Following a chemical reaction using high-harmonic interferometry

Reaction tracking with high-harmonic spectroscopy New methods are emerging that aim to image chemical reactions as they occur, using X-ray diffraction, electron diffraction or laser-induced recollision. But none of these methods offer spectral selection, which allows a laser pulse with light of one wavelength to initiate a reaction, and a second pulse with another, appropriately selected wavelength to monitor the reacting molecules. Wörner et al . now show that this apparent limitation offers exciting opportunities for recollision-based high-harmonic spectroscopy: due to the coherent nature of the attosecond high-harmonic pulse generation, unexcited molecules can act as local oscillators against which structural and electronic dynamics is observed on an attosecond timescale. High-harmonic spectroscopy thus seems ideally suited to measure coupled electronic and nuclear dynamics in fast photochemical reactions, or to characterize short-lived transition states. New methods are emerging that aim to image chemical reactions as they occur using X-ray diffraction, electron diffraction or laser-induced recollision, but spectral selection cannot be used to monitor the reacting molecules for these methods. These authors show that this apparent limitation offers opportunities for recollision-based high-harmonic spectroscopy, in which unexcited molecules can act as local oscillators against which structural and electronic dynamics is observed on an attosecond timescale. The study of chemical reactions on the molecular (femtosecond) timescale typically uses pump laser pulses to excite molecules and subsequent probe pulses to interrogate them. The ultrashort pump pulse can excite only a small fraction of molecules, and the probe wavelength must be carefully chosen to discriminate between excited and unexcited molecules. The past decade has seen the emergence of new methods that are also aimed at imaging chemical reactions as they occur, based on X-ray diffraction 1 , electron diffraction 2 or laser-induced recollision 3 , 4 —with spectral selection not available for any of these new methods. Here we show that in the case of high-harmonic spectroscopy based on recollision, this apparent limitation becomes a major advantage owing to the coherent nature of the attosecond high-harmonic pulse generation. The coherence allows the unexcited molecules to act as local oscillators against which the dynamics are observed, so a transient grating technique 5 , 6 can be used to recon