Coupling between phonons and quantum rotors

Quantum free-rotors in crystalline solids are exceedingly rare, but have recently been found for NH, groups in several Hofmann clathrates: M(a)(NH3)(2)M(b)(CN)(4) . 2C(6)D(6), where M(a) and M(b) are first-row transition metal elements. Above 4.0 K the spectrum of the NH3 J = 0 <--> J = 1 transitions broadens, and at around 30 K the inelastic peaks are Lorentzian-shaped with appreciable intensity spreading beyond 1 meV. Because the rotational constant for NH3 approximate to 0.71 meV, any inhomogeneous broadening of the J = 0 <--> J = 1 peaks cannot extend beyond this value. A model based on resonance of the rotational system with phonons suggests interaction with librational motions of the benzene molecules.