Poster

Theory of rotational inelastic electron tunneling for physisorbed H₂

T. Sugimoto¹, Y. Kunisada², and K. Fukutani³

¹Department of Chemistry, Graduate School of Science, Kyoto University, Japan

²Center for Advanced Research of Energy and Materials, Hokkaido University, Japan

³Institute of Industrial Science, The University of Tokyo, Japan

A quantum rotor of molecular H₂ retains its rotational motion in a physisorbed state [1]. Rotational and vibrational spectroscopy at the single molecule level has been recently observed for para-H₂ (J: 0→2, Fig. 1a) weakly physisorbed on surfaces [2,3]. Despite the remarkable progress of inelastic electron tunneling (IET) spectroscopy technique with scanning tunneling microscope (STM), mechanism of IET mediated by rotational excitation of H₂ remains to be clarified.

Here we propose a new microscopic IET model based on the resonant coupling [4] through rotation-electron interactions between the STM-tip, physisorbed H₂, and a metal surface (Fig. 1b,c). In this model, an electron with σ symmetry in the tip tunnels into the 2pσ_u or 1sσ_g state of H₂ through virtual negative or positive ion formation, respectively. In this formalism, the anisotropic term of the electron transfer t and the weak hybridization U induce the rotational excitation of H₂, which leads to a rotational selection rule of ΔJ=+2, ΔJ_z=0.

With this model, we analyzed the observed rotational spectrum in the IET spectra of H₂ physisorbed on Au(110) [2]. Potential anisotropy derived from the peak shift is in good agreement with our DFT calculation showing that rotational symmetry of H₂ in the nanocavity between the STM-tip and Au(110) is significantly broken.

Figure 1. (a) Rotational energy diagram of nuclear spin isomers of para-H₂ and ortho-H₂ [1]. (b) Schematic diagram of molecular-axis angle (Θ) dependent transfer matrix elements t between STM-tip and H₂, and weak hybridization interaction U between H₂ and substrate through the 2pσ_u orbital of virtual H₂^- state for Θ=0 and (c) Θ=π/2 configuration.

[1] T. Sugimoto and K. Fukutani, Nature Physics 7, 307 (2011); Prog. Surf. Sci. 88, 279 (2013); Phys. Rev. Lett. 112, 146101 (2014)

[2] S. Li et al., Phys. Rev. Lett. 111, 146102 (2013)

[3] F. D. Natterer et al. Phys. Rev. Lett. 111, 175303 (2013); ACS Nano 8, 7099 (2014)

[4] B. N. J. Persson, Phys. Rev. Lett. 59, 339 (1987); Physica Scripta. 38, 282 (1988)