Garcia Jacob M, Shaffer Ryan E, Sayres Scott G
School of Molecular Sciences, Arizona State University, Tempe, AZ 85287, USA.
Phys Chem Chem Phys. 2020 Nov 4;22(42):24624-24632. doi: 10.1039/d0cp03889j.
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. 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 high density of states, and a slow relaxation shows the formation of a bound excited state. The lifetime and relative ratio of the two pathways depend 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.
中性氧化铁团簇（FenOm，n，m≤16）在激光蒸发源中利用氦气中注入的氧气产生。中性团簇通过一系列飞秒激光脉冲电离，并使用飞行时间质谱进行检测。已证实小团簇在化学计量比（n = m）分布中最为突出，在n > 4时观察到m = n + 1的团簇。采用泵浦 - 探测光谱法研究由400 nm光子吸收驱动的电子从氧轨道转移到氧化铁团簇的铁非键合轨道的动力学。初始波包出现分叉，其中飞秒成分归因于通过核间振动弛豫和高态密度辅助的电子弛豫，而缓慢弛豫显示出束缚激发态的形成。两条途径的寿命和相对比例取决于团簇大小和铁的氧化态。飞秒寿命随着团簇大小的增加而减小，直到在n > 5时达到饱和时间尺度。长寿命激发态的相对丰度随着团簇大小而降低，这表明激发电子在铁原子上停留超过20皮秒。
