J. Phys. Chem. 1981, 85, 3739-3742
1 eV), the energy difference between the two emitting states could be small if there is a large energy difference between the corresponding ground states. This is supported by various calculations15J7~19 which showed the ground-state energy of the twisted form was higher than that of the planar form by about 0.7 eV. The fast equilibration rate reported here therefore is not inconsistent with energetic considerations. The possibility that the short wavelength emission is due to S2 fluorescence, as suggested by other authors for a similar compound p-(N,N-dimethy1amino)benzoicacid methyl ester,18J9can be eliminated for DMABN molecule based on our new findings. We have shown that the state responsible for the short wavelength emission reaches equilibrium with the CT state within 20 ps, and then both decay with a lifetime of 2.3 ns. Since this 2.3-11s lifetime includes all the radiative and nonradiative decay channels for both states other than the equilibration process, it is highly unlikely that the S2-S1 vibronic relaxation rate can be slower than 2.3 ns. We therefore conclude that the short wavelength emission is due to the S1 state which borrows
3739
intensity from S2 by vibronic coupling.
Conclusions We have studied the dual fluorescence of DMABN molecule in propanol. The existence of an equilibrium between the emitting states, arising from different geometries, is demonstrated by our data. The sum of the forward and backward reaction rates is 19 f 3 ps with the forward rate estimated to be 20 ps. This latter value, 20 ps, is interpreted as the time required for the molecule and solvent to relax conformationally to the twisted chargetransfer geometry. The lifetimes of both excited states after equilibration are 2.3 f 0.2 ns. The short wavelength emission is due to the S1 state which is vibronically coupled to the close-lyingS2state and is not due to direct emission from S2. Acknowledgment. We gratefully acknowledge the support of the Air Force Office of Scientific Research, the National Science Foundation, and the Joint Services Electronics Program (DAAG29-79-007921011).
Mass-Selective Two-Color Photoionization of Benzene Clusters J. B. Hopklns, D. E. Powers, and R. E. Smalley" Rice Quantum Institute and Department of Chemistry, Rice Univers/& Houston, Texas 7700 1 (Received: Juk 24, 198 1; I n Final Form: August 26, 198 I )
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Using two-color mass-selective photoionization,we have obtained the S1 Soexcitation spectrum of the benzene dimer, trimer, and tetramer in a supersonic beam. All clusters show a weakly induced 0; band red shifted from the forbidden monomer origin by an amount which increases monotonically with cluster size. The 66 band for all clusters is split into two features corresponding to a crystal-field-induced lifting of the degeneracy of this e2gbenzene vibration. Exciton splitting of the 0; band in these species was found to be less than 6 cm-l. The trimer and tetramer spectra show strong progressions in van der Waals modes which ultimately correspond to the optically active phonon side bands in the bulk benzene crystal. Excited-statelifetime, fluorescence quantum yield, and band profile measurements indicate the dimer rearranges from a T-shaped geometry into a sandwich excimer within several picoseconds after laser excitation from the ground T-shaped van der Waals well. This process does not occur readily in the higher clusters.
Introduction One of the principal advantages of the newly emergent technique of resonant two-photon ionization (R2PI)I is the ability to produce the molecular ion without fragmentation. This mass information is particularly valuable when the spectrum to be probed is for one species in a mixture of others with similar spectral properties. Perhaps the most demanding such application of R2P1 is the study of individual members of a distribution of molecular van der Waals clusters such as the benzene species discussed in this paper. Here it is particularly important that fragmentation not accompany the ionization process so that the complicated spectral features arising from higher clusters not appear as well in the spectrum of the particular cluster under study. (1) (a) T.G.Dietz, M. A. Duncan, M. G. Liverman, and R. E. Smalley, J. Chern.Phys., 73,4816 (1980); (b) M.A. Duncan, T. G. Dietz, and R. E. Smalley, ibid.,in press; (c) M. A. Duncan, T. G. Dietz, M. G. Liverman,and R. E. Smalley, J. Phys. Chern., 85, 7 (1981); (d) T.G.Dietz, M. A. Duncan, and R. E. Smalley, J. Chem. Phys., submitted for pub-
lication.
