  How molecules convert light energy to perform a specific transformation is a
fundamental question in photophysics. Ultrafast spectroscopy reveals the
kinetics associated with electronic energy flow, but little is known about how
absorbed photon energy drives nuclear or electronic motion. Here, we used
ultrabroadband transient absorption spectroscopy to monitor coherent
vibrational energy flow after photoexcitation of the retinal chromophore. In
the proton pump bacteriorhodopsin we observed coherent activation of hydrogen
wagging and backbone torsional modes that were replaced by unreactive
coordinates in the solution environment, concomitant with a deactivation of the
reactive relaxation pathway.
