A new technique for following chemical reactions in real time makes a virtue of necessity, by using the radiation from non-reacting molecules as part of the detection method. As reported in Nature (doi: 10.1038/nature09185), this implementation of “high-harmonic interferometry” can be used to monitor both molecular structure and electron dynamics, the latter with attosecond time resolution.

Most methods for studying chemical reactions on short timescales use laser pulses to excite molecules, followed by “probe” pulses to interrogate them. In these methods, the wavelength of the probe pulses is selected to discriminate the excited (reacting) molecules from the background of unexcited molecules. Some newer methods—including high-harmonic spectroscopy, based on the laser-induced removal of an electron from a molecule and its subsequent recollision—do not use probe pulses, making it impossible to eliminate the background emission.

Now Paul Corkum and colleagues have turned this limitation into an advantage. In the same way as a radio receiver uses a “local oscillator” to decode the incoming signal, the authors use the emission of the unexcited molecules as a reference against which the amplitude and phase of the excited molecules’ emission can be measured.

The authors use their technique to monitor the dissociation of the bromine molecule into two bromine atoms, tracking both the separation of the atomic nuclei and the changing electronic structure of the transient molecule. In future, the authors anticipate using high-harmonic interferometry to image electron orbitals in chemical reactions dynamically.