J. Phys. Chem. 1982, 86. 4714-4778
4714
is consistent with the idea that water is held on two kinds of Lewis acid surface sites and that coordinatively unsaturated Ti4+ions on two different crystal planes comprise these sites. C 0 2 and CO Adsorption. The C02 adsorption results demonstrate the surface basicity of oxidized titania surfaces and, significantly, the loss of this property after reduction with hydrogen. Since the C02results on oxidized anatase are in general agreement with earlier work, no additional disctission is given here. Turning now to CO adsorption, we found that it is clear that surface reduction took place to a small extent at room temperature when CO was exposed to an oxidized surface. This is indicated by the formation of bicarbonate species. No adsorption was found for coordinated C02indicating that, if formed, it was rapidly converted to carbonates. The loss of CO giving the 2185-cm-' band implies that the product carbonate species are held at sites which control the adsorption of CO. There are no previous reports of a CO band at 2115 cm-' on anatase. We ascribe this to adsorption on an oxidized surface and that it is active for the reduction of the surface. To support this idea we note that the behavior of the 2115-cm-' species parallels that of the carbonate during evacuation and that its concentration is independent of CO pressure and is therefore a possible reactive intermediate.
Conclusions From the results presented in this paper we draw the following conclusions 1. Two kinds of chemisorbed OH and two kinds of chemisorbed H 2 0 are found on anatase. The OH species
at 3676 cm-' is assigned to the (001) plane while that species at 3715 cm-' is assigned to the (100) and/or (010) planes. The two types of adsorbed water showed paired IR absorptions at 3694,3495 and 3660,3465 cm-l. These pairs are assigned to the (100) and (010) planes, respectively. 2. Four-coordinate Ti4+ions at the surface of anatase are proposed as the sites for adsorption of both OH and H20 while five-coordinate ions are suggested as sites for OH adsorption only. 3. Water is more strongly held on oxidized compared with reduced titania. The results suggest that water adsorption is enhanced by the presence of four-coordinate Ti4+ions and of surface oxygen ions. 4. Adsorption of C02 on oxidized anatase produces a coordinated C02 species which converts slowly to surface bicarbonate until equilibrium is reached which, under our conditions, is a state involving significant concentrations of both species. 5. The surface bicarbonate species thermally decomposes to produce water molecules accompanied by the loss of surface OH. The water molecules formed in this process occupy sites formed during the reaction and retard the subsequent readsorption of C02into the coordinated state. 6. Lattice oxygen is involved in a room temperature reduction reaction with CO to form a bicarbonate species. During this reaction, two kinds of adsorbed CO are observed: the 2185- and 2115-cm-' bands are assigned to ordinary chemisorbed CO and to the intermediate leading to the bicarbonate, respectively.
Acknowledgment. This work was supported in part by the Office of Naval Research.
Analysis of the Electron Spin Resonance Spectrum of Manganese( I I ) Impurity Centers in the Layered Compound CdPS3 E. LHshHz and A. H. Francis' Department of Chemistry, Univers& of MlchMn, Ann Arbor, Michigan 48109 (Received: April 30, 1982; In Final Form: August 2, 1982)
The X-band electron spin resonance spectra of Mn(I1) in the monoclinic lattice (CL) CdPS3has been studied at room temperature. The spectrum consists of a set of 30 lines corresponding to a single Mn(I1) environment. All details of the spectra are satisfactorily described by a spin Hamiltonian appropriate for Mn(I1) in a crystal field of rhombic or lower symmetq. The observed angular variation of the fme struthwe of the allowed transitions is found to be in good agreement with theory. The parameters of the spin Hamiltonian have been determined. The degree of covalency is determined in terms of the nuclear hyperfine coupling parameter, A . The overall results are discussed in relation to the previous optical results.
Introduction Many transition-metal chalcogenophosphates (MPS3) crystallize in a layered structure of the CdC12type, in which the metal and P-P pair substitute for the Cd(I1) ions in the ratio 2:l and sulfur atoms substitute for the chloride atoms. The structure of the MPS, compound has been reported in detail only for FePS,,l which belongs to the C&(C,,,)
monoclinic space group with two formula units per unit cell. Each transition-metal ion is surrounded by six sulfur atoms forming a trigonally distorted octahedron. In the layered MPS3 structure, adjacent planes of sulfur atoms are weakly bound by van der Waals (VDW) interactions. CdPS3 and MnPS, belong to the same monoclinic system as FePS3 and have very similar lattice parametem2 We assume that the structures of all three materials are isomorphous.
