Dynamics for CH3F trapped in rare gas crystals and spectroscopic consequences

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Abstract
The dynamics of CH3F molecule trapped in a rare gas crystal is determined in order to explain the infrared spectra and vibrational relaxation data which had led to controversial qualitative interpretations. It is shown that the orientational motions of the molecular axis are strongly coupled to the translational dynamics of the molecule and, in a smaller extent, to the lattice vibrations. As a first consequence, the molecular axis remains nearly anticollinear to the axis joining the molecular center of mass and the site center, and the molecule behaves as a slightly hindered rotor implying both the molecular axis and its center of mass; the spinning motion appears to be a tunneling motion which considerably narrows the splitting between the k=0 and k=1 levels. As a second consequence, the orientational signals are strongly broadened by the translational dynamics of the molecule and of the crystal. So, the only observed sharp signal with a large foot is interpreted as the superimposition of a pure vibrational Q(1) branch and of broadened rotational structures connected to R( j0) and R( j1) ( j=0,1,2) signals. Moreover, there is not a dominant channel for the vibrational relaxation mechanism of CH3F trapped in argon matrix, since the transfers to the orientational modes, to the local or to the bulk phonon modes are shown to be equally efficient, with times (∼10 μs) in agreement with experimental data. In xenon matrix, the direct transfer to the lattice vibrations seems to be the most efficient mechanism of relaxation.