The ground state harmonic frequencies of gas phase H/(DSiCl)-Cl-35 and H/(DSiBr)-Br-79 have been determined by exciting single vibronic bands of the (A) over tilde (1)A " - (X) over tilde (1)A' electronic transition and recording the dispersed fluorescence. The jet-cooled radicals were produced in a pulsed discharge jet using H/DSiX3 (X = Cl or Br) precursors. The emission data were fitted to an anharmonic model and a normal coordinate analysis of the harmonic frequencies allowed the determination of five of the six force constants of each molecule. Using previously obtained upsilon" = 0 rotational constants and the improved force fields, average (r(z)) and estimated equilibrium (r(e)(z)) structures were calculated for both monohalosilylenes. The validity of the force constants was evaluated by comparing calculated and observed zero-point inertial defects and by simulating the Franck-Condon profiles of the observed emission spectra in the harmonic approximation.