J. Phys. Chem. 1984,88, 4655-4660 titative picosecond laser studies of such intramolecular exciplex systems-will be reported in a forthcoming paper.”
4655
also by Grants-in-Aid (no. 58430003,no. 58045097) from the Japankse Ministry of Education, Science, and Culture to N.M. The support by a Grant-in-Aid from the Yamada Science Foundation to N.M. is also gratefully acknowledged.
Acknowledgment. This work was supported in part by a Grant-in-Aid for Special Project Research on Photobiology and
Registry No. EEP, 87883-43-0; TQ, 553-97-9; FTFP, 91384-77-9;
(17) Karen, A.; Mataga, N.; Okada, T.; Nishitani, S.; Kurata, N.; Sakata, Y . ;Misumi, S., to be submitted for publication.
THF, 109-99-9; acetone, 67-64-1; benzene, 71-43-2.
Phosphorescence Spectra in Mixed Naphthalene Crystals: A Transition from Site to Crystal States J. P. Lemaistre,* A. Blumen,? Centre de MZcanique Ondulatoire AppliquPe, F- 75019 Paris, France
F. Dupuy, Ph. Pee, R. Brown, and Ph. Kottis Centre de Physique MoMculaire Optique et Hertzienne, Universitd de Bordeaux I, F- 33405 Talence, France (Received: March 8, 1984; In Final Form: May 8, 1984)
Phosphorescence excitation spectra of both guest and host excitons in isotopically mixed naphthalene crystals are presented over the whole concentration range. The concentration variation from low to high values leads to eigenstates whose character changes from trap to band states. To provide a quantum picture of the mixed crystal we evaluate numerically the eigenstates and their localization indices in a two-dimensionalfinite lattice with appropriate boundary conditions. The phosphorescence line shapes are calculated from the triplet spin quantization and the spin-orbit coupling. Our calculations show very good agreement with experimental data and allow us to follow, in the optical response of the mixed crystal, the transition from small aggregates to the crystal.
I. Introduction Low-temperature spectra of triplet excitons in isotopically mixed naphthalene crystals are a subject of intense interest.’-13 Protonated naphthalene (Nh,) molecules act as traps in the deuterated (Nd,) crystal; in their triplet states they are an important system for the study of energy transfer and triplet-triplet annihilation p r o c e s ~ e s . ~We ~ , ~emphasize ~ that these effects depend on the spatial distribution of both guest and host molecules. From the measured excitation spectral6 the distribution of clusters formed by the traps has been previously obtained.”J8 In ref 19 the Davydov components of the triplet exciton band were used to infer the distribution of host molecules: the calculation of the eigenstates of a finite two-dimensional model with triplet interactions reproduced the naphthalene data; the model incorporated random distributions of traps with various energies and also appropriate boundary conditions to remove the edge effects due to the finite size of the lattice. In the present work we display the low-temperature phosphorescence excitation spectra of both guest and host excitons for a series of Nhs/Nds mixed crystals, in which the Nh, concentration increases from low to high values. At low trap concentrations (Le., less than 10%) the line shapes can be analyzed in terms of the trap states of small molecular clusters, a representation which corresponds to localized wave functions. For high trap concentrations, however, the Nh, molecular states evolve into crystal states represented by delocalized bands. The purpose of this paper is to analyze the spectroscopic properties of mixed crystals for trap concentrations ranging from 10 to 90%. Thus we have to evaluate the line shapes of heavily doped mixed crystals also for concentrations where neither the site representation nor the band-states description is very adequate. In this concentration range the identification of the numerous geometrical configurations of large clusters becomes an inextricable problem and the calOn leave from Lehrstuhl fiir Theoretische Chemie, Technische Universitat Miinchen, D-8046 Garching, West Germany.
