In this talk, we will discuss the observation of intramolecular vibration dynamics using electrons rescattered during the process of high-order harmonic generation.[1] 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).