13470
J. Phys. Chem. 1994, 98, 13470-13475
Photodissociation Spectra of Naphthalene Cluster Ions (CloH&+, n = 2-7: Evidence for Dimer Core Structure and Comparison with Neutral Clusters Hiroyuki Saigusa* and E. C. Lim? Department of Chemistry, The University of Akron, Akron, Ohio 44325-3601 Received: June 11, 1994; In Final Form: September 12, 1994@
The photoabsorption spectra of naphthalene cluster ions, (CloH&+ with n = 2-7, in the visible and near-IR (500- 1100 nm) spectral regions have been observed by mass-selected photodissociation spectroscopy. These cluster spectra show two distinct absorption maxima centered at 1pm and 570 nm and match well the absorption spectra of the dimer ion previously obtained in solutions. The near-IR band is assigned to the characteristic intervalence transition of the dimer ion produced in these clusters, while the visible band is attributed to a local excitation Corresponding to the monomer D2 Do transition. Both absorption bands are found to be independent of cluster size, which is explained by assuming a core structure involving a strongly bound dimer ion. The stability and dissociation dynamics of the ionic clusters are compared with those of the corresponding neutral clusters.
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Introduction Stable molecules and rare gas atoms form weakly bound clusters which are held together by van der Waals (vdw) forces. When such homogeneous species are electronically excited, the excitation, which is initially delocalized throughout the constituents, will eventually collapse around a dimeric core. This dynamical pathway is termed excimer formation and accompanies large structural rearrangement of the cluster framework. In a series of we have demonstrated that the vdW clusters of naphthalene, (C~OHS),,with n = 3 and 4, produce a strongly bound dimeric species when they are excited into S1 vibronic levels at the vdW geometries. The resulting dimer core (excimer) then relaxes to the repulsive ground state by radiative transition, leading to the appearance of a strongly red-shifted fluorescence analogous to the excimer fluorescence observed in solutions. For these clusters including n = 2, a strong absorption band has been found in the visible and near-IR regions centered at 700 nm. This absorption has been assigned to an intervalence transition between the bound excimer state and an upper state of repulsive ~haracter.~ The observation that the intervalence transition of (C1OH8)Z remains intact in (ClOH8)3 and (C1OH& has provided strong evidence for the existence of a dimer core onto which additional ground-state molecules associate. The idea of such a core structure has been documented before, particularly for homogeneous cluster ions involving inert gas atoms and small molecules (e.g., Ar, Nz, CO;?,and 02).5*6 In these systems, characteristic intervalence transition bands of the dimer ions are found to be unaffected by further clustering, which leads to the conclusion that the higher cluster ions must involve dimer ion cores. A similar dimer core model has also been used to interpret the photodissociation spectra of benzene cluster ions. The observation that the intervalence absorption of (C&&+ appearing near 950 nm is essentially intact in (C6H6)3+, as well as in the higher clusters, has indicated that a (CsH&+ core is responsible for the photoab~orption.~-~ The present work investigates the intervalence absorption bands of the naphthalene cluster ions (CloH8)nf for n = 2-7 by mass-selective photodissociation spectroscopy. Naphthalene t Holder of the Goodyear Chair in Chemistry at the University of Akron. @
Abstract published in Advance ACS Absrrucrs, December 1, 1994.
0022-365419412098-13470$04.5010
is highly favorable for such investigations because the corresponding transitions for the neutral clusters have been obtained previously by fluorescence depletion spectroscopy.3 By comparing the data for the neutral (excimers) and ionic clusters, direct information can be obtained on the core structure of these clusters and the effect of delocalization on the cluster stability. Although a similar comparison was made for small benzene clusters by Krause et al.,l0 the data for the neutrals were acquired at the vdW geometries. Since excimer states of the neutral species are not directly accessible from their vdW ground states, only time-resolved experiments permit the study of intermolecular potential curves at the excimer geometries. An important aspect of the present study is to compare the short-range chemical forces (as opposed to vdW forces) of the neutral and ionic clusters as a function of cluster size. The experimental technique used to measure the intervalence absorption bands of (CloH&+ involves mass-selective photodissociation spectroscopy, which provides a method for photodepletion of the parent ions and time-of-flight (TOF) mass analysis of resulting fragmentation processes. Unlike previous photodissociation experiments, the ions are prepared near threshold via resonant two-photon ionization (R2PI) and hence are expected to exist in their ground state with relatively small internal energies. A complementary study, concerned principally with intracluster reaction dynamics that take place following excitation through the intervalence transitions, will be described elsewhere."
