Since the discovery of X-rays by Roentgen in 1895, its use has been ubiquitous, from medical and environmental applications to materials sciences^1,2,3,4,5. X-ray characterization requires a large number of atoms and reducing the material quantity is a long-standing goal. Here we show that X-rays can be used to characterize the elemental and chemical state of just one atom. Using a specialized tip as a detector, X-ray-excited currents generated from an iron and a terbium atom coordinated to organic ligands are detected. The fingerprints of a single atom, the L_2,3 and M_4,5 absorption edge signals for iron and terbium, respectively, are clearly observed in the X-ray absorption spectra. The chemical states of these atoms are characterized by means of near-edge X-ray absorption signals, in which X-ray-excited resonance tunnelling (X-ERT) is dominant for the iron atom. The X-ray signal can be sensed only when the tip is located directly above the atom in extreme proximity, which confirms atomically localized detection in the tunnelling regime. Our work connects synchrotron X-rays with a quantum tunnelling process and opens future X-rays experiments for simultaneous characterizations of elemental and chemical properties of materials at the ultimate single-atom limit.

自 1895 年伦琴发现 X 射线以来，它的应用无处不在，从医学和环境应用到材料科学^1,2,3,4,5。X 射线表征需要大量原子，减少材料数量是一个长期目标。在这里，我们表明 X 射线可用于表征一个原子的元素和化学状态。使用专门的尖端作为检测器，检测到与有机配体配位的铁和铽原子产生的 X 射线激发电流。单个原子的指纹，L _2,3和 M _4,5在 X 射线吸收光谱中可以清楚地观察到铁和铽的吸收边沿信号。这些原子的化学状态通过近边 X 射线吸收信号表征，其中 X 射线激发共振隧穿 (X-ERT) 对铁原子占主导地位。只有当尖端非常接近地直接位于原子上方时才能感测到 X 射线信号，这证实了隧道区域中的原子定位检测。我们的工作将同步加速器 X 射线与量子隧穿过程联系起来，并开启了未来的 X 射线实验，以在最终的单原子极限下同时表征材料的元素和化学性质。