J. Phys. Chem. 1995,99, 15172-15180
15172
A Family of Trapped Electron Centers on Alkali Metal Vapor Doped Magnesium Oxide Damien Murphy and Eli0 Giamello" Dipartimento di Chimica Inorganica, Chimica Fisica e Chimica dei Materiali, Universitb di Torino, Via P. Giuria 9, 10125 Torino, Italy Received: March 30, 1995; In Final Form: July 23, 1 9 9 9
The interaction of alkali metal vapors with the surface of MgO has been studied by EPR spectroscopy. After the previously described initial step of interaction leading to the formation of surface F,+ and F, color centers, higher amounts of metal vapor generate new paramagnetic centers which are characterized by a distinctive hyperfine structure. Two types of centers have been identified. The first one, found on Na, K, and Rb doped MgO, contains a single alkali metal atom which shares its valence (unpaired) electron with a suitable site on the MgO matrix. The spin density on the metal atom (percent s character) varies from about 0.2 (for Na) to about 0.5 (for Rb). A small but significant contribution of the metal p orbitals in the center is observed in terms of spectral anisotropy. The second type of center (observed for Li, Na, and K) contains three alkali atoms. Similar to the previous case, the unpaired electron density is not totally confined on the alkali cluster but is partially delocalized toward the matrix (especially for Li and Na). A model which gives a common explanation for the observed results, and which also incorporates the known matrix morphology, is discussed in detail. The observed phenomena are compared with similar examples, including the interaction of alkali metals with zeolites and the interaction of alkali metals with liquid ammonia or similar solvents.
Introduction
F2+-Mg0
The interaction of low ionization energy metals with ionic solids and in particular with metal oxides and zeolites is currently a subject of relevant interest in fundamental and applied research. The metal vapor-metal oxide interaction has mainly been investigated in the case of the alkaline earth (principally MgO) and in various zeolitess-I6 exchanged with monovalent alkali cations. Due to the frequent formation of paramagnetic centers upon metal vapor-metal oxide interaction, the electron paramagnetic resonance (EPR) technique has played a fundamental role in this experimental area. When the whole oxide is involved in the interaction with the metal atoms, the released electrons are essentially trapped at suitable sites (or traps) available in the solid lattice. In the particular case of the nanoporous zeolitic systems the interaction occurs in the internal cages where exchangeable cations, which form the electron trapping sites, are located.s-'6 In non-zeolitic oxides like MgO the bulk interaction only occurs at high temperature (usually higher than 1200 K), and under such conditions the electrons are trapped in bulk anion vacancies so that the resulting species are color centers also called F centers. When the anion vacancy in MgO (which can be labeled F2+ indicating the lack of two negative charges) contains one electron, a paramagnetic F+ center is formed, while the trapping of two electrons at this anion vacancy produces a diamagnetic center described as an F center. The F+ or F center in bulk MgO can be essentially regarded as a box surrounded by six octahedrally disposed Mg2+ cations containing one or two electrons. The metal vapor-metal oxide interaction that produces these centers can be written as follows (for a monovalent metal) F2+-Mg0
+ M - F+-MgO + M'
(1)
* To whom correspondence should be addressed. @Abstract published in Advance ACS Abstracts, September 15, 1995. 0022-3654/95/2099-15172$09.00/0
+ 2M -.F-MgO + 2M+
(2)
As previously mentioned, dehydrated zeolites exchanged with alkali cations also contain sites (which are easily accessible due to the open porous stgcture of the solid) with electron trapping abilities. Although these electron trapping sites are not strictly anion vacancies, they have some analogy with the P+vacancies in the bulk of ionic solids. Small groups of M+ exchanged cations (where M+ = Na+, K+,or Rb+) in the sodalite cages of Y zeolite or other faujasite type aluminosilicates trap the released electron to form entities which are often regarded as ionic clusters of general formula Mn("-l)+. The reaction can be expressed as shown in eq 3 for the particular case of sodium interacting with a Na+ containing Y zeolite (nNa+-Y), nNa+-Y -tNa
-
Na,("-')+-Y
+ Na+
(3)
A characteristic feature of these metal addition reactions is that the trapped electron centers can be generated regardless of the nature of M, the incoming metal atom. Sproull et al.I7 demonstrated, for instance, that bulk color centers were generated in BaO after doping with an excess of Ca or Ba metal vapors. In zeolites, Edwards et a1.I0 have clearly shown that N a 3 + or b3+ clusters were formed in the sodalite cages of Na+ or K+ exchanged Y zeolites, respectively, independently of the nature of the doping metal (Na, K, Rb). It is therefore clear that all these reactions are characterized by complete ionization of the metal atoms, accompanied by electron transfer to the electron "traps". The metal vapor-metal oxide interaction can however be confined to the surface of ionic oxides like MgO by operating at lower t e m p e r a t ~ r e . ' ~ -In~ ~this case surface F centers (Fs+ and F,) are formed when high surface area polycrystallineMgO is exposed to small amounts of alkali and alkaline earth metal vapors.18-20 At these low levels of added metal vapor the trapped electrons (now confined in surface anion vacancies which are less symmetric than the bulk vacancies) and the resulting alkali cations are well separated so that the spinHamiltonian parameters of the paramagnetic centers are scarcely 0 1995 American Chemical Society
Alkali Metal Vapor Doped Magnesium Oxide affected by the nature of M, which seems to mainly influence the line width.20 However, at higher levels of added alkali metal vapors, unusual paramagnetic centers are formed which are dependent on the nature of M, the doping metal. Two examples of such trapped electron centers have been the subjects of preliminary reports by The first one was observed in the case of Na doped MgO which was indicated as the sodium monoalkali cation center, formally written as Nad+(trap)d-, and modeled as an. association formed between a partially ionized alkali atom and a suitable surface site capable of trapping the electron.22 Another center observed on the surface of Li doped MgO, which is characterized by an appreciable interaction between the ionized atoms and the trapped electrons, is the lithium trimeric cation center.23 In the chemistry of the metal vapor-metal oxide interaction, these novel surface centers on alkali metal doped MgO are unique because the trapped electron and the parent cation maintain significant contact, as indicated by the presence of a clear hyperfine structure in the EPR spectrum. The present paper reports a thorough description of the results obtained by interaction of alkali metals (Li, Na, K, and Rb) with the surface of MgO as detected by EPR in that region of the interaction (medium levels of added metal) as opposed to the region where surface F centers are uniquely formed (Le., low levels of added metal) reported in previous paper^.'^,^^ The whole set of paramagnetic centers reported below can be regarded as a novel family of surface trapped electron centers.
