J. Phys. Chem. 1901, 85, 4079-4084
cannot deviate from linearity by more than perhaps 5O. The empirical relationship between deuterium quadrupole coupling constant and 0-e0distance is found to hold in the general case for departures from linearity no greater than perhaps 30°.26
4079
Acknowledgment. This research was supported by the National Institutes of Health via research grant GM-23395 and by the National Science Foundation through research grant DMR-77-23999 with the Materials Research Laboratory, University of Illinois.
Photostimulated Reorientation of PO,- in KCI Halide Ion Vacancies A. E. Gentry and A. H. Francis' Department of Chemistv, The University of Michigan, Ann Arbor, Michigan 48 109 (Received: May 22, 1981; In Final Form: August 27, 1981)
The temperature dependence of the rate of photoorientation of POz-anion impurity centers in potassium chloride single crystals has been determined over the temperature range 2-50 K. The temperature dependence of the photoorientation rate constant has been interpreted in terms of a kinetic model for thermal and photophysical reorientation of the molecular axes in a sixfold orientationally degenerate orthorhombic site. Analysis of the experimental data yields values for the potential barrier to reorientation in the 'A, electronic ground state of 783 cm-l and in the first electronically excited 3B, state of 325 cm-l. Several mechanisms are discussed for the photoorientation process.
Introduction Small molecular ions have been observed to reorient between crystallographically equivalent sites in alkali halide The motion of ions such as OH-, NO;, Sz-, 02-, and POz- in various of the alkali halide lattices has been investigated by dielectric constant measurements,4-6uniaxial stress effects:+ magnetic resonance,l+14 Raman,16-21 infrared,16~22-24 and optical spectroscopy.2,16-18,Z1,25-29
(1) K. K. Rebane and L. A. Rebane, Pure Appl. Chem., 37,161 (1974). (2)S.J. Hunter, K. W. Hipps, Richard Bramley, and A. H. Francis, Chem. Phys., 45,149 (1980). (3)F. Luty, Phys. Rev. B,10,3667 (1974). (4)U. Bosshard, R. W. Dreyfus, and W. Kiinzig, Phys. Kondens. Mater., 4,254 (1965). (5)W.Kiinzig, H. R. Hart, Jr., and S. Roberta, Phys. Rev. Lett., 13, 543 (1964). (6)H. S.Sack and M. C. Moriarty, Solid State Commun., 3,93(1965). (7)M. Ikezawa and J. Rolfe, J. Chem. Phys., 58,2024 (1973). (8)W. Kiinzig, J. Phys. Chem. Solids,23,479 (1962). (9)W.E.Bron and R. W. Dreyfus, Phys. Reu. Lett., 16,165 (1965). (10)S. J. Hunter, K. W. Hipps, and A. H. Francis, Chem. Phys., 40, 367 (1979). (11)S.J. Hunter, K. W. Hipps, and A. H. Francis, Chem. Phys. Lett., 51, 287 (1977). (12)J. R. Brailsford and J. R. Morton, J.Magn. Reson., 1,575(1969). (13)W.Kiinzig and M. H. Cohen, Phys. Rev. Lett., 3, 509 (1959). (14)A. Suisalu and R. Avarmaa, Phys. Status Solidi b, 97,69(1980). (15)R. Callender and P. S. Pershan, Phys. Reo. Lett., 23,947(1969). (16)A. R. Evans and D. B. Fitchen, Phys. Reu. B , 2, 1074 (1970). (17)A. Freiberg and P.Kukk, Chem. Phys., 40,405 (1979). (18)K. K. Rebane, L.A. Rebane, T. J. Haldre, and A. A. Gorokhovskii, Adu. Raman Spectrosc., 1, 393 (1972). (19)L.A. Rebane, A. B. Treshchalov,and T. Yu. Khal'dre, Sou. Phys. Solid State, 16,1460 (1975). (20)L. A. Rebane, T. Yu. Khal'dre, A. E. Novik, and A. A. Gorokhovskii, Sou. Phys. Solid State, 15, 2129 (1974). (21)C.A. Sawicki and D. B. Fitchen, J. Chem. Phys., 65,4497(1976). (22)M. V. Klein, B. Wedding, and M.A. Levine, Phys. Rev., 180,902 (1969). (23)V. Naravanamurti, W.D. Seward, and R. 0. Pohl, Phys. Reu., 148, 481 (1966). (24)B.Wedding and M. V. Klein, Phys. Reu., 177, 1274 (1969). (25)A. M. Freiberg, A. B. Treshchalov, and 0. I. Sil'd, Zh. Prikl. Specktrosk., 28,808 (1978). (26)L. A.Rebane and A. B. Treshchalov, J.Lumin.,12/13,425(1976). (27)A. B. Treahchalov, A. M. Freiberg, and L. A. Rebane, Sou. Phys. Solid State, 17, 1884 (1975). (28)A. Brun, A. C. Boccara, and N. Moreau, J.Phys., 37,111(1976). 0022-3054/81/2085-4079$01.25/0
When the point symmetry of the ionic impurity center is lower than the symmetry of the site of the perfect lattice where the defect is located, then several crystallographid y equivalent orientations of the defect are possible. The degeneracy associated with the different orientations, called orientational degeneracy, may be removed by application of an electric field, a magnetic field, a uniaxial stress, or any perturbation which effectively lowers the site symmetry of the perfect lattice. The barriers to reorientation of small ions in alkali halide lattices range from near zero to several thousand wavenumbers; under these circumstances, thermally activated reorientation times range from nanoseconds at room temperature to many years at the temperature of normal boiling liquid helium, 4.2 K. In the majority of situations encountered experimentally, the barriers to reorientation of the impurity center are sufficiently high that reorientational motion is