On the growth dynamics of neutral vanadium oxide and titanium oxide clusters

Cluster growth dynamics of vanadium oxide and titanium oxide clusters produced by laser ablation of vanadium and titanium metal in a He gas flow seeded with up to 2% O-2 are studied by covariance mapping time-of-flight mass spectrometry. Covariance mapping enables the recognition of two different distribution components in the overall homogeneous mass spectra for both vanadium oxide and titanium oxide cluster systems. The oxygen-rich component Or shows small correlated fluctuations while the oxygen-poor component Op shows large correlated fluctuations. These two cluster distribution components are observed at low ablation laser powers and low expansion gas concentrations. Fluctuations of small vanadium oxide clusters (V2O, V2O2, and V2O3) and small titanium oxide clusters (Ti2O2 and Ti2O3) are covariance determining. The less fluctuating V2O3 and Ti2O3 clusters are "nuclei" for the oxygen-rich components Or. The more fluctuating V2O and Ti2O2 are "nuclei" for the oxygen poor components Op. Correlated fluctuations or covariances within each distribution component are constant. Covariances for the different distribution components are different. Studies of mass spectra and covariances as functions of ablation laser power and expansion gas concentration imply that V2O and Ti2O2 clusters are formed in different regions of the ablation plasma plume than V2O3 and Ti2O3. We suggest that V2O3 and Ti2O3 are formed in the hot and optically dense region near the ablated metal surface and that V2O and Ti2O2 are formed in the colder plasma region farther away from the ablated metal surface. Larger vanadium oxide and titanium oxide clusters grow from these small clusters by very specific pathways which involve only uptake of VO or VO2, and TiO2, respectively. (C) 1999 American Institute of Physics. [S0021-9606(99)00345-1].