We present the results of a combined theoretical and experimental study of the growth of extended surface vibrational modes during the initial oxidation of Si(100)-(2x1). Controlled incorporation of oxygen into the first two layers of the silicon surface is achieved via sequential water (H2O) exposures and stepwise annealing in ultrahigh vacuum. Transmission infrared spectroscopy is used to monitor the transition from vibrational modes characteristic of isolated Si-O structures, so-called silicon epoxides, to extended modes with the character of transverse optical and longitudinal optical phonons in an extended SiO2 film. Quantum chemical calculations on two-dimer silicon clusters identify these modes as arising predominantly from coupling between species on adjacent dimer units, consistent with the thermodynamic driving force for local oxygen agglomeration. The vibrational signature of this surface is proposed to offer a link between well-characterized local surface Si-O structures and the extended SiOx film that exists between Si(100) and SiO2 after thermal oxidation of silicon. (C) 2003 American Institute of Physics.