Single vibronic level emission spectra of jet-cooled HSiI and DSiI have been recorded by laser excitation of selected bands of the (A) over tilde (1)A"-(X) over tilde (1)A' electronic transition. The data have been used to derive the ground state harmonic frequencies and anharmonicities for both isotopomers. A normal coordinate analysis of the harmonic frequencies yielded reliable values for five of the six force constants. Using previously determined ground state rotational constants and the force field data, average (r(z)) and approximate equilibrium (r(e)(z)) structures were calculated, with r(e)(z)(SiH)=1.5151(2) Angstrom, r(e)(z)(SiI)=2.4610(1) Angstrom, and theta(e)(z)(HSiI)=92.5(1)degrees. A comparison of trends in the structural parameters and vibrational frequencies of the monohalosilylenes shows that the bond angle increases significantly and the Si-H bond length decreases slightly with heavier halogen substitution. These trends have been rationalized based on the inductive effect and the electronegativity of the halogen substituent. (C) 2003 American Institute of Physics.