0022-3654/84/2088-4655$01.50/0
culation of the phosphorescence emission arising from the eigenstates of such clusters is not easy to perform. In the following we evaluate the phosphorescence intensities where we use the triplet spin quantization of the electronic states of the mixed crystal together with the spin-orbit coupling These
(1) E. I. Rashba, Sou. Phys. Solid State, 4, 2477 (1963); 5,757 (1963). (2) D. M. Hanson, J . Chem. Phys., 52, 3409 (1970). (3) F. W. Ochs, P. N. Prasad, and R. Kopelman, Chem. Phys., 6, 253 (1974). (4) R. Kopelman, E. M. Monberg, and F. W. Ochs, Chem. Phys., 19,413 (1977). ( 5 ) C. L. Braun and H. C. Wolf, Chem. Phys. Lett., 9, 260 (1971). (6) H. Port and H. C. Wolf, Z . Naturforsch. A , 30, 1290 (1975). (7) H. Port and D. Rund, J. Mol. Structure, 45, 455 (1978). (8) J. Jortner, S.A. Rice, J. L. Katz, and S. I. Choi, J . Chem. Phys., 42, 309 (1965). (9) B. S. Sommer and J. Jortner, J . Chem. Phys., 50, 839 (1969). (10) J. Hoshen and J. Jortner, Chem. Phys. Lett., 5, 351 (1970). (11) J. Klafter and J. Jortner, J. Chem. Phys., 71, 2210 (1979). (12) U. Doberer, H. Port, and H. Benk, Chem. Phys. Lett., 85,253 (1982). (13) D. Rund and H. Port, Chem. Phys., 78, 357 (1983). (14) R. Brown, J.-P. Lemaistre, J. Megel, Ph. Pee, F. Dupuy, and Ph. Kottis, J . Chem. Phys., 76, 5719 (1982). (15) Ph. Pee, Y . Rebiere, F. Dupuy, R. Brown, Ph. Kottis, and J.-P. Lemaistre, J . Phys. Chem., 88, 959 (1984). (16) F. Dupuy, Ph. Pee, R. Lalanne, J.-P. Lemaistre, C. Vaucamps, H. Port, and Ph. Kottis, Mol. Phys., 35, 595 (1978). (17) J.-P. Lemaistre, Ph. Pee, R. Lalanne, F. Dupuy, Ph. Kottis, and H. Port, Chem. Phys., 28, 407 (1978). (18) Ph. Pee, R. Brown, F. Dupuy, Ph. Kottis, and J.-P. Lemaistre, Chem. Phys., 35, 429 (1978). (19) Ph. Pee, J.-P. Lemaistre, F. Dupuy, R. Brown, J. Megel, and Ph. Kottis, Chem. Phys., 64, 389 (1982). (20) H. Sternlicht and H. M. McConnel, J . Chem. Phys., 35,1793 (1961). (21) R. M. Hochstrasser, J . Chem. Phys., 47, 1015 (1967).
0 1984 American Chemical Society
4656
The Journal of Physical Chemistry, Vol. 88, No. 20, 1984
Lemaistre et al.
r
c. sax
1
c
i
i
V
i
H
i
I I I
-r
I
C I 70%
c.
97%
V
V
H
H
1
4 I
Figure 1. Measured polarized phosphorescence excitation spectra of guest and host excitons in mixed naphthalene crystals. The different Nh8 concentrations are c = 0.3%, 15%, SO%, 70%, and 97%. The vertical lines indicate the energy positions of the Nh, and Nds monomers. V (or H) indicate that the spectra are mainly b (or a) polarized (see text).
calculated line shapes, averaged over the different statistical configurations due to the randomly distributed traps, are then compared to the measured spectra. Furthermore, we quantify the degree of localization of the eigenstates of the system by means of localization indices. The paper is arranged as follows: section I1 is devoted to the experimental findings on the phosphorescence excitation spectra of the guest and host; we pay special attention to the concentration dependence of the line shapes. In section I11 we present the theoretical expressions for the phosphorescence intensity of the mixed crystal and for the degree of localization of the eigenstates. (22) J.-P. Lemaistre and Ph. Kottis, J. Chem. Phys., 68, 2730 (1978). (23)J.-P. Lemaistre and A. H. Zewail, J . Chem. Phys., 72, 1055 (1980). (24) J.-P. Lemaistre, Ph. Pee, M. Beguery, F. Dupuy, J. Megel, R. Brown, and Ph. Kottis, Chem. Phys. b i t . , 89,207 (1982).