(1) W. Klingen, G. Eulenberger, and H.Hahn, 2.Anorg. Allg. Chem., 401,97 (1973).
(2) W. Klingen, R. Ott, and H. Hahn, 2.Anorg. Allg. Chem., 396, 271 (1973).
0022-365418212086-47 14807.2510
0 1982 American Chemical Society
Mn(I1) Impurlty Centers in CdPS3
The two-dimensional MPS3 compounds which are built from neutral layered matrix unita conneded by weak VDW interactions can accommodate guest species (I) between the layers to form intercalation compounds MPS3(I),. In the course of the intercalation process the guest diffuses from the outer surface into the bulk of the solid. The reaction is topotactic and reversible. Frequently, nonstoichiometric intercalation compounds are obtained with partial occupation of the accessible empty sites in the VDW gap or partial replacement of Cd(1I) ions in the host l a t t i ~ e . Moreover, ~ lamellar compounds, and particularly the chalcogenides, can easily be nonstoichiometric due to a metal excess in the VDW gap. The intercalate species in MPS3 that have been reported in the literature are electropositive alkali species (NiPS3:Li,),a organic molecules with Lewis-base character (pyridine, n-alkylamine),7s8and organometallic complexes with low first ionization potential (cobaltocene, bis(benzene)chromium).39*12 The intercalated species are weakly bonded and, in some cases, dynamically disordered at room temperature? The physical properties of many MPS3compounds such as transport phenomena, superconductivity, magnetic and optical properties, phase transitions, etc., may be modified significantly by intercalation. For example, intercalation with organometallic compounds leads to a decrease in the two-dimensional Heisenberg exchange interaction and a corresponding decrease in the NBel temperature of the material. Similarly, electrical and optical measurements have shown6J3that MPS3 compounds are broad-band semiconductors with gap values ranging from 1.3 to 3.5 eV, which may be changed as a result of intercalation. The majority of chalcogenide intercalation compounds have been characterized only by elemental analysis and TGA-DTA, or by determination of the interlayer spacing from X-ray powder diffraction. X-ray powder patterns all showed sharp (001) reflections that change as a result of intercalation. These changes correspond to the dimensions of the intercalated species. In contrast, (hko)reflections exhibited no change, indicating that the interatomic distances within a layer are almost unchanged upon intercalati~n.~J~ Historically, electronic and vibronic spectroscopy have been of considerable utility in the characterization of materials. We have done preliminary work13 investigating the electronic spectra of manganese in MnPS3 and CdPS3 lattices in order to provide the necessary information for subsequent studies of the corresponding intercalation compounds. That study has shown that there is high-order covalent bonding between manganese and the sulfur ligands within a layer of a host crystal. Additionally, spectroscopic evidence suggests that Mn(I1) ions in CdPS3 (3)R. Schollhorn, Physica, SSB, 89-99 (1980). (4)A. H. Thompson and M. S. Whittingham, Mater. Res. Bull.,12,741 (1977). (5)R. Clement, J. Chem. Soc., Chem. Commun., 647 (1980). (6)R. Brec, D.M. Schleich, G. Ouvrard, A. Louiey, and J. Rouxel, Inorg. Chem., 18, 1814 (1979). (7)J. Shoji Yamanaka, H. Koboyashi, and M. Tanaka, Chem. Lett., 329-32 (1976). (8)J. S. Otani, M. Shimada, F. Kanamaru, and M. Koizumi, Inorg. Chem., 19, 1249-51 (1980). (9)J. P.Audiene, R. Clement, U. Mathey, and C. Maziers, physic^, SSB, 133 (1980). (10)R. P.Clement, W. B. Davies, K. A. Ford, M. M. L. H. Green, and A. J. Jacobson, Inorg. Chem., 17,2754 (1978). (11)R. Clement and M. L. H. Green, J. Chem. Soc., Dalton T r a m . , 1566 (1979). (12)R. Clement, J. J. Girerd, and I. Morgenstern-Badarau, Inorg. Chem., 19, 2852 (1980). (13)J. Boerio-Goates, E. Lifshitz, and A. H. Francis, Inorg. Chem., 20, 3019 (1981).
