Monitoring molecular dynamics using coherent electrons from high harmonic generation

1. 

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   Fig. 1.

   Schematic of experiment. At time 0 (t ₀), the femtosecond laser excites the Raman-active vibrations in SF₆ by ISRS. The vibrational wave packet then evolves in time, until times t = t ₁ when a probe femtosecond laser pulse generates high-order harmonics from the vibrationally excited molecule.

   
   
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   Fig. 2.

   The intensity of the 39th harmonic generated from vibrationally excited SF₆ as a function of time delay between the pump pulse and the EUV-generating probe pulse. The red curve shows the high harmonic emission with the pump pulse present. The black curve shows the emission without the pump pulse present.

   
   
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   Fig. 3.

   Comparison of vibrational frequencies. (A) Discrete Fourier transform of the 39th harmonic emission from Fig. 1, showing the three Raman-active modes of SF₆ that are excited by our ISRS pump pulse. (B) Stimulated anti-Stokes Raman scattering of a 400-fs probe pulse centered at 400 nm from SF₆, after excitation by the same ISRS pump pulse used to excite vibrations in Fig. 1. (C) Discrete Fourier transform of the harmonic emission data from Fig. 1 for 0.3-ps time intervals centered at different times after the pump pulse (0.45, 0.75, and 1.05 ps).

   
   
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   Fig. 4.

   Normal modes of vibrations for SF₆. The wavenumber, period, degeneracy, and activity of each mode is shown (36). SF₆ has three Raman-active modes, two IR-active modes, and one forbidden mode (which are not excited in our experiment).

   
   
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   Fig. 5.

   High harmonic signal for all observed harmonic orders. (Upper) Harmonic orders 23–47 are shown as a function of time delay. The high harmonic signal has been normalized by the signal without the pump pulse present. (Lower) Amplitude of the high harmonic modulation by the excited Raman-active modes as a function of harmonic order. The amplitude of all modes significantly increases with harmonic order. The modulation of the high harmonic signal due to the ν₂ mode is above the noise level only for harmonic orders >37.

   
   
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   Fig. 6.

   Fully quantum numerical simulations for a 1D triatomic molecule showing the high harmonic signal as a function of delay after the vibrations are initiated. The symmetric and antisymmetric vibrational modes of the molecule modulate the high harmonic signal by approximately equal amounts, although only one mode would be Raman-active. This result indicates that HHG should be sensitive to both Raman- and IR-active vibrational modes and, in general, to any type of molecular motion.