| AVS 62nd International Symposium & Exhibition | |
| Surface Science | Wednesday Sessions |
| Session SS+AS-WeA |
| Session: | Surface Dynamics, Non-Adiabaticity, and Single Molecule Phenomena |
| Presenter: | Katsuyuki Fukutani, University of Tokyo, Japan |
| Authors: | H. Kobayashi, University of Tokyo, Japan S. Ohno, University of Tokyo, Japan M. Wilde, University of Tokyo, Japan M. Matsumoto, Tokyo Gakugei University, Japan S. Ogura, University of Tokyo, Japan K. Fukutani, University of Tokyo, Japan |
| Correspondent: | Click to Email |
Molecular hydrogen is physisorbed on flat metal surfaces via van der Waals interaction. By taking advantage of the fact that molecular hydrogen exists in nuclear-spin isomers of ortho and para species [1], we have shown the interaction potential on Ag(111) is anisotropic with a slight perpendicular preference [2]. The Pd(210) surface has a step-like structure consisting of alternately aligned (100) and (110) terraces, and it has been shown that H2 is rather strongly adsorbed on H-covered Pd(210) with a significant contribution of orbital hybridization [3]. On the other hand, it has been suggested that molecularly adsorbed species could be important for hydrogen absorption into the interior of Pd surfaces [4]. In the present study, we have investigated the adsorption of H2 and D2 on the Pd(210) with temperature-programmed desorption (TPD) combined with resonance-enhanced multi-photon ionization (REMPI).
When Pd(210) was exposed to H2 at 115 K, TPD revealed a desorption peak at 180 K (α-peak) originating from the absorbed state as well as a peak at 280-320 K (b-peak) due to chemisorbed H. From the uptake rate of the α-peak, the absorption probability of H on Pd(210) was estimated to be 3×10-3. When the surface was exposed to either H2 or D2 at 45 K, on the other hand, an additional TPD peak was observed at about 70 K (γ-peak), which was attributed to molecular adsorption. While a small difference between H2 and D2 was observed for the b-peak, the γ-peak temperature of D2 was found to be higher than that of H2 by 9 K, which corresponds to the difference in the adsorption energy of about 20 meV. Assuming that this difference is due to the zero-point energy difference in the adsorption potential, the adsorption potential was analyzed in terms of the Morse potential. By applying REMPI-TPD, furthermore, the TPD spectra of ortho-H2 in the rotational state of J=1 and para-H2 in J=0 were state-selectively measured. The desorption temperature of ortho-H2 was found to be higher than that of para-H2 by about 4 K, which corresponds to a difference in the adsorption energy of about 10 meV. We discuss that this large energy difference between the ortho and para species originates from the potential anisotropy on the basis of the first-order perturbation.
[1] K. Fukutani, T. Sugimoto, Prog. Surf. Sci. 88, 279 (2013).
[2] T. Sugimoto, K. Fukutani, Phys. Rev. Lett. 112, 146101 (2014).
[3] P. K. Schmidt et al., Phys. Rev. Lett. 24 87, 096103 (2001).
[4] S. Ohno et al., J. Chem. Phys. 140, 134705 (2014).