Direct observation of non-fully-symmetric coherent optical phonons by femtosecond x-ray diffraction

We directly measure by femtosecond time-resolved x-ray diffraction the E${}_g$ symmetry coherent phonon excited in bismuth by strong optical excitation. The magnitude of the E${}_g$ mode observed is 0.2 pm peak-to-peak, compared against the 2.7 pm initial displacement of the fully-symmetric A${}_{1g}$ mode. The much smaller motion of the E${}_g$ mode is a consequence of the short lifetime of the electronic states that drive the atomic motion. The experimentally measured magnitude of the E${}_g$ motion allows us to rule out a previously suggested scenario for explaining the dynamics in bismuth that relies on strong coupling between these modes.