MED I UM WEAK STRONG,W I OE
144 consequences with regard to the validity of crystal field
calculations.
Pair lines on the low energy side of the visible band have been found. Since the ground state spin is quenched, a spin-spin interaction is not possible. As observation of a no-phonon transition requires a finite
zero-point distortion of the surroundings of the transition ion, pair no-phonon lines may be due to vibrational coupling between the energy levels of two pair ions.
THE SPECTRUM OF Co++ - ZnO
8 . 1 The d7 Configuration in Near Tetrahedral Symmetry
In several respects the d7 configuration of Co++ is
more complicated than d8 • The most obvious point is that an odd number of electrons is involvedr necessitating the use of
double groups when spin-orbit interaction is considered.
Another factor making identification of levels less simple is that the ground state in a tetrahedral field is one from which transitions to all excited states are allowed, so that we
cannot immediately identify the strongest transitions as cubic allowed levels . This lowest level is a degenerate one , and
is split by the trigonal field , so transitions can occur from both states . Finally , there are two multiplets , 2G and 4P , contributing to the visible band absorption . The spin-allowed 4P can be expected to give the strongest absorption , and the
2G to give some sharp lines .
The observed spectra show , for the visible band , a large
number of components , those of lowest energy being sharpest.4 ,5
As for Ni - ZnO , anisotropy of the spectrum indicates that a
trigonal field is present . Cubic field calculations5 indicate
appreciable 4P - 2G mixing, which will relax the spin se lect
ion rules .
The ground state cubic level , G - 4A2 (F) , has been shown
146 two Kramers doublets, an E112 ground state and an E
312 state
Transitions from both of these levels are seen in the optical spectra, verifying the ground state splitting value. Pulsed field Zeeman study of the very sharp lines of the visible band4 0 has been used to find the g values for the two components of the ground state and for the lowest level in the band. From these results it has been concluded that the two lowest lines of the visible band are transitions from the
3
two lowest states to the level E112 - 2E(e4t 2 ) .
Spin-orbit and trigonal parameters can be estimated from the g values and optical spectra, but the values found depend on the criteria used, and unique values cannot be obtained. In particular, a high value of the spin-orbit parameter seems to be necessary. With this uncertainty complete trigonal calculations, which involve matrices of order 42, are hardly justified.
8 . 2 The Spectra Taken for Co - ZnO++
Four good cobalt-doped crystals with concentrations from
0.17 to 1% were available to study axial spectra. As with the nickel impurity, only one fragment usable for polarisation study was found for all the samples grown. This was unfort- unately thick and small, and of reasonably high concentration. These three factors made use of the Bausch and Lomb almost impossible, due to the great reduction in intensity of the sample beam. Results were taken, but the accuracy was very
low. Attention was therefore restricted to study of the
on the low energy side of the visible band using polarised light . Temperature dependence could of course be studied for all cases.
As for nickel, a low resolution absorbance spectrum for
a low concentration sample is useful to obtain an overall picture of the spectrum. Figure 8 .1 shows under these con ditions most of the visible band. Below this range there is no absorption before an infrared band at 7000 cm-1 • The structure at higher energies is shown inserted on a different wavelength scale . Once again the strongest and sharpest components appear on the low energy side of the band. Apart from the three lines above 17000 cm-1, the structure becomes progressively more coalesced as energy increases . Slit limit
_ing is probably taking place for the 15153 and 15195 cm-1
lines. Figure 8 . 2 gives the spectrum for a higher concentrat ion crystal.
Figure 8 . 3 shows the spectral temperature dependence for the low concentration . It is to be noted that the lowest energy line shows clearly the rise in intensity and falling again characteristic of inadequate resolution . This and the strongest line show the same temperature dependence as the no-phonon lines of nickel. The next line, at 15260 cm-1,
148
ure, so identification as a vibronic line is not ruled out by temperature dependence. The behaviour of the rest of the band indicates that no other no-phonon lines are present, so consid eration of the phonon structure will be necessary.
The Jarrell-Ash axial spectra for two different concen trations are given in Figure 8. 4. The high resolution shows that both of the no-phonon lines are in fact doublets, of
different splitting. Study of the temperature dependence could show whether any lines were from an excited ground state, but
the merging of the lines above 10°K may reduce accuracy. Several
lines in the 15 100 cm- 1 region are strong in the high concen tration spectra. A marked decrease in intensity with increasing temperature, as for the weak lines of Ni - zno, rules out a
vibronic sideband origin. The � spectrum of a small sample is shown in Figure 8. 5. This shows two further sharp lines
at 15161 and 15 18 2 cm- 1, the latter apparently coming from a thermally excited state. As in the axial spectrum, the com ponents above 15200 cm- 1 are much broader at low temperatures, and do not show the strong dependence of width on temperature which characterises the sharp lines.
Tables of the estimated components of the spectra follow.
The format is as for the Ni - ZnO tables of section 6. 4 . 8. 3 Concentration Dependence
Samples 7, 2 2 and 25 give a range of spectra for concen tration dependence. Only the weak lines in the 15100 cm- 1