Reporting in the March 31 issue of Nature, scientists from the University of California, Berkeley, Lawrence Berkeley National Laboratory, also in Berkeley, Korea University in Seoul, South Korea, and Arizona State University in Tempe describe the use of two-dimensional femtosecond infrared spectroscopy at 805 nm to directly observe the electronic couplings in a molecular complex. As the technique is further refined to be compatible with shorter wavelengths in the visible range, it is expected to enable the study of dynamic electronic transitions in proteins and DNA bases.

Two-dimensional spectroscopy employs three excitation pulses to yield information on the electronic couplings between energy levels in a specimen at two time intervals of interest. In their work, the scientists employed 50-fs-long pulses of 805-nm radiation from a homebuilt 3-kHz Ti:sapphire regenerative amplifier laser system to probe a photosynthetic light-harvesting protein from a green sulfur bacterium. They introduced the time delays between excitation pulses using movable glass wedges, achieving a precision of better than λ/100.

Their study revealed that the excitation energy does not simply cascade down from one energy level to the next. Rather, energy transport in the protein is dependent on the spatial properties of the molecule, so that energetically intermediate states are skipped if they do not sufficiently overlap spatially with neighboring potential transfer partners.