For ordinar molecules such as the alkylbenzenesl" or naphthalene,': resonant two-photon ionization cleanly produces the parent ion as long as the lasers are not sufficiently intense to fragment this ion after its initial generation. This continues to be true even if the R2PI excitation reaches far above the ionization threshold to energies where ion fragmentation would be possible. Fragmentation does not occur in these cases because the geometry of the parent ion is nearly the same as that of the neutral molecule and the Franck-Condon factors for photoionization strongly favor AIJ = 0 transitions. Photoionization of vibrationally cold neutrals therefore produces vibrationally cold ions which cannot fragment, and most excess energy involved in the R2PI process is taken off as translational energy of the electron. The situation is not nearly so favorable in the case of van der Waals clusters, however. The neutral cluster is bound (at least asymptotically) by weak instantaneous dipole-induced dipole dispersion forces while the parent ion of this cluster is bound much more strongly by monopole-induced dipole forces. In this case the Franck-
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Condon factors for photoionization favor large changes in vibrational quanta of the intermolecular modes and a large fraction of the excess energy can end up in vibrational degrees of freedom of the complex ion. In order to avoid fragmentation of this ion, one must arrange the R2PI lasers such that excitation reaches just above the ionization threshold. Unfortunately, for such clusters the absorption cross section near threshold is not a sharp step function,2 but shows rather a gradual rise for several thousand cm-l before leveling off in the range of 10-la to lo-'' cm2. Even so, it has been possible in the following experiment with the clusters of benzene to configure a two-color R2PI scheme such that parent ion mass selectivity is largely preserved. The spectral results of this scheme have much to say both about the future of the technique and about the particular issues surrounding the benzene dimer, excimer, and higher clusters.
Experimental Study The supersonic beam apparatus used in this work was similar to that discussed in earlier publications from this group on bromobenzenela and the benzene triplet lifetime.lc In order to produce a stable source of benzene clusters with a repeatable distribution of cluster sizes, a continuous nozzle was used identical in design with that employed earlier in our alkylbenzene works3 The free jet produced by this source (50-pm diameter orifice operated at 20 "C with a 6.5-atm backing pressure of 0.2% benzene in helium) was skimmed by a 10 cm long 30" included angle conical skimmer (Beam Dynamics Corp., Minneapolis, MN) located 3 cm downstream on the expansion axis. The nozzle expansion chamber was evacuated by a Varian VHS 10 diffusion pump to a steady-state pressure of -2 X torr, and the skimmed beam passed into a second chamber evacuated by a VHS 6 diffusion pump which in turn maintained a steady pressure of 2 X torr while the beam was on. This second chamber housed the flowthrough time-of-flight mass spectrometer (TOF MS) described in our earlier experiments.' The ionization region of this TOF MS was centered on the molecular beam axis, 33 cm downstream of the nozzle. Two Quanta-Ray Nd: YAG pumped dye laser systems provided the ultraviolet excitation and ionizing laser beams which overlapped at the intersection with the molecular beam in the center of the TOF MS ionizing region. The time delay between the two laser probes was computer controlled with an Evans Associates Model 4145-2 digital delay generator. By varying the time delay between the pump and probe (ionizing) laser and by monitoring the two-color photoion signal, we found it possible to determine the excited-state lifetime for the various benzene clusters. In order to correct for pulse-to-pulse timing jitter the exact time of arrival of each laser pulse was monitored with two RCA IP28 phototubes which activated two fast rise-time compensated discriminators (Lecroy 825) feeding a Lecroy 2228A CAMAC-based time-to-digital converter (TDC). The resulting lifetime data are influenced only by the overlap of the -3-115 pulse width of each lasere4 Results and Discussion Since the S1 state of benzene and its clusters is more than half the energy of the first ionization potential, IP1, (2) For typical photoionization threshold behavior of van der Waals clusters, see Y. Ono, S. H. Linn,H. F. Prest, M. E. Gress, and C. Y. Ng, J. Chem. Phys., 73, 2523 (1980). (3) J. B. Hopkins, D. E. Powers, and R. E. Smalley, J. Chem. Phys., 72. -,5039 - - (1980).
Letters
25, 1981
(4)J. B: Hopkins, D. E. Powers, and R. E. Smalley, J. Chem. Phys., 73, 683 (1980).
-200 -150 -100 -50 0 RELATIVE FREQUENCY (cm-') Flgure 1. One-color RPPI spectrum of the benzene cluster beam
monitoring the photoion signal in the benzene dimer channel. The frequency scale is relative to the monomer orlgin at 38 080.1 cm-'. Note by comparison to Figure 2 that most spectral features in this scan result from fragmentation of the trimer into the dimer signal channel.
