J. Phys. Chem. 1986, 90, 101 1-1014 in the figures as being the predominant radical species. Of course, there is the possibility of H M B reducing either the ortho or para nitro group of T N T to produce a nitroxide function and the spin density distribution would be significantly different in these two structures (vide supra). There are a number of other possible processes that can also be envisioned, such as intra- or intermolecular oxidations of the methyl substituent by a nitro group. Since similar features were also observed in the spectrum of neat T N T in the absence of kieselguhr and since this complication was not evident in the reaction of T N B with HMB, the presence of the methyl group must be a determining factor in the formation of the subordinate radicals during the T N T thermolysis. In an attempt to identify other chemical species present in reaction mixtures of neat T N T , 1-4-g quantities were heated (200-250 OC for varying periods) to produce black, polymeric “explosive coke“. Earlier thin layer chromatographic (TLC) workdahad identified the presence of TNT, tetranitroazoxytoluene, 2,4,6-trinitrobenzaldehyde,picric acid, and seven other unknown components when this material was eluted with polar solvent mixtures. (These workers employed two-dimensional TLC with the first elution using 5:l petroleum ether/acetone and the orthogonal elution using 9: 1:1 petroleum ether/ethyl acetate/ methanol.) We observed color-producing reactions during extraction of the explosive coke with polar solvents (acetone or alcohols), probably due to Meisenheimer-type compound formation.I0 Consequently, we restricted our chromatographic studies to elution with nonpolar solvents of the fraction resulting from Soxhlet extraction of the explosive coke with benzene. TLC on silica gel, with benzene as the eluent, revealed the presence of at least seven mobile aromatic products in this extract. Visualization was accomplished with 5: 1 dimethyl sulfoxide/ ethylenediamine or with ultraviolet light. The two principal components were 4,6-dinitroanthranil and 2,4,6-trinitrobenz( I O ) E. Buncel, A. R.Norris, and K. E. Russell, Q.Rev. Chem. SOC.,22, 123 (1968).
1011
aldehyde which had also been identified by other worker^.^^.^ In addition, a heretofore undetected free radical product was observed in the benzene extract by EPR. This radical did not exhibit a discrete TLC spot but appeared to be associated with the closely spaced anthranil and trinitrobenzaldehyde spots. Column chromatography was unsuccessfully used in an attempt to isolate this radical as a solid although the effluent solution did produce an enormous EPR signal, some four orders of magnitude more intense than others recorded during this study. While this radical was not observed in the EPR spectra of the in situ thermolysis reactions discussed above, its hfs pattern is typical of a nitroxide having a major three-branch splitting (aN = 9.15 Oe), further split into 1:2:1 triplets (a2H= 3.25 Oe). (No further hfs was observed when this material was examined in highly diluted toluene solutions which had been vacuum degassed by repeated freeze-pump-thaw cycles.) This value of ax is significantly lower than any others reported in Table I and suggests that this unidentified radical might be a cyclic nitroxide such as an isatogen derivative’’,’*or perhaps a nitroxide derived from a substituted anthranilic acid;I2 the magnitude of the nitrogen hfs is too small to suggest a nitroxide derived from anthranilI3 and too large to suggest an acyl n i t r 0 ~ i d e . l ~Definitive determination of this structure and that of other T N T decomposition products is currently under investigation.
Acknowledgment. We are grateful to Mr. Richard Vierich and the staff of the University of California, Riverside Physical Sciences Library for use of the Landolt-Bornstein reference works. (1 1) L. Lunazzi, G. F. Pedulli, G. Maccaganasi, and A. Mangini, J . Chem. SOC.E , 1072 (1967). (12) G . A. Russell, C. L. Myers, P. Bruni, F. A. Neugebauer, and R. Blankespoor, J . Am. Chem. SOC.,92, 2762 (1970). (13) H . G. Aurich, G. Bach, K. Hahn, and W. Weiss, J. Chem. Res. ( M ) , 1544 (1977). (14) A. R. Forrester, M. M . Oglivy, and R. H. Thomson, J . Chem. SOC. C, 1081 (1970).
A Time-Resolved Fluorescence Observation of Intramolecular Vibrationally Redistribution within the Channel Three Region of S, Benzene David B. Moss and Charles S. Parmenter* Department of Chemistry, Indiana University, Bloomington, Indiana 47405 (Received: January 6, 1985)
Chemical timing has been used to obtain time-resolved fluorescence spectra from the vibronic levels 6’ l 3 and 7’ within the channel three region of SI benzene. The spectra in each case reveal a dynamic IVR with an initial dephasing time on the order of tens of picoseconds. These results are consistent with previous estimates of rovibronic state decay rates from absorption line widths. The measured IVR rates are approximately two orders of magnitude faster than the electronic state decay observed from the same levels by fluorescence measurements.
