Zntroductian In an alkali halide, an M-center consists of two neighbor F-centers. In the case of KCl crystals, the M-center is characterized by an optical absorption band centered at about 800nm (1.550eV) at liquid nitrogen temperature. These bands are present in most of the crystals of the alkali halides with colored centers [l]. It has been verified that these bands do not present a Gaussian shape [2, 31. Besides, von Doorn [2] determined that new M-centers are generated when the crystal is illuminated with F-band light, where this increment depends on the time of exposure to it. In addition, the M-band presents a shift when the KC1 crystal is roughly illuminated with F-band light; this shift ceases after 100 min of illumination, and its maximum goes from an initial value of 800 nm (1.550 eV) to a steady final value of 820 nm (1.512 eV). The M-band is highly asymmetrical during the shift. This asymmetrical aspect suggests that the M-band would be formed by at least two bands, as Fig. 29 in [2] indicates. As a consequence, two kinds of M-centers are defined: thp properly called M-center, that absorbs at 800 nm; and the so called M'-center, that absorbs at 820 nm. The existence of these two kinds of M-centers has also been observed by Kanzaki [4]. A shift of the M-band has been detected when the crystal is set under high pressures ranging around 52.800 atm; but in this case, the shift goes to the high energies of the spectrum [5].Experimental procedure In order to make an exhaustive study of the M-band, a 4.5 x 4.5 x 23 mm3 KCl crystal, bought from Harshaw Chemical Co., was prepared, and additively colored. The coloration was done at 500 "C, for 90 min, within a potassium atmposphere. Except for one extreme, all the faces of the crystal were blocked to the coloration. This fact led to attain a density gradient of M-centers along the crystal. Thus, four absorption spectra were obtained, each in a particular longitudinal location of the crystal, generating different spectra for different concentrations of M-centers between 1014 and 1015 M-centers/cm3. These spectra are illustrated in Fig. 1. The spectra were obtained at room temperature right after coloring the crystal, in a UV-VIS spectrometer model 552 A, from Perkin-Elmer, with a slit of 1 nm. It is necessary to point out that this spectrometer has a wavelength accuracy of f0.5 nm, a repeatability of kO.1 nm, and a resolution of 0.25 nm. The presence of maxima was detected in each of the measurements between the 750 and 900 nm range where the M-band is located, indicating that the M-band is formed by several other constitutive bands. These constitutive bands lose resolution when the spectrum is produced with low density of M-centers, and the entire resolution approximates I) Casilla 110-V, Valparaiso, Chile.
