J. Phys. Chem. 1981, 85, 7-9
Kreevoy6 suggest that large barriers lead to values of 4 greater than 0.5. Preliminary calculations on the potential energy surface of (MeOH0Me)-, using the approximate molecular orbital method, PRDD0,20also suggest either no central maximum or a very small maximum for the hydrogenic coordinate. As in the case of the model used by Kreevoy,6this surface only deals with the linear onedimensional motion of the proton in the well. For detailed theoretical calculations of 4@,it is perhaps more appropriate to examine K5explicitly and we intend to pursue such studies when good potential energy surfaces for the proton-bound dimer are available. We are presently completing experiments to determine 4 for (MeOH0Me)- and (MeOH0Et)-. Preliminary results
7
suggests that the value of 4 for these species are somewhat lower, on the order of 0.2. We are also pursuing more extensive calculations of the potential surface including bending modes in order to develop a more realistic model for the gas-phase isotope exchange equilibria in protronbound dimers.
Acknowledgment. We acknowledgepartial support from the Graduate School of the University of Minnesota. Mark R. Ellenberger acknowledges the support of a Eastman Kodak Predoctoral Fellowship. We acknowledge the assistance of Monica Hendewerk on some of the tetramethylsilane experiments and thank Professor M. M, Kreevoy for many helpful discussions.
Photoionization Measurement of the Triplet Lifetime of Benzene M. A. Duncan, T. G. Dietz, M. 0. Liverman, and R. E. Smalley” Rice Quantum Institute and Deparrment of Chemistty, Rice UniversiQf,Houston, Texas 7700 1 (Received: September 22, 1980)
The lifetime of triplet benzene produced by laser excitation of the 6l level of the SI state in a supersonic beam has been measured by photoionization with an ArF excimer laser. This collision-free lifetime pertaining to the vibrationally unrelaxed triplet was found to be 470 f 50 ns. Measurement of a component of the photoion signal due to decay of SIpermitted, in addition, an estimate of the quantum yield for intersystem crossing.
Introduction Triplet production and decay have long been recognized as key factors in photochemical reacti0ns.l A prime reason for this importance is that the triplet state is often sufficiently long lived to participate in reactive collisions. The lowest triplet state of benzene, for example, is known to live in a pure crystal lattice at 4 K for times in excess of 15 s . ~In the gas phase, this triplet is known to be formed in a purely intramolecular radiationless decay process from the excited singlet state, and its presence can be deduced indirectly by its ability to cause photochemical reactions (such as the cis-trans isomerization of 2 - b ~ t e n e ) . ~ The gas-phase triplet lifetime of benzene, however, is much shorter than that of the cold molecule in a crystal. The reason for this is that the triplet is initially formed from the singlet state in a unimolecular process which has to conserve energy. The new-born triplet molecule therefore must contain a large amount of vibrational excitation, E,, in order to make up for the difference between the singlet and triplet electronic energies (approximately 8600 cm-l for the S1 to T1 spacing in benzene). The radiationless decay process which returns this triplet molecule to the ground electronic state is strongly accelerated by large amplitude vibrational motion and the triplet decay rate is therefore a steep exponential function of the internal vibrational energya4 Such highly vibrationally excited polyatomics are hypersensitive to collisional perturbation. In normal vapor or condensed phase photochemical situations, triplet stabilization by collision-inducedvibrational relaxation is a major kinetic factor competing with photochemical reaction and unimolecular decay to the ground state in determining the overall triplet lifetime.
*Alfred P. Sloan Fellow.
