Exploring photoelectron angular distributions emitted from molecular dimers by two delayed intense laser pulses

Václav Hanus, Sarayoo Kangaparambil, Seyedreza Larimian, Martin Dorner-Kirchner, Xinhua Xie (谢新华), Andrius Baltuška, and Markus Kitzler-Zeiler

Phys. Rev. A 102, 053115 – Published 25 November 2020

Abstract

We describe the results of experiments and simulations performed with the aim of extending photoelectron spectroscopy with intense laser pulses to the case of molecular compounds. Dimer frame photoelectron angular distributions generated by double ionization of $N_{2} - N_{2}$ and $N_{2} - O_{2}$ van der Waals dimers with ultrashort, intense laser pulses are measured using four-body coincidence imaging with a reaction microscope. To study the influence of the first-generated molecular ion on the ionization behavior of the remaining neutral molecule we employ a two-pulse sequence comprising of a linearly polarized and a delayed elliptically polarized laser pulse that allows distinguishing the two ionization steps. By analysis of the obtained electron momentum distributions we show that scattering of the photoelectron on the neighboring molecular potential leads to a deformation and rotation of the photoelectron angular distribution as compared to that measured for an isolated molecule. Based on this result we demonstrate that the electron momentum space in the dimer case can be separated, allowing us to extract information about the ionization pathway from the photoelectron angular distributions. Our work, when implemented with a variable pulse delay, opens up the possibility of investigating light-induced electronic dynamics in molecular dimers using angularly resolved photoelectron spectroscopy with intense laser pulses.

Received 7 August 2020
Revised 4 November 2020
Accepted 9 November 2020

DOI:https://doi.org/10.1103/PhysRevA.102.053115

Physics Subject Headings (PhySH)

Multiphoton or tunneling ionization & excitation Photodissociation Photoemission Strong-field-induced spectra

Clusters Ions

Strong-field approximation

Atomic, Molecular & Optical

Authors & Affiliations

Václav Hanus ^1,2,*, Sarayoo Kangaparambil¹, Seyedreza Larimian¹, Martin Dorner-Kirchner ¹, Xinhua Xie (谢新华)^1,3, Andrius Baltuška¹, and Markus Kitzler-Zeiler ^1,†

¹Photonics Institute, Technische Universität Wien, 1040 Vienna, Austria, European Union
²Wigner Research Centre for Physics, Institute for Solid State Physics and Optics, 1121 Budapest, Hungary, European Union
³SwissFEL, Paul Scherrer Institute, 5232 Villigen PSI, Switzerland

^*vaclav.hanus@tuwien.ac.at
^†markus.kitzler-zeiler@tuwien.ac.at

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Issue

Vol. 102, Iss. 5 — November 2020

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Images

Figure 1
(a) Measured photoion-photoion coincidence (PIPICO) distributions obtained by application of both the pump and probe pulses. Indicated are the three dimer species that are generated and detected in our experiment. (b) Kinetic energy distributions (KER) that result from Coulomb explosions initiated by laser double ionization of the three dimer species shown in (a) when both the pump and probe laser pulses were applied. (c) Schematic representation of the experiment. A linearly polarized and an elliptically polarized pulse delayed by 33 ps each remove one electron (cyan and pink electric field-vector traces) from one of the two molecules in the dimer. Depicted is the example of a dimer consisting of an $N_{2}$ molecule with $σ$ geometry of the highest occupied molecular orbital (HOMO) and an $O_{2}$ molecule with a $π$ geometry of the HOMO. (d) Measured electron momentum distribution in the laser polarization plane for $N_{2}$ after interaction with the double-pulse sequence. The distributions for the dimers look similar. The narrow elongated structure in the cyan box corresponds to electrons emitted during the linearly polarized pulse, for which electron emission happens dominantly along the laser polarization axis [cf. cyan sphere in (c)]. The arc segment-shaped parts in the pink boxes are due to electron emission in the elliptically polarized delayed pulse [cf. pink sphere in (c)].
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Figure 2
Measured molecular frame and dimer frame photoelectron angular distributions (MFPADs, respectively DFPADs) with statistical error bars for the (a) $N_{2}$ molecule, (b) $N_{2} - N_{2}$ dimer, and (c) $N_{2} - O_{2}$ dimer. The lines are to guide the eye and represent fits to the measured data. Depicted are the angular distributions of the second electron released during the elliptically polarized pulse. For the DFPAD in (c) the electrons for both orientations of the $N_{2} - O_{2}$ dimer were superposed. The molecular, respectively dimer, axes retrieved from the ions' momenta are aligned horizontally, which coincides with the major axis of the polarization ellipse, see the cartoon between (b) and (c), which also depicts the clockwise rotation of the electric field vector.
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Figure 3
(a)–(d) Results of simulations detailed in the text and explanatory sketches. The gray line in (b) shows the simulated angular distribution of photoelectrons emitted from a molecular-type potential depicted in (a). The assumed internuclear distance of 2.46 a.u. corresponds to that of neutral $N_{2}$ . The red line in (b) shows the photoelectron angular distribution when the electrons tunnel from the up-field potential well towards the left. This situation, depicted in (c) with a red circle indicating the tunneling electron, occurs when the laser electric field vector points to right (as indicated by an arrow). The green line in (b) shows the angular distribution for the opposite case, sketched in (d), when the electric field vector points to the left and the electron (green circle) is ejected to the right. (e) Measured angular distribution of the photoelectron emitted from the $N_{2} - O_{2}$ dimer during the elliptically polarized probe pulse. The black line shows the data when the dimer is oriented such that $N_{2}$ is on the right side. For clarity, only one representative statistical error bar is shown (around $90^{\circ}$ ). The red line is added for reference and shows the angular distribution of the nonoriented dimer, duplicated from Fig. 2. See text for further details.
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Figure 4
Simulated electron momentum distributions in the polarization plane for the molecular model potential shown in Fig. 3 and the dimer model potential shown in Fig. 3, reproduced for convenience in the inset. The data in (a) were used to plot the gray distribution in Fig. 3, while the data in (b) were used for the green distribution in Fig. 3. (c) and (d) Example trajectory (blue line) of an electron tunneling from the up-field site of the model dimer in position space (c) and momentum space (d). The green dot marks the beginning of the trajectory while the red dot marks the final momentum value, indicated in both (b) and (d). The color density in (c) depicts the Coulomb potential of the doubly charged dimer. The dashed arrow in (b) denotes the clockwise rotation of the laser electric field vector.
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