These expressions are evaluated numerically in section IV; this section also provides the comparison between the theoretical and the experimental results for the triplet line shapes. Finally, in section V, we summarize our main conclusions. 11. Experimental Section
In this work the photoexcitation procedure follows that of the ref 16 and 19; a tunable dye laser (Molectron DL 400 with Coumarin 102 dye) pumped by a nitrogen laser (Molectron UV 1000) was used for the excitation of the mixed Nh8/Nd8 crystals. In the region of the measurements (around 470 nm) the laser bandwidth was approximately 0.8 cm-' and the pulse energy was around 200 pJ. The triplet excitation spectra were observed behind a filter and measured with a time delay achieved by a mechanical chopper synchronized with the laser pulse. The use of the chopper also prevents the saturation of the photomultiplier by the laser
Phosphorescence Spectra in Mixed Naphthalene Crystals
The Journal of Physical Chemistry, Vol. 88, No. 20, 1984 4657
0.2
10 I
50 I
I
I
I
I
Figure 2. Variation with the Nh8 concentration c of the observed level splittings for guest and host excitons in mixed naphthalene crystals. The full width (fwhh) of the excited line shapes has been indicated (dotted lines) for the region where the splitting becomes unobservable. flash. All the phosphorescence excitation spectra were recorded a t 4.2 K. The naphthalene single crystals were prepared by the Bridgeman method from zone-refined materials (over 100 passes). The concentrations of Nh, and Nds in the mixed crystal were checked by mass spectroscopy. The crystals were cut in the ab-cleavage plane and irradiated perpendicular to this plane. The interest of this work lies in the high Nh8 concentration region. As stressed by Rund and Port in their recent article on the triplet state of a n t h r a ~ e n e ’previous ~ spectroscopic investigations on highly concentrated isotopic mixed aromatic crystals studied the singlet states only. Our study of the triplet excitations provides the corresponding results for naphthalene. Phosphorescence excitation spectra of Nh, and Ndg excitons are shown in Figure 1, where the Nh, concentration ranges from 0.3 to 97%. At low Nh, concentrations the two Davydov components of the Nds host crystal are observable: the lower component is mainly b polarized and its line width is smaller than that of the upper component. The different peaks observed on the low-energy side have been already assigned to Nd7h shallow traps.lg Part of the spectrum due to the Nh, molecules (which act as traps) arises mainly from monomer and dimer states. At higher Nhs concentrations (around 15%) the phosphorescence intensity of the Nh8 monomers decreases while the intensity of the dimer states increases; there are two components due to translationally inequivalent dimer states. By going with the Nhs concentration up to 50%, the splitting of these two dimer lines continuously increases, reaching for even higher concentration the Davydov splitting of the Nhs excitonic band. We note that the Nhs and Nds triplet exciton bands are similar: they have comparable bandwidths and also show the same polarization dependences. Furthermore, beyond a Nhs concentration of 50%, the Nds line shapes show a behavior similar to that of the Nh, traps in the low concentration range. We have drawn in Figure 2 the observed level splitting of the Nh8 and Nds excitonic bands as a function of the Nh, concentration. These findings strongly suggest that for triplet excitations a transition from localized to extended (Nh,) states takes place. Such a transition parallels the results known for the singlet absorption and f l u o r e s ~ e n c eas ~ ~well , ~ ~as for the vibrational exciton (25)
1
H.K. Hong and R. Kopelman, J. Chem. Phys., 55, 724 (1971).
O.’
0
0.5
Figure 3. The number density n of Nhs and Nd8 eigenstates with a localization index equal to L for various Nhs concentrations c = 15%, 25%, 50%, and 70%. features in the Raman spectra of mixed naphthalene crystals.27 We thus prcceed to analyze theoretically the localization properties of the triplet states. 111. Theoretical Approach In an isotopically mixed crystal the sites are occupied by either protonated or deuterated molecules. Let us consider first a deuterated crystal (host) in which a fraction, c, of protonated molecules (guests), of lower energy, is randomly substituted. The total Hamiltonian is then
where the sums run over all sites. The E, are the site energies and the denote the interactions between sites. In what follows we shall assume that the guest (E,) and the host ( E h )energies
v,
(26) R. Kopelman in “Excited States”, Vol. 2, E. C. Lim, Ed., Academic Press, New York, 1974. (27) P. N. Prasad and R. Kopelman, J. Chem. Phys., 57, 856 (1972).
4658
The Journal of Physical Chemistry, Vol. 88, No. 20, 1984
Lemaistre et al.
1
c = 50% 0.5
I 0
0.25
0.50
0.75
0
1
0.50
0.25
0.75
,
L
1
1
t
I
8
c
I
C = 25%
0.5
C = 70%
I
-
I I
I I
I I I
I
t I I
I
I
L
I
1 I 0.50 0.75 0 0 0.25 0.50 0.75 1 Figure 4. Plot of X(L),the relative number of eigenstates having a localization index less than L, as a function of L. X(L) is calculated in the energy domain of Nhg molecules (full line) and Ndg molecules (dotted lines) for various Nhs concentrations c = 1595, 25%, 50%, and 70%.
