We present experimental measurements of the resonance of the large intensity dependence of refractive index in InSb near the band-gap energy at 77 K and derive a semiempirical theory for this effect which fits the measurements well, both in absolute magnitude and wavelength dependence, using only measurable parameters. The size of the effect in other nondegenerate direct-band-gap III-V compound semiconductors and at other temperatures is predicted.

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Recoilless nuclear resonance absorption of the 6.2-keV y rays of 181 Ta was used to study the hyperfine interaction of 181 Ta in Re metal. From the completely resolved Mbssbauer spectra measured at room temperature parallel and perpendicular to the [0001] axis of the single-crystalline m W(Re) sources, we derived the sign and magnitude of the nuclear quadrupole interaction, e 2 qQ(^) = ^ (2.15 ±0.02) x 10" 6 eV, the quadrupole-moment ratio Q(y)/Q(|.) = 1.133±0.010, and the isomer shift of 181 Ta(lte) relative to Ta metal, -14.00 ±0.10 mm/sec.The recent observation of large isomer shifts of the 6.2-keV y rays of 181 Ta between sources of 181 W diffused into single crystals of the secondand third-row transition metals and a Ta-metal absorber has demonstrated the high resolution inherent in this y resonance, when applied to solidstate problems. 1 The total range of these isomer shifts is by far the largest in terms of the natural . linewidth T-H/r (~10 4 T) or even in terms of the presently observed minimum experimental linewidth W (~ 500W), 2 compared to all the y resonances ever observed. The sensitivity of the 6.2-keV y resonance for magnetic hyperfine studies has already previously been demonstrated by measurements of the magnetic moment of the excited state in externally applied magnetic fields of the order of 1.5-3.0 kOe. 3 » 4 Likewise, a very high resolution is expected for the measurement of electric quadrupole interactions (EQI) because of the large electric quadrupole moment of the ground state of 181 Ta, Q(-J) = 3.9±0.4 b. 5 Despite these features, the 6.2-keV y resonance has not been applied to studies of magnetic dipole or electric quadrupole hyperfine interactions in solids up to now. This is at least partly connected with the difficulties in observing the resonance at all, a fact which is a direct consequence of its high sensitivity.The present paper reports on a measurement of the electric quadrupole splitting of the 6.2-keV y rays of 181 Ta in sources of 181 W diffused into single-crystalline rhenium metal. This is the first application of the 181 Ta y resonance to nuclear quadrupole interaction studies. From the completely resolved split spectra, we derive the sign and magnitude of the electric field gradient (EFG) at the 181 Ta site in rhenium metal, the ratio of the quadrupole moments of the excited state to that of the ground state, and the isomer shift of 181 Ta(/te) relative to Ta metal.The experimental technique employed was similar to the one described previously. 1 ' 4 From a high-purity single crystal of rhenium metal (p 300 / p 42 = 10 500), oriented to within 1°, thin disks (about 1 mm thick) were cut with a spark cutter parallel and perpendicular to the [0001] axis. After electropolishing them in a solution of 6% perchloric acid in methanol, the dissolved 181 W activity, produced by neutron irradiation of 93% enriched 180 W metal, was dropped onto the disks, dried and reduced in an atmosphere of H 2 at 850°C. Finally the diffusion was done in highvacuum (at 10 " 7 Torr) for 1 h at a te...