Mass Spectrometric Investigation of the Influence of Water Vapour and Oxygen on Gas‐Phase Reactions of Aluminium Acetylacetonate

ABSTRACT The controlled synthesis of aluminium oxide (Al ₂ O ₃ ) thin films by metal–organic chemical vapour deposition (MOCVD) critically depends on the gas‐phase decomposition behaviour of metal–organic precursors. Aluminium acetylacetonate (Al(acac) ₃ ) is widely used owing to its low toxicity and thermal stability; however, its gas‐phase chemistry in the presence of reactive carrier gases such as water vapour and oxygen remains incompletely understood. Here, we present an in‐situ mass spectrometric investigation of the gas‐phase decomposition of Al(acac) ₃ using vacuum ultraviolet (VUV) synchrotron radiation coupled with double imaging photoelectron photoion coincidence spectroscopy (i ² PEPICO). Temperature‐dependent spectra were recorded in humid (Ar+H ₂ O) and oxygen‐containing carrier gas environments, enabling identification of key decomposition products and intermediates. In the presence of water vapour, Al(acac) ₃ decomposes at lower temperatures than under inert conditions, consistent with a water‐mediated weakening of the Al–acetylacetonate bond. This pathway enhances the formation of acetylacetone and acetone, while suppressing characteristic intermediates. In contrast, oxygen exhibits a fundamentally different behaviour: the precursor signal remains nearly constant over a broad temperature range. Only above 700 K does oxygen promote alternative gas‐phase decomposition routes. These results highlight the distinct, non‐equivalent roles of water vapour and oxygen in MOCVD and their impact on deposition kinetics.