Strong-Field Ionization: Fragmentation of ${\mathrm{HeH}}^{+}$ by Intense Ultrashort Laser Pulses

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Heteronuclear Limit of Strong-Field Ionization: Fragmentation of ${HeH}^{+}$ by Intense Ultrashort Laser Pulses

Philipp Wustelt, Florian Oppermann, Lun Yue, Max Möller, Thomas Stöhlker, Manfred Lein, Stefanie Gräfe, Gerhard G. Paulus, and A. Max Sayler

Phys. Rev. Lett. 121, 073203 – Published 17 August 2018

Abstract

The laser-induced fragmentation dynamics of this most fundamental polar molecule ${HeH}^{+}$ are measured using an ion beam of helium hydride and an isotopologue at various wavelengths and intensities. In contrast to the prevailing interpretation of strong-field fragmentation, in which stretching of the molecule results primarily from laser-induced electronic excitation, experiment and theory for nonionizing dissociation, single ionization, and double ionization both show that the direct vibrational excitation plays the decisive role here. We are able to reconstruct fragmentation pathways and determine the times at which each ionization step occurs as well as the bond length evolution before the electron removal. The dynamics of this extremely asymmetric molecule contrast the well-known symmetric systems leading to a more general picture of strong-field molecular dynamics and facilitating interpolation to systems between the two extreme cases.

Received 19 April 2018

DOI:https://doi.org/10.1103/PhysRevLett.121.073203

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.

Published by the American Physical Society

Physics Subject Headings (PhySH)

Atomic & molecular processes in external fields Electron correlation calculations for atoms & ions Electronic excitation & ionization Electronic structure of atoms & molecules Multiphoton or tunneling ionization & excitation

Ions Molecules

Photoionization Schroedinger equation Semiclassical methods

Atomic, Molecular & Optical

Authors & Affiliations

Philipp Wustelt^1,2,*, Florian Oppermann³, Lun Yue⁴, Max Möller^1,2, Thomas Stöhlker^1,2, Manfred Lein³, Stefanie Gräfe⁴, Gerhard G. Paulus^1,2,†, and A. Max Sayler^1,2

¹Institute of Optics and Quantum Electronics, Friedrich Schiller University Jena, D-07743 Jena, Germany
²Helmholtz Institute Jena, Fröbelstieg 3, 07743 Jena, Germany
³Institut für Theoretische Physik, Leibniz Universität Hannover, Appelstraße 2, 30167 Hannover, Germany
⁴Institute of Physical Chemistry, Friedrich Schiller University Jena, Helmholtzweg 4, D-07743 Jena, Germany

^*philipp.wustelt@uni-jena.de
^†gerhard.paulus@uni-jena.de

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Vol. 121, Iss. 7 — 17 August 2018

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Figure 1
Left: Extremal cases of molecular bonds: The highest occupied molecular orbital for the asymmetric ${HeH}^{+}$ (top), where the direction of the dipole moment is depicted by the red arrow, compared to the situation of the perfectly symmetric molecule $H_{2}^{+}$ (bottom). In the right panel, the potential energy curves of ${HeH}^{+} (X^{1} Σ^{+})$ , ${HeH}^{+} (A^{1} Σ^{+})$ , ${HeH}^{2 +}$ , and ${HeH}^{3 +}$ are displayed. Also shown is the population density of the ground state (blue curve) and the most likely ionization pathways. At low intensities, ${HeH}^{+}$ needs to stretch to 4 a.u. before ionization sets in (red arrow). Double ionization requires much higher intensities. The first ionization step, therefore, can take place already at slightly more than 2 a.u. while stretching to 10 a.u. may occur before the second ionization step (dark red). The middle panel zooms into the ground state and displays its vibrational levels. An 800-nm laser pulse (red arrow) will couple these in contrast to a 400-nm pulse (blue).
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Figure 2
Single ionization of ${HeH}^{+}$ . (a) Measured fragmentation yield as a function of intensity (log. scale) and KER (lower axis) or internuclear distance $R$ (upper axis) for 800-nm pulses. (b) Same as (a) for focal volume and vibrationally averaged TDSE results ( $I_{\max } = 10^{16} W / {cm}^{2}$ ). The yield is normalized for each intensity. At $I < 10^{15} W / {cm}^{2}$ substantial ionization is only possible after the bond has stretched to $R ≳ 4 a . u .$ , while ${HeH}^{+}$ can be ionized without appreciable stretching for $I \geq 10^{16} W / {cm}^{2}$ . At 800 nm, the higher vibrational states can be resonantly excited. This can be seen in panel (c), where computed population probabilities of the vibrational states of ${HeH}^{+}$ are displayed after interaction with 800-nm and 400-nm pulses ( $1 0^{15} W / {cm}^{2}$ ), which is representative for the excitation dynamics during the pulse where ionization quickly ionizes the population in the higher vibrational states. The correspondingly reduced stretching dynamics at 400 nm (cf. Ref. [13] for calculations in the context of high-harmonic generation at 600 nm) results in slightly larger KER at low intensities. This is shown in panel (e), which compares KER spectra for both wavelengths at $1 0^{15} W / {cm}^{2}$ . The dashed curves pertain to TDSE simulations. Stretching can also be impeded by increasing the reduced mass, which is accomplished by using the ${HeD}^{+}$ isotopologue in panel (f), where the KER spectra for 800 nm are displayed at a peak intensity of $\approx 10^{17} W / {cm}^{2}$ . The larger mass of ${HeD}^{+}$ inhibits stretching, which results in less population of the excited vibrational states as compared to ${HeH}^{+}$ , see (d). Note, that for the larger reduced mass of the molecule, higher vibrational levels were preferentially excited and correspond to similar extent along $R$ .
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Figure 3
Double ionization of ${HeH}^{+}$ : (a) Measured KER-dependent double ionization yield (solid green line) and, for comparison, the single ionization yield (solid red line) at $I \approx 10^{17} W / {cm}^{2}$ peak intensity. The corresponding computed spectra are represented by dashed curves ( $I_{max} = 6 \times 1 0^{15} {W/cm}^{2}$ ). (b) Joint $R_{1} − R_{2}$ distribution extracted from DSH simulations. The first ionization step occurs at $R_{1} \approx 2 a . u .$ , while the second electron can be ionized during an extended time interval as reflected by the broad $R_{2}$ distribution, see also Fig. 1. The fringes reflect the fact that the second ionization takes place only at the extrema of the laser field in each half-cycle. Panel (c) displays double ionization of ${HeH}^{+}$ in the time domain. The gray curve represents the instantaneous laser intensity. First and second ionization are plotted in the same colors as in panel (a).
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Physical Review Letters

Heteronuclear Limit of Strong-Field Ionization: Fragmentation of HeH+ by Intense Ultrashort Laser Pulses

Philipp Wustelt, Florian Oppermann, Lun Yue, Max Möller, Thomas Stöhlker, Manfred Lein, Stefanie Gräfe, Gerhard G. Paulus, and A. Max Sayler

Phys. Rev. Lett. 121, 073203 – Published 17 August 2018

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Heteronuclear Limit of Strong-Field Ionization: Fragmentation of ${HeH}^{+}$ by Intense Ultrashort Laser Pulses