Experimental Section and The experimental setup has been described only a short description specific to the photodissociation experiment will be given here. The cluster ions of naphthalene were formed by R2PI of the corresponding neutral clusters through their 8; transitions (using the notation of Stockburger et a1.12). The excess energy available from this threshold ionization is estimated to be less than 0.3 eV? resulting in the formation of cluster ions having internal energies between room temperature and the tens-of-Kelvin region. The R2PI method is insufficient to ionize the monomer through this intermediate (g; = 32 455 cm-', IP = 65 666 cm-l),13 and the resulting TOF spectra exhibit only a weak peak corresponding to the monomer ion. 0 1994 American Chemical Society
J. Phys. Chem., Vol. 98, No. 51, 1994 13471
Spectra of Naphthalene Cluster Ions
[ A h l [a) N=2
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Spectral Shift/cm-' Figure 1. Resonant two-photon ionization (R2PI) spectra of (CloHs), for n = 2-7 in the 8; region. The spectral shift is relative to the corresponding monomer transition at 32 455 cm-'.
The cluster ions were intersected, in the ionization region of a Wiley-McLaren-type TOF chamber, by a photodissociation laser pulse with a temporal delay of 0-200 ns with respect to the ionization laser. Since, when using this configuration, most of the parent ions can interact with the dissociation laser pulse, no significant size selectivity is achieved for the photodissociation process. The delay between the ionization and photodissociation laser pulses allowed the photofragments to be separated in time with respect to the parent ions. Thus, photofragment mass spectra were obtained by employing laser onoff subtraction of the TOF spectra. Depletion of a parent ion was obtained as a function of the photodissociation laser wavelength with a fixed pulse energy, resulting in a photodepletion spectrum. The fundamental output as well as Raman-shifted output of a YAG-pumped dye laser, which provide tunable radiation over the wavelength range from 500 to 1100 nm, passed through the ionization region slightly focused. The pulse energy (15-500 pJ) was controlled by varying the Q-switch delay of the YAG laser and measured by a Scientech power meter.
Results and Discussion The one-color R2PI spectra of the neutral clusters (C1OH&, n = 2-7, for the 8; vibronic band region are compared in Figure 1. The spectral features vary dramatically with the cluster size, which suggests that cluster geometry differs from cluster to cluster. This clearly demonstrates that the R2PI. method cause efficient ionization without substantial fragmentation. Among them, the pentamer spectrum shows the most redshifted feature from the monomer 8; band at 32455 cm-', which is still well above the corresponding absorption band observed at 31 976 cm-' for the bulk crystal of na~htha1ene.l~ These spectra contain important information on the cluster structure and the nature of excitation delocalization, as discussed by Wessel and Syage15for n = 2-4 based on the weak exciton interaction model.
Figure 2. (a) TOF mass spectrum obtained following R2PI of (ClOH& through the most intense peak at -242 cm-' shown in Figure Id. (b) TOF mass spectrum obtained upon photodissociation at 950 nm, with a time delay of 150 ns with respect to the ionization laser. (c) Difference mass spectrum, (b) - (a). The pulse energy of the photodissociation laser is 100 pJlpulse.
