We performed inelastic neutron scattering experiment to study magnetic excitation of O$_2$ molecules adsorbed in microporous compound. The dispersionless excitation with characteristic intensity modulation is observed at $hbar omega = 7.8$ meV at low temperature. The neutron cross section is explained by spin dimer model with intradimer distance of 3.1 AA . Anomalous behaviour in the temperature dependence is discussed in the context of enhanced magnetoelasticity in the soft framework of O$_2$ molecule.
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