Light molecules such as H2O are the systems in which we can have access to quantum mechanical information on their constituent atoms. Here, we have investigated electron transport through H2O@C60 single molecule transistors (SMTs). The H2O@C60 SMTs exhibit Coulomb stability diagrams that show multiple tunneling-induced excited states below 30 meV. Furthermore, we have performed terahertz (THz) photocurrent spectroscopy on H2O@C60 SMTs and confirmed the same excitations. From comparison between experiment and theory, the excitations observed below 10 meV are identified to be the quantum rotational excitations of the water molecule. Surprisingly, the quantum rotational excitations of both para- and ortho-water molecule are observed simultaneously even for a single water molecule, indicating that the fluctuation between the ortho- and para-water states takes place in a time scale shorter than our measurement time (∼1 min), probably by the interaction between the encapsulated water molecule and conducting electrons.
Keywords: molecular electronics; nuclear spin isomer; single electron tunneling; single molecule transistor; terahertz spectroscopy; water molecule.