Coherent spectroscopy of a hydrogen molecule in the STM junction

My talk will focus on the study of the precise energy landscape of an H₂ in the STM junction enabled by time-resolved femtosecond THz STM. Previous studies have shown fascinating behaviors of an H₂ in the junction, including the strong negative differential resistance, the stochastic resonance, the enhanced imaging ability due to its short-range Pauli repulsion. All these peculiar features can be explained by a simple double-well two-level system, except that the low energy modes are hard to precisely calculate from DFT theory because of the lightest mass of H₂. Our experiments observe coherent oscillatory conductance of hydrogen junction by changing the delay of two THz pulses. By transforming the delay spectrum into the frequency domain, we found that the energy spacing of the distinctive two levels of hydrogen is around 2 meV (~0.5 THz). These results combined with the STM inelastic electron tunneling spectroscopy measurement provide an insightful view of the energy landscape of hydrogen molecules in the STM junction and how the photon, the rotational and vibrational degree of freedom get involved in the dynamics of H₂ in the junction.