Experimental Section At least two distinctive types of trapped electron centers have been identified by EPR spectroscopy on the surface of alkali metal vapor modified MgO (hereafter indicated as M/MgO).'8-23 These include the surface F,+ color centers and the novel excess electron centers discussed below. The experimental conditions used to generate the respective surface centers have been thoroughly reported el~ewhere,'~-*~ and therefore only a brief summary is outlined here. The MgO powder was supplied by Johnson Matthey (Puratronic, Grade 1). This material was transformed into the corresponding hydroxide, Mg(OH)2, by overnight stimng in purified water at 350 K and finally slowly decomposed under vacuum at 523 K to produce the high surface area MgO (180220 m2 g-l). The oxide was further annealed under vacuum at 1073 K for 1-2 h in a specially designed cell used for sample preparation and containing an EPR tube into which the sample is placed for spectra recording. The alkali metals were all supplied by Johnson Matthey (Grade 1, 99.95%). A small piece of metal was transferred to that part of the cell containing the activated sample under vacuum, and the temperature was slowly raised to that required for metal vapor production (Le., 570-590 K for Li, 533-553 K for Na, 413433 K for K, and 363-383 K for Rb). Under a 10-4-10-5 Torr dynamic vacuum the metal produced vapors at these temperatures. The onset of metal vapor production was recognized by an increase of pressure in the vacuum line and by the appearance of a metallic mirror on the cooler cell walls immediately outside the furnace. The powder and metal were constantly agitated to ensure thorough mixing of the sample and vapors. At the first appearance of vapor production the cell was immediately removed from the fumace, leaving a pale blue MgO powder. (The EPR spectra have shown this sample to contain F,+ centers 0 n l y . ' ~ 3 ~For ~ ) longer mixing times of vapors and sample (about 10 s) a deep blue oxide is produced. This sample contains the monomeric and trimeric trapped electron centers depending on the exposure time to metal vapors.
J. Phys. Chem., Vol. 99, No. 41, 1995 15173
ll
Figure 1. (a) Experimental and (b) computer simulated EPR spectrum of WMgO at 298 K. (c) Calculated profile of the monomeric center only (see text).
(Longer exposure times favor trimeric center formation, and shorter exposure times favor monomeric center formation.) The EPR spectra were recorded, usually at 298 K, on a Varian E-109 spectrometer equipped with a dual cavity operating in the X-band mode and with a 100 kHz field modulation. Varian pitch (g = 2.0028) was used for g value calibration, and the EPR spectra are reported in gauss (1 G = 0.1 mT). Computer simulations of the EPR powder spectra were obtained by means of a particular version of the SIM14S program adapted for PC. The program performs the spin-Hamiltonian calculations to second order.
Results
1. Monoalkali Cation Centers on M/MgO. The first type of paramagnetic centers formed when very low levels of alkali metal vapors are exposed to MgO are, as already described, the F,+ color centers. The EPR spectra of these centers are characterized by a narrow axial signal at g values of around 2.000-2.001 and a line width that varies between 1.5 and 3.4 G (depending on the metal used).20 In addition, these centers also show a peculiar saturation behavior.20 No hyperfine structure, which is indicative of an appreciable interaction between the released electron and the parent cation, was observed by CW-EPR. The only very weak hyperfine pattern which was observed in the spectra of these F,+ centers on M/MgO is due to the interaction of the trapped electron with one 25Mg nucleus (1 = 5 / 2 , abundance 10.13%) belonging to the surface cationic trap. As the amounts of Na, K, or Rb vapors are progressively increased and simultaneously the blue color of the sample intensifies, a multiplet of lines is observed in the EPR spectra of exactly 21 1 lines for each atom, where 1 is the nuclear spin ( I = 3/2 for 23Naand 39Kand I = 5 / 2 for s5Rb).24No spectra characterized by a 21 1 multiplet are observed on Li/MgO doped with similar levels of Li vapors (I = 3/2 for 'Li). The EPR spectrum of a potassium doped sample (KIMgO) is shown in Figure la. Despite the complexity of the spectrum due to small amounts of potassium trimeric cation centers (discussed later in the next section) which generate an underlying signal, a dominant structure composed of two very similar quartets, whose features are distinct in the external lines only, can be
+
+
15174 J. Phys. Chem., Vol. 99, No. 41, 1995 B 500 0
t
,
1
“Rb
I Figure 2. EPR spectrum at 298 K of R b N g O at two different scan ranges (a, b). (c) Computer simulation of (b) excluding the *’Rb component indicated with the asterisk.
clearly seen in the figure. Analysis of the spectrum at various microwave power levels indicates that the intensities of the two quartets behave similarly upon power variations and differently from all the other lines present in the spectrum. The most abundant isotope of potassium is 39Kwith A0 = 81.25 G. Similar to the previously reported spectrum22on Na/MgO, the hyperfine quartet on IUMgO suggests the interaction between an unpaired electron and a single 39Knucleus. The average hyperfine coupling constant of about 40 G suggests as a first approximation about 50% 4s character for the unpaired electron (Le., the percentage spin density in the 4s orbital of 39K, expressed as 40/81.25 x 100). This means that a much larger spin density is present in the K 4s orbital compared to the 20% 3s character previously observed for the corresponding spectrum on Na/Mg0.22 When thermally activated MgO is exposed to Rb vapors, the EPR spectrum shown in Figure 2 is observed. The spectrum in Figure 2, recorded at two different scan ranges (a and b), can be understood in terms of the simultaneous presence of a minor quartet signal with large hyperfine splitting (seen in Figure 2a) and a dominant sextet (magnified in Figure 2b), both of which are centered at a g value of about 1.995. The quartet and sextet both arise from the interaction of the electron with a single s7Rb ( I = 3/2 and 27.82% abundance) and 85Rb( I = 5/2 and 72.5% abundance) nucleus, respectively. Due to the wide field covered by the 87Rbstructure, the four lines of the quartet have remarkably different intensities. In particular, the outer ones are weak and broad while that corresponding to the I = ‘ / 2 transition (close to the spectrum center and indicated with an asterisk) is quite intense. The line separation of the Rb hyperfine structure increases dramatically from low field to high field (especially noticeable in Figure 2a) due to the secondorder effects described by the Brite-Rabi equation.25 The average splitting is 610 G for the quartet (87Rb)compared to 178 G for the sextet (due to x5Rb). The two experimentally determined values of A0 for the 87Rband 85Rbatoms are 1220 and 361 G, re~pectively.~~ The ratio of these latter values is about 3.38, which is very close to the 3.42 ratio measured for the average observed hyperfine separation (Le., 610/178). This
