J. Phys. Chem. 1986, 90. 1528-1530
1527
Kasai and Jones'but varying the angle 8, the results of which are given in Table Ill. Comparison of the computed and estimated values of p(3s),, and p(2pJC suggest a value of0 somewhat greater than I I O o whereas that ofp(3px)Al is consistent with the lower value. Interestingly, our I N D O calculations give p(2p,), > p(2pJC whereas Kasai and Jones' calculation' gives similar unpaired spin populations in the 2p, orbitals of carbon and oxygen. A possible alternative structure for AI(CO), is 2, Le., a structure
AI
Ix-
ob0ob
to the 'A," representation of C, symmetry. Although we cannot completely discount this structure it seems unlikely because the unique "AI and 'IC anisotropic tensors are parallel and not perpendicular to each other as would be expected for structure 2. This conclusion is supported by consideration of the simplified molecular energy level schemes for AI(CO), with C2, and C, symmetry (Figure 5). In the bent C,, structure the 3s orbital ( a , ) interacts with the 5a(al) ligand orbitals and the 3p,(h,) orbital interacts with two 2a*(b,) ligand orbitals (the p,(al) and p,(b2) orbitals are not shown for clarity). The energy of the b, MO falls below that for the antibonding a , MO and the odd electron is placed in the former. In the sandwich C, structure the lowest-lying a , orbital interacts with two Ir(a,) ligand orbitals and the 3p, orbital interacts with two 2 r * orbitals and the odd electron in placed in the latter MO. The C , structure is clearly more favorable than the C, structure.
which is analagous to the one for CU(C,H,),'~ and which belongs
Acknowledgment. C.A.H thanks SERC for a studentship. We thank NATO for a collaborative research grant (No. 442/82) and Dr. J. S. Tse for performing the I N D O calculations. Registry No. AI(CO),, 12691-52-0 AI. 7429-90-5; CO. 630-08-0.
(16) Kasai, P. H.: McLead. D..Jr.; Watanabe.T.J. Am. Chem.Soe. 1980. IOZ. 179-190.
(17) Jorgcnscn. W.L.:Salem. L. The Organic Chemisf Bwk of Orbilals: Academic Press: New York. 1973; p 78.
2
Infrared Spectrum of the Tritiated Hydroxyl Ion (Or)in a Neutron-Irradiated LiF Crystal Yasuyuki Aratono, Mikio Nakashima, Masakatsu Saeki,* and Enzo Tachikawa Department of Chemistry. Japan Atomic Energy Research Institure. Tokai-mum. 1baraki-ken 319- / I . Japan (Receiued: August 13. 1985)
Infrared absorption of the tritiated hydroxyl ion (OT-) in a LiF crystal has been studied. The dominant absorption occurs at 2225 cm-'. Spectroscopic constants are determined on the basis of the anharmonic oscillator mcdel for a diatomic molecule. The results suggest a smaller anharmonicity of the O-H and 0-T stretching vibrations in the LiF crystal compared to those in TiO,, a-Al2OJ.and KTaO,.
Introduction In the course of the study on tritium centers in neutron-irradialed LiF crystals,' the authors attempted to observe an infrared spectrum due to the tritiated hydroxyl ion, OT-. which is the isotopic analogue of OH- and OD-. Though three reports have been published on the infrared spectra of O T , they always dealt with oxides such as Ti02? a-Al,O,.' and KTaO,? in which tritium was introduced thermally. The present paper will describe the experimental observation of the O T spectrum, being the first report on O T in alkali halide crystals, as far as the authors know. Experimental Section Single crystals of LiF were purchased from the Horiba Co. Ltd. Impurity analysis by inductively coupled plasma atomic emission spectroscopy detected 144 and 67.5 ppm of Mg and Ca. Thermal neutron irradiation was carried out in the S-pipe of a JRR-4 with a flux of 5.5 X I O " m-'.s-' for 6 h a t ambient temperature. The size of a sample for neutron irradiation was typically 5 X 10 X I mm'. The concentration of tritium in a ( I ) Y. Aratono. M. Nakashima. M. Sacki, and E. Tachikawa, Radioehim. A m . 37, 101 (1984). (2) J. B. Bates and R. A. Perkins, Phys. Reo.. 16, 3713 (1977). (3) H. Engstrom, J. R. Rala. J. C. Wang. and M. M. Abraham. Phys. Reo.. 21. I520 (1980). (4) H. Engstram. J. B. Bates, and L. A. Boatner. J. Chem. Phys., 73, 1073 (1980).