The Journal of Physical Chemistry, Vol. 86, No. 24, 1982 4715
substitutionally replace octahedrally coordinated Cd(I1) ions in the transition-metal plane and may also occupy octahedrally or tetrahedrally coordinated sites in the VDW gap. There is still a lack of characterization of the intercalation compounds with regard to reproducible synthesis, nature of bonding between guest species and host lattice, process of intercalation, transport properties, etc. It has been suggested9J0J4that the intercalated species appears in ionic form within the host lattice and the process of intercalation involves electron transfer from the guest species into the conduction band of the host lattice. In addition to the pure scientific interest in intercalation compounds, a strong motivation is provided by certain potential technological applications: use of intercalation compounds with strongly electropositive metals as electrode materials for high-energy secondary batterie~,'~ heterogeneous catalysis,16 nonemissive optical display system,17 etc. ESR measurements can provide information about crystal environment, electronic structure, coordination, ion formation, and the nature of the bonding of the intercalated species in the VDW gap or in a substitutional site within the layer. It is the purpose of this work to report the ESR measurements of a diamagnetic, stoichiometric host lattice CdPS3 doped with Mn (93,3d5, S = 512, I = 512) at room temperature.
Experimental Section Sample Preparation. Single crystals of Mn(I1)-doped CdPS3 were prepared by vapor sublimation.'Je The synthesis of Mn(I1)-doped CdPS3 was accomplished by mixing a stoichiometric mixture of CdS and the remaining constituent elements with sufficient metallic manganese to give an initial 1mol % concentration. The mixture was then heated in an evacuated Vycor tube in a vertical two-zone furnace where a 670-600 "C temperature gradient was established over 30 cm. A good yield of yellowish crystals in the form of platelets could be obtained after 8-10 days. The dimensions of crystals used for ESR measurements were 2 X 2 X 0.5 mm3. Elemental analyses of Mn(I1)-doped CdPS3 (% Cd = 45.89, % P = 12.24, % S = 41.83) crystals prepared by the above methods were in agreement with the empirical formulas. Spectrometer. A Brucker ER 2OOE-SRC X-band spectrometer with 100-kHz field modulation was used to record the spectra at room temperature. The microwave cavity was a Brucker TMllo mode cylindrical cavity with the cavity axis parallel to the same rotation axis and perpendicular to the direction of the magnetic field. The microwave frequency of the Klystron source was determined by using a DPPH standard sample with g = 2.0036. Sample Orientation. Since CdPS3 has a monoclinic crystal structure with the space group C ( 2 / m ) ,the cation, and therefore Mn(II), in a substitutional site should lie on the twofold axis ( b axis), which could easily be identified optically. The crystal was optically aligned externally by using the method described by Rasmussen et al.19 and ~~~
~~~
(14)Y.Mathey, R. Clement, C. Sourisseau, and G. Lucazeau, Inorg. Chem., 19, 2773-9 (1980). (15)M. S.Whittingham, Prog. Solid State Chem., 12,41 (1978). (16)L. B.Ebert, Annu. Reu. Mater. Sci.,6 , 181 (1976). (17)H. N.Hersh, W. E. Kramer, and J. H.McGee, Appl. Phys. Lett., 27,646 (1975). (18)B. E. Taylor, J. Steger, and A. Wold, J.Solid State Chem., 7,461 (1 ,-".972) _,.
(19)E. J. Hornyak, K. M. Beem, and P. G. Rasmussen, Reu. Sci. Instrum., 40,224 (1969). (20)J. B. Chambers, W. R. Datars, and C. Calvo, J. Chem. Phys., 41, 806 (1964). (21)V. D. Jain and P. Venkateswarlu, J. Chem. Phys., 70,5168(1979).
4716
The Journal of Physical Chemistry, Vol. 86, No. 24, 1982
1
i
I
Lifshitz and Francis
I 2000
LN 5 2+.3
HI12
c
2
1 2c
I
1 40
I 60
Angle(8) in degrees
mounted on a rotating device which was then attached to the top of the microwave cavity with the axis of rotation parallel to the cavity axis and perpendicular to the magnetic field.
