Far-infrared absorption of pure and hydrogenateda-Ge anda-Si

The far-infrared spectra of pure and hydrogenated $a$-Ge and $a$-Si have been measured with a double-beam, optically compensated Fourier-transform method. In addition to the standard TA-LA-LO-TO absorption bands, "critical-point" structures are observed for the TA bands and also their suppression upon hydrogenation. A local mode, attributed to clusters of four hydrogen atoms, appears at the top of the TA band. These results are discussed in the light of lattice-dynamical calculations for mass defect clusters.