Fast ortho-to-para conversion of molecular hydrogen in chemisorption and matrix-isolation systems

Ueta, Hirokazu   ; Fukutani, Katsuyuki*; Yamakawa, Koichiro   

Molecular hydrogen has two nuclear-spin modifications called and . Because of the symmetry restriction with respect to permutation of the two protons, the and isomers take only odd and even values of the rotational quantum number, respectively. The -to- conversion is promoted in condensed systems, to which the excess rotational energy and spin angular momentum are transferred. We review recent studies on fast -to- conversion of hydrogen in molecular chemisorption and matrix isolation systems, discussing the conversion mechanism as well as rotational-relaxation pathways.