Optical properties of cubic and tetragonal Cu- centers in alkali halides

In the excitation spectrum of KC1 :Cuthe A band due to 'A,,(4s2) + 3Tlu(4s 4p) is detected. Its flat-topped shape reveals equal coupling to e, and tZg modes, whereas the near-gaussian shape of the C band signifies preponderant coupling of the ITlu state to the t2, mode. Negligible quadratic coupling is deduced from a moment analysis of several C bands. A newly detected absorption band in the D band region and the splitting of the D, band of tetragonal Cu, centers seem to confirm Tsuboi's D band assignment to 3d1° + 3d9 4p. C band splitting, induced dichroism, and polarized emission of Cu, centers prove their close analogy with Agi centers. In contrast with the usual behavior of s2-type cen41. For excitation spectra a 1000 W xenon lamp and ters in alkali halides only one of the known absorpa double monochromator have been used. tion bands of Cucenters is due to a s2 + sp type Figure 1 shows some of the spectra obtained on transition (C band : 'Alq-+ 'TI ), whereas at least cubic and tetragonal Cucenters in KC1 and CsBr. two absorption bands at higher energies (Dl and D2) belong to transitions into other electron configurations [I]. Based on the qualitative agreement of the D band spectrum with the energy level diagram of free Cu+, Tsuboi [Z] proposed the attribution to transitions of the type 3d1° 4s2--r 3d9 4s2 4p. In this paper we shall contribute some further arguments to support this new assignment. In its main part, however, we shall tackle some open questions concerning the 4s 4p states : 1. The location of the hitherto unknown A and B bands using excitation spectrometry because of the low center concentration available in single crystals (1016 cm-3 [I]) and of the low expected oscillator strengths (10-~-10-~ 111). 2. The absence of any structure within the C bands despite the Jahn-Teller effect (JTE ; here : T-t problem [3]), which manifests itself in a trigonal distortion of the relaxed excited Cucenter in KC1 [I]. 3. The investigation of perturbed Cucenters (e.g. Cu-(Na) in KI-,Na,Cl with x -0.02), which are expected to behave analogously to the corresponding Agi centers [I, 41. Experimental details like sample preparation and orientation, photochemical Cu--Cu, conversion, and optical techniques have been described previously [I, Fig. 1. Absorption (C, Dl , D,), emission (A', C'), and excitation spectra (A", C , D'i, D';) at LHeTof (a) Cu(-), Cu-(Na) (--------), and Cu-(Li) (. . . .) centers in KC1, and of (b) Cu( 1 and Cu-(K) (------) centers in CsBr, respectively. 0 6 5 f C 9