Phonon mode coupling and Fano-type antiresonance in polyethylene

Polyethylene (PE) is a nonpolar partially crystalline polymer. The crystalline regions have lateral dimension of several hundred Angstroms and are embeded in an amorphous polymer matrix. Crystalline polyethylene belongs to the orthorhombic crystal system with point group D21~. In the far infrared spectral region PE shows a strong absorption line at 73 cm -1 and a very weak band at 109 cm -1, which is observable only at low temperature up to about 120 K [1, 2]. Both bands are superimposed on a continuous background absorption. They have been recognized to result from external lattice vibrations of the crystalline portions of the polymer [3, 4]. The corresponding optical phonons belong to the Blu (73 cm -1) and B2u (109 cm -1) representations of D2h, where, as shown in figure 1, the two sublattices vibrate along the b-axis and in a-axis direction, respectively. The transition dipole moments of both modes are normal to the elongations of the PE-chains. A recent detailed investigation of the temperature dependence of the far infrared absorption of PE by Frank et al. has revealed several remarkable features of the structure of the absorption which have not yet been satisfactorily explained [5]. Figure 2 shows the absorption coefficient of PE as a function of frequency for several temperatures between 4 K and 414 K, just below the melting temperature Tin=430 K. The weak B2u-band decreases with rising temperature and vanishes at 120 K. The absorption strength of the Blu-band is constant up to 80 K. Above that temperature the absorption decreases gradually (3 ,,C~H