0
L
#
0.25
are unaffected by the disorder so that, in eq 1, Ei = E, or Ei = Eh, depending on the occupation of the site. Here we are led by the observation that in naphthalene the effect of the isotopic substitution on the distribution of the triplet energies is relatively small. Moreover, deuteration does not seem to affect much the site-site interactions since the triplet bandwidths of pure protonated and pure deuterated crystals are nearly identical. We will assume therefore equal guest-guest, host-host, and also guest-host interactions. The diagonalization of the full Hamiltonian (eq 1) gives the eigenenergies Ek and the eigenfunctions of both the guest and host states l$k)
=
xcikli)
(2)
Note that in the case of a perfect crystal the index k corresponds to a vector of the reciprocal lattice; here, however, we let it be a general index which distinguishes the different electronic eigenstates. This direct diagonalization for coupled guest and host eigenstates overcomes the difficultly of identifying and counting the numerous configurations of clusters, a procedure which gets quite cumbersome when the trap concentration becomes large. Furthermore, the method can also be used to evaluate the influence of impurities (such as X traps and natural traps) and also of local strains. For each fixed concentration value an averaging over several trap configurations is, however, absolutely mandatory in order to ensure the proper statistics. For a given trap concentration and configuration the method presented here allows us to obtain the guest and host eigenfunctions and thus to study their localization behavior. This behavior determines under which physical conditions the mixed crystal may be described in terms of molecularlike (or clusterlike) wave
functions or in the opposite case, in terms of crystallike (bandlike) wave functions. Here we have to settle for an appropriate criterion. As localization criterion we use the inverse participation ratio Lk defined through the relati~n*~-~O (3) and calculated for both guest and host eigenstates. We note that for a given eigenstate I+k) the localization index L k is close to unity for a state localized over one single site while it drops to l / n for a state delocalized evenly over n sites. As a result of the diagonalization of the Hamiltonian (eq 1) we obtain the coefficients C l whose knowledge allows us to compute the localization indices Lk for each eigenstate and their distribution over the energy range of guest and host excitons. In order to get an easy to view description we also introduce the cumulative number of states with localization index L, below a certain value L X(L) = xO(L - L v ) / N
(4)
V
The sum in eq 4 extends over N eigenstates ( N = Ng + Nh,where Ng and Nh denote the number of guest and host states, respectively). In eq 4 O(x) is the Heaviside step function, O(x) = 0 for x < 0 and O(x) = 1 for x > 0. The derivative of X(L) gives the normalized density of localization indices n(L). After having formulated the Hamiltonian of the mixed crystal and having discussed how to determine the localization behavior (28) S. Yoshino and M. Okazaki, J . Phys. SOC.Jpn., 43, 415 (1977). (29) W. Y. Ching and D. L. Huber, Phys. Rev. E , 25, 1096 (1982). (30) J.-P. Lemaistre and A. Blumen, Chem. Phys. Lett., 99, 291 (1983).
Phosphorescence Spectra in Mixed Naphthalene Crystals
The Journal of Physical Chemistry '01. 88, No. 20, 1984 4659
of the eigenstates let us consider the phosphorescence line shape of guest and host excitons. A thorough derivation of an analytical expression for the line shape has been given in ref 19. The perturbed (by spin-orbit coupling mechanisms) triplet spin electronic eigenfunctions I3!bkaM(K)) may be expanded in terms of molecular single site functions as follows:
In eq 5 we have separated the contributions arising from the two kinds of differently oriented molecules which we denote by A and by B. We have assumed that the intermolecular resonance interaction is much larger than the zero-field splittings of the triplet spins. This assumption, which is valid for the molecular systems considered here, allows us to define triplet spin functions uM(k ) for each electronic eigenstate I3qk). Straightforward calculations using the symmetry properties of the crystal lead to the following expressions for the phosphorescence intensities polarized along the a and b axes:I9
1Ocm-1
c = 15 ' A
- 1
I:" H
Z,(k) = ( T x / A E ) 2 ( Z a ) 2 { [ 1( b ~ ) ~ ] C ++~ (bx)2C-2(k)} (k) (6a)
+
= ( 7 x / A E ) 2 ( Z b ) 2 ( -[ 1( b ~ ) ~ ] C - ~ ((bx)2C+2(k)) k) (6b) In eq 6 AE is the energy difference between the triplet and the singlet state through which the spin-orbit coupling mechanism occurs with T~ as matrix element; (Zu) is the direction cosine of the molecular transition moment (2) with the crystal axes ( b and a ) ; (bx) is the direction cosine of the b axis of the crystal with the component of the spin-orbit operator x; finally the coefficients C,(k), defined as Zb(k)
C i2 5 %
-
I'
A H
ii
lOcn-'
C.bO%
are obtained from a numerical diagonalization procedure. Expressions 6a and 6b giving the polarized phosphorescence intensities arising from the cluster states are valid for any cluster configuration going from the monomer to the crystal. If we denote by Yk the homogeneous line widths of the various cluster states ( k ) , the polarized phosphorescence line shapes can be represented by
I,(E) = CYkZ,(k)/((Ek- E)' 4- Ykz)
t~ =
a, b
(8)
k
with Z,(k) being given by eq 6.
