Neutral titanium oxide clusters of up to 1 nm in diameter (TiO 2) n , with n < 10, are produced in a laser vaporization source and subsequently ionized by a sequence of femtosecond laser pulses. Using 400 nm pump, 800 nm probe lasers, the excited state lifetimes of neutral (TiO 2 ) n clusters are measured. All clusters exhibit a rapid relaxation lifetime of ~30 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 electron-hole pair 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. File list (2) download file view on ChemRxiv Sayres_TiO2_JPCL_MainText.pdf (2.79 MiB) download file view on ChemRxiv Sayres_TiO2_JPCL_Supplemental.pdf (272.55 KiB)
<div>Neutral titanium oxide clusters of up to 1 nm in diameter (TiO<sub>2</sub>)<sub>n</sub>, with n < 10, are produced in a laser vaporization source and subsequently ionized by a sequence of femtosecond laser pulses. Using 400 nm pump, 800 nm probe lasers, the excited state lifetimes of neutral (TiO<sub>2</sub>)<sub>n</sub> clusters are measured. All clusters exhibit a rapid relaxation lifetime of ~30 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 electron-hole pair 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.</div>
TD-DFT calculations were performed on neutral TinO2n, TinO2n-1, and TinO2n-2 clusters, where n ≤ 7. Our calculations show that the TinO2n clusters are closed shell systems containing empty d orbitals and that the partially filled d orbitals of the suboxide clusters have a profound effect on their structural, electronic, and topological properties. The low energy photoexcitations of TinO2n clusters are all O-2p to Ti-3d transitions, while the open-shell suboxide clusters are all characterized by d-d transitions that occur at a much smaller optical gap. Upon photoabsorption, the localization of the hole is accompanied by a local bond elongation, i.e., polaron formation, whereas d-electrons are generally delocalized among the cluster. Several of the compact of the TinO2n-2 structures contain higher symmetry which is reflected in their relative stability in the experimental cluster distribution. In particular, the tetrahedral symmetry of the optimized ground state structure for Ti4O6 inhibits charge carrier localization and therefore contains higher stability.
The ultrafast photodynamics of n-butyl bromide are explored with femtosecond time-resolved mass spectrometry. Absorption of two UV (400 nm) pump photons induces the direct dissociation of the C−Br bond from the A state within 160 fs. Absorption of three UV pump photons excites the molecule into the 5p Rydberg state which undergoes several relaxation pathways including to the ion-pair state. Relaxation to the ion-pair state is tracked through the transient of the C 4 H 9 + fragment and suggests an E state lifetime of 10.8 ± 0.5 ps, in close agreement with the tunneling time of smaller molecules. Predissociation from the 5p Rydberg states leads to the β-elimination of H−Br and formation of C 4 H 8 + within 3.0 ± 0.25 ps. A portion of the excited parent molecule avoids the ion-pair formation and instead relaxes through the Rydberg excited state manifold into the D state within 30.2 ± 0.21 ps.
<div>Neutral titanium oxide clusters of up to 1 nm in diameter (TiO<sub>2</sub>)<sub>n</sub>, with n < 10, are produced in a laser vaporization source and subsequently ionized by a sequence of femtosecond laser pulses. Using 400 nm pump, 800 nm probe lasers, the excited state lifetimes of neutral (TiO<sub>2</sub>)<sub>n</sub> clusters are measured. All clusters exhibit a rapid relaxation lifetime of ~30 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 electron-hole pair 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.</div>
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