0022-3654/81/2085-0007$01 .OO/O
Due to this extreme sensitivity of the triplet lifetime to collisional effects, the true unimolecular lifetime has never been measured for the triplet state of any molecule. For example, triplet-triplet absorption measurements can detect the formation and decay of triplets, but this technique has not yet been refined to the point that true collision-freeunimolecular behavior can be guaranteed on the relevant time scale (microseconds) while at the same time preventing bias due to migration of the triplets out of the probe beam path.”’ The purpose of this short paper is to point out that the current level of development of laser and supersonic beam technology has now made it possible to cleanly measure true unimolecular triplet lifetimes. In as much as benzene has always been a popular molecule for the discussion of polyatomic spectra, triplets, and radiationless transitions, we have chosen to use it as the first illustration of this new technique. There is no fundamental reason for this choice, however. The method is perfectly general (for any mole(1) (a) J. Calvert and J. Pitta, “Photochemistry”, Wiley, New York, 1965. (b) N. J. Turro, “Modern Molecular Photochemistry”, Benjamin/Cummings, Menlo Park,1978. (2) (a) G. W. Robinson, J. Mol. Spectrosc., 6 , 58 (1961). (b) M. R. Wright, R. P. Frosch, and G. W. Robinson, J. Chem. Phys., 33,934 (1960). (c) P. M. Johnson and L. Ziegler, J. Chem. Phys., 66, 2169 (1972). (3) C. S. Parmenter, Adu. Chem. Phys., 22,365 (1972), and references cited therein. (4) For recent reviews on polyatomic radiationlesstransitions see: (a) P. Avouris, W. M. Gelbart, and M. A. El-Sayed, Chem. Reu., 77, 793 (1977). (b) K.F. Freed, Acc. Chem. Res., 11,74 (1978). (c) K.F. Freed, Top. Appl. Phys., 15, 23 (1976). (d) S.A. Rice, Excited States, 2, 111 (1975). (e) J. Jortner and S. Mukamel in “The World of Quantum Chemistry”, R. Daudel and B. Pullman, Ed., Reidel, Boston, 1974, p 145. ( f ) J. Jortner and S. Mukamel in “Molecular Energy Transfer”, Vol. 2, R. D. Levine and J. Jortner, Ed., Wiley, New York, 1976, p 178. (5) H. Schroder, N. J. Neusser, and E. W. Schlag, Chem. Phys. Lett., 48, 12 (1977). (6) B.Soep, C. Michel, A. Tramer, and L. Lindquist, Chem. Phys., 2, 293 (1972). (7) G. Porter and F. J. Wright, Trans. Faraday SOC.,51,1205 (1955).
0 1981 American Chemical Society
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The Journal of Physlcal Chemlstty, Vol. 85, No. 1, 198 1
t
Letters
Arl',,',"iER
D Y E LASJR 2590 A
SUPERSONIC B E N Z E N E BEAM
TIME OF FLIGHT MASS SPECTROMETER INTERACTION REGION
LN2- COOLED C R Y 0 BAFFLE
Flgure 1. Schematic of triplet detection apparatus. Triplets are produced In the supersonic benzene beam by intersystem crossing from the 6' vibrational level of the S1 state excited by the 2590-A dye laser. These electronically excited molecules (both S1 slnglets and trlplets) are then detected by photoionization with the ArF excimer laser.
cule that can be put into or made in a supersonic beam) and it is quite likely that the most significant early applications are yet to come.
Experimental Section In this experiment a collision-free,ultracold supersonic beam of benzene was excited to the first excited electronic state, S1 (lB,,(mr*)) by a pulsed dye laser propagating coaxially with the molecular beam. Triplet benzene molecules formed in the beam by intersystem crossing from the laser-excited S1 state were detected by direct photoionization with an ArF excimer laser. The triplet lifetime was then determined by monitoring the intensity of this photoionization signal as a function of delay time between the pump and probe lasers. Figure 1 displays a schematic of the interaction region of the apparatus. The S1 pump dye laser was a NdYAG-pumped dye laser (Quanta-Ray) tuned to the 6; transition of the S1 So absorption spectrum of cold benzene.8 This laser output consisted of 3-11s 0.5-mJ pulses with a bandwidth of approximately 1 cm-l, collimated to a roughly parallel 0.2-cm diameter beam and directed down the molecular beam axis. Optimum alignment of the pump laser position and frequency was easily accomplished by adjusting for maximum photoionization signal due to this laser alone. As shown in Figure 2, the S1 state of benzene is energetically more than halfway to the first ionization potential, IP1. Successive absorption of two pump laser photons (resonance-enhanced twophoton ionization, RBPI) therefore efficiently ionizes benzene to produce the parent The excimer laser was a Lumonics TE-861 unit which delivered 10-ns pulses of ArF laser radiation at 1930 A. As shown in Figure 1the unfocused output from this laser was collimated to a 0.3-cm diameter beam and adjusted to cross the molecular beam in the center of the ionization region of a time-of-flight mass spectrometer. The effective energy +
(8)S.M. Beck, M. G . Liverman, D. L. Monts, and R. E. Smalley, J. Chen. Phys., 70,232 (1979). (9)T. G.Dietz, M. A. Duncan, M. G. Liverman, and R. E. Smalley, Chem. Phys. Lett., 70, 246 (1980). (10)(a) S.Rockwood, J. P. Reilly, K. Hohla, and K. L. Kompa, Opt. Cornnun., 28,175(1979). (b) J. P. Reilly and K. L. Kompa, Adu. Mass. Spectra, 8 (1979). (11)T. G.Dietz, M. A. Duncan, M. G. Liverman, and R. E. Smalley, J. Chem. Phys., in press. (12) U. Boesl, H. J. Neusser, and E. W. Schlag, Z . Naturforsch. A, 33, 1546 (1978). -_._ ~
(13)Triplets have been detected in room-temperature vapors by this same technique of two-laser photoionization. See V. S. Antonov, I. N. Letokhov, V. M. Matiuk, V. G. Movshev, and V. K. Potapov, Opt. Lett., 3 , 37 (1978).
