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Probing Proton Dynamics in Molecules on an Attosecond Time Scale

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Science  21 Apr 2006:
Vol. 312, Issue 5772, pp. 424-427
DOI: 10.1126/science.1123904
  • Fig. 1.

    Pump-probe method for measuring proton dynamics in molecules. (A) The experimental setup required to observe high-harmonic emission. (B) Ionization serves as the pump process because it launches an electron wavepacket into the continuum simultaneously with a nuclear wavepacket on the H2+ ground state potential surface (σg). The electron wavepacket then moves in response to the laser field, returning to the parent ion with an increased kinetic energy at some later time. The recollision acts as the probe of the nuclear motion that has occurred in the time delay since ionization occurred.

  • Fig. 2.

    Encoding of nuclear dynamics within harmonic spectra. Upper panel: The trajectory of the ionized electron differs depending on the exact time of ionization. Three possible electron trajectories labeled 1, 2, and 3 are shown, which recollide with the molecular ion after delays Δt1, Δt2, and Δt3, with increasing kinetic energy E1, E2, and E3, resulting in the emission of increasingly higher frequency photons after recombination (shown as the 17th, 25th, and 33rd harmonics for the purpose of this illustration). Note: Although the curves in the lower panel are physically accurate, the electron trajectories shown in the upper panel have been slightly altered to improve clarity.

  • Fig. 3.

    Harmonic emission in H2 and D2. (A) Raw CCD images on a common intensity scale (red represents brightest signal, blue weakest) revealing that at all orders observed, harmonic emission is weaker in H2 than in D2 at the same density. (B) Ratio of harmonic peak intensities for D2 and H2 (black). Vertical errors represent SEM for 400 laser shots. Horizontal errors are estimated from quantum mechanical energy-time uncertainty. The control ratio of two harmonic spectra from H2 taken separately is also shown (red) and is seen to be unity for all harmonic orders, as expected. The blue line is a calculation of harmonic ratio (described in text). (C) The nuclear motion reconstructed from the experimental data by multiple runs of a genetic algorithm (red curves) converges closely to the exact result (blue curves) calculated using the exact Born-Oppenheimer potentials for H2+ and D2+.

  • Fig. 4.

    Probing structural rearrangement in CH4 and CD4. (A) Ratio of harmonic signals in CD4 and CH4 (black). The error represents SE over 200 laser shots. Also shown is the control ratio of two harmonic spectra from CD4 taken separately (red). (B) Known structures of CH4 and CH4+ at equilibrium. Upon removal of an electron, it is anticipated that CH4 will rapidly evolve toward the CH4+ structure shown.

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  • Probing Proton Dynamics in Molecules on an Attosecond Time Scale
    S. Baker, J. S. Robinson, C. A. Haworth, H. Teng, R. A. Smith, C. C. Chirilă, M. Lein, J. W. G. Tisch, J. P. Marangos

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