Introduction The nature of the so-called “channel three” nonradiative decay of SI benzene has been a longstanding problem in excited electronic state dynamics.’ Channel three is an additional (and fast) nonradiative electronic state decay occurring with a rather steep onset as one climbs through the region 2500-3000 cm-’ of the vibrational ladder of lBzu (SI) benzene. Much of the recent discussion of channel three has dealt with what role (if any) intramolecular vibrational redistribution (IVR) plays in channel three relaxation. Although evidence for the extensive state mixing that is prerequisite to IVR exists in col(1) J. H. Callomon, Faraday Discuss. Chem. Soc., 75, 417 (1983).
lision-free fluorescence spectra (see below), a definitive statement about the presence or absence of a dynamic IVR requires timeresolved m e a s ~ r e m e n t s . ~ -We ~ have therefore begun a fluorescence study using the technique of chemical timing6 to obtain (2) K. Von Puttkamer, H. Dubal, and M. Quack, Faraday Discuss. Chem. SOC.,IS, 197 (1983); M. Quack in Energy Storage and Redistribution in Molecules, J. Hinze, Ed., Plenum, New York, 1983. (3) S. Mukamel and R. E. Smalley, J . Chem. Phys., 73, 4156 (1980). (4) K. F. Freed and A. Nitzan, J . Chem. Phys., 73,‘4765 (1980). ( 5 ) S. Mukamel, J . Chem. Phys., 82, 2867 (1985). (6) R.A. Coveleskie, D. A. Dolson, and C. S. Parmenter, J . Phys. Chem., 89,645 (1985); J . Phys. Chem., 89,655 (1985); K. W. Holtzclaw and C. S. Parmenter, J . Chem. Phys., 84, 1099 (1986).
0022-3654/86/2090-1011$0l.50/00 1986 American Chemical Society
1012 The Journal of Physical Chemistry, Vol. 90, No. 6, 1986 12-
&
&
0
Figure 1. The rate constants for nonradiative decay of single vibronic levels in SI benzene plotted as a function of SIvibrational energy. Open symbols denote measurements of S, electronic state decay, either as fluorescence decay rates (circles; ref 15) or fluorescence quantum yields
(triangles; ref 13). Closed symbols denote measurements of rovibronic state decay from line widths in absorption (circles; ref 10) or line widths in resonant multiphoton ionization (triangles; ref 12). (Note that some of the line width measurements are only upper limits, as indicated by the downward pointing arrows.) The open squares denote our measurement of the dephasing rate constant k , (see text) for IVR from the levels 7' (c,,, = 3077 cm-') and 6'13 (3288 cm-I). explicit evidence concerning IVR dynamics from levels within the channel three region. This report presents our first results. A number of previous experiments concerning channel three sets the stage for our work. The early description of channel three appeared in connection with the first report concerning tuned single vibronic level (SVL) excitation of a polyatomic molecule, the molecule being benzene.' Whereas S V L fluorescence spectra from SI levels up to t b , b = were obtained with reasonable S", 2370 cm-', no spectra were produced by tuning to a level just 235 cm-I higher, or to other levels ranging up to tVlb= 3600 cm-I. It was thus evident that some process decreased the fluorescence yield precipitously over a relatively narrow vibrational energy range. Quantitative measures of S V L fluorescence yields soon confirmed the onset and the persistence of channel three from higher SI The most impressive revelation of channel three, however, has come from measurements of lifetimes of SI levels within the channel three region. The results of lifetime studies from four laboratories are compared in Figure 1. Such a plot is the contemporary view of channel three dynamics. It shows dramatically the rapid onset of this nonradiative decay. Channel three lifetimes were first determined quantitatively by the classic work of Callomon, Parkin, and Lopez-Delgado.'O They observed that the characteristic sharp rotational structure in the high-resolution lBzu 'Ag absorption spectrum" becomes diffuse for most absorption bands involving levels above the channel three threshold. (Exceptions were levels that contained one quantum of a C-H stretch, mode v,.) The diffuseness was interpreted in terms of lifetime broadening and estimates were made of the rovibronic line widths necessary to produce the diffuseness. The equivalent nonradiative decay rate constants were typically more than three orders of magnitude greater than those determined by direct fluorescence lifetime measurements for levels below the channel three threshold. With the completely different technique of resonant multiphoton ionization, Johnson and co-workersI* have
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(7) M . W. Schuyler and C. S. Parmenter, Transitions NonRadiat. Mol., Reun. SOC.Chim. Phys. ZOth, 1969, 92-100 (1970) (Suppl. to J . Chem. Phys.). (8) Michael W. Schuyler, Ph.D. Thesis, Indiana University, 1970. (9) K. G . Spears and S. A. Rice, J . Chem. Phys., 55, 5561 (1971). (10) J. H. Callomon, J. E. Parkin, and R. Lopez-Delgado, Chent. Phys. Lett., 13, 125 (1972). ( I 1) J. H. Callomon, T. M . Dunn, and I . M. Mills, Phil. Trans. R . SOC. London, Ser. A , 259, 499 (1966). ( 1 2 ) K . Aron. C. Otis, R. E. Demarey, and P. Johnson, J . Chem. Phys., 73. 4167 (1980).