In order to measure size-dependent photodissociation behaviors of cold cluster ions, R2PI has been undertaken through an appropriate spectral feature of the 8; band for each cluster, to generate a corresponding cluster ion with a higher abundance than other cluster ions. A typical TOF mass spectrum, obtained subsequent to ionization of (C~OH&through the most prominent feature at -242 cm-', is shown in Figure 2a. It is to be noted that the resonance excitation enhances a peak corresponding to (CloH&+ with respect to other cluster ions. The n = 2 peak also appears in the spectrum because (C1OH8)2 exhibits a broad absorption band corresponding to 8; (see Figure la). When photodissociation is carried out at 950 nm with a delay time of 150 ns after the formation of the parent ions, we obtain a mass spectrum showing photofragment peaks at lower masses (Figure 2b). This photodissociation behavior is seen more clearly in the difference mass spectrum shown in Figure 2c, which is obtained by subtracting the mass spectrum of the primary ions (Figure 2a) from the photofragment mass spectrum (Figure 2b). The downward peaks correspond to signal depletion of the primary ions, while the upward peaks, appearing 150 ns later than the corresponding primary ions, are due to the photofragments. It is seen that (CloH&+ exhibits a similar strong depletion, which indicates that the dimer ion also absorbs strongly at this wavelength. Under the same condition, no photodepletion is observed for the monomer ion. We have obtained photodissociation spectra for (CloH&+ by measuring photodepletion signals in the 500- 1100-nm wavelength region. The pulse energy of the photodissociation laser has been reduced below 50 pJ/pulse to avoid multiphoton effects, and thus the spectra obtained by this method presumably mimic the absorption spectra of (CloH&+. When the photodissociation laser is employed at higher fluences than -1OOpJ/ pulse, fragmentation of naphthalene itself takes place to produce photofragments such as CloH7' and CsH6+.L6317The spectra for this entire series of clusters are shown in Figure 3. Two distinct bands are clear in all cases: a visible band near 570 nm and a more intense band in the near-IR (-1 pm), both being
Saigusa and Lim
13472 J. Phys. Chem., Vol. 98, No. 51, 1994 80 60 -
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Figure 3. Photodepletion efficiency curves of (ClOHS)n+as a function of photodissociation wavelength: (a) for n = 2-4; (b) for n = 5-7. Each cluster ion is produced by R2PI through the most strong peak of the corresponding neutral cluster in Figure 1. The pulse energy of the photodissociation laser is kept at 50 pUpulse.
Figure 4. TOF mass spectra showing photodepletion behaviors for (CIOHS)~+ (marked N29, [(CIOH~)(C~OHI~)I+ (ND+), and (ClOH14)2+ (D2+),obtained with the photodissociation laser (a) at 560 nm and (b) at 950 nm and (c) with the photodissociation laser off.
shifted relative to the first optically allowed absorption band of the monomer ion located near 700 nm.19-24The monomer band corresponds to the D2 Do transition arising from the excitation of an electron from the inner MO ~ ( b 3 to ~ )the half-occupied essentially independent of cluster size. These photodissociation MO x(A,) on the basis of the HMO description. Despite of spectra are remarkably different from those of the monomer the large blue shift, we assign the 570-nm band of (ClOHs)n+ to ion obtained previously in the gas phase16J8and in a supersonic a local excitation correlating with the D2 monomer ion and the jet. l7 The monomer photodissociation spectra correspond well neutral monomer. The spectral blue shift of the visible band with the absorption spectra measured in y-irradiated s o l ~ t i o n ~ ~ - ~in~the dimer ion as compared to the monomer ion indicates a and in solid argon.22-24 larger stabilization of the Do state relative to the D2 state in the The