IV. Results and Discussion We start by considering the eigenvalues and the eigenfunctions of our model Hamiltonian given by eq 1 . As a model for the naphthalene lattice we consider a square arrangement of N = 81 molecules coupled by triplet interactions and illustrated by Figure 5 of ref 19. As repeatedly discussed, the use of a two-dimensional lattice seems to be justified in the study of the triplet interactions of naphtha1e11e.l~In order to limit the influence of the boundaries, the square is closed edge to edge, using periodic boundary conditions, as described and illustrated in ref 19. The nonzero nearest-neighbor triplet interactions used are V,, = +1.2 cm-' if i and j are translationally inequivalent molecules and V,, = +0.7 cm-' if i and j are translationally equivalent molecules along the b axis of the crystal. The energy separation between Nds (Eh) and Nhs (E,) molecules is Eh - E, = 95 cm-I. For a given concentration c of Nh, molecules, we then generate random distributions of Nhs and Nd, molecules and consider in every case some 50 realizations of molecular configurations. The eigenenergies Ek and the eigenvectors C,k of both Nh, and Nds states are calculated by numerically diagonalizing the Hamiltonian of eq 1 . For each eigenstate we calculate the localization index Lk according to eq 3. In Figure 3 we display for various Nhs concentrations c histograms of the eigenstate populations n(L). From Figure 3 it can be seen that at c = 15% the Nds eigenstates are delocalized states while the Nh, eigenstates are localized states stemming from clusters of various configurations. Increasing the Nh, concentration leads to an increase
:i H
"8
Figure 5. Calculated phosphorescence line shapes of guest and host excitons for various Nhs concentrations c = 15%, 25%, 50%, and 70%. The V and H notation is as in Figure 1.
of the populations of such clusters. Within our definition of the localization behavior and for short-ranged interactions the calculations show that the Nh, eigenfunctions remain localized up to high values of c (say 30%). Another important point arising from these calculations is the high density of eigenstates with localization indices equal to 0.5. These eigenstates are delocalized over two molecules and can be assigned to dimers (Le., two guest molecules coupled through nearest-neighbor interactions) or pairs (Le., two guest molecules separated by several host molecules and coupled through superexchange interactions). These molecular pairs may be seen as possible centers for the triplet-triplet annihilation process between traps at the trap energy (i.e., within the inhomogeneous line width of the monomer line). Note that
J. Phys. Chem. 1984,88, 4660-4664
4660
for this process the cluster eigenstates are out of resonance with the monomers. The number of such pairs obviously decreases with increasing concentration, as may be seen from Figure 3. Furthermore, we note that the transition between the localized and the delocalized behavior occurs gradually.30 This fact may be illustrated by considering the cumulative number of states X(L) with localization index L, below a certain value L as defined in eq 4 and depicted in Figure 4 for the Nhs and Nds exciton eigenstates. On a cautionary note, we remark here that localization is more sensitive to long-range fluctuations than, say, the evaluation of line shapes. It may thus well happen that an increase in the size of the systems considered will somehow sharpen the gradual transition between localized and delocalized behavior which was found here. On the other hand, the combined effects of additional degrees of freedom, such as phonons (not considered here), and of the experimental limitations (finite spatial and time scales) will again lead to an erosion. We thus expect our localization results to mirror the prevailing physical situation rather closely. The calculated phosphorescence line shapes have been obtained from eq 10 and are presented in Figure 5 for various Nhs concentrations. The molecular transition moment is assumed to be perpendicular to the plane of the molecule. Line widths of 1.2 and 1.8 cm-’ for the lower and upper Davydov components, respectively (for both Nhs and Nds), have been evaluated from the experimental data. The experimental spectra presented here (Figure 1) are mainly b polarized (V) and mainly a polarized (H). A comparisan between theory and experiment gives the mixing coefficients; we have Zv = 0.751bll 0.25Zb, and ZH = 0.25Zb,, 0.75Zbl. The concentration dependence of the calculated peak positions and of the integrated intensities for the Nhs and Nds bands agrees well with the measured ones. Furthermore, the concentration variation of the line shapes can be envisaged as follows: At relatively low Nhs concentration (below 10%)the line shape arises mainly from monomer and dimer states. Increasing the concentration leads to the formation of larger-size clusters whose eigenstates with nonzero transition moments split off from the monomer states. The corresponding line shapes are located on the lower- (higher-) energy side of the lower (higher) dimer states. Consequently, the energy separation between the two peaks increases and the two lines get more intensity. A further increase of the Nhs concentration leads to the disappearance of the mo-
+
+
nomer lines; the two peaks shift away from each other and evolve into the Davydov components. This behavior emphasizes the role of the clusterization in determining the splitting of the crystal energy levels: the splitting is due to the resonance coupling between the (possibly randomly distributed) molecules and does in no way presuppose any translational symmetry.25~26J’For example, at a concentration of 50% (equal concentration of Nhs and Nds molecules) we observe and the calculation shows two “apparent” Davydov components corresponding to Nhs and Nds subbands, although for such random distributions no translational symmetry exists.
