Coherent electronic-vibrational dynamics in deuterium bromide probed via attosecond transient-absorption spectroscopy

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Coherent electronic-vibrational dynamics in deuterium bromide probed via attosecond transient-absorption spectroscopy

Yuki Kobayashi, Kristina F. Chang, Sonia Marggi Poullain, Valeriu Scutelnic, Tao Zeng, Daniel M. Neumark, and Stephen R. Leone

Phys. Rev. A 101, 063414 – Published 22 June 2020

Abstract

Ultrafast laser excitation can trigger complex coherent dynamics in molecules. Here, we report attosecond transient-absorption experiments addressing simultaneous probing of electronic and vibrational dynamics in a prototype molecule, deuterium bromide (DBr), following its strong-field ionization. Electronic and vibrational coherences in the ionic $X {}^{2}Π_{3 / 2}$ and $X {}^{2}Π_{1 / 2}$ states are characterized in the Br- $3 d$ core-level absorption spectra via quantum beats with 12.6-fs and 19.9-fs periodicities, respectively. Polarization scans reveal that the phase of the electronic quantum beats depends on the probe direction, experimentally showing that the coherent electronic motion corresponds to oscillation of the hole density along the ionization-field direction. The vibrational quantum beats are found to maintain a relatively constant amplitude, whereas the electronic quantum beats exhibit a partial decrease in time. Quantum wave-packet simulations show that decoherence from vibrational motion is insignificant because the $X {}^{2}Π_{3 / 2}$ and $X {}^{2}Π_{1 / 2}$ potentials are nearly parallel. A comparison between the DBr and HBr results suggests that rotational motion is responsible for the decoherence since it leads to initial alignment prepared by the strong-field ionization.

Received 9 April 2020
Accepted 22 May 2020

DOI:https://doi.org/10.1103/PhysRevA.101.063414

Physics Subject Headings (PhySH)

Atomic & molecular processes in external fields Molecular spectra Multiphoton or tunneling ionization & excitation Ultrafast phenomena

X-ray absorption spectroscopy

Atomic, Molecular & Optical

Authors & Affiliations

Yuki Kobayashi ^1,*, Kristina F. Chang ¹, Sonia Marggi Poullain ^1,2, Valeriu Scutelnic ¹, Tao Zeng ³, Daniel M. Neumark ^1,4,†, and Stephen R. Leone ^1,4,5,‡

¹Department of Chemistry, University of California, Berkeley, California 94720, USA
²Departamento de Qumica Fsica, Facultad de Ciencias Qumicas, Universidad Complutense de Madrid, 28040 Madrid, Spain
³Department of Chemistry, York University, Toronto, Ontario, Canada M3J1P3
⁴Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
⁵Department of Physics, University of California, Berkeley, California 94720, USA

^*ykoba@berkeley.edu
^†dneumark@berkeley.edu
^‡srl@berkeley.edu

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Vol. 101, Iss. 6 — June 2020

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Images

Figure 1
(a) Pump-probe scheme of the experiment. A NIR pulse drives strong-field ionization (SFI), and a XUV probe pulse records the dynamics via Br- $3 d$ core-level absorption signals. (b) Potential energy curves of DBr computed for the neutral ground state (bottom), ionic valence states (middle), and ionic core-excited states (top). Different colors indicate the associated quantum numbers ( $Ω = 1 / 2, 3 / 2, and 5 / 2$ ) of the ionic states. The red arrows show the ionization step by the strong NIR pulse, and the blue arrow shows the core-to-valence transition by the attosecond XUV pulse. (c) Experimental transient-absorption spectrum of DBr at 200 fs delay time. (d), (e) Simulated absorption signals for the $X {}^{2}Π_{3 / 2}$ and $X {}^{2}Π_{1 / 2}$ states. Decomposition into each probe state is denoted.
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Figure 2
(a) Experimental delay-dependent transient-absorption spectra of DBr. The pump and probe pulses are parallelpolarized. Multiple quantum beats are resolved, showing the electronic and vibrational coherences induced by the strong-field ionization. (b) Fourier transformation (FT) of the experimental spectra along the delay axis. Main FT components are marked by dashed boxes with the numbers indicating the beat frequencies in units of eV.
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Figure 3
(a) Simulated delay-dependent transient-absorption spectra for the coherently prepared $X {}^{2}Π_{3 / 2}$ and $X {}^{2}Π_{1 / 2}$ states of ${DBr}^{+}$ . (b) Fourier transformation of the simulated spectra along the delay axis. The 0.217 eV (vibrational) and 0.327 eV (electronic) components successfully reproduce the experimental quantum beats. (c), (d) Illustration of the probing mechanisms of vibrational and electronic coherences.
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Figure 4
Comparison between the parallel-polarization results [blue, Fig. 2] and the perpendicular-polarization results [yellow, Fig. S3(a)]. Three lineouts of the absorption signals are taken at (a) 70.15 eV, (b) 66.69 eV, and (c) 65.96 eV. The shades represent one standard deviation. The inset in (b) shows an expanded view of the early-time electronic quantum beats. Quantum beats originating from the vibrational coherences are identical between the two measurements, whereas those originating from the electronic coherence show clear contrast.
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Figure 5
(a)–(c) Time-window Fourier transformation analysis of the DBr results for the (a) parallel measurements, (b) perpendicular measurements, and (c) simulations. Time evolution of the oscillation amplitude from the electronic and vibrational coherences is extracted. (d) Normalized signal amplitude for the electronic quantum beats (orange) and vibrational quantum beats (blue) of the DBr results. The electronic quantum beats in the parallel (solid curve) and perpendicular (dotted curve) measurements exhibit a partial decrease, whereas the simulated result (dashed curve) maintains a constant amplitude. (e) A comparison of the electronic quantum beats between the DBr (blue) and HBr (red) results. The dots show the experimental absorption signals at 66.7 eV, and the solid curves show the fitting with a cosine function and an exponential decay. The fitted decay times ( $τ_{1 / e}$ ) are $177 \pm 44$ fs for DBr, and $119 \pm 24$ fs for HBr, which yields a ratio of $1.5 \pm 0.5$ . (f) Time evolution of the anisotropy parameter $r (t)$ calculated for the Boltzmann distributions of DBr and HBr at room temperature. The decay times are 180 fs for DBr and 127 fs for HBr, and the predicted ratio is 1.42. The horizontal dashed line shows the value of $r (0) / e$ .
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