Rotational dynamics of n-H2 in porous Vycor glass

The rotational kinematics of normal H-2 confined to the pores of Vycor glass have been studied via incoherent inelastic neutron spectroscopy. At low temperature we observe two distinct rotational transitions. One is centered near 14 meV, corresponding to the free rotor value of molecular HZ, while the other is centered at similar to 10 meV. Measurements taken at filling fractions ranging from 10% to 95% allow us to ascribe the two transitions to H-2 in the center of the pore and H-2 strongly bound to the pore surface, respectively. The molecules bound to the surface are modeled as rotationally hindered rotors, and a distribution of orientation-dependent interaction potentials is extracted. The bound molecules can further be divided into a monolayer that is in direct contact with the pore wall, and a second, less tightly bound layer. The ortho to para conversion rate has also been measured and is higher than the bulk rate of 1.9%/h. The rate for the molecules bound to the pore wall is 2.1%/h, while the rate for molecules in the center of the pore is 8.5%/h. [S0163-1829(99)03920-X].