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If a monochromatic light beam absorbed by a semiconductor generates enough carriers to fill band states up to and including those of the optical transition, the optical absorption will be saturated and a condition of transparency should result. Analysis shows that in this regime the absorption coefficient varies inversely with light intensity.

The far infrared and Raman spectra of polyethylene oxide (PEO) between 5 and 150 cm-1 has been measured for three crystalline samples with different degrees of polymerization. Lines are observed at 37, 52, 81, and 107 cm-1 in the infrared and at 37, 56, and 78 cm-1 in the Raman spectrum. Theoretical calculations show that the 81 and 107 cm -1 lines are assignable to the torsional motion of the PEO helical chain about the C-O bond. The absorptions at 37 and 52 cm -1 have not been previously reported or predicted by any normal mode calculations of the isolated 712 helix although they seem to arise from intramolecular rather than lattice vibrations. FIG. 1. Infrared spectra of PEO 1500 (sample thickness =0.1 mm).

Values of carrier concentrations and mobilities in large monocrystalline samples of bismuth at 4.2 °K have been derived from measurements of all the galvanomagnetic tensor components through second order in magnetic field. Fields of the order of one gauss and less are required, and a superconducting chopper amplifier detects the minute voltages involved. The electron concentration is found to be 2.5X10 17 cm~3, and the concentration of "light" holes is very nearly the same. When compared with cyclotron resonance and de Haas-van Alphen experiments, the results are consistent with a three-ellipsoid model for electrons and a single ellipsoid for "light" holes. There is no indication of the presence of "heavy" holes in a concentration comparable to the other carriers. Electron and "light"-hole relaxation times are isotropic to within a factor of 2; for electrons, r = 2X 10~1 0 sec, and for holes, r == 5X 10~1 0 sec. Some of the effects of temperature and of larger magnetic fields are discussed.

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