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

Abstract

The rotational state and ortho-para conversion of ${\mathrm{H}}_{2}$ 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 ${\mathrm{D}}_{2}$ than that for ${\mathrm{H}}_{2}$, which is ascribed to the rotational and zero-point vibrational energies. The RS-TPD data show that the desorption energy of ${\mathrm{H}}_{2}$($J=1$) ($J$: rotational quantum number) is higher than that of ${\mathrm{H}}_{2}$($J=0$). This is due to the orientationally anisotropic potential confining the adsorbed ${\mathrm{H}}_{2}$, which is in agreement with theoretical calculations. Furthermore, the ${\mathrm{H}}_{2}$ 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.