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)
Neutral iron oxide clusters (FenOm, n,m ≤ 16) are produced in a laser vaporization source using O2 gas seeded in He. The neutral clusters are ionized with a sequence of...
<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>
<div><p>Neutral iron oxide clusters (Fe<sub>n</sub>O<sub>m</sub>, n,m ≤ 16) are produced in a laser vaporization source using O<sub>2</sub>gas seeded in He. The neutral clusters are ionized with a sequence of femtosecond laser pulses and detected using time-of-flight mass spectrometry. Small clusters are confirmed to be most prominant in the stoichiometric (n = m) distribution, with m = n + 1 clusters observed above n = 4. Pump-probe spectroscopy is employed to study the dynamics of an electron transfer from an oxygen orbital to an iron nonbonding orbital of iron oxide clusters that is driven by absorption of a 400 nm photon. A bifurcation of the initial wavepacket occurs, where a femtosecond component is attributed to electron relaxation assisted through internuclear vibrational relaxation, and a slow relaxation shows the formation of a bound excited state. The lifetime and relative ratio of the two pathways depends on both the cluster size and iron oxidation state. The femtosecond lifetime decreases with increased cluster size until a saturation timescale is achieved at n > 5. The relative population of the long-lived excited state decreases with cluster size and suggests that the excited electron remains on the Fe atom for > 20 ps. </p></div>
<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>
<div><p>Neutral iron oxide clusters (Fe<sub>n</sub>O<sub>m</sub>, n,m ≤ 16) are produced in a laser vaporization source using O<sub>2</sub>gas seeded in He. The neutral clusters are ionized with a sequence of femtosecond laser pulses and detected using time-of-flight mass spectrometry. Small clusters are confirmed to be most prominant in the stoichiometric (n = m) distribution, with m = n + 1 clusters observed above n = 4. Pump-probe spectroscopy is employed to study the dynamics of an electron transfer from an oxygen orbital to an iron nonbonding orbital of iron oxide clusters that is driven by absorption of a 400 nm photon. A bifurcation of the initial wavepacket occurs, where a femtosecond component is attributed to electron relaxation assisted through internuclear vibrational relaxation, and a slow relaxation shows the formation of a bound excited state. The lifetime and relative ratio of the two pathways depends on both the cluster size and iron oxidation state. The femtosecond lifetime decreases with increased cluster size until a saturation timescale is achieved at n > 5. The relative population of the long-lived excited state decreases with cluster size and suggests that the excited electron remains on the Fe atom for > 20 ps. </p></div>
Excited state lifetimes of neutral titanium oxide clusters (TinO2n-x, n < 10, x < 4) were measured using a sequence of 400 nm pump and 800 nm probe femtosecond laser pulses. Despite large differences in electronic properties between the closed shell stoichiometric TinO2n clusters and the suboxide TinO2n-x (x = 1-3) clusters, the transient responses for all clusters contain a fast response of 35 fs followed by a sub-picosecond excited state lifetime. In this non-scalable size regime, subtle changes in the sub-ps lifetimes are attributed to variations in the coordination of Ti atoms and localization of charge carriers following UV photoexcitation. In general, clusters exhibit longer lifetimes with increased size and also with addition of O atoms. This suggests that removal of O atoms develops stronger Ti-Ti interactions as the system transitions from a semiconducting character into a fast metallic electronic relaxation mechanism.
Excited state lifetimes of neutral titanium oxide clusters (TinO2n-x, n < 10, x < 4) were measured using a sequence of 400 nm pump and 800 nm probe femtosecond laser pulses. Despite large differences in electronic properties between the closed shell stoichiometric TinO2n clusters and the suboxide TinO2n-x (x = 1-3) clusters, the transient responses for all clusters contain a fast response of 35 fs followed by a sub-picosecond excited state lifetime. In this non-scalable size regime, subtle changes in the sub-ps lifetimes are attributed to variations in the coordination of Ti atoms and localization of charge carriers following UV photoexcitation. In general, clusters exhibit longer lifetimes with increased size and also with addition of O atoms. This suggests that removal of O atoms develops stronger Ti-Ti interactions as the system transitions from a semiconducting character into a fast metallic electronic relaxation mechanism.
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