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RECENT RESULTS ON ZERO-PHONON LINES IN
NaF
G. Baumann, F. Lanzl, W. von der Osten, W. Waidelich
To cite this version:
JOURNAL DE PHYSIQUE Colloque C 4, supplkment au no 8-9, Tome 28, Aozit-Septembre 1967, page
C
4-72RECENT RESULTS ON ZERO-PHONON LINES IN NaF
G. BAUMANN, F. LANZL, W. VON DER OSTEN and W. WAIDELICH
(I. Physikalisches Institut der Technischen Hochschule Darmstadt)
R&um6.
-
Les raies B zero phonon de NaF ont ktk Ctudites, en absorption et en emission sous tension uniaxiale. On discute des centres colores correspondant et de certaines de leurs propriktks.Abstract.
-
Zero-phonon lines in NaF have been studied in absorption and emission by uniaxial stress. The corresponding color centers and some of their properties are discussed.The effect of uniaxial stress on optical zero-phonon line spectra of color centers in the alkali halides has proved to be useful in the determination of symmetry properties of P aggregate centers (see e. g. [l]). In colored crystals under uniaxial stress a splitting of the lines is observed that may be due to two reasons :
1. One reason raises from the fact that a Faggregate center, because of its anisotropy, exhibits several equivalent orientations in the cubic crystal. The number of these orientations is determined by the symmetry of the color center. This so-called " orien- tational degeneracy " may be lifted by uniaxial stress because then some of the orientations are no longer equivalent with respect to the stress axis. As a result one gets different changes of the transition energies which can be detected sensitively by studying the zero-phonon line.
2. The second reason for the lines to be split may be due to the removal of orbital degeneracies of the electronic states involved in the transitions. Degenerate electronic states are expected for color centers with trigonal, tetragonal and cubic symmetries.
The quantities of interest in the uniaxial stress measurements are the numbers, the energetic shifts, the intensities and polarizations of the components observed with stress applied in a symmetry direction of the crystal. From the numbers and the energetic shifts of the components an unique determination of the color center symmetry is possible. From the intensities and the polarizations the directions of the dipole moments associated with the transitions can be found.
The paper presented here briefly summarizes the results of stress splitting experiments with zero- phonon absorption and emission lines in NaF.
In the absorption spectrum of X-, y- or electron- irradiated NaF crystals a great variety of zero-phonon lines are observed at liquid helium temperature [2]. A number of these lines are found in emission if the crystal is excited with light of different wavelengths. As far as the lines are studied in the uniaxial stress experiments their spectral positions in absorption and emission, respectively, are listed in table 1. Figure 1
FIG. l. - 5 754 A zero-phonon line in NaF and multipho-
non bands ;
a) excitation spectrum ; b) emission spectrum.
presents as one example the 5 754 A zero-phonon line which is observed in absorption as well as in emission. The lines are accompanied by broad multiphonon bands, the band positions in absorption and emission being rather mirror symmetric with respect to the zero-phonon line position. This is expected from the Configurational Coordinate Model if the transitions in absorption and emission occur between the same pair of electronic states. Instead of the absorption band the excitation spectrum is shown as measured in the emission band maximum. The absorption band is superimposed by several absorption bands due to transitions of other aggregate centers.
RECENT RESULTS ON ZERO-PHONON LINES IN NaF C 4 - 7 3
Stress splitting results on zero-phonon lines in NaF
I
Position (A)abs. em. Symmetry class Rotation axis Dipole moment of electrons Numbers 3 2 (even) (even) (even) (even) (even) (even) (even) (even) (even) (even) (even) trigonal trigonal rhombic
< 110 >
rhombic < 110 > monoclinic < 110 > monoclinic<
100 > rhombic < 110 > rhombic < 110 > monoclinic< 110
> monoclinic< 110
> monoclinic < 110 > monoclinic< 110 >
monoclinic < 110 >The uniaxial stress measurements are performed at 4.2 OKby means of a pressure system making possible a variation and a measurement of the magnitude of stress. The crystals are stressed along the
<
100 >-,<
111 >-, < 110 >- and in some cases along the<
210 >-directions with maximum stresses of about 20 kp/mm2.In figure 2 some typical densitometer traces of the prominent 5 803
A
zero-phonon line are shown as obtained from a photographic plate taken with a highresolution optical spectrograph. The components of the 5 803 A line are presented for different directions and magnitudes of stress P, different directions of observation L and light polarized parallel (X) and
perpendicular (c) to the direction of stress. The schematically drawn components labelled a, b, c and d are calculated with respect to their positions and inten- sities assuming a
< 112 > dipole moment and mono-
clinic symmetry of the center. The rotation axis of the center is oriented along< 110 >
,
the< 112
>C 4 - 7 4 G. BAUMANN, F. LANZI, W. VON DER OSTEN AND W. WAIDELICH dipole thereby lying in a plane perpendicular to the
rotation axis.
