Due to the Coulomb blockade effect, electrons typically tunnel into a localized state one by one, showing antibunching behavior. However, theory predicts that strong vibronic coupling can generate electron bunching locally, yet direct atomic-scale evidence has remained elusive. In this Letter, we report an atomically resolved shot-noise study probing the transport statistics of single-electron tunneling in an atomic impurity coupled to a vibrational mode. Conventional time-averaged conductance spectroscopy reveals clear signatures of vibron-assisted tunneling in this atomically sized nanoelectromechanical system. Simultaneous shot-noise measurements at the impurity center show super-Poissonian noise, providing direct evidence that vibronic coupling drives electron bunching during tunneling. As a future outlook, if coherence between electrons can be implemented, vibron-mediated electron bunching at single atomic sites may be exploited as a local injection source of $N$-paired electrons.

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• Charge dynamics
• Coulomb blockade
• Electron-phonon coupling
• Vibronic effects
• Semiconductors
• Topological insulators
• Noise measurements
• Scanning tunneling microscopy