Attosecond atoms

Atoms reacting on the attosecond timescale can now be observed with unprecedented detail using a new laser spectroscopy technique reported recently in Nature.

Most methods for studying chemical reactions on short timescales use laser pulses to excite the target molecules and then apply a probe pulse to investigate their nature. In this approach, the wavelength of the probe pulse is selected so as to discriminate the reacting, excited, chemical species, from the background of unexcited molecules. Alternative approaches such as high-harmonic spectroscopy use a laser to remove an electron from the molecule and then observe its subsequent recollision but avoiding the probe pulses means such techniques cannot eliminate the background emission.

Now, a new technique developed by Paul Corkum and colleagues at the Joint Laboratory for Attosecond Science, National Research Council of Canada and University of Ottawa in Ontario, Canada, together with researchers at the Institute for Photonics at the Technische Universitaet in Vienna, Austria, can be used to track chemical reactions in real time by using the radiation from non-reacting species as part of the detection method and at the same time eliminate background noise without the need for pulses. The team describes details in the journal Nature of high-harmonic interferometry and suggests that it might be used to monitor both molecular structure and electron dynamics; the latter on the attosecond (10^18 second) timescale.

The researchers point out that their technique augments the work of at least the past decade into techniques that can image chemical reactions as they occur, based on X-ray diffraction , electron diffraction, and laser-induced recollision, but brings with it spectroscopic selection not available for any of those methods. The new technique essentially turns a limitation into an advantage in the same way that a radio receiver uses a local oscillator to "decode" the incoming signal, the researchers use the emission of the unexcited molecules as a reference against which the amplitude and phase of the excited molecules' emission can be measured.

"In the case of high-harmonic spectroscopy based on recollision, [the] limitation becomes a major advantage owing to the coherent nature of the attosecond high-harmonic pulse generation," the team explains. "The coherence allows the unexcited molecules to act as local oscillators against which the dynamics are observed, so a transient grating technique can be used to reconstruct the amplitude and phase of emission from the excited molecules."

They have now demonstrated proof of principle using the dissociation of bromine molecules into two bromine atoms as a model reaction. They were able to track both the separation of the atomic nuclei and the changing electronic structure of the transient intermediate. In future, the authors anticipate using high-harmonic interferometry to image electron orbitals in chemical reactions dynamically.

"In our experiment, we are able to document a temporal shift of the high-harmonic field of less than an attosecond between the stretched and com- pressed geometry of weakly vibrationally excited Br₂ in the elec tronic ground state," the team says.

The researchers add that being able to probe structural and electronic features of molecules, combined with such high time resolution, make high-harmonic spectroscopy an ideal candidate for measuring coupled electronic and nuclear dynamics in photochemistry. It should also be applicable to the characterization of the electronic structure of transition states in such reactions.