absorption spectra of the naphthalene dimer cation dimer cation. When two chromophores interact via the charge produced upon y-irradiation were reported by Badger et al. 19,20 delocalization interaction, both Do and D2 states split into the and by Shida and Iwata.21 More recently, Yamamoto et al.25v26 bound and repulsive combination states @+. One may expect have employed a laser photolysis technique to obtain transient the charge delocalization in the D2 state to be smaller than that absorption spectra of this species. In all cases, two absorption in the Do state because the D2 state canies a positive charge in bands at 570 and 1100 nm are observed. The near-IR band, the inner MO ~ ( b 3 ~ Therefore, ). it is reasonable to expect that which finds no match in the monomer ~ p e c t r u m , l ~has - ~ been the @-(D2) @-(Do)transition shifts to the blue upon dimer assigned to the intervalence transition between bound and formation. Although the bare ion possesses two other weaker repulsive combinations of the naphthalene monomer states. absorption features at 488 and 395 no attempt has been These two combinations are expressed as @y= (1/2)1'2[4+4 made for measuring photodepletion efficiencies in this spectral region. f@+I, where and 4 are the ionic and neutral wavefunctions of naphthalene, respectively. The photodissociation spectra A critical test for this assignment is provided by photodisshown in Figure 3 bear striking resemblance to the absorption sociation of mixed cluster ions where charge delocalization spectra of (CloH&+ previously obtained in solutions. On the should be less significant than the case of pure cluster ions. basis of their similarity, we assign the near-IR band to the Figure 4 compares photodepletion behaviors of the mixed dimer intervalence absorption bands of (CloH&+. This observation ion involving durene (1,2,4,5-tetramethylbenzene),[(CloH!& indicates that the characteristic intervalence absorption of (CloH14)]+ (denoted ND'), with those of the pure dimer ions, (cl0&)2+ persists upon further clustering, which can be regarded (ClOH&+ (Nz+) and (CloH14)2+ (D2+), at two different photoas evidence for the existence of a dimer core. Recent photodissociation wavelengths. The top mass spectrum taken at 560 dissociation results on the benzene clusters have also been nm shows a significant depletion for both N2+ and ND+ peaks explained by invoking a similar core Although the with respect to the corresponding peaks in the spectrum obtained D1 Do transition of the monomer cation, predicted by the without photodissociation (Figure 4c). Since the mixed complex Hiickel molecular orbital theory at 8550 cm-1,23 could appear ( C ~ O H ~ ) ( C ~has O Ha ~broad ~ ) absorption band in the vicinity of upon cluster formation, we disregard this possibility since the 8; band of (C~OH&,~ both N2+ and ND+ ions are produced oscillator strength is expected to be small due to the forbidden from R2PI at this excitation wavelength (309.5 nm). It is also nature of the transition. important to note that a peak corresponding to D2+ appears in the TOF spectrum, which is assigned as arising from a In contrast, no clear assignment has been made for the visible nonresonant two-photon ionization process. The D2+ peak absorption band.19*26327 The visible band near 570 nm matches exhibits no depletion at 560 nm, providing evidence that the those of the dimer ion observed in solutions but is strongly blue-
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J. Phys. Chem., Vol. 98, No. 51, 1994 13473
Spectra of Naphthalene Cluster Ions
cs F
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Wave 1eng t h/nm Figure 5. Comparison of photodepletion efficiency curves for (a) the neutral clusters (C,&Jn* and (b) the cluster ions (Cldf&+, with n = 2 and 4. The data for the neutrals are taken from refs 3 and 26, while those for the ions are reproduced from Figure 3.
