  We derive fully covariant expressions for all two-point scattering amplitudes
involving closed string tachyon and massless strings from Dirichlet brane in
type 0 theories. The amplitude for two massless D-brane fluctuations to produce
closed string tachyon is also evaluated. We then examine in detail these string
scattering amplitudes in order to extract world-volume couplings of the tachyon
with itself and with massless fields on a D-brane. We find that the tachyon
appears as an overall coupling function in the Born-Infeld action.