0
0,
BAND
TRIMER
-200 -150 -100 -50 0 50 RELATIVE FREQUENCY ( cm-'
-
Flgure 2. Twocolor RPPI scan of the absorption bands of the benzene dimer, trimer, and tetramer In the region of the 'B.&T*) '% orlgln of benzene monomer. The intensity for each spectrum has been normalized here to a constant level. Actualty, the two-color observed peak photoion signals were in the ratio 1:0.05:0.03 for dimer:trimer:tetramer, respectively. The zero of the relative frequency scale is set to the position of the forbidden origin of the benzene monomer at 38 086.1 cm-'. The red shifts of the most prominent features from the monomer origin are -40, -115, and -149 cm-' for the dimer, trimer, and tetramer, respectlvely. The strong features in the trimer spectrum centered 22 and 40 cm-' to the blue of the origin are due to progression activity in the van der Waals modes of the cluster. Similar features appear in the tetramer spectrum and constitute the main observed optical phonon modes in the bulk benzene crystal.
the laser that excites this state can also cause efficient ionization in a sequential R2PI process. For the benzene monomer, the second laser photon reaches 2300 cm-' over IP1, while for the higher clusters at least 4500 cm-' of excess energy is available. As discussed in the Introduction, this results in substantial fragmentation in the photoion product distribution. Figure 1 illustrates what is obtained in such a one-color R2PI probe of the spectrum of the benzene cluster beam. Even though only the dimer mass channel is monitored here, most of the spectral features observed are in fact due to the trimer. In order to minimize this fragmentation it is necessary to convert to a two-color R2PI scheme where the ionizing laser is tuned to excite closer to the ionization threshold. Figure 2 shows the effect of such a scheme on the dimer spectrum of benzene. Here the ionizing laser was a dou-
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TABLE I : Two-Color RPPI Measurements o f the S, Lifetimes of the Benzene Clusters
BAND
S, lifetime: cluster (C6H6)1 (C6H6)2
(C,H,),
ns
OD
6l
102.gb
81.5b
36.6 * 1.2 81.3 i- 3.8
40.4 i- 2.4 51.5 f 2.4
Indicated errors are estimated 95% confidence limits. Values t a k e n f r o m t h e fluorescence measurements o f ref 5. a
b
DIMER
l
"
'
~
l
"
~
~
appears to be abnormally low, therefore. Table I presents the result of lifetime measurements of the excited S1 states of these clusters obtained by varyin the time delay between the S1pump laser and the 2789ionizing laser. Note that the lifetime of the dimer Oo level is much lower than that of the monomer' or any higher property of the dimer is that ~cluster.g ' l ~ Another ~ ~ ~unique l ' ~ the two-color R2PI signal for this species was found to be 20 times larger than that of the trimer even though Figure 1 shows that the dimer and trimer concentrations are similar in the molecular beam. The absorption cross section for the ionizing laser transition from the S1 state into the ionization continuum near threshold appears therefore to be much larger for the dimer than the trimer. These data are consistent with the following simple and appealing model. The van der Waals dimer of benzene has long been believed to be T shaped. This is the geometry of the closest interaction in the benzene crystalg and explains nicely the observation by Janda et al.1° that the dimer has a substantial dipole moment as well as our observation that the 6l vibration is split in the dimer by nearly the same amount it is split in the crystal,'lJ2 and that the 0: band is induced nearly as strongly in the dimer as it is in the crystal.'l The simple absorption spectrum we see for the dimer, shifted only 40 cm-l from the monomer bands, indicates that the van der Waals well in the T-shaped region of the S1state is nearly identical with that of the ground state. This impression is further reinforced by the absence of progressions in the van der Waals modes of the dimer. However, even though the dimer is initially excited into a T-shaped van der Waals well, the abnormal properties observed here for the dimer S1 state are characteristic of a far different arrangement where the two rings have formed a "sandwiched" excimer with net D6h (or &) symmetry. Such an excimer state of benzene has been extensively studied in solution,13 and has been the subject of several calculation^.^^-^^ Since the lowest excimer state is of lB1, symmetry, the radiative rate is quite low-the transition to the lAl, ground state being allowed
Bf
l
'
~
~
~
l
~
400 500 RELATIVE FREQUENCY ( c m - ' )
300
Figure 3. Two-color R2PI scan of the absorption bands of small benzene clusters In the region of the 6; band of the benzene monomer. The frequency scale is relative to the monomer origin at 38 086.1 cm-I. The photoion intensity scale for each scan has been adjusted by using the same normalizing factors as In Figure 2. The midpoint of the observed 4-cm-' splitting of the 6; band of the dimer is measured to be 521 cm-' from the 0; band shown in Figure 2. This is within experimental error of the known monomer 6' interval of 522.4 cm-' and is close to the crystal value of 519 cm-I (see ref 11 and 13). The corresponding features of the trimer and tetramer appear at 522 and 518 cm-I, respectively, when measured from their origin features as seen in Figure 2.
