Ultrafast Pump-Probe Spectroscopy of Finite-Sized Neutral Iron Oxide Clusters

Abstract

<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>

Oxygen Deficiencies in Titanium Oxide Clusters as Models for Bulk Defects

“…Detailed information on the experimental setup was previously described. 49 The static mass spectrum was published previously 29 and reveals the presence of neutral clusters containing several stoichiometries, with the TinO2n-1 series generally being the most prominent. The TinO2n series are the least intense cluster series in the distribution obtained through femtosecond ionization of neutral clusters produced via laser ablation.…”

Oxygen Deficiencies in Titanium Oxide Clusters as Models for Bulk Defects

“…Detailed information on the experimental setup was previously described. 49 The static mass spectrum was published previously 29 and reveals the presence of neutral clusters containing several stoichiometries, with the TinO2n-1 series generally being the most prominent. The TinO2n series are the least intense cluster series in the distribution obtained through femtosecond ionization of neutral clusters produced via laser ablation.…”

“…The neutral cluster distribution was subsequently measured using a home-built Wiley–McLaren type time-of-flight mass spectrometer coupled to a femtosecond laser (<35 fs) for ionization. Detailed information on the experimental setup was previously described. , Briefly, the ionic clusters produced through laser ablation are deflected from the fs laser pulse interaction area through the use of electrostatic fields. Neutral clusters are ionized using temporally overlapped 400 nm (3.1 eV) and 800 nm (1.55 eV) pulses from a Ti:sapphire laser at intensities of 9.9 × 10 14 and 3.1 × 10 15 W/cm 2 respectively.…”

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

“…Experimental lifetimes were measured using a home-built Wiley-McLaren 18 type time-of-flight mass spectrometer (TOF-MS) that was previously described. 19 A sub-35 fs Ti:Sapphire laser pulse was synchronized to the cluster beam to ionize the neutral clusters for detection. Mass spectra were recorded using 400 nm pump and 800 nm probe pulses with intensities of 9.9 x 10 14 and 3.1 x 10 15 W/cm 2 , respectively.…”

“…Transient signals were fit using two exponential decay functions convoluted with a Gaussian function described elsewhere. 19,20 All clusters contain a fast (~35 fs) and a sub-picosecond relaxation component (Figure 2). The fast lifetime is attributed to an increase in signal that occurs through non-resonant excitation/ionization.…”