Oscillation in Excited State Lifetimes with Size of Sub-nanometer Neutral (TiO₂)_nClusters Observed with Ultrafast Pump-Probe Spectroscopy

Neutral titanium oxide clusters of up to 1 nm in diameter (TiO2)n, with n < 10, are produced in a laser vaporization source and subsequently ionized by a sequence of femtosecond laser pulses. Using a 400 nm pump and 800 nm probe lasers, the excited state lifetimes of neutral (TiO2)n clusters are measured. All clusters exhibit a rapid relaxation lifetime of ∼35 fs, followed by a sub-picosecond lifetime that we attribute to carrier recombination. The excited state lifetimes oscillate with size, with even-numbered clusters possessing longer lifetimes. Density functional theory calculations show the excited state lifetimes are correlated with charge carrier localization or polaron-like formation in the excited states of neutral clusters. Thus, structural rigidity is suggested as a feature for extending excited state lifetimes in titania materials.