bled rhodomine 6G dye laser at 2789 A,producing a fluence of 100 mJ ern-?- in the ionizing region of the TOF MS. In order to optimize the contrast between the one- and two-laser signal, the S1 excitation laser was kept at the relatively low fluence of 1mJ cm-2. Under these conditions the dimer signal enhancement due to the addition of the ionizing laser field was a factor of 20. Also shown in Figure 2 is the spectrum obtained by this two-color technique while monitoring photoions produced in the trimer and tetramer mass channels. As seen in the figure, the two-color technique has largely succeeded in preserving the mass selectivity required to study the spectrum of each cluster separately. Figure 3 shows the same information for the next strong spectral band of the clusters: the 6; band -520 cm-l to the blue of the origin. Careful scans of the intervening regions have revealed no features stronger than 5% of the 0: intensity. The 0; band is induced by asymmetry in the "crystal field" of the benzene clusters. During a previous free-jet experiment in our group5we had searched for laser-induced fluorescence signals from the benzene dimer. We were surprised that this experiment failed even though we were confident that substantial conversion to benzene clusters was occurring in the free jet. Recently, however, Langridge-Smith et al. have succeeded in seeing weak laser-induced fluorescence features by this techniquea6 The spectral features they see correspond well to what is reported here for the trimer and tetramer. Remarkably, the fluorescence excitation spectrum of the dimer appears to be weaker in their spectrum by at least an order of magnitude than that for the trimer and tetramer. The dimer fluorescence quantum yield (5) S. M. Beck, M. G. Liverman, D. L. Monts, and R. E. Smalley, J. Chem. Phys., 70, 232 (1979). (6) P. R. R. Langridge-Smith, D. V. Brumbaugh, C. A. Haynan, and D. H. Levy, J.Phys. Chem., 85, 3742 (1981).
'
(7) (a) W. M. Gelbart, K. G. Spears, K. F. Freed, J. Jortner, and S. A. Rice, Chem. Phys. Lett., 6, 345 (1970); (b) K. G. Spears and S. A. Rice, J. Chem. Phys., 55, 5561 (1971). (8) To the best of our knowledge the fluorescence lifetime of benzene in a cold benzene crystal has never been accurately measured. In a wide variety of other cold matrices, the lifetime is -100 ns. See E. P. Gibson, R. Narayanaswamy, A. J. Rest, and K. Salisbury, J. Chem. SOC., Chem. Comrnun., 20, 925 (1979). (9) E. Cox, Rev. Mod. Phys., 30, 159 (1958). (10) K. C. Janda, J. C. Hemminger, J. S. Winn, S. E. Novick, S. J. Harris, and W. Klemperer, J. Chem. Phys., 63, 1419 (1975). (11) (a) V. L. Broude, Pure Appl. Chem., 37, 21 (1974); (b) V. L. Broude, Opt. Spectrosc., 30,46 (1971); (c) V. L. Broude, Sou. Phys. Vsp.,
4, 584 (1962). (12) E. R. Bernstein, J. Chem. Phys., 50, 4842 (1969). (13) R. B. Cundall and S. McD. Ogilvie in "Organic Molecular Photophysics", Vol. 2, J. B. Birks, Ed., Wiley-Interscience, New York, 1975, Chapter 2. (14)M. T. Vala, I. H. Hillier, S. A. Rice, and J. Jortner, J. Chem. Phys., 44, 23 (1966). (15) A. K. Chandra and E. C. Lim, J. Chem. Phys., 49,5066 (1968). (16) T. Azumi and H. Azumi, Bull. Chem. SOC. Jpn., 39,2317 (1966).
