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The optical absorption spectra of substitutional ${\mathrm{Cr}}^{2+}$ ($3{d}^{4}$) in single crystals of ZnS (cubic and mixed cubic-and-hexagonal), ZnSe, ZnTe, CdS, and CdTe have been measured for Cr concentrations of ${10}^{18}$-${10}^{20}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ and at 2, 15, and 300\ifmmode^\circ\else\textdegree\fi{}K in the wave-number range $5\ensuremath{\le}\overline{\ensuremath{\nu}}\ensuremath{\le}30000$ ${\mathrm{cm}}^{\ensuremath{-}1}$. The room-temperature spectrum in the range $500\ensuremath{\le}\overline{\ensuremath{\nu}}\ensuremath{\le}30000$ ${\mathrm{cm}}^{\ensuremath{-}1}$ is dominated by a broad absorption band at 5500 ${\mathrm{cm}}^{\ensuremath{-}1}$ arising from the $^{5}T_{2}\ensuremath{\rightarrow}^{5}E$ transition. These two levels originate from the crystal-field splitting of the $^{5}D$ free-ion ground state. At helium temperatures distinct lines appear on the low-energy side of the $^{5}T_{2}\ensuremath{\rightarrow}^{5}E_{2}$ absorption band in all samples except for CdTe. These lines are associated with zero-phonon transitions and phonon-assisted transitions. As the temperature is increased, the relative intensities of some of the lines change due to the thermal population of low-lying levels belonging to the orbital ${T}_{2}$ state. In the case of ZnSe these levels have been studied in the far infrared in the range $5\ensuremath{\le}\overline{\ensuremath{\nu}}\ensuremath{\le}200$ ${\mathrm{cm}}^{\ensuremath{-}1}$, both with and without a magnetic field. An interpretation of the results is made based on crystal-field theory with a strong, static, Jahn-Teller distortion of $E$ symmetry in the $^{5}T_{2}$ orbital ground state. There is little or no Jahn-Teller effect in the $^{5}E$ orbital excited state. The optical transitions exhibit a strong coupling to $E$-mode phonons of average energy \ensuremath{\sim}70 ${\mathrm{cm}}^{\ensuremath{-}1}$.

The optical absorption in Cr doped ZnSe has been measured for photon wavenumbes,, ν , from 2 cm−1 to the bandedge at about 20 000 cm−1 for temperatures from 1.2 to 300°K. The Cr produces an extremely sharp absorption line at ν = 7.43 cm −1 , additional broad absorption lines between 50 and 100 cm−1, a broad absorption band at 5500 cm−1, and some further structure at higher energies. As we will show, these results support the conclusion of earlier paramagnetic resonance studies that a static Jahn-Teller distortion of E symmetry is present in the ground state for Cr2+ ions in ZnSe.

The results of a series of experiments undertaken to examine the statistical theory of inhomogeneous strain broadening of optical lines in crystals are described. The system chosen for this investigation was the sharp N 1 colour centre absorption line in sodium chloride, the random strains being provided by doping the crystals with other alkali and halide ions. The general theory of strain broadening is summarized, and it is shown that in the isotropic continuum approximation the predictions of the theory regarding the shape, halfwidth and shift of the line as a function of impurity concentration involve no unknown parameters. The theory predicts a Lorentzian contribution to the lineshape from the impurity strain fields, and a shift and halfwidth proportional to the fractional change in lattice parameter and its modulus respectively. On the whole good agreement between theory and experiment is found. The experimentally measured lineshapes can be well fitted by a convolution of the Gaussian shape of the pure crystal and the Lorentzian contribution of the impurities, but the measured factors of proportionality linking the shift and halfwidth to the impurity concentration and lattice parameter change are 25 per cent larger than the theoretical predictions. Some possible reasons for this discrepancy are discussed in terms of the breakdown of the isotropic continuum approximation, the accuracy of the experimentally determined strain coupling parameters of the N 1 centre and the effects of defects ( F centres, interstitials, etc.) produced along with the N 1 centres. The origins of two groups of satellite lines observed in the doped crystals are discussed with reference to the structure of the N 1 centre, and interpreted as being due to N 1 centres having nearby impurity ions.

The shape and moments of the F+ absorption band in SrO have been measured between 4.2K and 300K. Analysis of the moments gives the effective frequency of the vibrational modes interacting with the centre as h ω = 236 ± 20 cm -1 , and the Huang-Rhys factors for cubic and non-cubic modes as SC = 3 ± 1 and SNC = 4 ± 1 respectively.

The strain broadening theory of Stoneham6 is applied to the results of Clayman et al .1 on the shift broadening of the KBr:Li+ resonance upon alloying with KCl and KI. Good agreement is found for the shift of the resonance frequency as a function of lattice parameter change, but the observed halfwidth suggests that the Li+ ions and impurity anions may be correlated in position.

The effect of magnesium impurities on the thermal stability of an irradiation-induced colour centre in LiF is described. Measurements of the splitting of a zero-phonon line at 8328 A under uniaxial stress show that the centre has trigonal symmetry. These results and evidence for the production of these defects by conversion from R centres support the identification as an R' centre.

Optical and infrared spectra of U centres formed by proton and deuteron irradiation of KCl are reported. The efficiency of formation of U centres is roughly one U centre per ten incident protons. U → F conversion by ultraviolet bleaching confirms that the implanted U centres behave in the same way as in chemically doped crystals.

ESR and ENDOR have been used to investigate the F+ centre in MgO crystals enriched in the isotope 17O. A study has been made of the hyperfine interactions between the trapped electron and 17O nuclei at the first four anion shells neighbouring the oxygen vacancy. The results are consistent with a wavefunction in which most of the unpaired spin is in 2p orbits on the twelve nearest O2- ions with smaller admixtures of 3s and 3p orbitals in the six nearest Mg2+ ions. There is also evidence for small 2p admixtures at further O2- sites.