(CloHg)+ chromophore is responsible for the absorption at this wavelength. It is therefore reasonable to regard the visible absorption as a blue-shifted local excitation of the monomer ion. In contrast, a strong depletion is discernible for D2+ excited at 950 nm (Figure 4b). This suggests that D2+ possesses an intervalence band similar to N2+ in this wavelength region. These homodimer ions are likely to be more stable than the heterodimer ions, since stabilization energy due to charge delocalization is expected to increase with decreasing difference in the ionization potentials between two chromophores.28 Thus, the intervalence band of the heterodimer ion ND+ (AIP = 0.1 eV) may be more red-shifted than those of the homodimers, thus explaining the less efficient photodissociation cross section observed at 950 nm. The transition energy of an intervalence band can be used to estimate the stability of a dimer core produced in a cluster. Previously, we have reported the corresponding intervalence transition bands for the neutral naphthalene clusters (CloHg)n (a = 2-4).3.29 Following vibronic excitation, these cluster produce a strongly bound dimer core, leading to localization of initial excitation. The dimer core (excimer) then absorbs a visible or near-IR photon, resulting in rapid dissociation into monomer fragments. The broad absorption band centered at 700 nm, obtained by measuring fluorescence depletion as a function of dissociation wavelength, has been assigned to an intervalence transition between the lowest and upper excimer states, = ( ' / ~ ) ' ' ~ [ q 5 * q 5 & #@*I, where q5* and q5 are the S2 and SO wavefunctions of the monomer, respectively. The observed absorption curves for (ClOH8)2* and (ClOH8)4* are compared with the present results for the corresponding cluster ions and displayed in Figure 5. We show the curves only for n = 2 and 4 since no excimer formation takes place for n = 3 excited at this vibronic band.' The curve for ( C l ~ H g ) 4 *displays a similar absorption band to (CloH8)2*. This supports the previous conclusion4 that the tetrameric cluster consists of a
Interplanar Distance
Figure 6. Qualitative representation of the potential energy curves of the excimer and dimer ion produced in the naphthalene clusters. The energy separations shown in electronvolts are discussed in the text.
dimeric core (excimer), in which the excitation is immobilized, and two weakly bound ground-state molecules, Le., (clOH8)2*(ClOHd2. An important observation in Figure 5 is that the intervalence transition bands shift to the red in going from the neutral to ionic clusters. The spectral shift is estimated to be in the range 0.5-0.6 eV, suggesting a larger stabilization for the neutral clusters (excimers) than for the cluster ions. The binding energy of (CloHg)2+ has been obtained from a thermodynamical measurement to be 0.77 eV.30 This value is approximately half the observed intervalence transition energy (- 1.24 eV), which demonstrates that the splitting between the bound and repulsive combination states is almost symmetrical. The dissociation energy of the naphthalene excimer (C&g)2* in solutions has been derived from temperature dependence of the fluorescence inten~ities.~'However, this value (0.25 k 0.04 eV) is apparently much smaller than half the intervalence transition energy (-1.77 eV). This discrepancy is explained by the previous assertion32 that the excimer stability arises from the exciton-resonance interaction between two monomer states that can be correlated to the S2 and SO states. On the other hand, the thermodynamical value can be associated with dissociation into two monomers, one in S1 and the other in SO. Thus, the observed intervalence transition energy must be correlated with the dissociation energy by taking account of the energy separation between S1 and S2 (-0.48 eV). This yields an excimer stabilization energy of -0.73 eV with respect to S2, which is roughly in accord with half the intervalence transition energy. Figure 6 represents a qualitative picture of the potential energy curves of the excimer and dimer ion produced in the naphthalene clusters. Photodissociation behaviors of size-selected benzene cluster ions have been studied extensively following excitation into the intervalence transition bands as well as into the locally excited state^.^-^,^^ In the present study, the lack of size-selectivity in photodissociation precludes a detailed analysis of the photofragmentation pathways. Nevertheless, we can obtain some
Saigusa and Lim
13474 J. Phys. Chem., Vol. 98, No. 51, 1994 ~~~~
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Figure 7. Difference TOF mass spectra for (a) (C1&)3+, (b) (Cl&I8).++,and (c) (CloHs)s+, obtained following photodissociation at 580 nm (left panel) and 950 nm (right panel). Each parent cluster ion is produced by R2PI of the corresponding neutral cluster through the strongest peak appearing in Figure 1. The photodissociation laser (30 pJ/pulse) is fired with a 150-11s delay.
