Mechanisms Underlying a Quantum Superposition Microscope Based on THz-Driven Coherent Oscillations in a Two-Level...

Area of Science:

• Quantum Mechanics
• Molecular Spectroscopy
• Surface Science

Background:

• Scanning tunneling microscopy (STM) enables atomic-scale surface investigations.
• Terahertz (THz) spectroscopy probes molecular vibrations and electronic transitions.
• Understanding molecular behavior at surfaces is crucial for catalysis and materials science.

Purpose of the Study:

• To investigate the quantum dynamics of hydrogen molecules (H_{2}) at a surface using pump-probe measurements.
• To characterize H_{2} as a two-level system (TLS) under THz excitation.
• To explore the influence of tip-sample interactions on molecular spectroscopy.

Main Methods:

• Pump-probe measurements utilizing ultrashort terahertz (THz) pulses.
• Coupling THz pulses with a scanning tunneling microscope (STM) tunnel junction.
• Analysis of THz-induced dc tunneling current oscillations.

Main Results:

• Observed coherent oscillations of the THz-induced dc tunneling current at ~0.5 THz, confirming H_{2} as a two-level system (TLS).
• Identified two components in the oscillatory signal, reflecting both photon and field aspects of the THz pulses.
• Evidence of loosely bound states for the upper TLS state through coherent revival of the oscillatory signal.

Conclusions:

• The study demonstrates THz-driven pump-probe spectroscopy as a method to probe molecular quantum states in STM junctions.
• Spectroscopic features of H_{2} were analyzed with different tips, providing insights into the TLS of H_{2} at the surface.
• The findings contribute to understanding light-matter interactions at the nanoscale.