V. Concluding Remarks In this paper we have studied the quantum properties of an isotopic mixed crystal for arbitrary concentrations of the components. From our model we have computed the quantum eigenstates together with localization indices for the naphthalene system in which the molecules are coupled by short-ranged exchange interactions. By taking into account the triplet spin quantization and the spin-orbit coupling the calculated eigenstates provide a good description of the triplet spectroscopic properties (absorption and emission) of naphthalene mixed crystals over the full range of concentrations. The data show the important role of the clusters and that the energy splitting, which is due to the resonance coupling between the molecules, is little influenced by the existence or the lack of translational symmetry. Furthermore the evaluation of the localization indices shows, in the absence of diagonal disorder, the presence of a concentration-dependent population of molecular pairs, Le., nonnearest-neighbor identical molecules coupled by superexchange interactions. With diagonal disorder or/and exciton-phonon coupling these molecules can act as efficient centers in the exciton annihilation process. We thus view the calculation of quantum eigenstates and localization indices as a basic step toward the understanding of dynamical properties such as exciton transfer or exciton fusion at lower temperatures. Acknowledgment. The support of the Deutsche Forschungsgemeinschaft and of the Fonds der Chemischen Industrie is gratefully acknowledged by A.B. Registry No. Nhs, 91-20-3;Nds, 1146-65-2. (31) J. Hoshen and J. Jortner, J . Chem. Phys., 56, 933, 5550 (1972).
Interaction of Physlsorbed Species with Chemisorbed Species As Studied by Infrared Spectroscopy John T. Yates, Jr.,* and Gary L. Hallert National Bureau of Standards, Washington, D.C. 20234 (Received: March 8, 1984) Infrared spectroscopy has been used to study the physical adsorption of CO onto a Rh/A1,03 surface. In addition to absorption bands related to monolayer and multilayer physisorbed CO species, an interaction between the physisorbed species and chemisorbed CO has been observed causing a decreaseof the chemisorbed CO wavenumber. Similar effects between physisorbed Xe and chemisorbed N2 on Rh surfaces have also been observed, suggesting that the effect is a general one. Correlation of these measurements with measurements of CO trapped in CO matrices suggests that inductive and dispersive effects are the main factors responsible for the negative shift in chemisorbed species wavenumber. It has been found that physisorbed CO preferentially adsorbs in the vicinity of ionic sites, and that -AHads= 5.6 2 kcal mol-’ for CO physisorption near Rh1(CO)2 species. Introduction The investigation of the infrared spectrum of adsorbed species is one of the most useful methods for understanding their structure
and bonding at surfaces.Ia* Infrared spectroscopy, because of its high sensitivity for frequency measurement, is also a superior tool for measuring weak interactions in the adsorbed layer where
*Present address: Department of Chemistry and Surface Science Center, University of Pittsburgh, Pittsburgh, PA 15260. t Department of Chemical Engineering, Yale University, New Haven CT
(1) (a) R. P. Eischens and W. A. Pliskin, Adv. Cutul., 10, 1 (1958). (b) L. H. Little, “Infrared Spectra of Adsorbed Species”,Academic Press, New York, 1966. (c) M. L. Hair, “Infrared Spectroscopy in Surface Chemistry”, Marcel Dekker, New York, 1967.
06520.
0022-3654/84/2088-4660$01.50/0
0 1984 American Chemical Society