qualitative features of the photodissociation behaviors. In Figure 7, we compare the difference mass spectra for n = 3-5 obtained by employing the laser on-off subtraction. The most striking observation is that the photodissociation at 950 nm produces mostly dimer ions, while fragmentation into monomer ions dominates the photofragment spectra obtained at 580 nm. Since the photodissociation of (CloH8)4+ at 950 nm results in (ClOH8)2+, this excitation energy is sufficient to evaporate two monomers from the cluster. However, the same excitation does not lead to two monomer loss for (CloH8)3+. This photodissociation propensity is very similar to that of the benzene cluster ions reported by Nakai et al.,33where (C6H6)4+ dissociates into (c6&)2+ following excitation through the intervalence transition at 880 nm. The most likely explanation for this popensity is that, as proposed by Beck and Hecht? photodissociation occurs via sequential evaporation of neutral monomers until the bare dimer ion is formed. According to this mechanism, the energy supplied by the 950-nm photon is sufficient to evaporate two naphthalene monomers from (cldI8)4+ but insufficient to further dissociate the resulting stable dimer ion. However, this mechanism ignores a possible contribution of charge delocalization effects to binding energies of cluster ions. If the positive charge is delocalized throughout all component molecules in clusters, the stability should be greatly affected by the cluster size and structure. Thus, the difference in photofragmentation pattern between (ClOH8)3+ and (clOH8)4+ could be explained by this charge delocalization model, and, in fact, Krause et al.1° suggested a similar model to rationalize the size dependence of the binding energies for the benzene cluster ions. Whereas this type of binding model can, of course, explain the stability of cluster ions, the intervalence transition band should change from cluster to cluster. However, the present observation that all naphthalene cluster ions up to n = 7 exhibit similar absorption bands in the intervalence transition region is not what would be expected for the case of charge delocalization.
The photodissociation of (CloH8)3+ at the visible band produces only (ClOHs)', as displayed in the left panel of Figure 7a. This behavior may be viewed, based on the dimer core model, as evidence that the available energy from this excitation is so large that even the bare dimer is dissociated. For parent cluster ions with n = 4 and 5 , photofragments involving more than one monomer appear in their TOF spectra (see Figure 7b,c). This observation is also consistent with the photodissociation results reported by Ohashi and Nishi for the benzene cluster ions.' They detected both (C6H6)2+ and (C6H6)3+ fragments subsequent to photodissociation of (C6H6)4+ at the local excitation band. These photodissociation behaviors suggest that the energy required to evaporate one monomer increases with increasing the cluster size and thus can be rationalized by the statistical dissociation model that evaporation of neutral monomers occurs sequentially following complete randomization of the excitation energy. The present results indicate that when an aromatic molecular cluster is ionized, the resulting positive charge tends to localize on a pair of molecules leading to the formation of a dimer ion core. Similarly, electronic excitation of the cluster can produce a tightly bound dimer core where the initial excitation is trapped. Therefore, these species can be regarded as dimers which are surrounded by other weakly bound monomers. This situation is quite analogous to the case of clusters involving rare gas atoms or small molecules. For example, the absorption spectra of (C02),+ are similar to the dimer ion spectrum up to n = 10 and interpreted as indicating that these homologous clusters consist of a dimer s u b ~ n i t . However, ~ very little has been known about the bonding nature of the neutral excited states of such clusters since the large Franck-Condon displacements preclude direct optical preparation of the species from the vdW ground state. It is therefore only recently that the transient absorption spectra of the rare gas dimers have been obtained by Killeen and Eden34 and assigned to transitions from the excimer states ns3Eu+ to upper Rydberg states. They have demonstrated that at small interplanar distances the potential
Spectra of Naphthalene Cluster Ions energy curves of the neutral dimers (excimers) and dimer ions are qualitatively similar. The present work suggests that such a similarity also exists for clusters of aromatic molecules where the general features of low-lying electronic energy surfaces are described qualitatively by those of dimer cores.