Murphy and Giamello indicates that the quartet and sextet multiplets in Figure 2 are due to the same type of paramagnetic center characterized by the presence of a single 85Rb or 87Rb atom. The unpaired electron in this center has approximately 50% 5s character (Le., 1781361 x 100 for s5Rb or 61011220 x 100 for 87Rb)similar to the value measured in the WMgO spectrum (Figure la). The most intense line in the central part of the Rb/MgO spectrum (Figure 2) at g RZ 1.96 can probably be attributed to small Rb metal particles since the signal does not saturate over the available microwave power range analogous to the trends observed and discussed in the spectrum of Na/MgO due to Na metal particles.22 This line overlaps the I = ‘12 hyperfine line of the s5Rb sextet. In addition, both the g value and line width of this signal did not vary greatly between 77 and 298 K as expected for conductance paramagnetic resonance of small metal particle^.^^,^^ A narrow and less intense resonance at g w 2.00, indicated in Figure 2b by an arrow, can be attributed to residual F,’ centers based on the expected g values20 for these color centers and on the similarities in saturation profile for this signal with the signal of the color centers on MgO doped with lower amounts of Rb vapors.20.21 It is clear from the spectra in Figures 1 and 2 that the dominant paramagnetic centers formed on both samples contain a single alkali metal nucleus. The same conclusion was reached in the case of N a g 0 for similar levels of metal addition.22 The shape and relative intensities of the individual hyperfine components are not however as expected for a pure contact interaction of a 4s or 5s electron with the nuclei of K and Rb, respectively. In both spectra in fact the intensities of the lines is not constant but decreases toward the wings of the spectra, accompanied by broadening of the outer resonance lines, a situation also found in the case of the N a g 0 spectrum22and in solutions of alkali metals in ethylamine and hexamethylphosp h ~ r a m i d e . ~The ~ . ~reason ~ for these observed spectral features was discussed in detail previously22 and shown to arise from the combined effects of (a) surface structural inhomogeneity with consequent dependence of the multiplet line width upon m: and (b) small but significant anisotropic contributions in both the g and A tensors. A similar combination of effects appears to dictate the shape of the present IUMgO (Figure 1) and Rb/ MgO (Figure 2) spectra. The computer simulations of these WMgO and Rb/MgO spectra are shown in Figures l b and 2c, respectively. Since the program at our disposal does not automatically compute the effect of a statistical distribution around a given g or A value, the spectra were calculated by assuming the presence (for both species I and II) of several paramagnetic centers very marginally differing in the g and A values in order to simulate the statistical distribution and obtain the ml-dependent line widths. However, similar to the Na/MgO case, acceptable computer simulations could not be achieved unless a contribution from anisotropy (axial) in both g and A tensors is introduced (Table 1). Furthermore, to correctly simulate the effects of the statistical distribution around the average values of the parameters for species I and 11, it was necessary to input a relevant number of species in the simulation program. As a consequence of this, the number of species employed to simulate the remaining spectral features was seriously limited, and the simulation of these details was therefore not completely satisfactory (Figure lb). For the sake of clarity, we have thus evidenced in Figure IC the calculated spectral trace of the two quartets (I and 11) which are buried in the more complex simulated spectrum of Figure lb. The simulated features of species I and I1 (Figure IC) closely recall the “cleaner” spectra observed for the monomeric centers on and on Rb/MgO (Figure 2).
J. Phys. Chem., Vol. 99, No. 41, 1995 15175
Alkali Metal Vapor Doped Magnesium Oxide
TABLE 1: Spin Hamiltonian Parameters Derived by Computer Simulation for the Monomeric Alkali Centers on Na, K, and Rb DoDed Me0 center
gll
gl
Ail
Al
Ao
ais0
Bo
B
CS2
Cp2
total spin density
Na/MgO" m g o
2.0017 1.9992 1.9992 1.9970 1.9965
2.0010 1.9995 1.9993 1.9948 1.9951
68 39.8 36.7 179.5 188
63 41.5 38.5 169 182
316 82.5 82.5 361 361
64.66 40.9 37.9 172.5 184
32.2 10.6 10.6 42.6 42.6
1.66 0.60 0.60 3.5 2.0
0.205 0.49 0.46 0.48 0.51
0.051 0.057 0.057 0.082 0.047
0.26 0.55 0.52 0.56 0.56
Rb/MgO a
~
~~
The values for N a g 0 were taken from ref 22.
Similar to the above case for IUMgO, the simulation of the monomeric Rb species in Figure 2c is also incomplete since the 87Rblines have not been included in the simulation in order to improve the fitting of the 85Rbstructure. The average values of the spin-Hamiltonian parameters derived from the simulation of the monomeric centers in IUMgO and R b N g O are reported in Table 1 together with the corresponding values observed on NalMg0.22 The calculated spectrum of the Rb center on R b N g O is based on the presence of the 85Rbsextet, combined with the additional resonances due to residual Fs+centers (g = 2.0003), impurity bulk Mn2+cations (g = 1.99, A = 85.75 G present in the background spectrum of activated MgO), and Rb small metal particles (g = 1.966); as previously indicated, the s7Rb quartet was not included in the simulation. From the principal values of the axial A tensor (A1 = aisoB and All = aiso 2B) both ais0 (the isotropic Fermi contact term) and B (the anisotropic dipolar term) can be derived so that the total spin density on the metal cations, which is expressed as C? :C (where aisdAO = Cs2and BIB0 = C2) can be calculated. The total spin density on the alkali atom of both centers for K and Rb was thus calculated to be 0.55 and 0.52 for species I and II of WMgO and 0.56 for species I and I1 of RbNgO. These calculated values are therefore significantly larger than the corresponding value of 0.26 observed for the Na/MgO center.22 Since in all three alkali metal centers the extent of the anisotropy remains rather small (Le., %5-8% C: character for Na+, K+, and Rbf centers), the observed increase in spin density from Na to Rb is essentially due to the increased s character of the centers built up by the two larger K and Rb atoms. On the basis of these results, we can conclude that (a) the surface monoalkali centers on WMgO and Rb/MgO are analogous to the center observed on Na/Mg0,22(b) in all three cases the ionization of the atom is not complete, and (c) a fraction of the electron spin density is still associated with the metal orbitals of the atom itself while the remaining fraction is delocalized out of the alkali atom orbitals. 2. Trialkali Cation Centers on M/MgO. When exposed to moderate levels of lithium vapors between 570 and 590 K, the microcrystalline MgO powder attains an intense blue color and simultaneously exhibits an EPR spectrum characterized by a scarcely resolved 10-line multiplet centered at g = 2.0011. This spectrum was assigned to the presence of a surface ionic cluster involving three equivalent Li cations on Li/Mg0.23 When activated MgO is exposed to similar levels of Na or K vapors, a IO-line hyperfine multiplet is also observed in the EPR spectra of both samples. The EPR spectra of Na and K doped MgO are shown in Figures 3 and 4a. The K/MgO spectra of this type are usually more intense and better resolved than the corresponding Na/MgO ones. The K/MgO spectrum at first sight appears to be composed of a IO-line hyperfine multiplet together with several other lines due to minor amounts of other paramagnetic species such as residual F centers, monomeric alkali centers (as described in the previous section), and small metal particles, whose presence and relative abundance in the
+
Figure 3. Experimental EPR spectrum of the trimeric centers on Na/ MgO.
