The recrystallisation rate of amorphous strontium titanate is dramatically enhanced by thermal annealing in water vapour as compared with a dry ambient. Time resolved optical reflectivity (TRR) has been combined with Rutherford backscattering spectrometry (RBS) and ion channelling to investigate this effect. Thin amorphous films were produced on single crystal substrates of (100) strontium titanate by ion bombardment. Specimens were annealed under controlled ambient conditions and the solid phase epitaxial recrystallisation monitored in situ by TRR. The TRR data were calibrated ex situ by RBS measurements. Isotope substitution, nuclear reaction analysis and secondary ion mass spectrometry were utilised to reveal the uptake of hydrogen and oxygen into the implanted layer.;Hydrogen is identified as the species which penetrates to the crystal/amorphous interface. It is shown that the recrystallisation rate is proportional to the concentration of diffusing hydrogen at the interface. The data show that the effect of water vapour, or more precisely hydrogen, is to reduce the activation energy of recrystallisation from 2.1 to 1.0 eV. It is concluded that hydrogen, provided by the dissociation of water molecules at the surface, is a catalyst in the crystallisation of strontium titanate.;The approach used in this study is seen to have application to a wider class of thin film phase changes and reactions. We have applied TRR to measure the temperature dependance of the amorphous to gamma and the gamma to alpha phase transitions in Al{dollar}sb2{dollar}O{dollar}sb3{dollar} (sapphire). TRR has also been utilised to measure the thermal oxidation rate of single crystal silicon in a water vapour ambient.
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