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 a strong optical pulse. 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. DOI: 10.1103/PhysRevB.87.054301