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Letters
measured SI So absorption line widths for the two-photon transitions 14A1: (n = 0-5). They found widths quite similar to those of Callomon et al. for rovibronic levels within the channel three region. Measurements based on fluorescence decay have yielded other sets of channel three SI decay constants. Schlag and co-workers13 used the integrated yield of the (weak) fluorescence from the higher two-photon levels (14'1" with n = 2-5) to extract rate constants for the decay of the SI electronic state. Otis et a l l 4 have measured SI depopulation rates via resonant multiphoton ionization to obtain nonradiative SI decay rates for two levels within the channel three region (7l and 6Il3). Yoshihara and co-workersI5 have extended SI decay measurements to more than 20 levels reaching well above the channel three threshold by direct determinations of fluorescence lifetimes. The nonradiative decay constants in Figure 1 fall into two distinct sets. Those that measure decay of the SI electronic state (Schlag,I3 Yoshihara,Is and also the depopulation measurements of JohnsonI4) give nonradiative rate constants in the range IO' to 1O1OS K I . (They are plotted with open symbols.) On the other hand, the line width measurements of Callomonlo and Johnson'* (closed symbols) monitor decay of individual S I rovibronic states. These decays are much faster with rate constants in the range 1011-1012 s-I. In fact, the sets generally differ by about two orders of magnitude for a given level or energy region. The obvious conclusion, stated by several of these researche r ~ , is~that ~ -there ~ ~ is some relaxation process taking place within the SI state on a time scale significantly shorter than that for decay of the Si electronic state. Thus line width studies indicate the faster nonradiative decay. The most logical candidate for this relaxation process is intramolecular vibrational redistribution (IVR). Much indirect evidence can be found to support such a proposal. Resolved fluorescence from levels within the channel three region contains a congested background that is characteristic of extensive vibrational level mixing in the excited state.I6 Resonant ionization photoelectron spectra show similar congestion which is consistent with ionization from a vibrationally mixed intermediate state." Perhaps the most convincing evidence is the sub-Doppler twophoton excitation spectrum of the 1461; band obtained by Riedle, Neusser, and Schlag.'* In this spectrum, only levels with K = 0 are observed to fluoresce with appreciable intensity. It is apparent that a rotationally selective nonradiative process is occurring, presumably Coriolis vibration-rotation coupling that mixes isoenergetic vibrational levels within the S I electronic state. Our experiments are intended to provide more direct information about IVR within the channel three region by using time-resolved fluorescence spectra to search for IVR after pumping each of two levels, 6'13 and 7' ( t V l b= 3288 and 3077 cm-', respectively). The level 7l is of particular interest since some have s ~ g g e s t e d ' ~that , ' ~ it is not involved in IVR. Experimental Section The experimental apparatus is only slightly modified from that described previously.6 The primary change is the replacement of the temeprature-tuned A D P crystal by an angle-tuned KPB crystal for frequency doubling of the dye laser. The lower efficiency of the KPB crystal resulted in an increased interference from scattered visible light. Discrimination against this light was (13) L. Wunsch, H. J. Neusser, and E. W. Schlag, Z . h'arurforsch. A . 86, 1340 (1981). (14) C. E. Otis, J . L. Knee, and P. iM. Johnson, J . Phys. Chem., 87, 2232 (1983). (15) M. Sumitani, D. V. OConnor, Y . Takagi, N. Nakashima, K. Kamogawa, Y. Udogawa, and K. Yoshihara, Chem. Phys., 93, 359 (19S5). (16) D. O'Connor, M. Sumitani, Y . Takagi, M. Nakashima, K . Kamogawa, Y . Udagawa, and K . Yoshihara, J . Phys. Chem., 87, 4848 (1983). (17) Y. Achiba, A . Hiraya, and K. Kimura, J . Chem. P h j s . , 80. 6047 (1984). (18) E. Riedle, H. J. Neusser, and E. W. Schlag, J . Phys. Chem.. 86.4847 (1982). (19) M. Sumitani, D. V . O'Connor, Y . Takagi, and K . Yoshihara. Chejn. Phys. L e t t . , 108. 1 1 (1984).
