  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.
