Hafnium oxide ultra thin films on Si are being developed to replace thermally grown SiO2 gates in complementary metal-oxide semiconductor devices. In this work, a specially designed attenuated total reflectance (ATR) Fourier transform infrared spectroscopy reaction cell has been developed to observe adsorption of hafnium (IV) t-butoxide (HtB) onto a Si (100), Si (111), and Ge ATR crystal heated to temperatures between 60 and 250 degrees C and under 0.4 torr of vacuum allowing the observation of initial reaction pathways in real time. Spectra generated by density functional theory calculations of monodentate and bridging adsorbed precursors as well as a spectrum of an applied liquid drop of HtB precursor were used to identify the chemisorbed species. Two symmetric O-tBu umbrella modes from bridged Si-O-Hf(O-tBU)(2)-O-Si located at 1226 and 1016 cm(-1) present in the chemisorbed spectra show the precursor dissociates and is present as a bridging ligand on both Si(100) and Si(Ill). Surface concentration of the chemisorbed species was dependent on the substrate temperature allowing for determination of adsorption activation energies of 11 (2) kJ/mol for HtB on Si (100) for the temperature range of 110-185 degrees C and 1 (+/- 0.1) kJ/mol on Si(100) and Si(111) at temperatures between 60 and 110 degrees C. Precursor decomposition on the surface is observed at temperatures above 185 degrees C. (c) 2007 American Vacuum Society.