The Journal of Physical Chemistry, Vol. 90, No. 6,1986 1013
Letters
5 , psec
1c
0.450
0.350
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.
l
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4 1
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35,000 cm-1
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Figure 2. Time-resolved fluorescence spectra from the level 6'1' (t,ib = 3288 cm-') in SI benzene. The mean fluorescence lifetime and the pressure of 0, used to achieve that lifetime (in parentheses) are indicated to the left of each spectrum. The benzene pressure is 2 torr in the unquenched spectrum and 50 torr in the quenched spectra. Fluorescence resolution is -60 cm-'. The dashed lines show some of the corresponding structure in the three spectra.
provided by placing a visible absorbing filter (Corning 7-54) in front of the monochromator entrance slit.
Results Time-resolved fluorescence spectra have been obtained from the levels 6'13 and 7' in SIbenzene by addition of the excited-state quencher 02.Some of the timed spectra for 6 I l 3 are presented in Figure 2 . They are quite reminiscent of the timed spectra previously obtained from p-difluorobenzene6 (pDFB) and p fluorotolueneZ0(pFT). Structure present in the absence of timing is assignable to the zero-order level populated in absorption.16 This structure is enhanced and only slighly perturbed as timing is imposed by added 02.The dashed lines of Figure 2 show that the structure in the timed spectra is essentially identical with that in the untimed spectrum. The spectra from the 7' level (not shown here) behave similarly, except that an initial decrease in structure is produced by collision-induced vibrational relaxation, followed by an increase in structure as the 0,pressure is further increased. This competition between quenching and vibrational relaxation is a well-understood aspect of the method.6 The analysis of chemically timed spectra has been discussed elsewhere.6 Briefly, the spectra are divided into structured and unstructured components, the two components are integrated, and (20) D. A. Dolson, C . S. Parmenter, and B. M. Stone, Chem. Phys. Lett., 81, 360 (1981).
TABLE I: IVR Parameters for Two Levels in S, Benzene Obtained from Fitting Time-Resolved Fluorescence Data to the Kinetic Model of Ref 6 k., k,,,, 1Olo lo6 torr-' lolo dm3 k+F 1OIo level s-I N s-I mol-' s-' S-1 T w n b PS 6'1'
71
5.1 3.3
4.1
4.8
1.3
6.7
8.9 12.4
6.3 5.8
16 17
" k + is the initial dephasing rate constant, given by k+ = k,,,(N + l)/N. *T,,, = l / k + is the most accurate representation of an IVR lifetime. It corresponds to rovibronic line widths.
the ratio of structured to total fluorescence (S/T) is obtained as a function of O2 concentration. These data are then compared to a kinetic model derived from radiationless transition theory. The comparison requires the oxygen-benzene quenching rate constant (&), which we estimate to be 8 X lo6 torr-' s-l from data of White2' and of Brown and Phillips.22 It also requires the collision-free SI electronic decay rates (kl), which were taken from Yoshihara and co-workers.15 The S / T data and best fits to the model for both levels are shown in Figure 3. From those fits, we derive the IVR rate constant (k,,,), the average number of coupled levels participating in IVR (N), and the 0,-benzene vibrational relaxation rate constant ( k J . All are presented in Table I.
Discussion The transformations of SVL fluorescence spectra upon timing are completely consistent with the expectations for a dynamic IVR and with comparable IVR studies in pDFB and pFT. It is thus certain that a dynamic IVR is associated with the decay of the SI levels 7l (3077 cm-I) and 6'13 (3288 cm-I), both lying within the channel three region. The analysis of these spectral changes (21) Anne H. White, Ph.D. Thesis, lndiana University, 1973. ( 2 2 ) R. S. Brown and D. Phillips, J . Chem. SOC.,Faraday Trans. 2, 70, 630 (1974).
