Hindered Rotation of Adsorbed Hydrogen Isotopes: A Hybrid Approximation to the Quantum-Statistical Partition Function

A new hybrid partition function is used to calculate the quantum-statistical behavior of the various isotopes and nuclear-spin isomers of diatomic hydrogen interacting with a plane structureless surface. Problems of convergence usually encountered are thereby avoided, and the rather pronounced effects of hindered rotation become readily apparent and are seen to be separable into two components which we designate as “adrotation” and “libration”. System measureables calculated from the new partition function are seen to be in excellent agreement with available experimental data. It is shown that use of the commonly employed Kirkwood–Wigner formulation can lead to serious errors in fitted energy parameters, errors usually ascribed to the polarizability differences of the isotopes.