In this talk, we will discuss the observation of intramolecular vibration
dynamics using electrons rescattered during the process of high-order
harmonic generation. We excite coherent vibrations in molecules (both
spherically-symmetric and non-spherically-symmetric molecules) using
impulsive Raman scattering (ISRS) with a short laser pulse. A
second, more-intense laser pulse generates high-order harmonics of the
fundamental laser, at wavelengths of ~20-50 nm. The high-order harmonic
yield is observed to oscillate, at frequencies corresponding to all the
Raman-active modes of the molecules. In the case of SF6, an asymmetric
breathing mode is most visible. This is in contrast to conventional
ISRS, where only the symmetric breathing mode of the molecule is
observed. The SF6 data also show evidence of relaxation dynamics following
impulsive excitation of the molecule. Our results indicate that high
harmonic generation is a very sensitive probe of vibrational dynamics and
yields more information simultaneously than conventional ultrafast
spectroscopic techniques. Since the de Broglie wavelength of the
recolliding electron is on the order of interatomic distances, i.e. ~ 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 new spectroscopic
technique for probing ultrafast internal dynamics in molecules, and in
particular on the chemically-important ground state potential surface that
is difficult to probe using other techniques. High harmonic generation from
excited molecules is also sensitive in theory to Raman-active as well as
infrared-active vibrational modes.
1. Nick Wagner et al., to be published in PNAS (2006).
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