n
+
I1
I
Y Figure 4. (a) Experimental and (b) computer simulated EPR spectrum of WMgO (containing the trimeric centers) at 298 K. (c) Calculated profile of one single trimeric species.
system depends on the amount of evaporated metal. The spectral features of such species are better understood by recording the spectra at various levels of microwave power since they are usually characterized by different saturation behaviors. The 10-line multiplet, which was found to be best observed at 10 mW, is centered at g = 1.997. This multiplet exhibits an increase in line width toward the wings of the spectrum similar to the spectra of the Na, K, and Rb monomeric cation centers discussed above. Since 39K and 4'K (both with I = 3/2) are the only I f 0 nuclei present on WMgO (with the exception of the
Murphy and Giamello
15176 J. Phys. Chem., Vol. 99, No. 41, 1995
TABLE 2: Spin Hamiltonian Parameters Derived by Computer Simulation for the Trimeric Alkali Centers on Li, Na, and K Doped MgO center .ill a Aii Ai Ao also Bo E C,? Cp2 total spin density 0.17-0.27 Li/MgO" 2.001 l b 2.001 I b 143.3 8.26 0.173 ~O.lOc N a g 0
WMgO a
2.0010 1.9979
2.0018 1.9993
50 17.5
55 20.0
316 82.5
53.33 19.16
32.2 10.6
1.66 0.84
0.506 0.697
0.155 0.238
0.661 0.935
The LilMgO values were taken from ref 23. glso. Estimated value; see text.
10.13% of 25Mg, I = 5 / 2 ) , we can assume that the 10-line multiplet arises from a hyperfine interaction with three potassium nuclei. Spectral analysis by computer simulation indicates that a mixture of both surface heterogeneity effects and small anisotropic contributions in both g and A determines the final shape of the 10-line structure in W g O . The effects of isotopic mixing between 39K (with I = 3/2 and A0 = 82.5 g) and 41K( I = 3/2 and A0 = 45.3 G) does not seriously influence the spectral shape which is dominated by the signal of the more abundant 39K trimeric center. In analogy to what was found for the monomeric centers, simulation of a spectrum with isotropic tensors and a given distribution of A values around a mean value produced broadened resonances in the outer transitions but with resulting significant base line crossing. When the isotropic tensors were replaced with slightly axial tensors, a more closely duplicated experimental spectrum was obtained. However, in this case also, owing to the intrinsic complexity of the spectrum and to the combined presence of other paramagnetic centers, the complete experimental spectrum was very difficult to simulate accurately. Nevertheless, the simulation in Figure 4b and the derived values reported in Table 2 for the W g O trimer likely reflect, in our opinion, an objective description of the multiplet. For the sake of clarity we also report the spectral profile of one of the 10-line K signals (Figure 4c) included in the whole simulation in Figure 4b. Using the spin-Hamiltonian parameters reported in Table 2, the total spin density (including the isotropic and anisotropic contributions) over the three K+ atoms may be evaluated. Since also= 19.16 G and B = 0.84 G (Table 2), then C4? = 0.697 (i.e., 19.6/82.5 x 3) and C4p2 = 0.238 (0.84/10.6 x 3); the total spin density over the three cations is therefore approximately 0.935. Due to the small value of B and the consequent relative error that likely affects its evaluation by computer simulation of the spectrum, the C4p2 value is probably also affected by a certain error, higher than that affecting C4?. In the case of Na/MgO the isotropic coupling constant also, estimated at about 53 G, indicates a 0.506 spin density over the 3s orbitals of the three Na+ cations (A0 is 316 G for the Na atom). The dipolar anisotropic contribution was estimated from preliminary computer simulations to be B = 1.7 G, indicating about 0.15 p-type spin density (BO= 32.23 G for Na). The total spin density over the three Na+ atoms is therefore 0.66, which is lower than the value for the corresponding trimeric K center on KhlgO. As reported previously for the 10-line multiplet on Li/Mg0,23 the percentage s character in the trimeric Li species was 17.3% (since also= 8.26 G and A0 = 143.3 G for 'Li). The anisotropic hyperfine term due to the presence of one electron in the 2p orbital of the Li atom is very small (about 1 G23) so that no anisotropic contribution could be detected in the Li/MgO spectrum, which appears in fact to be very symmetric. It is however absolutely reasonable that the contribution of the 2p orbital to the total spin density remains lower than that of the 2s contribution (in analogy with the observation of the Na and K trimeric species). The total spin density over the three Li atoms should not therefore exceed a value of around 0.27-
0.30, so that the remaining unpaired electron spin (0.73-0.70) is not directly associated with Li-based orbitals. On the basis of these EPR results, we can conclude that (a) the trialkali paramagnetic centers formed on Na/MgO and WMgO are similar to the center observed on Li/Mg023and (b) the extent of electron localization over the three constituent metal atoms of the trimer varies significantly from Li/MgO (0.27-0.30) to Na/MgO (0.66) to WMgO (0.935).
Discussion 1. The Monomeric Centers. It is clear from the above experimental results that spectra characterized by 21 1 lines are observed on Na, K, and Rb doped MgO and that these spectra are due to surface species containing a single alkali center. The nature of the Na monomeric species, in particular, was discussed in a previous publication.22 The hypothesis which was proposed involves the partial ionization of the metal atom, which is the reason for the presence of a residual electron spin density on the metal orbitals, and with the corresponding delocalization of the remaining spin density into a suitable surface trap located in close proximity to the adsorbed metal atom. The center was formally indicated as Nad+(trap)d-,with 0.26 spin density on the Na and the remaining 0.74 spin density delocalized onto the MgO matrix. Different structural models of the center were advanced, having in common the stabilization of the electron-cation pair at a particular morphological defect where they can maintain a significant degree of contact.22 Although no additional information on the precise structure of the proposed center is available from the present data, the idea of a monomeric Md+(trap)6- type center is reinforced since a high percentage of the total spin density in the K and Rb monomeric centers (approximately 50%) is also delocalized away from the alkali atom. Another peculiarity of the monomeric centers containing Na, K, and Rb which reinforces the idea that they all belong to a unique family is the slight degree of spectral anisotropy (unambiguously proved by computer simulation) which corresponds to a small but definite p contribution to the unpaired electron orbital. In all these cases, however, most of the observed spin density remains associated with the valence s orbitals of the alkali cations (Table 1). The reason for the observed difference in the total spin density on the three respective alkali cations (Le., 26% for Na, 5255% for K, and 56% for Rb) may be reasoned as due to the increased sizes of the respective cations in the centers and their fixed location on the surface. The relationship between the size of the M+ cationic radii and the extent of electron interaction with the valence s orbitals (the percentage s character) is shown in Figure 5 (line a). It should be outlined here that the actual radius of the partially ionized atoms (Md+)in the present centers is not easy to define. The choice of reporting the ionic radius in Figure 5 is therefore arbitrary. However, Figure 5 is not meaningless since a similar diagram could also be obtained by employing the atomic radius (rMO) value because of the relatively constant ratio between the atomic and ionic size in the alkali metal group. It is clear from this graph that the percentage s character on the alkali cations increases in parallel to the increase in ionic radius.