sample was (8.2 f 2.0) X IOs Bqmg-' which was determined by dissolving the irradiated sample in a nitric acid solution. (This concentration corresponds to (1.2 f 0.3) X T atoms.cm-J.) The irradiated sample was annealed in a quartz tube equipped with an electric furnace in a stream of H e for 15 min at various temperatures. Subsequently, the sample was subjected to infrared absorption measurement. Spectra were obtained with a conventional double-beam infrared spectrophotometer, Hitachi Model 260-50, in the frequency range 250 to 4000 cm-l at room temperature. The stated resolution was 1.5 cm-' at 1000 c d . +radiation was carried out with a *Co source at room temperature. The dose rate was 4.1 X IO' C.kg-'.
Results Figure I shows the infrared spectrum o f a sample annealed at 650 OC after neutron irradiation in the frequency region of OHand O T together with that of a nonirradiated sample. In the latter spectrum, two strong absorption bands at 3578 and 3614 cm-' and a weak one at 3649 cm-' were observed in the OH- region (scatrum Al. while in the O T reeion no absorotion was detected (spectrum i j . I n the samole. tritium atoms were mainlv formed by the 6Li(n,,,a)T reaciion. The resulting a and triton particles initially possess kinetic energies of 2.06 and 2.73 MeV, respectively. Thus, radiation damage in the sample was caused by these particles as well a s by y-rays and neutrons in the reactor. As a result, the irradiated sample was black due lo color centers. However, almost
~.
-
0022-3654/86/2090-1528%01.50/0 0 1986 American Chemical Society
IR Study of OT-
The Journal of Physical Chemistry, Vol. 90, No. 8, 1986 1529 (B)
(A) OH- region v---%-
OT- region
@ 3578 c m
(-)
2225 cm-' (----)
-
cn
v
0" 1.20 I
1.00 2225 cm-'
z
-
20-
OLh 450
m
1
I
500 550 600 650 Ann e a Ii n g t e m p e r a t ur e ( 'C)
I
.
700
Figure 2. Annealing behavior of the 3578- and 2225-cm-' bands.
all of radiation damage was quenched by thermal annealing at 500 OC. With thermal annealing above 500 O C , the absorption bands a t 3578, 3614, and 3649 cm-' were gradually restored and a new band appeared at 3625 cm-' (spectrum C). Furthermore, an absorption pattern similar to spectrum C was observed in the OT- region, that is, 2225, 2242, 2254, and 2282 cm-l (spectrum D). The full width at half-maximum (fwhm) was 6.8 f 0.2 cm-l for the 3578-cm-' band and 5.2 f 0.2 cm-' for the 2225-cm-' band. The restoration behavior of the 3578-cm-' band against annealing temperature is shown in Figure 2 together with the formation behavior of the 2225-cm-' band. In this figure, the relative intensity of the 3578-cm-' band denotes the ratio of the absorbance of the sample annealed at various temperatures to that of the nonirradiated sample. The intensity of the 3578-cm-' band was restored completely at 650 OC. Therefore, for the 2225-cm-' band, the absorbance at 650 OC was chosen as the reference standard. Both bands coincide well each other, suggesting isotopic analogues. The decrease in the relative intensities at 700 "C is possibly due to the change of the surface condition caused by partial evaporation of the sample. The effect of y-irradiation on the intensities of 3578- and 2225-cm-' bands was also examined. The survival fraction, defined as the ratio of the absorbance before and after y-irradiation, is plotted in Figure 3 as a function of the irradiation dose on a semilogarithmic scale. The plots give straight lines which are expressed by the least-squares fitting as log
SOH-
= -(S.SI A 0.22) x 10-4x
+ (1.97 f 0.02)
X
10-4X
1
'.=
I
4 6 8 1 0 1 2 1 4 Dose (C.kg-') (xlO-*)
Figure 3. Radiolysis behavior of the 3578- and 2225-cm-' bands by 60Co y-ray.