Figure 2. Energy level diagram relevant to triplet detection by photolonizatlon. Dye laser radiation at 2590 A (4.8 eV) excites the cold benzene molecules to the 6' Vibrational level of S, which then decays by fluorescence (at a rate k,)and by intersystem crossing (at a rate kST)to isoenergetic levels of the triplet manifold. The resultant vlbrationally hot triplet, Tit, then decays (at a slower rate, kTs)into highly vibrationally excRed ground state levels, Sot. Franck-Condon factors for photoionizlng transltlons In benzene are vanishingly small unless the resultant ion has the same vlbratlonal excltation as the molecular state to be ionized. Thus, for the Tlt hot triplet, only the ArF excimer laser radlation (6.4 eV) can produce efficient ion formation by exciting above the first ionization threshokl (IPit) which can produce CeHe+ with 1.1 eV of vibrational energy.
in this 0.3-cm excimer laser beam was approximately 10 mJ/pulse. Pulse-to-pulse energy variations of both the S1 pump dye laser and the excimer laser were detected by photodiodes and recorded for each shot with a CAMACbased minicomputer system. Ions produced in the molecular beam by photoionization was accelerated out of the ionization region by a 175 V cm-l dc field, then further accelerated to 3000 eV and directed down a 1.2-m drift tube to be detected by a Johnston MM-1 electron multiplier. The time-of-flight mass spectrum resulting from each laser shot was recorded by a Lecroy 22568 transient digitizer. Further details of this computer-based time-of-flight photoionization spectrometer have been published elsewhere in our R2PI study of bromo- and iodobenzene." The supersonic benzene beam was produced by a pulsed nozzle of our own design. The characteristics of intensity and cooling obtained from this source have been publ i ~ h e d . ~ For J ~ the objectives of this work on triplets, only a moderate expansion was necessary to obtain sufficient cooling. Consequently, a 5-atm backing pressure of helium was used, resulting in rotational cooling to -1 K, and vibrational cooling sufficient to keep hot band intensities to less than 1% the intensity of the 6; absorption band in the S1 Sospectrum. In order to minimize benzene dimer formation in the supersonic expansion, we kept the concentration of benzene below 200 ppm. The absence of dimers and higher oligomers in the beam was verified by looking for ions of the corresponding masses upon ionization with either the dye laser pump or the excimer probe laser. At benzene concentrations higher than roughly 0.1%, oligomer ion signals are easily observed in the supersonic beam. At 2% benzene concentration, singly ionized clusters of up to 12 benzene units were detected. The mass selective R2PI excitation spectrum of such highly clustered beams may be quite interesting. The interaction region where the triplet molecules are made and probed in this experiment is situated 1.2-m downstream of the supersonic nozzle in a carefully cryotrapped chamber maintained at lo-* torr. The helium +
(14)M. G.Liverman, S. M. Beck, D. L. Monta, and R. E. Smalley, J. Chem. Phys., 70, 192 (1979).