J. Phys. Chem. 1981, 85.3742-3746
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only through vibrational excitation. The combination of a somewhat enhanced nonradiative decay with this much-decreased radiative rate produces a fluorescence quantum yield which is much reduced from that of the monomer. Apparently, the T-shaped dimer S1 state initially populated by the pump laser is only a local minimum on the upper state hypersurface. The excited dimer is then free to rearrange over a low barrier (less than the zero point energy) into the deeper excimer well. In fact, the excitation profile of the dimer 0; bands does appear to be broadened to -4 cm-' (fwhm) by such a nonradiative process occurring on a picosecond timescale (see Figures 1-3). Since the (cGH&+ ion is also likely to have a parallel sandwich geometry, the Franck-Condon factors for the ionizing laser transition near the ionization threshold are expected to be far higher when the S1 state is in the excimer well as compared to the T-shaped van der Waals well, thus explaining the higher two-color ionization cross section for the dimer compared to the trimer. The most likely configuration of the van der Waalsbound trimer is a triangular pinwheel, if one assumes the same sort of bonding interactions are active in the trimer as in the dimer. Such a structure must have a substantial activation barrier toward excimer formation since one of the three major van der Waals bonds must be broken to allow rotation of one ring until it is parallel to another.
Absence of sufficient activation energy prohibita the trimer from entering the excimer region of the S1 hypersurface (at least for 0: excitation). Similar arguments will apply to the tetramer and higher clusters. The strong activity of progressions in the van der Waals modes of the higher clusters-particularly in the trimer-indicates the SI surface in these clusters is somewhat different from the ground-state surface. Even though these higher clusters may adopt a slightly different geometry in the S1state, the measurements of the fluorescence lifetime and quantum yield as well as the large difference in ionization cross section near threshold compared to the dimer suggest quite strongly that the larger clusters do not relax into a sandwich excimer well.
Acknowledgment. We thank D. H. Levy and P. R. R. Langridge-Smith of the University of Chicago for helpful and stimulating discussions, particularly with regard to their fluorescence data on the benzene clusters and the possibility (and consequences) of excimer formation. The initial single-color photoionization observation of benzene clusters was performed in our group by M. A. Duncan and T. G. Dietz. This research was supported by the National Science Foundation and The Robert A. Welch Foundation. Acknowledgment is also made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research.
Ultraviolet Spectra of Benzene Clusters Patrick R. R. Langridge-Smith,+ Donald V. Brumbaugh, Christopher A. Haynam, and Donald H. Levy" The James Franck Institote and The Department of Chemlstry, The Unlverslty of Chlcago, Chicago, Illinols 60637 (Recelved: August 26, 1981; I n Flnal Form: October 19, 1981)
-
Clusters of benzene have been prepared in a supersonic expansion, and their lBzu lAl, ultraviolet spectra have been observed. The fluorescence excitation spectra of the clusters have features at the 6; vibronic band and at the origin. Dimers, trimers, and tetramers show sharp structure in the fluorescence excitation spectrum, while larger clusters produce a broad, unresolved band. The dispersed fluorescence spectra of the clusters is the same for excitation to either the 6' or the origin level. This is due to intramolecular vibrational relaxation from the mode into the low-frequency cluster vibrations. The dispersed fluorescence spectrum is broad even following excitation to the origin, and this broadening is interpreted as evidence for relaxation to an excimer state following excitation to a more weakly bound van der Waals state. Benzene shows anomalously little vibrational cooling in a supersonic expansion.
Introduction In a recent paper we described the fluorescence excitation spectrum of the gas-phase dimer of dimethyltetrazine (DMT).l The origin of the dimer spectrum was shifted by 548 cm-' to the low-energy side of the origin of the monomer spectrum, and a new progression with w,' = 78.7 cm-l, w p i = 1.21 cm-l was observed. The spectral shift gives the increase in dimer binding energy when the dimer is electronically excited, and the new vibrational progression was interpreted as the stretching motion of the two halves of the dimer. The overall interpretation was that in the excited electronic state the dimer was somewhat more tightly bound with a somewhat shorter bond distance. ~~
~~
NATO Postdoctoral Fellow.
This behavior is to be contrasted to that of aromatic hydrocarbons whose solution spectra have been studied as a function of concentration. Essentially all aromatic hydrocarbons are observed to form excimers a t higher concentration, these excimers being observed as strongly red-shifted bands in the fluorescencespectrum.2 Excimers have been described as dimers which are unbound (neglecting van der Waals forces) in their ground electronic state but rather tightly bound in their excited electronic state. The red shift in the fluorescence spectrum provides a measure of the excited-state binding energy, and this binding energy is thought to be several thousand ~ m - l , ~ (1) D. V. Brumbaugh, C. A. Haynam, and D. H. Levy,J. Chern. Phys., 73, 5380 (1980). (2) J. B. Birks, Rep. h o g . Phys., 38, 903 (1975).
0022-3654/81/2085-3742$01.25/00 1981 American Chemical Society
