Rotational state modification and fast ortho-para conversion of H2 trapped within the highly anisotropic potential of Pd(210)

The rotational state and ortho-para conversion of H2 on a Pd(210) surface is investigated with rotational-state-selective temperature-programmed desorption (RS-TPD) and theoretical calculations. The isotope dependence of TPD shows a higher desorption energy for D2 than that for H2, which is ascribed to the rotational and zero-point vibrational energies. The RS-TPD data show that the desorption energy of H2(J=1) (J: rotational quantum number) is higher than that of H2(J=0). This is due to the orientationally anisotropic potential confining the adsorbed H2, which is in agreement with theoretical calculations. Furthermore, the H2 desorption intensity ratio in J=1 and J=0 indicates fast ortho-para conversion in the adsorption state, which we estimate to be of the order of 1 s.