+
Alkali Metal Vapor Doped Magnesium Oxide
J. Phys. Chem., Vol. 99, No. 41, I995 15177
A preliminary model of the paramagnetic Li trimer on Li/MgO was previously discussed23based on the presence of a surface Li3*+ ionic cluster. However, this original model must now be reexamined in order to accommodate the additional results 40 I observed for NalMgO and WMgO which are reported in the I I / i present paper. An appropriate and general model of the trimeric centers must explain the experimental fact that the extent of electron localization on the metal orbitals increases significantly from the Li (6-9% on each Li) to Na (22% on each Na) to K (31% on each K) trimers. The hypothesis that the distinctive 10-line hyperfine pattern in the M/MgO spectra arises from a hyperfine interaction between a typical surface F,+ center on MgO (an electron trapped by five Mg2+ cations) and three nearby alkali M+ cations originated by metal ionization must be disregarded as a possible interpretation for the spectra since 0 20 40 EO 60 100 120 140 160 the observed coupling constants are by far higher than those IONIC RADIUWpm expected for such a model.20 Figure 5. Ionic radius (in pm) versus percentage s character on (a) the Na, K, and Rb monomeric centers and (c) Li, Na, and K trimeric Similar to the monomeric centers described above, the centers. The percentage s values were taken from Table 1, C?, and assignment of the three Li, Na, and WMgO spectra to a unique Table 2, C,2/3. (b) The same values for bulk F centers in Li, Na, and family of surface centers, each individual member having a K fluorides. particular electron density, seems very likely. This is suggested A somewhat comparable effect is also observed for the by line c in Figure 5, which shows that the unpaired electron hyperfine interaction measured for bulk F centers in a series of density on the outer s orbital of each alkali atom in the trimeric alkali halides. It was shown that the spin densities in the ns center increases with the size of the atom itself. This size effect orbitals of the cations surrounding the trapped unpaired electron is similar to that observed for F centers in alkali fluorides (line increase with increasing the cation size for a fixed a n i ~ n . ~ ~ * ~b)' and also for monomeric centers in MlMgO (line a). While This effect is also shown in Figure 5 (line b) where the percent the spin density values of the trimeric Li centers are comparable s character for P centers in Li, Na, and K fluorides is reported.31 to those of F centers in alkali halides, those of the sodium and The series of fluorides has been chosen due to the similarity potassium centers are definitely higher. between the ionic radius of the fluoride ion and that of the oxide The spin density values recorded for the trimeric centers must ion in MgO. The trend observed in Figure 5 , line b, can in a also be compared with those of the ionic clusters observed in sense be understood in terms of a competition for the electron the framework of various zeolite^.^^'^ The zeolitic ionic clusters between the bulk vacancy and the six cations as the "volume" are generally regarded as excess electron traps having substantial of the vacancy is reduced by the increasing cation size. Thus, electron delocalization into the space between the ions and the need to use the outer levels of the cations increases.30 By therefore characterized by spin densities of less than 1 on the analogy, on the MgO surface, with fixed anion (02-) and cation surrounding cations. These ionic clusters have the general (Mg2+)sizes, the observed variations in s character on the Nad+, formula Mn("-l)+,and although no Li32+clusters have ever been Kd+, and Rbd+ species could likewise be attributed to the reported, several Na and K ionic species have been formed decreased "volume" of the surface trap as the constituent alkali including Nas2+ 32 and K32+,113'3both observed in X and A cation size increases. It must be stressed however that in Figure zeolites. The structures of these latter clusters have recently 5 the values recorded for the hyperfine constants (and the related been obtained by X-ray d i f f r a ~ t i o n .The ~ ~ spin density on the spin densities) of the alkali monomeric centers on MgO remain alkali centers for sodium and potassium is 0.3732 and 0.60," definitely higher (2-3 times) than the corresponding values respectively, i.e., slightly lower than those observed in the observed for bulk F centers in alkali fluorides and, a fortiori, present case (Table 2). in alkali halides. The positively charged alkali metal clustes in zeolites possess The analysis of the experimental facts and the above only a single valence electron, and so they can be regarded as discussion on this first family of alkali metal centers can be a trap containing a single electron which is shared by the array summarized by the following points: (a) Monomeric centers of surrounding cations. It must be noted that these ionic positive on MgO can be collectively understood if we are dealing with traps are present in the zeolite prior to metal doping since they the same center described as (Md+(trap)d-) in all three cases are part of the zeolitic structure. This is particularly evident in for Na, K, and Rb doped MgO. (b) The three alkali metal the case of Nq3+ and G3+which are formed in X and Y zeolites cations can be regarded as stabilized in the same fixed position when an ionized electron is stabilized in the array of the four on the MgO surface in close proximity to the electron trapping tetrahedrally disposed Na+ (or K+) cations present in the defect (probably an anion vacancy type trap); this allows the sodalitic cages of these zeolites. Again, similar to the observed larger metal orbitals to increasingly interact with the trapped situation for the bulk F centers, the percentage s character (i.e., electron which explains the observed increase in percentage s the extent of the ns orbital occupation by the unpaired electron) character. (c) The Md+(trap)d- centers generated on Na, K, increases with increasing the cation size (passing from 40% to and Rb doped MgO are therefore best described as an associa80% for N q 3 + and G3+, respectivelyI0). This increase can be tion between a suitable surface defect and an adsorbed atom/ understood in terms of a reduction in the free space available cation whereby the unpaired spin is shared between the surface for the unpaired electron within the confined space of the trap and the partially ionized atom (see section 3 of the present sodalite cage due to the increased cation size. discussion). Although the spin density values reported for zeolitic ionic 2. The Trimeric Centers. From the above experimental clusters are not dramatically different from those observed in results (Figures 3 and 4)and the previously reported spectra,23 the present case for the trimeric centers on M/MgO, several it is evident that paramagnetic centers composed of three alkali facts indicate that the structural model corresponding to the ionic nuclei are formed on the surface of Li, Na, and K doped MgO. 4: S CHARACTER
50,
Murphy and Giamello
15178 J. Phys. Chem., Vol. 99, No. 41, 1995