Results and Discussion Spin Humiltonian. Since ESR, in general, does not distinguish between orthorhombic or lower symmetry, the following Hamiltonian is suitable for the present problem of Mn(I1) 6S5j2ground statez8 7f8 = /3*H*gs+ D[Szz- l/S(S + l)] + E(SX2- S:) + f/su[SX4+ S: + Sz4- l/S(S + 1)(3S2+ 3 s - l)]+ AZZSJZ + AXZSXIX + A,,S$y (1)
where the first term representa the electronic Zeeman interaction, the second and third terms represent respectively the axial and rhombic parts of the fine structure (zero-field splitting), the fourth term represents the cubic part of the crystal field, and the last term represents the nuclear Zeeman (hyperfine) interaction S = I = 512 for Mn(II) (d5). It was assumed that the electron Zeeman interaction, the zero-field interaction, and the hyperfine interaction all have the same set of principal axes ( X , Y , and 2). In the free ion Mn(I1) all six possible orientations of the electron spin are degenerate in the absence of magnetic field. The zero-field term splits the six electronic levels into three Kramers’ doublets labeled by M = f5/2, f3/2, f 1 / 2 whose degeneracy can be lifted by the magnetic field. Each of these fine-structure levels is further split into a sextet by the hyperfine interaction with the nuclear spin I = 512 ( m = f5/2, f3/2, f1/2). The spacing within each sextet is constant and the only allowed transitions are AM = f l , = 0.29 (22) C. Calvo, J. S. Leung, and W. R. Datars, J.Chem. Phys., 46, 796 (1967). (23) C. V. Stager, Can. J . Phys., 46, 807 (1968). (24) V. K. Jain, Phys. Status Solidi B , 97, 337 (1980). (25) V. J. Folen, Phys. Rev., 139, 1961 (1965). (26) G. L. Bir, L. V. Dmitrieva, and S.A. Kizhalva, Sou. Phys.-Solid State (Engl. Transl.), 14, 18 (1972). (27) G. C. Upreti, J . Magn. Reson., 13, 336 (1974). (28) V. K. Jain and P. Venkateswarlu, J . Phys. C, 12, 865 (1979). (29) B. Bleaney, and D. J. E. Ingram. Proc. R. SOC.,London, Ser. A , 206, 336-56 (1951).
-.
4 IX
1 80
Figure 2. Angular variation of the allowed fine-structure transition in the ZX plane in the ESR spectrum of CdPS3:Mn(II) at 300 K: (0) experimental positions, (-) theoretical curves.
For an arbitrary orientation, the ESR spectrum consists of lines corresponding to the allowed (AM = f l , Am = 0) and forbidden transition (AM= f l , A m # 0). When the magnetic field is aligned parallel or perpendicular to one of the principal axes of the spin Hamiltonian, we obtain a characteristic 30-line spectrum, with each of the five fine-structure lines split into a hyperfine sextet, corresponding to the allowed transitions of a single Mn(I1) site. Figure 1 shows the ESR spectrum of Mn(I1) for the magnetic field (H)parallel to the 2 axis. When one of the principal axes of 7 f , parallels the magnetic field, the energy-level separations of the spin system correspond to an extreme value. The orientation of these axes can be found by searching for the magnetic field direction where the fine-structure lines lie at an extreme in field. From a comparison of the transitions in the five hyperfine sextets shown in Figure 1, it was found that the inhomogeneous broadening of the M = f5/2 f 3 / 2 transitions was not significantly larger than that of the central M = +1/2 s -112 transition, which indicate crystalline perfection. The line width is 12 G. The Y axis of the spin Hamiltonian is chosen parallel to the twofold crystallographic axis ( b axis). The X and 2 principal axes then lie in the crystallographic uc plane perpendicular to b. This axis convention is the same as that used in a number of other ESR studies of monoclinic systems reported in the literature: Zn2P207,20 (NH4)2Se04?lp-MgPz07,nCdzVz07,23 KzCz04.Hz0,24 C0203,25 LN03?6 Cd(CH3C00)2.3H20.27 The angular variation of the allowed fine-structure transitions was found by rotating about one of the spinhamiltonian principal axes and recording the spectra at (30) C. Kikuchi and G. H. Azarbayejani, J. Phys. SOC.Jpn., Suppl. B-1, 17, 453-5 (1962). (31) K. Falkowski, Acta Phys. Pol., 32, 831-8 (1968). (32) R. S. Title, Phys. Reu., 131, 2503 (1963). (33) N. F. Deigen, V. M. Maevskii, V. Ya. Zevin, and N. T. Vitrikhovskii, Radiospektrosk. Tuerd. Tela, Dokl. Vses. Soueshch., 1964,317-20 (1967). (34) D. Matamura, J. Phys. SOC.Jpn., 14, 108 (1959). (35) H. H. Woodbury and G. W. Ludwig, Bull. Am. Phys. Soc., 6,118 (1961).