+ (1.98 f 0.02)
Discussion The infrared absorption of OH- in a LiF crystal was studied with a Mg-doped sample by S t ~ e b e . H ~ e showed that the broad band centered a t 3731 cm-I was observed only in the specimen containing less than 10 ppm of Mg and originated from free OH-. On the other hand, a new band with a very narrow fwhm (5 cm-I) emerged at 3571 cm-' from the sample with more than 50 ppm of Mg and was assigned to the stretching vibration of 0 - H complexed with Mg.5 The sample used in the present experiments contained 144 ppm of Mg. Thus, it was expected that a very sharp band would appear near 3571 cm-I. The infrared spectrum showed no absorption near 373 1 cm-l but a strong absorption at 3578 cm-' with a fwhm of 6.8 f 0.2 cm-l. Though the small differences in frequencies and fwhm between the present work and Stoebe's work cannot be well explained, the different conditions of sample preparation may affect the spectroscopic properties to some extent. It is also considered that the difference may be caused by the complex formation of OH- with Ca as well as with Mg. Such a complex has been suggested to have an infrared absorption near 3570 Therefore it can be considered that the 3578-cm-' band corresponds to the 3571-cm-' band reported by Stoebe and is assigned to the stretching vibration of 0 - H complexed with Mg and possibly with Ca. The other absorption bands centered a t 3614, 3625, and 3649 cm-', therefore, may be attributed to various forms of Mg2+- and Ca*+-OH- vacancy complexes, as suggested by S t ~ e b e . ~ As was shown in Figures 2 and 3, the behavior of the 2225-cm-' band against thermal annealing and y-radiolysis is very similar to that of the 3578-cm-' band, suggesting that the 2225-cm-* band is assigned to the isotopic analogue of OH-, Le., the tritiated hydroxyl ion OT-, complexed with Mg2+ or Ca2+. The infrared absorption around 2000 cm-' has been suggested to result from protium centers in a LiF However, they decay out a t 350 O C . Therefore it is very unlikely that the absorption between 2225 and 2282 cm-' originates from protium impurity centers. If we assume harmonic vibration, the frequency shift among the isotopic analogues relative to the OH- frequency, p,, can be predicted by the following equation:'O p i = y,har/YOH.har
=
(pOH-/pi)
'I2
(3)
where ?V and v ~ are the ~ harmonic ~ frequencies ~ ~ and hOH-and the reduced masses. The atomic masses for H , T, and 0 are 1.0078, 3.0160, and 15.9949 amu, respectively.'' Thus,
pi are
(1)
for the 3578-cm-l band and log SOT= -(6.62 i 0.26)
I
1
perimental uncertainty. This fact also implies that both bands originate from isotopic analogues.
i
01
2
I
(2)
for the 2225-cm-' band, where SOHand SOTare the survival fractions of 3578- and 2225-cm-' bands and X i s the irradiation dose (C-kg-I). The slopes, which correspond to G values for the radiolysis, agree well with each other within the range of ex-
(5) T. G. Stoebe, J . Phys. Chem. Solid, 28, 1375 (1967). (6) B. Fritz, F. Luty, and J. Anger, 2.Phys., 174, 240 (1963). (7) T. Kamikawa, Phys. Status Solidi E , 68, 639 (1975). (8) Z. G. Akhvlediani, K. J. Berg, and G. Berg, Cryst. Lattice Defects, 8, 167 (1980). (9) Z . G. Akhvlediani, J. G. Akhvlediani, and L. G. Bakhsbetsyan, INIS-SU-94, 1981, p 74. (10) G. Hertzberg, Molecular Spectra and Molecular Structure. Vol. I, Spectra of Diatomic Molecules, Van Nostrand, New York, 1953, p 141.
J . Phys. Chem. 1986, 90, 1530-1534
1530
TABLE I: Observed Frequencies, Harmonicity Constants ( p , ~ , ) , Anharmonicity Constants (p:wJx,), and Dissociation Energies (Do’) for OH- and O T in a LiF Crystal” obsd freq, cm-’ p i w e , cm-’ p12wJe, cm-’ D,,’. eV
where plwe is the harmonicity constant, p12wJe and p13weYeare the anharmonicity constants, and c is the vibrational quantum number. Generally, terms higher than third order are very small compared to first and second order and can be neglected. Thus, for the transition from the ground to the first excited state, eq 4 can be reduced to
”OH
3578 3614 3625 3649
3738 3753 3787 3867
2225 2242 2254 2282
2285 2294 2314 2363
80.5 69.5 81.0 109.0
5.12 5.56 5.26 3.02
30.0 26.0
5.25 6.13 5.39 4.07
vi = piwe -
2pi2wJe
(5)
The theory also predicts dissociation energies of OH- and OT-, Doi,as
”or
‘p,
30.0 40.5
= 1 .OOOO for OH- and 0.6 1 12 for OT-
isotopic frequency shift for OT- can be estimated to be 0.6 1 12. The observed isotopic frequency shifts were calculated from the frequency pairs 2225 and 3578 cm-I, 2242 and 3614 cm-’, 2254 and 3625 cm-l, and 2282 and 3649 cm-l and obtained to be 0.6219, 0.6204, 0.621 8, and 0.6254, respectively. The observed values coincide very well with each other and are relatively in good agreement with the predicted value. In conclusion, the present results suggest that the four absorption bands centered a t 2225, 2242,2254, and 2282 cm-l can be reasonably assigned to the 0-T stretching vibration. Theoretical calculation of the spectroscopic constants of OHand OT- was attempted using the methods described by Hertzberg.1° The term value of the anharmonic stretching vibration for a diatomic molecule of the isotopic analogues can be expressed as follows:
Gi(V = Piwe(v + Yz) - P i * w J e ( v +
72)2+ ~ i ~ w e Y e+( v‘/,I3... (4)
(1 1) A. H . Wapstra and N. B. Gove, Nucl. Data Tables, 9, 265 (1975).
