Quantum states of rigid linear rotors confined in a slit pore: quantum sieving of hydrogen isotopes and extreme one dimensional confinement

The quantum states of molecular hydrogen isotopes confined in carbon slit pores of varying width are calculated by direct diagonalization of the single particle Hamiltonian, evaluated with a model potential. The results are used to calculate the zero-pressure isotopic selectivity that can be expected from adsorption on these nanostructures, and the contribution from the rotational degrees of freedom is analysed in detail. It is shown that the role of the rotational degrees of freedom is far from negligible, resulting in values of the isotopic separation 8 to 50 times higher than those calculated using spherically symmetric models for the hydrogen molecule. It is also shown that if the pores are narrow enough, the molecular axis in the ground state is found preferentially aligned along the slit planes, and the conditions when this happens will be analysed in detail. This behaviour, termed extreme one dimensional confinement, is analogous to a similar behaviour observed in carbon nanotubes.