clusters in zeolites is not suitable to describe the family of trimeric alkali metal centers reported here. In the present case, the trimeric surface centers on MgO are directly built up by the particular metal atoms added to the oxide. The formation of ionic M32+ entities would involve the ionization of two electrons (i.e., 3M M32+ 2e-) whose fate, on a surface where the originally existing anionic vacancies are “saturated” by the electrons released in the early stages of the interaction, is not easy to understand. Furthermore, due to the unique valence electron in MA”-’)+ and the corresponding low bond order, such ionic clusters are probably highly unstable entities if not encapsuled in the sodalitic cage of a zeolite. It is hard to imagine that the open ionic surface of MgO could play the same role as the tridimensional zeolitic framework in stabilizing the M32+moieties. Another fact which conflicts with this possible ionic cluster assignment is the high spin density (close to 1) observed for the trimeric K species on MgO, clearly higher than that observed for K32+ in zeolites.” The other possible candidate which would account for the 10-line multiplets are the trimeric neutral M3 clusters which are paramagnetic species characterized by relatively high bond order, large spin density on the ns metal orbitals, and, last but not least, the formation of which does not involve ionization. Neutral alkali metal clusters have been prepared by cryogenic isolation in inert mat rice^.^^-^^ Both the a,,,, and g values for K3 and the trimeric species on WMgO discussed here are quite similar (Le., g = 1.999, also= 24.5 G for the matrix-isolated “dynamic” K3 neutral species35at 25 K and g = 1.9988, also= 19.16 G for the trimeric species on WMgO). This means that the total spin density on the three K atoms of WMgO (93%) is close to the approximately 100% spin density observed for matrix-isolated K3.35 However, the presence of true neutral alkali M3 clusters on metal vapor doped MgO seems rather unlikely for several reasons. First, only the spin density values of W g O are in agreement with those of the trimeric neutral clsuters while the spin density values for Li/MgO and Nah4gO are definitely lower. Second, it is hard to explain the stabilization of a small neutral aggregate of reactive alkali atoms on the highly ionic MgO surface. Third, there is a clear contrast between the temperature interval in which the neutral alkali clusters are stable (a limited range at very low temperature related to the existence of the inert solid matrix) and the relatively high temperature (300 K) to which trimeric centers on MgO have been observed in the present work. An alternative description of the trimeric centers on MgO that provides an overall understanding of the various data is a model which is intermediate between the two limiting possibilities discussed above (Le., a model intermediate between the ionic and the neutral clusters). The model can be described in terms of a neutral cluster which is partially ionized and in contact with a suitable surface trap. The trap itself is not necessarily preexisting but can be built up by incorporation of the adsorbed alkali atoms. This model (described in detail in the next section) which can be written as ( M # + ( t r a ~ ) ~ -is analogous to the monomeric centers described above. Such a model is favored over the purely ionic cluster model since according to the bonding scheme in M3,35the stability of the neutral species is higher compared to the ionic M32+species because of the higher number of electrons with bonding character. In addition, using the new model the species remains partially ionized and therefore more easily stabilized on the highly ionic MgO surface. Indeed, slightly charged (6+) metal clusters of Ir4 have been previously reported on thermally activated MgO, formed by decarbonylation of the adsorbed metal carbonyl or other carbonyl anionic precursor^.^' As discussed at the beginning of this paragraph, the difference in the extent of electron
-
+
w-
Figure 6. Cationic trap constituting the Fs+centers on M/MgO. Small spheres = Mg2+; large spheres = 02-.
delocalization from Li3df to Na3&+to K3df can be related to the size of the cations as seen in Figure 5 (line c), in analogy to the size-related trends observed for the monomeric species (Figure 5, line a). 3. A Family of Trapped Electron Centers. The experimental EPR results provide detailed factual information on the state of the partially ionised metal atoms themselves, Le., the Ma+ and (M#+ entities. The principal values of the tensors and respective spin densities on each atom have been evaluated from the EPR spectra. From this analysis we can directly infer that the remaining (unobserved) spin densities not associated with Ma+ and (M#+ are likely delocalized onto the surface, for example, onto a surface defect site (or trap). This inference (Le., the presence of a surface trap) is justified since the surface morphology of the unmodified high surface area (HSA) MgO is characterized by the presence of various surface defects, some of which (Le., the anion vacancies) are abundantly formed during the dehydration of the solid. We can therefore confidently describe the observed monomeric and trimeric centers simply as Md+(trap)d-and (M#+(tra~)~-even though a comprehensive structural elucidation of such centers remains difficult to achieve based on the present results. However, some possible structural models of these novel surface centers can be advanced on the basis of the knowledge of both the chemistry of the metal vapor-metal oxide interaction (described in this and previous papers) and on the basis of the surface morphology of MgO. Considering the first of the two factors, one must bear in mind that the degree of electron interaction with the parent atom or cation increases as the concentration of metal at the surface increases. In other words, the ability of the ionic surface in separating and stabilizing the electron and the associated cation (which is maximum in the initial step of the interaction with the formation of surface F, centers on the MgO matrix) is reduced as the concentration of metal on the surface is increased (see below). Considering the second factor, it is well-known that the surface of an ionic oxide like MgO possesses several kinds of defects, some of which are point defects such as anion vacancies while others are larger in scale such as ledges, steps, and kinks. The presence, properties, and availability of these defects will ultimately influence the type of trapped electron center formed at the metal vapor-metal oxide interface. The anion vacancies are well-known electron traps producing the F, class of color centers. The lack of one 02-anion from the planar face (or, alternatively, from the edge of a cubic crystal) produces a positive site of five Mg2+ (or four Mg2+) lattice cations which act as a surface trap for one or two electrons (Figure 6). In these centers, which are formed at very low levels of added metal vapors, the electron and adsorbed cation do not maintain any significant contact (i.e., the atom is completely When the amount of doping metal is increased, however, the EPR signal due to the monomeric centers begins to grow until the intensity of this signal is significantly greater than the intensity of the F,+ signal. This implies that a point must be reached when all the available anion point defects are
Alkali Metal Vapor Doped Magnesium Oxide
J. Phys. Chem., Vol. 99, No. 41, 1995 15179
b
“
J
Figure 7. (a, b) Two possible models for the monomeric alkali centers on MiMgO (see text). Large dashed sphere = M6-.
