Absorption Bands and Lines in Irradiated LiF

Abstract

The paper deals with a number of phenomena observed in lithium fluoride after formation of color centers by exposure to x-rays, Van de Graaff electrons, or neutrons and not observed in other alkali halides under similar conditions. The most important of these phenomena are: (1) The appearance of a new band at 222 mμ, when the crystal is bleached with light of wavelength 254 mμ absorbed in the F band, and the inverse effect by subsequent irradiation with light of shorter wavelengths. (2) The production of F centers in a crystal heated to temperatures up to at least 450°C. The higher the temperature at which the F centers were produced, the greater is the intensity of the band at 222 mμ obtained by subsequent optical bleaching of the F band at room temperature. (3) The appearance of fine structure in a relatively weak band with peak at 380 mμ and of a very weak line at 523 mμ when the spectrum is recorded at the temperature of liquid nitrogen. (4) The very strong increase in the intensity of the band at 380 mμ and the line at 523 mμ by irradiation with short wavelength ultraviolet. (5) The sharpness at low temperatures of the first line of the band at 380 mμ and the line at 523mμ. At the temperature of liquid helium their half-widths are of the order of 1A.

F centers in LiF: A nuclear magnetic resonance study

“…An alternative mechanism of the observed effect could be the formation of F 2 center complexes showing a diamagnetic ground state [45]. The formation of such centers will lead to the observed decrease of 1=T 1 . However, the explanation of the surprising effect is still an unsolved problem.…”

“…The optical spectra were investigated, and the density of the different kinds of crystal defects like F, F 2 or F 3 centers can be determined from the intensity of the respective absorption lines [4,15,17,18]. Some dynamical studies and annealing experiments were performed [1,19], which show that the F centers are stable at room temperature, but recombine at temperatures higher than 360 K [19]. NMR studies showed clear evidence that besides the F centers molecular fluorine and metallic lithium clusters were created [12].…”

“…electrons at an empty anion site, are formed relatively easily. F centers in alkali halides have widely been studied over the last fifty years [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] by optical absorption spectroscopy, nuclear magnetic resonance (NMR), electron paramagnetic resonance (EPR), and other techniques. As these studies have shown, the F center EPR spectra show orientation dependent hyperfine splittings due to the interaction with the neighboring lithium and fluorine atoms.…”

Ab initio calculations of electronic and optical properties in O-doped LiF crystal

“…Meanwhile, the calculated optical absorption band is about 200 nm, and its shoulder peak is around 150 nm. It can be conceived that the peak at 200 nm should be corresponding with absorption band at 196 nm, which is related to the O 2− Va + dipole absorption band [37]. An earlier investiga- tion has revealed that the O-doped LiFcrystal may induce some absorption bands, such as 143 and 200 nm [33].…”

M-Center Spin Resonance and Oscillator Strength in LiF

“…Quantum-mechanical expressions for the rotational magnetic moment [2][3][4] do not lend themselves to numerical evaluation because of the great difficulties involved in obtaining electronic wave functions. However, for diatomic ionic molecules such as the alkali halides an approximation can be made which assumes that the molecule is composed of singly charged ions whose electrons are undisturbed by the presence of the other ion.…”