On the titanium oxide neutral cluster distribution in the gas phase: Detection through 118 nm single-photon and 193 nm multiphoton ionization

Titanium oxide clusters are generated in a supersonic expansion by laser ablation of the metal and reaction with oxygen (0.1-6%) in He expansion gas. Mass spectra of the titanium oxide clusters are observed by photoionization with lasers of three different wavelengths: 118, 193, and 355 nm. Only the 118 nm (10.5 eV) light can ionize Ti _mO _n neutral clusters without fragmentation. Both the 193 nm (6.4 eV) and 355 nm (3.5 eV) multiphoton ionization cause fragmentation of the neutral clusters during the ionization process and, thus, can complicate the determination of the stable neutral Ti _mO _n gas-phase species. Employing 118 nm single-photon ionization and line-width data, the Ti _mO _2m and Ti _mO _2m+1 series are found to be the most stable neutral cluster species for high oxygen content in the expansion gas. Fragmentation during the multiphoton ionization process for 193 nm light yields the cluster ions Ti _mO _2m-1,-2 ⁺. These ions are formed by the loss of one or two oxygen atoms from Ti _mO _2m,2m+1 neutral species. The dominant cluster growth process is suggested to be through the addition of TiO ₂ species. For low oxygen content (<2%) in the expansion gas, oxygen-deficient clusters of the form Ti _mO _2m-1,-2 are also observed. These latter series are not fragmented by the 193 nm ionization process.