Weak infrared bands in solid SiH4 and GeH4

The infrared side bands associated with the vibrational modes of SiH4 and GeH4 have been studied in the solid phase. At low T, all of the observed bands exhibit librational transitions. The tunneling splitting EA–T has been extracted from the librational energies. It turns out to be 0.2 μeV in SiH4 II, in excellent agreement with previous nuclear magnetic resonance determinations, and about 5×10−2 μeV in GeH4 (IV). A broad but distinguishable librational absorption is still present in the plastic phases at high T. The observation of overtone and combination bands is also reported.