The Journal of Physical Chemistry, Vol. 86, No. 24, 7982
Mn(1I) Impurity Centers in CdPS3
4717
TABLE I: Spin-HamiltonianParameters for Mn(I1)Doped in Various Single Crystalsa compd parameters
(NH4
K,C,O,.H,O
)zSe04
CdS
CaTe
2.0018 t 0.0005 2.0018 f 0.0005 2.0018 f 0.0005 8.2 f 0.1
2.003 2.003 2.003
2.002 2.002 2.002
67.2 67.2 67.2 34 cubic 300
55 55 55 35 tetragonal 300
GaP
CdPS,
~~~~~~
2.0039 2.019 f 0.001 2.005 t 0.005 Gx x 2.0039 2.003 0.005 2.081 f 0.001 GY Y 2.0065 2.000 f 0.001 2.002 f 0.005 G, z -383t 2 D 460+ 1 - 1 1 8 0 t 10 E 75 f 0.05 31t 2 5t 2 a 0.34 -87.8 + 0.5 -84f 1 88f 2 Ax X -87.4 f 0.5 88+ 2 -84t 1 A, Y -85.3 f 0.5 901 2 -82.8 i 0.5 A,, ref 21 24 22 remarks monoclinic monoclinic monoclinic temp, K 300 3 00 300 All the crystal field and hyperfine parameters are in units of
3.3 ? 0.4 -64.3 f 0.5 -64.3 t 0.5 -64.3 t 0.5 30-33 cubic 77
2.006 t 0.002 2.007 f 0.002 2.001 f 0.001 365 f 2 -6t 1 0.7 t 0.2 -69.2 t 1 -69.4 * 1 -70.5 i 1 present monoclinic 3 00
cm-'.
5' f 1' increments in the angle subtended by the magnetic field vector with a chosen principal axis and plotting the 4500 positions of the centers of M: f5/2 f3/2, f3/2, f3/2 f 1 / 2 , +1/2 transitions. These plots showed two ext trema separated by 90°, corresponding to positions where In vi 2cn 4000 one of the principal axes parallels the magnetic field. The Z axis was then chosen as the direction with the largest f field separation among the transitions. I Figure 2 shows the angular variation of the allowed 2 3500 fine-structure transitions (AM= A l ) in the ZX plane (4 !? u= 0, B = variable). The solid line represents the theoretical curve and the circles represent experimental data. The angular variation of the fine-structure transitions in the ZY plane (4 = 90,O = variable) is nearly the same as in the Z plane, except for the spread of the spectrum along the Y axis, which is less than along the X axis. Figure 3 shows the angular variation of the allowed finestructure transitions (AM= f l )in the XY plane (0 = 90°, 4 = variable). The angular variation in this plane is en0 20 40 60 80 tirely different from that in the ZX and ZY planes. Angle@) in degrees The ESR spectra were analyzed by computer diagonalization of the matrix of the spin Hamiltonian given in Flgure 3. Angular variation of the allowed fine-structure transitions in eq 1. The resultiag best-fit parameters for the experithe X Y plane in the ESR spectrum of CdPS,:Mn(II) at 300 K: (0) experimental positions, (-) theoretical curves. mental data are as follows: D = (365 f 2) X cm-', E = (-6 f. 1) X cm-l, a = (0.7 f 0.2) X lo-" cm-', g,, = The ESR spectra of octahedrally coordinated Mn(I1) in 2.001 f 0.001, g,, = 2.006 f 0.002, gw = 2.007 f 0.002, A,, the monoclinic lattices of some transition-metal phosphates = (-70.5 f 1) X cm-l, A,, = (-69.2 f 1) X cm-l, and vanadates are also qualitatively similar to the results AuY= (-69.4 f 1)X lo4 cm-'. The uncertainties associated reported here, with the principal differences being the with the best-fit values of these parameters were not demagnitude of the zero-field splitting parameters. rived from the mathematical analysis but rather represent The ESR spectrum of manganese has also been studied only subjective estimates. The signs of D and E are relin the family of cubic metal chalcogenide lattices MX:Mn ative and were deduced by comparing the second-order in which the Mn(I1) impurity ion substitutionally replaces shifts in the separation between hyperfine components for the metal M = Cd, Zn, Mg, Ca and is octahedrally coorthe various electron spin transitions. Here A is assumed with six