IR absorption spectrum of molecules trapped in matrices : I . The libron - phonon coupling in 1 ~

Theoretical expressions are obtained for the infrared absorption coefficient of a one-dimensional monatomic lattice containing one diatomic dubstitutional impurity. The diatomic molecule interacts with the environment introducing high rotational barriers that reduce the molecular rotation to a libration coupled to the host-lattice vibrations (libronphonon coupling). This is equivalent to the explicit consideration of static and dynamic interactions on the lineshape function. It is found that the absorption coefficient contains three main components associated with the pure vibration and libration spectra and with the combination spectrum of libration-vibration. These expressions are worked out in the harmonic and anharmonic approximation of the system Hamiltonian. Broadening of the absorption lines is obtained when the libration frequency falls inside the phonon band. The anharmonicities in the dynamical interaction (only third-order terms have been kept) give rise to an important temperature dependence of the shift and width of lines. Numerical results of the absorption coefficient of CO trapped in linear chains of Ne, Ar, Kr and Xe interacting via molecular-pair potentials are discussed. The near-infrared spectrum is qualitatively in agreement with the measured vibrational frequencies.