thermally quenched at cryogenic temperatures, and the residual motion is predominantly librational in character. Recent spectroscopic studies of impurity molecules and ions in both crystalline lattices and in vitreous matrices have demonstrated that, under certain circumstances, it is possible to stimulate a change in the orientation of the impurity by electronic excitation of the impurity center. The phenomenon of photoreorientation is of interest on several accounts: first, it may provide the solid-state chemist/physicist with an interesting and novel system in which to study a variety of electron-phonon interactions important to the theories of lattice dynamics. Research on reorientation processes in solid matrices provides information on the structure, orientation, and local symmetry of centers in a crystal, the shape of the potential surface of the crystal near a center, etc. Second, since only a change in molecular orientation occurs and not a change in chemical bonding, the process is reversible under the action of either heat, light, or electric and magnetic fields. Therefore, the phenomenon provides a potential basis for high-density information storage by optical methods.30 (29)R. Avarmaa and L.Rebane, Phys. Status Solidi,35,107 (1969).
0 1981 American Chemical Society
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The Journal of Physical Chemistry, Val. 85, No. 26, 1981
We have previously reported results of extensive spectroscopic investigations of the PO, molecular ion impurity center in potassium chloride single crystals. These investigations and those of other r e s e a r ~ h e r shave ~ , ~ ~established the vibrational assignments for the ground electronic state and the orbital symmetry of the first excited singlet (lB1) and triplet (3B1)states. Optically detected magnetic resonance (ODMR) spectroscopy in the presence of an external magnetic field was employed to establish the orientation of the molecular axes with respect to the crystallographic axis ~ y s t e m . ' ~ J lAn J ~ earlier study of the reorientation dynamics in the temperature range 30-45 K2 yielded a single potential energy barrier of 783 cm-l between crystallographically equivalent orientations of PO,. A simple kinetic scheme was developed in the previous work that could qualitatively predict the kinetic behavior observed. The present paper reports results obtained over a broader temperature range, 2-50 K. An additional potential energy barrier was observed at low temperatures, forcing a modification of the kinetic model. The two barriers were assigned to the ground and excited electronic states by using the temperature dependence of the photoorientation rate constant and the temperature dependence of the polarization ratios. The kinetic model was then fit to the experimental data.
Experimental Section Large single crystals of phosphorus-activated potassium chloride were prepared in a vertical furnace. High-purity alkali halide (99.99%) together with about 0.01 wt % of red phosphorus was sealed under vacuum torr) in a vitreous silica tube. The sample tube was then lowered at a rate of 1cm/h through a Bridgeman furnace operated a t about 800 "C. Samples prepared in this manner were found to be uniformly activated with POz- centers and could be stored for several months a t 0 "C without deterioration. Crystal samples were cleaved parallel to the principal cleavage planes of the cubic lattice to approximate dimensions of 2 mm X 2 mm X 2 mm and mounted on a temperature-regulated sample holder. The sample was cooled by a flow of helium gas from a 4.2 K liquid helium reservoir in a Janis Model lODT cryostat. Liquid helium from the reservoir was vaporized by a resistively heated copper diffuser assembly, the temperature of which was regulated by a rate/proportional controller (Artronix Model 5301) to within 0.1 K over the 10-200-K range. Precise control of the temperature at the sample was achieved with an additional rate/proportional controller which regulated the temperature of the resistively heated sample block. The temperature of the sample block was measured with a Scientific Instruments Model N2G four-lead germanium resistance thermometer located 1mm behind the sample position. The excitation source used was a 1-kW high-pressure xenon lamp (Osram 900 B) filtered by a combination of an aqueous Ni/CoS04 solution and a Corning 7-54 glass filter. This combination of filters provided approximately an 800-A bandpass at 2950 A. The total radiation intensity at the sample was typically 0.19 W/cm2. Incident radiation was polarized with a 5 cm diameter 3M Co. Model PL-40 polarizing filter, which provided approximately 98% linearly polarized light between 2200 and 8000 8,and had an average transmittance of approximately 35%. The optical (30) H. Blume, T.Bader, and F. Luty, Opt. Cornrnun., 12,147(1974). (31) S.J. Hunter, K. W. Hipps, and A. H. Francis, Chern. Phys., 39,
209 (1979).
