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arXiv 1003.4452cond-mat.mes-hall
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分子电子结构在IETS中的作用:Ag(110)表面O_2的情况

Role of molecular electronic structure in IETS: the case of O_2 on Ag(110)

S. Monturet, M. Alducin, N. Lorente

摘要:

采用密度泛函理论(DFT)方法,计算了O_2分子在Ag(110)表面的振动非弹性电子隧穿谱(IETS).半局域DFT计算预测的自旋吸附分子,然而,包括U导致的极化的分子的自旋极化的分子轨道向费米能级移动。费米能级的分子共振可能意味着电导的降低,而在非共振的情况下,电导的增加是预期的IETS信号。我们使用最低阶展开的电子振动耦合,以评估的幅度和空间分布的非弹性信号。这使我们能够重现实验数据:(i)在O_2分子的[001]方向上观察到负电导变化,(ii)在O_2分子上记录到的O-O伸缩振动和反对称O_2-Ag伸缩振动的电导变化的空间分布,(iii)没有质心和受阻转动模式的信号,以及(iv)沿[1-10]方向化学吸附的分子缺乏IETS信号。此外,我们的结果让我们了解电子和振动对称性在发挥作用。振动频率需要超出谐波近似,以便与实验进行比较,因此我们提出了一个Morse势拟合的势能面,以评估准确的振动频率。最后的IET谱的自洽玻恩近似的帮助下进行评估和温度和调制电压的展宽的效果进行了探讨。结果表明,O_2的IETS不能归因于单轨道分子共振效应。

英文摘要:

Density functional theory (DFT) simulations corrected by the intramolecular Coulomb repulsion U, are performed to evaluate the vibrational inelastic electron tunneling spectroscopy (IETS) of O_2 molecules on Ag(110). Semilocal DFT calculations predict a spinless adsorbed molecule, however the inclusion of the U leads to the polarization of the molecule by shifting a spin-polarized molecular orbital towards the Fermi level. A molecular resonance at the Fermi level can imply a decrease in conductance while in the off-resonance case, an increase in conductance is the expected IETS signal. We use the lowest-order expansion on the electron-vibration coupling, in order to evaluate the magnitude and spatial distribution of the inelastic signal. This allows us to reproduce the experimental data in: (i) the negative conductance variation observed in the vibrational spectra of O_2 along the [001] direction, (ii) the spatial distribution of the conductance changes recorded over the O_2 molecule for the O--O stretch and the antisymmetric O_2--Ag stretch vibrations, (iii) the absence of signal for the center-of-mass and hindered rotations modes, and (iv) the lack of IETS signal for the molecule chemisorbed along the [1-10] direction. Moreover, our results give us insight of the electronic and vibrational symmetries at play. The vibrational frequencies need to go beyond the harmonic approximation in order to be compared with the experimental ones, hence we present a Morse-potential fitting of the potential energy surface in order to evaluate accurate vibrational frequencies. The final IET spectra are evaluated with the help of the self-consistent Born Approximation and the effect of temperature and modulation-voltage broadening are explored. This ensemble of results reveals that the IETS of O_2 cannot be ascribed to the effect of a single orbital molecular resonance.

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