On the instrument-dependent appearance of ion dissociation events in atom probe tomography mass spectra.

The successful application of atom probe tomography (APT) relies on the accurate interpretation of the mass spectrum (i.e.m/z histogram) from a sample.

Atom probe tomography using an extreme ultraviolet trigger pulse.

Atom probe tomography (APT) is a powerful materials characterization technique capable of measuring the isotopically resolved three-dimensional (3D) structure of nanoscale specimens with atomic resolution. Modern APT instrumentation most often uses an optical pulse to trigger field ion evaporation-most commonly, the second or third harmonic of a Nd laser is utilized (∼λ = 532 nm or λ = 355 nm). Herein, we describe an APT instrument that utilizes ultrafast extreme ultraviolet (EUV) optical pulses to trigger field ion emission.

Tuning the photodynamics of sub-nanometer neutral chromium oxide clusters through sequential oxidation.

Sub-nanometer neutral chromium oxide clusters were produced in the gas phase through laser ablation and their low-lying excited state lifetimes were measured using femtosecond pump-probe spectroscopy. Time-dependent density functional theory calculations relate the trends in experimental lifetimes to the cluster's electronic structure. The photoexcited (CrO) ( < 5) cluster transients with the absence of up to four O atoms (CrO, < 5) exhibit a ∼30 fs and sub-ps lifetime, attributed to instantaneous metallic e-e scattering and vibrationally mediated charge carrier relaxation, respectively.

Orbital-dependent photodynamics of strongly correlated nickel oxide clusters.

The ultrafast electronic relaxation dynamics of neutral nickel oxide clusters were investigated with femtosecond pump-probe spectroscopy and supported with theoretical calculations to reveal that their excited state lifetimes are strongly dependent on the nature of the electronic transition. Absorption of a UV photon produces short-lived (lifetime ∼ 110 fs) dynamics in stoichiometric (NiO) clusters ( < 6) that are attributed to a ligand to metal charge transfer (LMCT) and produces metallic-like electron-electron scattering. Oxygen vacancies introduce excitations with Ni-3d → Ni-4s and 3d → 4p character, which increases the lifetimes of the sub-picosecond response by up to 80% and enables the formation of long-lived (lifetimes >2.

Oxygen Deficiencies in Titanium Oxide Clusters as Models for Bulk Defects.

TD-DFT calculations were performed on neutral TiO, TiO, and TiO clusters, where ≤ 7. Calculations show the TiO 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 TiO 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.

Effect of oxidation on excited state dynamics of neutral TiO (n < 10, x < 4) clusters.

Excited state lifetimes of neutral titanium oxide clusters (TiO, 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 TiO clusters and the suboxide TiO (x = 1-3) clusters, the transient responses for all clusters contain a fast response of 35 fs followed by a sub-picosecond (ps) 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.

Increased Excited State Metallicity in Neutral CrO Clusters ( < 5) upon Sequential Oxidation.

Excited state lifetimes of neutral CrO ( < 5) clusters were measured using femtosecond pump-probe spectroscopy. Density functional theory calculations reveal that the excited state dynamics are correlated with changes in the cluster's electronic structure with increasing oxidation. Upon absorption of a UV (400 nm) photon, the clusters exhibit features attributed to three separate relaxation processes.

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

Neutral titanium oxide clusters of up to 1 nm in diameter (TiO), with < 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 (TiO) 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.

Ultrafast pump-probe spectroscopy of neutral FeO clusters (, < 16).

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 femtosecond laser pulses and detected using time-of-flight mass spectrometry. Small clusters are confirmed to be most prominent in the stoichiometric (n = m) distribution, with m = n + 1 clusters observed above n = 4.