Rotational dynamics of n-H2 in porous Vycor glass
The rotational kinematics of normal H2 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 H2 while the other is centered at ~10 meV. Measurements taken at filling fractions ranging from 10% to 95% allow us to ascribe the two transitions to H2 in the center of the pore and H2 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.
Brown, D.W., P.E. Sokol, and S.A. FitzGerald. 1999. "Rotational dynamics of n-H2 in porous Vycor glass." Physical Review B 59(20): 13258-13266.
American Physical Society
Physical Review B
Physics and Astronomy
Porous materials, Glass, Adsorption, Neutron spectra, Molecular reorientation, Solid hydrogen