Temperature-Dependent Inelastic Scanning Tunneling Spectroscopy and Population of Two Levels in a Double-well Potential of a Single Molecule

Vibrational spectroscopy is a key technique for molecular identification. Inelastic electron tunneling spectroscopy with the scanning tunneling microscope (STM-IETS) extends this capability to single atoms and molecules, allowing for the precise detection of their chemical and physical properties [1]. Typically, temperature broadens the spectroscopic features due to the broadening in energy of the electron distribution at the Fermi level. Pyrrolidine (C₄H₈NH) and its deuterated variant, pyrrolidine-d8 (C₄D₈NH), adsorbed on a Cu(001) surface, undergo conformational transitions between the two levels that can be thermally excited or vibrationally assisted [2,3]. In this study, we investigate the spectral line shape in STM-IETS of these molecules using a home-built, variable-temperature STM. By adjusting the temperature, we observe changes in the line shape that arise not only from thermal broadening of the Fermi distribution but also from temperature-dependent changes in the steady-state population of the two levels in a double-well potential. This work highlights how the temperature influences both the spectral features and the dynamic behavior of two-level systems to advance our understanding of thermally equilibrated two-level molecular systems in double-well potentials.