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

Abstract

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)

Increased Excited State Metallicity in Neutral Cr2On Clusters (n < 5) upon Sequential Oxidation

“…Here, we report the excited state transient signals of neutral Cr 2 O n ( n = 0–4) clusters using two-color pump–probe spectroscopy and apply theoretical calculations to understand the increasing metallic behavior as a function of sequential oxidation. A time-of-flight mass spectrometer (TOF-MS) , coupled to synchronized sub-35 fs laser pulses was employed to measure the excited state lifetimes of neutral Cr oxide clusters. A single 400 nm (3.1 eV) pump photon initiates a charge transfer and relaxation mechanism that is probed through ionization by a synchronized strong-field 800 nm (1.55 eV) probe beam.…”

“…Metal oxide clusters contain a larger splitting of the molecular orbitals (lower density of states), which decreases the number of unoccupied levels within the excitation energy, greatly reducing relaxation rates (longer lifetime). For example, the excited state lifetimes for (TiO 2 ) n , (ZnO) n , and (FeO) n clusters depend strongly on both cluster size and charge carrier localization. The instantaneous O-2p to Cr-3d e–e scattering processes (κ 1 contribution) recorded in Cr 2 O n clusters increases with oxidation (Figure ) and suggests that they become more metallic with increasing oxidation.…”

Laser Desorption/Ionization Mass Spectrometry as a Potential Tool for Evaluation of Hydroxylation Degree of Various Types of Titanium Dioxide Materials

“…There is a number of papers devoted to the gas-phase studies of neutral and charged titanium oxide clusters [23,24,[36][37][38][39][40][41][42], since they provide some insight into the properties of bulk titanium oxide at the molecular level. Therefore, the (TiO 2 ) n − and (TiO 2 ) n OH − ions were subjected to further analysis, to get the spectra of metastable ions (LIFT mass spectra).…”

Oxygen Deficiencies in Titanium Oxide Clusters as Models for Bulk Defects

“…Topological descriptors for the first excited state were calculated for the S0 and S1 geometries and compared against previously reported information for stoichiometric clusters. 36 Because photoexcitation involves contributions from many occupied and virtual orbitals, it is convenient to represent the location of the electron and hole as their charge densities. Therefore, we employed topological descriptors, such as deh (distance between centroid of the electron and hole densities), 37,38 σ (degree of delocalization of the RMS density of the electron and the hole), and Λ (percent overlap of the electron and hole wavefunctions) as analytical tools.…”