The results of the stress splitting experiments are summarized in table 1. It contains the symmetries and the rotation axes of the corresponding color centers, the directions of the dipole moments associated with the transitions and the numbers of electrons attributed to the centers.
So far we have found color centers belonging to three different symmetry classes, to monoclinic
< 110 > and
<
100 >, rhombic < 110 > and trigonal symmetries.Most of the centers exhibit monoclinic or rhombic symmetries. The transition dipole moments are orien- ted along both < 110 > and < 100 > for the centers with rhombic, and along
< 112
> and< 110
> for those with monoclinic symmetries. From the intensi- ties of the components one can obtain information about the \numbers of electrons, because for odd electron number there are components of the dipole matrix elements allowed in all three possible direc- tions perpendicular to each other while for even electron number at most two components are pos- sible. In no case components of the dipole matrix elements in all three directions have been found. The conclusion that the electron numbers of the corres- ponding centers are all even is not absolutely sure as certain components may vanish although they are allowed from group theoretical arguments.One can imagine a number of possible models for those centers. The 5 803
A
line is due to a transition of a monoclinic center with a < 112 > dipole. Pick's 4 F-center model of the NI-center [3] or slightly different models consisting of 2 or 3 Pcenters [4] might be possible models for the corresponding center.The 5 456
A
zero-phonon line turns out to be due to a F:-center 'transition [5]. From theory of linear combination of atomic orbitals (L. C . A. 0.) a twofolddegenerate excited state is expected for this center. The degeneracy can be revealed by studying the splitting of the line in absorption as well as in emission. A strong stress- and temperature-dependent dichroism is seen in the emission components which are starting from the split levels of the excited state. This dichroism is caused by different thermal populations of these levels corresponding to a Boltzmann distribution.
This result is supported by measurements of the F, + F:-center conversion at 16 OK. A linear
relation between the decrease of the R-center (F,-cen- ter) and the increase of the F:-center absorption is obtained when the R-center is ionized by irradiating the crystal with R, band light. The result gives strong support for the 5 456 A line to be due to a transition of the ionized R-center.
Several zero-phonon lines are accompanied by a detailed phonon assisted structure. From this structure in absorption and emission the same phonon fre- quencies are found to couple to the electronic states of different color centers. The detailed phonon structure observed with the 5 803 zero-phonon line is presented in figure 3. Corresponding to the coupled
FIG. 3. - Zero-phonon line 5 803 A in NaF
and multiphonon structure.
transverse and longitudinal optical and acoustic modes the peaks are labelled TO, TA, L 0 and LA. The peaks marked byf are caused by transitions which do not belong to the phonon structure of this center, as seen by annealing experiments.
The work was supported by the Deutsche For- schungsgemeinschaft.
References
[l] LANZL (F.), VON DER OSTEN (W.) and WAIDELICH (W.), Zero-phonon lines and multiphonon structure ; Lectures given at the N. A. T. 0. Summer School, Ghent, Belgium 1966.
[2] JOHANNSON (G.), VON DER &TEN (W.), SCHINKMANN
(M.), und WAIDELICH (W.), Phys. Letters, 1964,
11, 210.
[3] PICK (H.), 2. Physik, 1960, 159, 69.
14.1 JOHANNSON (G.), L A ~ L (F.), VON DER OSTEN (W.) and WAIDELICH (W.), Phys. Letters, 1965, 15, 110.
[ 5 ] BAUMANN (G.), LANZL (F.), VON DER OS-IEN (W.) and
