Jacques Claudel

Far Infrared Study of the Copper Halides at Low Temperatures

The far infrared spectra of the copper halides CuCl, CuBr, and Cul have been measured in transmission and in reflection at various fixed temperatures from room temperature to liquid-helium temperature. Marked changes of such parameters as resonance frquency and damping as a function of temperature have been observed, also a line-splitting in the case of CuCl. In addition, a considerable increase in the asymmetry of the transmission curves with decreasing temperature has been observed.

Sur le comportement différent des cristaux et des verres dans l’absorption de l’infrarouge lointain (40–1500μ) à la température de l’hélium liquide

Absorption spectra at liquid helium temperature in the far infrared are given for thick plates of CsI, CsBr, TlBr, TlCl, KI, NaCl, InSb, KBr, CuCl, Al2O3, SiO2, MgO, and LaCl3. For a thickness of about 3 mm all these crystals, completely opaque at room temperature, become quasitransparent at very low temperature. This supertransparency of crystals at low temperature gives several far infrared filters with remarkable cutoffs and a lot of available materials to the spectroscopist in a part of the spectrum where only quartz and polyethene were usable up to now. This is also evidence that the main part of the absorption at room temperature comes from the lattice phonons. It decreases with their number when the crystal is cooled (2 phonons difference process). The small absorption which remains in a few cases—KBr, —NaCl, KI—could arise from 2 phonons addition process nearly independent of temperature and much less efficient to give absorption. In the case of glasses the absorption coefficient is higher by 2 or 3 orders of magnitude and keeps constant when the glass is cooled. For quartz glass, the index of refraction also seemed invariant. An explanation is proposed in terms of one single phonon process, independent of temperature and allowed by the disorder characteristic of glassy states.

Far Infrared Reflection Spectra of AgCl, AgBr, and AgI at Low Temperatures

The reflection spectra of AgCl, AgBr, and AgI are measured at liquid hydrogen, liquid nitrogen, and room temperatures. For AgCl, they show distinctly two components. For AgBr and AgI, the structure, though complex, is not resolved. Nevertheless, in all cases, except for AgI, at low temperature, the Lorentz analysis needs two oscillators. The stronger one v(1) corresponds to the TO transition at point Gamma, the second one v(2), about an order of magnitude smaller, corresponds to a TA + TO addition band for the NaCl type structure, and to a TA + LA for the zinc blende or wurtzite structure, confirming preliminary measurements made by a completely different experimental approach (transmission of thin evaporated films instead of reflection on bulk crystals). They are, however, slightly distorted towards high frequencies. The proposed frequencies for the TO branch at point Gamma are, for AgCl: 105 cm(-1) (290 degrees K), 114 cm(-1), (80 degrees K), 119 cm(-1) (25 degrees K); for AgBr: 79 cm(-1) (290 degrees K), 84 cm(-1) (80 degrees K), 85 cm(-1) (25 degrees K), and for AgI: 103 cm(-1) (290 degrees K), 106 cm(-1) (80 degrees K), 106 cm(-1) (25 degrees K). The oscillator strengths and the damping of both oscillators are studied vs temperature. A large decrease of the damping is still observed between liquid nitrogen and liquid hydrogen temperatures. The refractive index and the absorption index are determined in the whole ir for the first time at liquid hydrogen, liquid nitrogen, and room temperatures, showing large variations, particularly a high transparency in the very far ir at liquid hydrogen temperature. The maximum values of n and k range from 2 to 10. The real and the imaginary parts of the dielectric constants are also computed for the whole ir range at room and lower temperatures down to liquid hydrogen temperature.

Spectres de transmission et de reflexion de I’iodure de potassium, pur et dopé, aux basses températures

The far ir reflectivity of a pure single crystal of KI at liquid helium, liquid nitrogen, and room temperature has been measured from 10 micro to 700 micro. Kramers-Kronig analysis, and a two-oscillators Lorentz analysis lead to close results concerning the refractive index n, and the absorption index k in the whole ir spectrum, with maximum values of n and k ranging from 4 to 8. The strongest Lorentz oscillator has a frequency at 80 degrees K (nu(1))= 110 cm(-1))) close to the transverse optic frequency recently determined from neutron scattering at 95 degrees K (TO = 107 +/- 1.3 cm(-1)). The strength of the second Lorentz oscillator decreases rapidly when the temperature is lowered. Its frequency (nu(2) = 150 cm(-1) at 80 degrees K) is such that it can be considered as the TO + TA combination at point L of the first Brillouin zone, which is ir active, and expected at 152.7 +/- 3 cm(-1) from neutron scattering. Absorption bands,locaized in the gap determined by neutron scattering as extending from 104-143 micro (96-70 cm(-1)), are induced by sodium, cesium, and chlorine ions impurities. A peak of absorption in the acoustic phonons spectrum is induced by sodium ion impurities at 63 cm(-1)) close to a maximum in the frequency distribution function for the normal modes of vibration of KI computed recently by Cowley et al. Bromine ions, on the other hand, lead to a general background absorption.