chalcogenideligands X = S, Se, Te. Thus, to be negative as in most of the previous ~ o r k s The . ~ ~ ~ dinated ~ ~ the manganese coordination in these lattices is similar to fact that the crystal field parameters (D, E, a) have nonzero that in CdPS3 and the nuclear hyperfine coupling paramvalues and that g departs slightly from the free-spin value eters obtained for CdS:Mn(II) and CaTe:Mn(II) are nearly and also shows a small anisotropy indicates that Mn(II), identical with those obtained for CdPS3. The spin-Hamwhich substitutionally replaces Cd ions in a distorted ociltonian parameters for GaP:Mn(II), in which the mantahedral hole, has nearly axial symmetry with a slight ganese ion is tetragonally coordinated, are included in rhombic distortion. Table I because of the similarity of the optical emission The spin-Hamiltonian parameters are assembled in and excitation spectra of Mn(I1) in this material and Table I for comparison with the parameters obtained for CdPS3.13 octahedrally and tetrahedrally coordinated manganese in The nearest-neighbor interactions can influence the other doped crystal lattices. The angular variation of the hyperfine splittings much more than the values of D, E , ESR spectrum in the monoclinic lattices of (NH4)$e04 a, and g, where long-range interactions can be i m p ~ r t a n t . ~ ~ and K2CzO4-Hz0 is substantially the same as that reported Therefore, the hyperfine splitting constant of Mn(I1) can here for the CdPS3 monoclinic lattice due to roughly simbe used to evaluate the nature of the bond between the ilar values of the zero-field splitting parameters D and E.
-
-
-
(36) W. Low, "Paramagnetic Resonance in Solids" (Solid State Phys., Suppl., 2), Academic Press, New York, 1960, pp 72.
(37) K. A. Muller, 'Proceedings of the 16th Ampere Congress, Bucharest, 1970", I. Ursu, Ed., Publishing House of the Academy of the Socialist Republic of Rumania, Bucharest, 1970.
J. Phys. Chem. 1982, 86. 4718-4725
4710
manganese ion and its immediate surroundings. Van Wieringen3s has shown that the hyperfine splitting of Mn(II) decreases with increasing covalency of the magnetic complex. The relationship between the mean nuclear hyperfine coupling constant and the manganese-ligand bonding was put phenomenologically on a quantitative basis by plotting the experimentally determined values of A for a variety of different ligand fields vs. the average covalency c l n , where c is the Pauling covalency parameter computed by using eq 2 and n is the number of ligands,32+34,39 c = 1 - 0.16(XMn- X,)
- 0.035(X~,- XL)'
(2)
where Xm and XLare the electronegativities of manganese and the nearest-neighbor ligand, respectively. With Xhln = 1.5 and Xs = 2.5, c / n = 18.75%. The measured averaged hyperfine constant of Mn(I1) in CdPS3 agrees with the value predicted from the plot of A vs. average covalency c / n given in ref 39 and 41 within experimental error. The A value is indicative of a high degree of covalency in the Mn-S bond which is in agreement with the current interpretation of the electronic spectra of MnPS3. In our previous work13we have shown that the emission spectrum of Mn(I1) in MPSBlattices is at a much longer wavelength than observed in any manganese compound previously investigated. It closely resembles the phosphorescence of Mn(I1) in GaP where Mn substitutionally replaces Ga with ~~~
~
(38)J. S. Van Wieringen, Discuss. Faraday SOC.,19, 118 (1955). (39)E.Simanek and K. A. Mnller, Phys. Chem. Solids, 31,1027-40 (1970). (40)A. T.Vink and G. G. P. Van Gorkom, J.Lumin., 5,379 (1972). (41)M.Schlaak and A. Weiss, 2. Naturjorsch. A, 28,1932-6 (1973).
