  We investigate coupled electron-lattice dynamics in the topological insulator
Bi2Te3 with time-resolved photoemission and time-resolved x-ray diffraction. It
is well established that coherent phonons can be launched by optical
excitation, but selection rules generally restrict these modes to zone-center
wavevectors and Raman-active branches. We find that the topological surface
state couples to additional modes, including a continuum of surface-projected
bulk modes from both Raman- and infrared-branches, with possible contributions
from surface-localized modes when they exist. Our calculations show that this
surface vibrational spectrum occurs naturally as a consequence of the
translational and inversion symmetries broken at the surface, without requiring
the splitting-off of surface-localized phonon modes. The generality of this
result suggests that coherent phonon spectra are useful by providing unique
fingerprints for identifying surface states in more controversial materials.
These effects may also expand the phase space for tailoring surface state
wavefunctions via ultrafast optical excitation.
