Liyhium-activated resonances in alkali-halide alloys

Abstract

Infrared-active lattice resonant modes have been observed as absorption lines in the far-infrared spectral region for a large number of lattice-defect systems. (1) Many of these absorption lines have been identified with the quasi-localized lattice modes which occur when the lattice impurity has a stable equilibrium position at the normal lattice site. (2) in particular, uniaxial stress experiments on the sharp absorption line centered at 16. 2 cm-1 in KBr:Li+ indicate that the lithium impurity in this system occupies the normal lattice site. (3) More recent studies of the stress-induced frequency shifts for the lattice resonant modes in Ki:Ag+ (17. 3 cm-1) and NaCl:Cu+ (23.4 cm-1) show similar characteristics. (4) in all these cases there is strong evidence for a singlet ground state being involved in the I. R. transition because the absorption strengths or integrated intensities in the various polarizations remain constant for frequency splittings which are large compared to the sample temperature of 2°K. Under these conditions, a degenerate ground state would produce dichroism resulting from thermal population effects.