Near infrared absorption bands of the hydroxyl ion in alkali halide crystals

The near infrared absorption bands due to OH- in NaC1, NaBr, KCI, KBr, KI, and RbC1 have been measured at temperatures ranging from 1.4~ to 380 oK. Energy levels of the hydroxyl dipole in the crystal lattice were determined from the spectra. The crystals were grown by the Czochra1ski m~thod in a purified argon atmosphere. The spectra were measured with a spectrophotometer assembled from connnercially available components. Three cryostats were used: 1) an innnersion cryostat which operated at 1.4°K with the sample in pumped liquid helium, 2) a conduction cryostat with exchange gas switch coupling to the liquid helium bath for sample temperatures between 4.SoK and 78°K, and 3) an exchange gas switch equipped liquid nitrogen cryostat for temperatures between 78°K and 380 oK. The helium temperature spectra consist of three classes of absorption bands: 1) the O-H stretching absorption (main band) near 3600 cm-1, 2) a side band at about 300 cm-1 higher energy due to torsional oscillation (libration), and 3) low-lying side bands at splittings ranging from about 4 cm-1 in KI:OH to 40 cm- l in KBr:OH. Spectra of KC1:0D and KBr:OD were measured to confirm the 1ibrationa1 assignment of the 300 cm- l band, and the rotational barrier was calculated using the Devonshire theory. The low-lying energy levels are not prediced by the Devonshire theory, and are strongly dependent on the host lattice. RbC1:0H, KC1:0H, and KBr:OH had side bands which peaked at 30, 32, and 40 cm-1 respectively, and these had similar features. KI:OH had bands at 3.7 and 7.5 cm-1 at 4.S0K; the one at lower energy was frozen out at 1.4°K. NaC1:0H had several bands between 3640 and 3670 cm -1, and some of these were strongly temperature and concentration dependent. At 1.4°K there was a summation side band at 11 cm-1 splitting, and at 4.S oK an additional summation band split by S cm-1 was present. Two difference bands split by Sand 11 cm-1 were evident in the SOK spectrum. The KC1:0H 32 cm-1 band shifted to 23 cm-1 with isotopic substitution, giving an energy ratio of 1.4. The shift ratio for KBr was much smaller, probably less than 1.1. The electric field induced dichroism was measured at 1.4°K for the three alkali chlorides. With parallel polarization, the 11 cm-1 band in NaC1:0H was enhanced, the 30 cm-1 band in RbC1:0H shifted to higher energy, and the 32 cm-1 band in KC1:0H was quenched. The half widths of the main band and 1ibrationa1 side band were measured at various temperatures to observe the thermal broadening of these excitations. Possible decay mechanism were proposed, but a good theoretical fit to all the data over the entire temperature range was not found. The oscillator strength of the main band at liquid helium temperature was about 10-3. Rotation-like motion of the dipole was not observed at any temperature.U of I OnlyThesi