Monitoring molecular dynamics using coherent electrons from high harmonic generation

Abstract

We report a previously undescribed spectroscopic probe that makes use of electrons rescattered during the process of high-order harmonic generation. We excite coherent vibrations in SF(6) using impulsive stimulated Raman scattering with a short laser pulse. A second, more intense laser pulse generates high-order harmonics of the fundamental laser, at wavelengths of approximately 20-50 nm. The high-order harmonic yield is observed to oscillate, at frequencies corresponding to all of the Raman-active modes of SF(6), with an asymmetric mode most visible. The data also show evidence of relaxation dynamics after impulsive excitation of the molecule. Theoretical modeling indicates that the high harmonic yield should be modulated by both Raman and infrared-active vibrational modes. Our results indicate that high harmonic generation is a very sensitive probe of vibrational dynamics and may yield more information simultaneously than conventional ultrafast spectroscopic techniques. Because the de Broglie wavelength of the recolliding electron is on the order of interatomic distances, i.e., approximately 1.5 A, small changes in the shape of the molecule lead to large changes in the high harmonic yield. This work therefore demonstrates a previously undescribed spectroscopic technique for probing ultrafast internal dynamics in molecules and, in particular, on the chemically important ground-state potential surface.

Fig. 1.

Schematic of experiment. At time 0 (t₀), the femtosecond laser excites the Raman-active vibrations in SF₆ by ISRS. The vibrational wave packet then evolves in time, until times t = t₁ when a probe femtosecond laser pulse generates high-order harmonics from the vibrationally excited molecule.

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.

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).

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).

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.

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.