The Journal of Physical Chemistry, Vol. 85, No. 1, 198 1 0
Letters
r l " r l l 1 1 1 ' 1 ' ' ~ ' ~ ~ ~ ' ' l ~ ' "
0
1000 TIME (NSEC)
2000
Figure 3. Photoion decay curve due to benzene SI singlets and TVt triplets In a supersonic beam. The vertical axis is the C6H photoion signal due to sequential excitation of benzene with a 2590-1 dye laser and a 1930-A ArF excimer laser. The horizontalaxis is the delay time between the two lasers. +
density in the beam at this point is equivalent to torr of room temperature gas, but here the translational temperature is less than 1 K. The triplet lifetime measurements reported here are therefore expected to be completely unperturbed by collisional phenomena with effective cross sections of less than lo6 w2. Lifetime data were obtained by scanning the delay time between the pump and probe lasers with a computerdriven digital delay generator (Evans Associates Model 4145-2). Photoion signal corresponding to the benzene parent ion mass was monitored for each laser shot by using the computer-driven transient digitizer and then normalized for fluctuations in the pulse energies of the two lasers. The normalization assumed the triplet photoion signal was proportional to the product of the pulse energies of the two lasers. Some parent ion signal was observed whenever either laser was fired alone, and the magnitude of these signals was monitored for each laser after every set of the 50 two-laser data shots required to scan from minimum to maximum delay. The data reported below was the result of summing computer-stored signals from 1000 scans over the delay range.
Results and Discussion Figure 3 displays the measured two-laser benzene signal as a function of delay time between the pump and probe. On the scale of the figure, the ion signal due to either laser alone was insignificant. The resultant decay is slightly biexponential with roughly 70-80% of the intensity falling off with a lifetime of 470 f 50 ns and the remainder decaying with a 100 f 50 ns time constant. The 6l level of benzene is known to have an 80-11s lifetime3J3and it is therefore likely that the faster decay component of our two-laser photoionization signal corresponds to S1decay. The 470-11s component, however, is definitely due to the triplets produced by intersystem ~r0ssing.l~ The fact that the two-laser parent ion signal was found to be much larger than that from either laser alone is
perfectly in accord with expectations derived from early studies with R2PI in our lab~ratory.~Jl The general rule (to which we have so far found no exceptions) is that extremely efficient ionization is obtained to form the parent ion whenever the light field can carry the molecule into the ionization continuum by a resonant process involving only states which are stable on the time scale of the laser pulse. As mentioned earlier, the pump dye laser meets this criterion since the 6l level of the SI state can be ionized directly with a 2590-A photon to produce the C6H6+ion with an equivalent amount of vibrational excitation. The one laser ion signal from this source was weak, however, since the pump laser intensity in this experiment was kept low. The 6l level of S1 can, of course, also be ionized directly by a 1930-A photon and the fact that this ArF excimer laser beam was 20 times more intense than the dye laser explains why the two-laser ion signal was so dominant. Very little C6&+ ion signal was observed from the ArF excimer laser alone since the R2PI mechanism in this case has a very rapidly predissociating resonant intermediate state. This situation changes drastically when benzene dimers and/or higher oligomers are in the beam. Apparently the dimers are able to transfer the 6.4-eV ArF photon energy into at least one stable electronic state and dispose of the excess energy by dissociation into two free benzene molecules prior to rupture of a chemical bond in one of the rings. The small component of intensity seen at early times to decay with an -100-ns lifetime is most likely due to radiative depopulation of the singlet. The currently accepted value for the fluorescence quantum yield from the 6l level of S1 is approximately 0.23J5and the cis-2-butene isomerization results indicate that intersystem crossing to the triplet accounts for the remaining 80% of the initial population? We note that, if it is assumed that the singlet and triplet have the same absorption cross sections (and photoion yields) for the ionizing excimer laser, careful measurement of the photoion decay curve will permit a direct measure of the singlet lifetime, the triplet lifetime, and the quantum yield for intersystem crossing. The rough figures given above for the observed decay curve of benzene are in accord with these expectations. Uncertainty in equivalence of the singlet and triplet absorption cross sections could be bypassed by increasing the excimer laser intensity past the point of saturating the ionizing transition of both species.
Acknowledgment. We thank A. Kaldor of Exxon Research and Engineering Corp. for providing the excimer laser essential to this experiment. Support from the National Science Foundation, the US.Department of Energy, and The Robert A. Welch Foundation is gratefully acknowledged. (15)K.G. Spears and S.A. Rice, J. Chem. Phys., 66,5561 (1971).