Figure 8. A possible model for trimeric centers on MiMgO.
filled with electrons so that the other morphological surface defects begin to play a role of greater importance in stabilizing the electrons and cations. Although the majority of anion vacancies are thought to occur on the planar surface of MgO (it is well-known that these vacancies are more stable on the surface than in the bulk3*),similar vacancies can also occur at the edges of the cubic microcrystal^.^^ These latter ones are formed in the final phase of the MgO dehydration process when only a few isolated surface hydroxyls are present, the removal of which implies the loss of one surface 02-ion.39 The model of the monomeric centers (in the case of Na/MgO) was earlier envisaged as an electron and cation stabilized at such anion type vacancies (i.e., at a step site having one 02-anion removed (Figure 7b)) but could also be at an anion-cation vacancy pair at the planar face (Figure 7a).22 These lattice defects were assumed to be partially filled by the adsorbed alkali atom (Figure 7) which, upon adsorption, delocalized a fraction of the 11s electron density toward the trap. However, in both of these related models (Figure 7, a and b) the alkali adatom interacts with an aggregate of four lattice Mg2+cations and together with them forms a five-membered positive cage. The partially ionized clusters of alkali atoms forming the (M#+(trap)6- centers are likely stabilized at the extended crystal defects, such as a step, on the MgO surface. The reasoning behind this assignment is as follows. At the higher levels of added metal (Le., in the region where the trimeric alkali centers are formed), the role of the extended morophological defects in stabilizing the electrons and cations should begin to predominate. At such an extended defect site (Figure 8) a “cagelike” symmetry (composed of three alkali atoms and two Mg cations), which is suitable for electron trapping, can be formed. The stabilization of the incoming alkali atoms at a crystal step is favored over stabilization on the flat planar face
of MgO. Theoretical calculations of isovalent cation substitutions on the MgO surface show that bulky cations preferentially occupy kinks where the lattice relaxation allows the larger cations to be accommodated more comfortably while doping at comers and ledges is “more favorable” than doping in the (100) plane.40 Experimental work has also demonstrated that aggregates of metal atoms or clusters may preferentially anchor at defect sites4‘ Long et al.42found by high-resolution electron microscopy that Oslo clusters on MgO lie preferentially along surface steps and ledges and (200) planes. The models reported in Figures 7 and 8 for mono- and trimeric alkali centers clearly show that these centers can also be seen as five-membered cationic cages containing a trapped electron, in analogy to the “classic” surface F, centers on the planar MgO face described in Figure 6. The progressive metal vapor-metal oxide interaction at increasing levels of added vapor as illustrated in, three distinct steps (Figures 6-8) is coherent with a view involving a progressive decrease in the ionization ability of the surface resulting in a progressive increase of electron localization on the parent atom(s). It should be clearly stated that the present results do not allow us to directly probe the nature of the surface traps which are certainly an inherent part of the M6+(trap)b- and (M3)d+(trap)dcenters. Therefore, the ultimate uncertainty of the proposed structures in Figures 7 and 8 lies in the uncertain nature of the surface defects themselves (i.e., the inherent or preexisting traps in Figure 7 and the surface step in Figure 8) which however clearly play a vital role in stabilizing the electron cation pairs. A more accurate description of these traps may be advanced with further experimental and/or theoretical studies. However, as stated above, a great deal of experimental and theoretical studies on surface m ~ r p h o l o g and y ~ ~on~the ~ ability of the MgO surface to capture electrons20 confirm beyond doubt the highly defective nature of HSA MgO and the presence of anion vacancies on these surfaces. In addition, it should be recalled that the extended surface defects (such as steps, edges) can stabilize small aggregates of metal atom^.^^.^^ The combined electron trapping ability (from the anion vacancies) and metal clusters stabilization ability (from the extended defects) of these sites could well be responsible for the presence of the described monomeric and trimeric centers on M/MgO. A final point for consideration concerns the difference between this new family of surface trapped electron centers on M/MgO and the well-known surface F,+ color centers. The schematic drawings of the monomeric and trimeric alkali centers (in Figures 7 and 8) would suggest that the new centers are somewhat analogous to modified F, centers (Le., with a reconstruction of the typical five-membered square pyramidal structure of surface F centers on MgO). This is certainly true under the structural point of view, but it must be clearly emphasized that certain properties of the new paramagnetic surface centers are clearly differ from those of the F centers. This difference can be seen for example in the respective relaxation times and saturation behaviors of the color centers and the new species on M/MgO. The F+ centers in the bulk of alkali halides and alkaline earth oxides (MgO and CaO) have typical relaxation times in the order of some microsecond^^^,^' and therefore a resulting ease of saturation with microwave power. An easy saturation with increasing microwave power has also been observed for various surface F,+ centers on MgO (Fsf centers generated on M/MgO and on y- or UV-irradiated MgO) as previously reported.20 On the contrary, the monomeric and trimeric centers have in general a peculiar saturation behavior which is unambiguously different from that of the surface F,+ centers and corresponds to a faster r e l a ~ a t i o n . ~ ’ - ~ ~
15180 J. Phys. Chem., Vol. 99, No. 41, 1995
Murphy and Giamello
These findings indicate that the novel centers described here are not merely composed of an electron trapped in mixed M+Mg2+ cages (“reconstructed” F centers) but have separate magnetic properties and can therefore be effectively considered a novel family of surface centers on M/MgO. A confirmation and a further improvement of the models proposed here should possibly be obtained by theoretical calculations, in particular on the geometric arrangement of the site and on the unpaired electron density.
Conclusions When thermally activated polycrystalline MgO is exposed to alkali metal vapors, a series of EPR spectra characterized by 21 1 and 21 x 3 1 hyperfine lines are observed depending on the amount and type of doping metal used. These EPR spectra have been assigned to a family of novel surface paramagnetic centers described as (M)d+(trap)d- and (M#+(trap)”, respectively. These surface centers can be visualized as a paramagnetic complex formed between one or three alkali atoms and a coordinatively unsaturated surface defect. The spatial arrangement of the adsorbed cations and the lattice Mg cations at these morphological defects is such that an unpaired electron can be partially delocalized into the space between this collective aggregate of cations. The nomenclature adopted in the above models tries to account for the observed fact that the electron spin density is partially localized on the cation and partially delocalized onto the MgO surface. The whole phenomena occurring upon contact of metal vapors with the ionic MgO surface can therefore be described by eqs 1 and 2, accounting for the early stages of the interaction (which produces surface F, centers), and by the following equation
+
xMg2’-Mg0
+
+ nM-(M,)G’(~Mg2’)d--Mg0 (n = 1 9 3 ) (4)
which accounts for the phenomena described in the present paper. As discussed above, the term xMg2+ indicates the two different arrays of Mg2+cations which are part of the monomeric and trimeric centers. The present results indicate the potential importance of the various surface defects in stabilizing metal atoms and small metal clusters. The interaction between alkali metals and MgO has some points in common with other phenomena in the field of “assisted” metal ionization as the contact of the alkali metals with the intemal structure of dehydrated zeolites or the ionization of the metals in liquid ammonia. In the former case, welldefined metal ionic clusters have been widely described in the literature.8-16 However, they are always formed by trapping an ionized electron in positive traps which were preexisting in the zeolitic framework so that the nature of the observed cluster is independent of the nature of the incoming metal. Surprisingly, the metal vapor-metal oxide interaction reported here (M/MgO) more closely resembles the phenomena of alkali metal ionization in liquid ammonia, leading to the formation of colored solutions containing solvated electrons and metal ions.43 In these solutions the ionized electrons and the parent cations exhibit different degrees of mutual interaction, depending on the metal concentration starting from the situation in which there is no electron-cation contact (loose electronion pair) to the one in which there is a relevant residual interaction (contact ion pairs). In the present system MgO can be regarded as acting like an ionic, bidimensional “solvent”, and the evaporated metal progressively undergoes the stages of total ionization (with formation of surface F centers), of partial ionization (monomeric and then trimeric centers), and
eventually that of the absence of any ionization when, for prolonged exposure periods to metal vapors, very small metal particles are nucleated at the surface.