Gentry and Francis
excitation
emission
e mission
absorption
Flgure 1. The experimental optical arrangement, illustrating the optically distinguishable molecular orientations, the directions of the molecular absorption (dashed vector), and emission (solid vector) transition dipole moment. The polarizer and analyzer notation used in the text is also illustrated. 80,
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Flgure 2. The variation of the first-order rate constant (keXp,) for the change in phosphorescence intensity (right ordinate) and the change in steady-state phosphorescence intensity (left ordinate) as a function of incident intensity at 10 K.
arrangement employed in all experimental measurements is diagrammed in Figure 1. Incident radiation was parallel to the [loo] axis and phosphorescence was monitored a t 90", along the [OlO] axis, using a sharp cut Corning 3-75 glass filter to separate the sample emission from the scattered excitation radiation. A 3M Co. 105UV polarizing filter was inserted as an analyzer between the sample and the EM1 9558 QB photomultiplier used to detect phosphorescence.
Results When the sample was irradiated at constant temperature with plane-polarized light normally incident upon a cubic face, the phosphorescence intensity was observed to decrease exponentially with time and approach an equilibrium level asymptotically. Both the rate and degree of the change in phosphorescence intensity are linearly dependent upon incident light intensity as illustrated in Figure 2. Additionally, the rate and degree of intensity change are dependent upon sample temperature as illustrated in Figure 3. The degree of linear polarization of phosphorescence is also found to vary dramatically with temperature as shown in Figure 4. The polarization ratios
The Journal of Physical Chernhtty, Vol. 85, No. 26, 198 1 4081
Photostlmulated Reorientation of PO2Temperature ( K )
~--.-.-.---..-----..-.---)
I \
I
I
l
o
k;
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Flgure 5. Kinetic model illustrating the various processes leading to reorientation of the molecular axes. The notation is discussed in the text.
o
Kinetic Model for Photoorientation. The molecular axis system is chosen with z axis parallel to the twofold axis of the C2”ion and x perpendicular to the molecular plane. We have previously determinedlo that the ions occupy positions in the KC1 lattice at halide ion vacancies such that the molecular z axis is directed parallel to the principal axes of the cubic unit cell and the molecular x and y axes are directed along cube face diagonals as illustrated in Figure 1. Accordingly, there are 12 crystallographically equivalent orientations of the POz- ion in the halide ion vacancy. These are represented in Figure 1 by the 12 possible orientations of the plane normal (x axis). The rate of photoorientation depends on the rms amplitu_deof the projection of the incident electric field vector (P)on the molecular transition (electric) dipole moment ji,therefore, it is not possible to determine the sign of the molecular axes. Experiments were conducted with radiation normally incident upon the (100)face and plane polarized parallel to either the [ O l O ] or [OOl] crystal axes. Under these circumstances, each of the 12 crystallographically equivalent orientations is distinguishable as belonging to one of three optically inequivalent sets. Each of the three optically inequivalent sets contains four crystallographically and optically equivalent ions. Reorientation of optically equivalent ions occurs by 90’ rotations about the molecular z axis, but does not effect the results of the optical experiment and therefore is not considered in the following kinetic model. Reorientation of optically inequivalent ions alters the optical properties of the sample and is detected as a change in the intensity or polarization of phosphorescence. Interconversion between sets can occur by three mechanisms: (1)a temperature-independent photon-assisted process, (2) a temperature-dependent reorientation in the ground state, and (3) a temperature-dependent reorientation in the electronically excited state followed by electronic relaxation. The overall kinetic model, illustrated in Figure 5, contains five independent first-order rate constants and leads to the following set of six linear, coupled, differential equations which describe the time dependence of the ground (N,”, N,”, and N,”) and excited state (Nl’,N i , and N3/)populations of each set of optically inequivalent ions: dNi”/dt = -(k1 + 2kT’’)Nl” + kT”N2 + kT‘‘N3 + hN,’ + kN2’ + kN