nearly tetrahedral phosphorus c o ~ r d i n a t i o n .In ~ ~both cases the analysis of the phosphorescence spectrum has yielded an exceptionally small value of the Rachah B parameter. This is also consistent with a high degree of covalent bonding between Mn(I1) and its nearest-neighbor surroundings. Because manganese in GaP is fourfold coordinated in GaP and sixfold coordinated in CdPS3, the average covalency parameter is greater in GaP. This is the principal reason for the substantially lower values of A cited for GaP:Mn(II) in Table I.
Conclusion The ESR spectra of Mn(II) in CdPS3have been recorded and the angular variation of the fine structure has been examined when the crystal was rotated about the principal axes of the spin Hamiltonian. A rhombic or lower symmetry Hamiltonian was used to calculate the theoretical ESR spectrum. The spin-Hamiltonian parameters were adjusted to give the best-fit with the experimental data and the results are compared with Mn(I1) in other lattices. The fact that the crystal field parameters (D, E , a ) are nonzero and that g has anisotropy and departs slightly from the free Mn ion value indicates that Mn(I1) substitutionally replaces Cd and has a nearly axial symmetry with a slight rhombic distortion. The comparatively low value of A indicates a high degree of covalency between Mn(I1) and the nearest-neighbor sulfur ligands in agreement with our preliminary invest i g a t i o n ~of~the ~ electronic spectra of this material. Acknowledgment. This work was supported in part by the donors of the Petroleum Research Fund, administered by the American Chemical Society.
Effect of Soivent-Induced Line Broadening on Resonance Raman Excitation Proflles and Depolarization Ratiost Wlllem Slebrand' and Marek Z. Zglerski Dlvlslon of Chemlstfy, National Research Councii of Canada, Ottawa, KIA OR6 Canada (Received: May 14, 1982; I n Final Form: Ju(v 21, 1982)
A theoretical study is reported of the effect of solvent-soluteinteractions on resonance Raman excitation profiles and depolarization ratios of polyatomic molecules. It is shown that these interactions do not only broaden vibronic lines but also affect the vibrational intensity distribution in the profiles and depolarization dispersion curves. To derive excited-state properties from such a distribution, it is therefore necessary to include solvent-induced broadening mechanisms in the theoretical model used to simulate the experimental observations. Several methods to accomplish this are compared and a general procedure is proposed based on qualitative analysis through simple, analyticalformulas, to be followed by a trial-and-error reproduction of observed profiles and depolarizationratios by numerical methods. The systems considered include totally and nontotally symmetric modes, homogeneous and inhomogeneous solvent-induced broadening, as well as models subject to strong interference. The treatment is used to reinterpret the excitation profile and depolarization dispersion curve of a nontotally symmetric fundamental of 3,9,12,18-tetra-tert-butyldidehydro[ 18lannulene. Introduction The study of resonance R~~~ excitation profiles may form of spectroscopy~i be regarded as a absorption spectroscopy one monitors transitions from a thermally relaxed set of ground-state levels to all excited-state levels that carry transition moment in the Issued as NRCC No. 20484. 0022-365418212086-47 18$01.2510
frequency range under investigation. Instead of measuring the absorption directly, one often measures the fluorescence excitation spectrum. If emission occurs from thermally relaxed excited-state levels, as is common for molecules in condensed phases or dense gases, this excitation spectrum will mimic the absorption spectrum in the low(1)W. Siebrand and M. Z. Zgierski in "Excited States",Vol. 4,E. C. Lim, Ed., Academic Press, New York, 1979,p 1, and references therein.
Published 1982 by the American Chemical Society