References and Notes (1) (2) (3) 25, 43. (4)
Weber, H. Z. Phys. (Munich) 1951, 130, 392. Ward, W. C.; Hensely, E. B. Bull. Am. Phys. SOC.1965, 10, 307. Kemp, J. C.; Ziniker, W. M.; Hensely, E. B. Phys. Lett. A 1967,
Dash, W. C. Phys. Rev. 1953, 92, 68. ( 5 ) Carson, J. W.; Holcomb, D. F.; Ruchardt, H. J . Phys. Chem. Solids 1959, 12, 66. (6) Henderson, B.; Wertz, J. E. Adv. Phys. 1968, 17, 749. (7) Wertz, J. E.; Orton, J. W.; Auzins, P. J . Appl. Phys. (Suppl.) 1962,
33, 322. (8) Kasai, P. H. J. Chem. Phys. 1965, 43, 3322. (9) Kasai, P. H.; Bishop, Jr., R. J. J . Phys. Chem. 1973, 77, 2308. (10) Edwards, P. P.; Hanison, M. R.; Klinowski, J.; Ramadas, S.; Thomas, J. M.; Johnson, D. C. J. Chem. SOC.,Chem. Commun. 1984,982. (11) Xu, B.; Kevan, L. J . Chem. Soc., Faraday Trans. 1991,87, 3157. (12) Xu, B.; Kevan, L. J . Chem. SOC.,Faraday Trans. 1991.87, 3843. (13) Anderson, P. A,; Singer, R. J.; Edwards, P. P. J . Chem. SOC.,Chem. Commun. 1991, 914. (14) Anderson, P. A.; Edwards, P. P. J. Chem. Sac., Chem. Commun. 1991, 915. (15) Anderson, P. A.; Edwards, P. P. J . Am. Chem. SOC. 1992, 114, 10608. (16) Edwards, P. P.; Woodall, L. J.; Anderson, P. A.; Armstrong, A. R.; Slaski, M. Chem. SOC.Rev. 1993, 305. (17) Sproull, R. L.; Bever, R. A.; Libowitz, G. Phys. Rev. 1953,92,77. (18) Zecchina, A.; Scarano, D.; Marchese, L.; Coluccia, S . ; Giamello, E. Surf.Sci. 1988, 194, 513. (19) Giamello, E.; Fererro, A.; Coluccia, S.; Zecchina, A. J. Phys. Chem. 1991, 95, 9385. (20) Giamello, E.; Murphy, D.; Ravera, L.; Coluccia, S . ; Zecchina, A. J. Chem. Soc., Faraday Trans. 1994, 90, 3167. (21) Murphy, D. Ph.D. Thesis, University of Dublin, 1993. (22) Murphy, D.; Giamello, E. J. Phys. Chem. 1994, 98, 7929. (23) Murphy, D.; Giamello, E.; Zecchina, A. J. Phys. Chem. 1993, 97, 1739. (24) Wertz, J. E.; Bolton, J. R. In Electron Spin Resonance Elementary Theory and Practical Applications; McGraw-Hill Series in Advanced Chemistry; McGraw-Hill: New York, 1972. (25) Breit, G.; Rabi, I. I. Phys. Rev. 1931, 38, 2082. (26) Anderson, M. R.; Edwards, P. P.; Klinowski, J.; Thomas, J. M.; Johnson, D. C.; Page, C. J. J. Solid State Chem. 1984, 508, 165. (27) Blazey, K. W.; Muller, K. A.; Blatter, F.; Schunacker, E. Europhys. Lett. 1987, 4 , 857. (28) Tuttle, J. R. J . Phys. Chem. 1975, 79, 3071. (29) Guy, S. C.; Edwards, P. P.; Catteral, R. J . Chem. Soc., Faraday Trans. 2 1984, 80, 1539. (30) Atkins, P. W.; Symons, M. C. R. In The Structure of Inorganic Radicals; Elsevier Science: Amsterdam, 1967. (31) Markham, J. J. F-Cenfers in Alkali Halides. Solid State Physics (Supplement 8. Advances in Research and Applications); Seitz, F., Tumbull, D., Eds.; Academic Press: New York, 1966. (32) Iu, K. K.; Liu, X.; Kerry Thomas, J. J. Phys. Chem. 1993,97, 8165. (33) Sun, T.; Seff, K. J . Phys. Chem. 1994, 98, 10156. (34) Howard, J. A.; Mile, B. Spec. Period. Rep.: Electron Spin Reson. 1989, l l B , 136. (35) Lindsay, D. M.; Thompson, G. A. J . Chem. Phys. 1982, 77, 1114. (36) Thompson, G. A.; Lindsay, D. M. J . Chem. Phys. 1981, 74, 959. (37) Triantafillou, N. D.; Gates, B. C. J. Phys. Chem. 1994, 98, 8431. (38) Catlow, C. R. A.; Faux, I. D.; Norgett, M. J. J . Chem. C (Solid State Phys.) 1976, 9, 419. (39) Coluccia, S . ; Lavagnino, S . ; Marchese, L. Mater. Chem. Phys. 1988, 18, 445. (40) Colbum, E. A. Surf.Sci. Rep. 1992, 15, 281. (41) Papile, C. J.; Gates, B. C. Langmuir 1992, 8, 74. (42) Long, N. J.; Gates, B. C.; Kelley, M. J.; Lamb, H. H. In The Physics and Chemistry of Small Clusters; Jana, P., Rao, B. K., Khanna, S. N., Eds.; Plenum: New York, 1987: p 831. (43) Edwards, P. P. Adv. Inorg. Chem. Radiochem. 1982, 25, 135.
JP950919I