from eq 5 and 6, we calculated the quantities piue,p,*wJe, and Do’ using the observed frequencies and the results are tabulated in Table I. The dissociation energies of OH- and OD- in KBr and KC1 were calculated to be 4.88 and 4.94 eV and 5.03 and 5.09 eV from the observed frequencies reported by Wedding et al.I2 These values suggest that the dissociation energies of OT- in the KBr and KCI crystals are about 5 eV which is relatively in good agreement with the present results. In crystal oxides such as Ti02,2 L Y - A I ~ and O ~ ,KTa03,4 ~ the infrared absorptions of OH- and O T occur a t lower wave numbers than in the present work. In addition, the isotopic frequency shifts are rather large (0.6303, 0.6301, and 0.6240 for TiO,, a-A1203, and KTa03, respectively), suggesting a relatively small anharmonicity of the 0-H and 0-T stretching vibrations in the LiF crystal compared to that in the crystal oxides.
Acknowledgment. The authors are indebted to Dr. K. Ohwada for his helpful suggestions and comments. Thanks are also due to Mr. N. Kuribayashi for analysis by inductively coupled plasma atomic emission spectroscopy. Registry No. OT-,20666-23-3; LiF, 7789-24-4; OH, 3352-57-6; neutron. 12586-31-1. (12) B. Wedding and M. V. Klein, Phys. Rec;., 177, 1274 (1969)
Theory of Raman Scattering by Aggregated Molecules D. L. Akins Department of Chemistry, The City College of The City University of New York, New York, New York 10031 (Received: August 15, 1985)
A Raman scattering enhancement theory, for aggregating dyes, is developed that is based on the formation of molecular vibro-excitonic levels. Herzberg-Teller (H-T) correction to the vibrational Raman problem is applied which in combination with commonly observed spectral changes which occur for certain dyes yields selection rules for Raman bands. This theory applies specifically to molecules, such as the cyanine dyes, which form ground-state and excited-state aggregate structures. We find that an intrinsic enhancement, upon formation of an aggregate containing N monomers, applies which is of magnitude N . A resonance effect on the Raman scattering intensity is also found when the exciting laser radiation’s frequency approaches the aggregate absorption band (e.g., the J-aggregate absorption). The net enhancement of dye Raman bands is attributed to an attenuation of fluorescence upon aggregate formation and the enhancement effects above. Discussion of H-T coupling between the exciton band and neighboring electronic states is also provided, and it is suggested that enhancement due to such couplings is unlikely for totally symmetric vibrational modes. For nontotally symmetric modes, no such proscription applies. Enhanced Raman spectra of 2,2’-cyanine, excited by 488- and 568-nm radiation, are provided to demonstrate the effect of resonance when the exciting radiation “overlaps” the aggregate absorption band.
I. Introduction The study of enhanced Raman scattering of molecules on surfaces has become an active research area within the past ten years. Several review articles have detailed the experimental techniques and systems in~estigated.’-~As a natural outgrowth 0022-3654/86/2090-l530$01.50/0
of interest in the enhancement mechanism, numerous theoretical treatments of proposed enhancement schemes have been put ( 1 ) Van Duyne, R. P. In “Chemical Applications of Lasers”; Moore, C. B., Ed.; Academic: New York, 1979; Vol. 4, p 101.
0 1986 American Chemical Society
