Université Paris-Est, Laboratoire Modélisation et Simulation Multi-Echelle, MSME UMR 8208 CNRS, 5 bd Descartes, 77454 Marne-la-Vallée, France.
Phys Chem Chem Phys. 2011 Sep 7;13(33):15233-43. doi: 10.1039/c1cp21566c. Epub 2011 Jul 15.
Three oxygen-containing gas-phase diatomic trications ReO(3+), NbO(3+) and HfO(3+) as well as the diatomic tetracation NbO(4+) have been observed by mass spectrometry at non-integer m/z values. These unusual triply charged molecular ion species, together with the corresponding diatomic dications ReO(2+), NbO(2+) and HfO(2+), were produced by energetic, high-current oxygen ((16)O(-)) ion beam sputtering of rhenium, niobium and hafnium metal samples, respectively, whose surfaces were dynamically oxidized by oxygen primary ion incorporation. In addition, NbO(z+) (z≤ 4) were generated by intense femtosecond laser excitation and photofragmentation (Coulomb explosion) of Nb(x)O(y) clusters and were detected through Time-of-Flight Mass Spectrometry (TOF). Our experimental results confirm previous reports on the detection of NbO(4+), NbO(3+), NbO(2+), HfO(3+) and HfO(2+) with Atom Probe mass spectrometry, whereas ReO(3+) and ReO(2+) apparently had not been observed before. In addition, these multiply charged molecular ions have been studied theoretically for the first time. Ab initio calculations of their electronic structures show that the diatomic trications ReO(3+), NbO(3+) and HfO(3+) are long-lived metastable gas-phase species, with bond lengths of 1.61 Å, 1.62 Å and 1.86 Å, respectively. They present large potential barriers with respect to dissociation of more than 2.7 eV. The corresponding diatomic dications are thermochemically stable molecules with very large dissociation energies (>3.5 eV). Our calculations predict the diatomic tetracation ReO(4+) to be a metastable ion species in the gas phase. We compute a potential barrier toward fragmentation of 0.6 eV; its formation requires a quadruple adiabatic ionization energy of 85.7 eV. Even though our calculations show that NbO(4+) is a weakly bound (dissociation barrier ∼0.1 eV) metastable molecule, it is here identified via linear time-of-flight mass spectrometry.
三种含氧气相三价阳离子 ReO(3+)、NbO(3+)和 HfO(3+)以及二价 NbO(4+)已通过非整数 m/z 值的质谱法观察到。这些不寻常的三重电荷分子离子物种，以及相应的二价阳离子 ReO(2+)、NbO(2+)和 HfO(2+)，是通过分别用高能、大电流氧((16)O(-))离子束溅射铼、铌和铪金属样品产生的，其表面被氧一次离子的掺入动态氧化。此外，通过高强度飞秒激光激发和 Nb(x)O(y)团簇的光解(Coulomb 爆炸)产生 NbO(z+) (z≤4)，并通过飞行时间质谱法(TOF)进行检测。我们的实验结果证实了以前关于用原子探针质谱法检测 NbO(4+)、NbO(3+)、NbO(2+)、HfO(3+)和 HfO(2+)的报道，而 ReO(3+)和 ReO(2+)显然以前没有观察到。此外，这些多电荷分子离子首次进行了理论研究。它们的电子结构的从头算计算表明，二价阳离子 ReO(3+)、NbO(3+)和 HfO(3+)是长寿命的亚稳态气相物种，键长分别为 1.61 Å、1.62 Å和 1.86 Å。它们的离解势垒大于 2.7 eV。相应的二价阳离子是热力学稳定的分子，离解能非常大(>3.5 eV)。我们的计算预测二价阳离子 ReO(4+)在气相中是一种亚稳态离子物种。我们计算出其碎裂的势垒为 0.6 eV；其形成需要四步绝热电离能 85.7 eV。尽管我们的计算表明 NbO(4+)是一种弱束缚(离解势垒约 0.1 eV)的亚稳态分子，但它是通过线性飞行时间质谱法鉴定的。