Acknowledgment. This work was supported in part by the division of Chemical Science, Office of the Basic Energy Sciences of the United States Department of Energy. References and Notes (1) Saigusa, H.; Sun, S.; Lim, E. C. J . Phys. Chem. 1992,96,2083. (2) Saigusa, H.; Sun, S.; Lim, E. C. J . Chem. Phys. 1992,97,9072. (3) Saigusa, H.; Sun, S.; Lim, E. C. J . Phys. Chem. 1992,96, 10099. (4) Saigusa, H.; Lim, E. C. Chem. Phys. Lett. 1993,211, 410. (5) Johkon, M. A,; Alexander, M. L.;-Lineberger, W. C. Chem. Phys. Lett. 1984,112, 285. (6) Deluca, M. J.; Johnson, M. A. Chem. Phys. Len. 1989,162, 445. (7) Ohashi, K.; Nishi, N. J . Chem. Phys. 1991,95,4002. (8) Ohashi, K.; Nishi, N. J . Phys. Chem. 1992,96,2931. (9) Beck, S. M.;Hecht, J. H. J. Chem. Phys. 1992,96, 1975. (10) Krause, H.; Emstberger, B.; Neusser, H. J. Chem. Phys. Left. 1991, 184, 411. (11) Saigusa, H.; Lim, E. C. J. Am. Chem. SOC., submitted for publication. (12) Sockburger, M.; Gattennann, H.; Klusmann, W. J. Chem. Phys. 1975,63,4519. (13) Duncan, M. A.; Dietz, T. G.; Smalley, R. E. J. Chem. Phys. 1981, 75, 2118.
J. Phys. Chem., Vol. 98,No. 51, 1994 13415 (14) McClure, D. S.; Schnepp, 0. J. Chem. Phys. 1955,23, 1575. (15) Wessel, J. E.; Syage, J. A. J. Phys. Chem. 1990,94,737. (16) Kim, M. S.; Dunbar, R. C. J. Chem. Phys. 1980,72, 4405. (17) Syage, J. A.; Wessel, J. E. J . Chem.Phys. 1987,87,3313. (18) Dunbar, R. C.; Klein, R. J . Am. Chem. SOC. 1976,98,7994. (19) Badger, B.; Brocklehurst, B.; Russell, R. D. Chem. Phys. Lett. 1967, I, 122. (20) Badger, B.; Brocklehurst, B. Trans. Faraday SOC.1969,65,2588. (21) Shida, T.; Iwata, S. J . Am. Chem. Soc. 1973,95,3473. (22) Andrews, L.; Blankenship, T. A. J . Am. Chem. SOC. 1981, 103, 5977. (23) Salama, F.; Allamadola, L. J. J . Chem. Phys. 1991,94, 6964. (24) Szczenpanski, J.; Roser, D.; Personette, W.; Eyring, M.; Pellow, R.; Vala, M. J . Phys. Chem. 1992,96, 7876. (25) Tsuchida, A.; Tsuji, Y.; Ito, S.; Yamamoto, M.; Wada, Y. J . Phys. Chem. 1989,93,1244. (26) Tsuji, Y.; Tsuchida, A.; Yamamoto, M. Macromolecules 1991,24, 4061. (27) Badger, B.; Brocklehurst, B. Trans. Faraday SOC. 1970,66,2939. (28) El-Shall, M. S.; Meot-Ner (Mautner), M. J. Phys. Chem. 1987,91, 1088. (29) Sun, S.; Saigusa, H.; Lim, E. C. J . Phys. Chem. 1993,97, 11637. (30) Meot-Ner (Mautner), M. J . Phys. Chem.1980,84, 2724. (31) Aladekomo, J. B.; Birks, J. B. Proc. R . SOC. London 1965,A284, 551. (32) Birks, J. B. Photophysics ofAromatic Molecules; Wiley: New York, 1970; pp 301-371. Mataga, N.; Kubota, T. Molecular Interactions and Electronic Spectra; Marcel Dekker: New York, 1970; pp 411-484. (33) Nakai, Y.; Ohashi, K.; Nishi, N. J . Phys. Chem. 1992,96, 7873. (34) Killeen, K. P.; Eden, J. G. J . Chem. Phys. 1986,84, 6048. See also Herzberg, G. Annu. Rev. Phys. Chem. 1987,38,27.