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The Journal of Physical Chemistry, Vol. 90, No. 6, 1986
gives IVR lifetimes of about 20 ps for each level (Table I). As a check on our procedures, we note that the values of the derived parameters in Table I are in reasonable correspondence with results of similar experiments. The IVR rate constants fall within the range of those observed for pDFB ((0.2-6.2) X 10" s - ' ) . ~ The values of k, are also within the range found in that study, albeit at the low end. In addition, our k, values also compare well with those obtained directly by Tang and Parm e r ~ t e rfor ~ ~relaxation of benzene by CO collisions for levels with = 1500-2000 cm-I. Finally, the values for N reflect, as they the S / T ratio in the untimed spectra. Our benzene IVR rate constants become most interesting when compared with the rate constants for relaxation of single rovibronic levels that come from line width measurements. The correct parameter for comparison is not k,,, itself but rather the initial dephasing rate constant k+ = k,,,((N l)/N). It is this dephasing rate that corresponds to the level width.6 Plotting our IVR results in the form of k , along with the decay constants of others in Figure 1 allows us to assess our findings. It is seen that the chemical timing IVR results (represented by open squares on the plot) are in agreement with the rovibronic state decay measurements. The value we obtain for 6'13 ( k , = 6.3 X 1O'O s-I) is indistinguishable from that of Callomon et a1.I' ( k n r= 5.6 X 1O'O s-I), considering the uncertainty in the two measurements. Although the value obtained here for 7' ( k , = 5.8 X IO'" s-l) exceeds the upper limit imposed by Callomon et al. (3.2 X 1O'O s-l), it is not so far above that these two measurements are necessarily outside of the respective experimental errors. The match between the IVR dynamics and line widths for these levels provides strong evidence that the broad homogeneous line widths observed in the studies of Callomon'' and Johnson'* indeed occur on account of IVR lifetime broadening of individual rovibronic transitions. We plan to expand these comparisons with a more complete set of measurements for additional vibrational levels both above and below the channel three threshold. IVR is only one of the two distinct nonradiative decays represented in Figure I . Historically, both decays have been discussed
+
(23) K . Y. Tang and C. S. Paramenter, J . Chem. Phys., 78,3922 (1983). (24) D. A . Dolson, K. W. Holtzclaw. D.B. Moss, and C. S. Parmenter. J . Chem. Phys., 84, 1119 (1986).
Letters together as "channel three". The second decay is the slower process of electronic state destruction as seen in the fluorescence lifetime measurements. The data in figure 1 suggest that both have similar thresholds, but debate continues on whether the two are connected. Several authors have proposed that the accelerated electronic state decay occurs on account of internal conversion to the So manifold, enhanced by excitation of "promoting modes" through IVR.'4J7,25 In contrast, Yoshihara and c o - ~ o r k e r s 'have ~ suggested that the electronic state channel three decay, which involves all levels with tVib > 2800 cm-', and IVR are unrelated. Their argument is based on the assertion that static fluorescence spectra from certain channel three levels, namely 7'1",contain no evidence of IVR. Our new data do not answer the general question of whether the two channel three decays are interrelated. Our data do, however, comment on the contention that the 7' level is aloof from IVR. First, we would find it difficult, based on the static spectra alone, to suggest that the 7'Inlevels are in general not involved in extensive vibrational mixing. The collision-free spectra of this study, as well as those published earlier,'6,'9*26 show a significant congested background, a necessary prerequisite for IVR and a characteristic that in other systems is associated with a dynamic IVR.6,27-29 The time-resolved spectra reported here, however, leave no doubt that 7l is vibrationally mixed and will, under conditions of coherent excitation, undergo IVR. Even in the absence of coherent excitation, however, the vibrational mixing will be present and could lead to enhanced internal conversion though the "promoting mode" character of the mixed state. Although this does not prove a connection between IVR and channel three, such a possibility cannot be dismissed.
Acknowledgment. We are grateful to the National Science Foundation and to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for financial support. D.B.M. also acknowledges fellowship support from the Procter & Gamble Company during part of this work. (25) H. Hornburger and J. Brand, Chem. Phys. Lett., 88, 153 (1982). (26) A. E. W . Knight, C. S. Parmenter, and M. W. Schyuler, J . A m . Chem. Soc., 97, 2005 (1975). (27) C. S . Parmenter, J . Phys. Chem., 86, 1735 (1982). (28) R. M. Hochstrasser and R. Moore, Chem. Phys. Left., 105, 359 ( 1984). (29) P. M. Felker and A. H. Zewail, J . Chem. Phys., 82, 2975 (1985).
