J. Phys. Chem. 1991, 95, 2364-2370
2364
283.3-nm line. The equation for this procedure is given by eq 7,
h exp[-~(w)l
~,(w)/~w =)
i- I
0
IO
20
30
4 0
50
60
70
80
Q(w) = sonlf;V(w) (7b) where so is the absorption cross sectioin at each line center, n is the Pb atom number density, 1 is the path length (0.14 cm) in the cell, and is the fraction of each isotopic and hyperfine component cfi = O.O75,f2 = O.OlS,h = 0.236, f4 = 0.523, fs= 0.151). This absorption profile was combined with the 750 K Doppler source function, lo(w), to obtain the intensity transmitted by the cell at each frequency, I,(w), for each argon pressure. The absorption spectra thus calculated for 0, 100,200,400, and 800 Torr of argon and nl = 1 X lo1* atoms/cm2 are presented in Figures 3-7. The source function, lo(w), from which the absorption occurs is also shown in Figures 3-7. As the argon pressure increases the composite absorption function broadens dramatically, becoming very flat at 800 Torr of argon. The procedure used here was to correct each experimental point on the calibration curve for the superlamp in Figure 2 by the calculated effect of a particular argon pressure utilizing eq 8.
9 0
nl(lOLII atomdcmh22)
Figure 8. Calibration curves for the superlamp for absorber in the presence of various pressures of argon: (0)no argon present; ( 0 ) 100 Torr of argon: (0)200 Torr of argon: (A) 400 Torr of argon; (A) 800 Torr of argon. T, = 773 K.
[(l,/lo)expt,Arl = H(4/lo)calc,Arl / [(l,/Io)calc,no Arll[(l,/l0)expt,no Arl (8)
retial work gave approximately the same d2 value for the thallium 377.6-nm transition as for the lead 283.3-nm transition in an acetylene-oxygen flame? which provides some support for our choosing a value for lead in argon that is the same as that for thallium in argon. Having obtained the Voigt function from eq 4 for a particular argon pressure, it was centered at each of the appropriate wo values in Table I and multiplied by the corresponding relative intensity values for each of the five isotopic and hyperfine components (see refs 1 and 2) to obtain the composite absorption profile for the ( 9 ) Lovett, R. J.;
(74
The calibration curves thus obtained are presented in Figure 8 for each argon pressure. It is seen from Figure 8 that as the argon pressure increases the Beer-Lambert plots become more linear and the sensitivity decreases, which is the expected pressure-broadening result. Acknowledgment. R.E.M. gratefully acknowledges the financial assistance of the N I H through its Minority Biomedical Research Support Program grant to NMSU. J.W.S. acknowledges the financial support of NMSU, the Research Corporation (Grant R - 5 9 , and LANL.
Parsons, M. L. Appl. Spectrosc. 1977, 31, 424.
Pumpprobe Fluorescence Studies of Excimer Formation and Dissociation for the van der Waals Dimer of Fluorene Hiroyuki Saigusa* and Edward C. Limt 1
Department of Chemistry, The University of Akron, Akron, Ohio 44325-3601 (Received: June 20, 1990; In Final Form: September 1 1 , 19901
Pumpprobe experiments have been performed on the excited-state dynamics of the fluorene van der Waals dimer isolated in a supersonic expansion. When the pump laser excites the dimer into its SI state, a spectral hole is observed in the dimer transitions due to the excited-state dimer undergoing an efficient excimer formation. This hole-burning technique has been used to demonstrate that the spectral congestion of the SI spectra of the dimer originates from homogeneous sources. The dissociation of the excimer in its ground electronic state has also been probed directly by observing monomer fragments of fluorene produced by the dissociation process. The results suggest the formation of vibrationally highly excited monomer fragments following the electronic relaxation of the excimer.
Introduction Studies of electronic excitations of aromatic clusters provide important information pertaining to the intermolecular interactions and dynamics. Ofspecial interest is the excitation of homodimers of aromatic molecules, such as benzene, since there may be additional nonbonded interactions besides the van der Waals (vdW) interactions, namely, the exciton and excimer interactions. Although these interactions are well-characterized in solutions and 'The inaugural holder of the Goodycar Chair in Chemistry at the University of Akron.
0022-3654/91/2095-2364$02.50/0
crystals,l~many-body effects due to solutesolute or solutesolvent interactions come into play in the condensed phase. In the collision-free environment attainable in a supersonic beam, it is possible to study these interactions in the absence of any interfering many-body effects. Benzene dimer is a particularly well-studied system. Previously, the structure of the benzene (1) 1970.
Birks, J. B. Phorophysics of Aromric Molecules; Wiley: New York, Mataga, N.; Kubota, T. Molecular Interactions and ElectronicSpectra;
Marcel Dekker: New York, 1970. (2) Bernstein, E. R.; Colson, S. D.; Kopelman, R.; Robinson, G. W. J . Chem. Phys. 1968,48,5596. Hanson, D. M.J. Chem. Phys. 1970,52,3409.
0 1991 American Chemical Society
The Journal of Physical Chemistry, Vol. 95, No. 6, 1991 2365
van der Waals Dimer of Fluorene homodimer in the SIstate has been probed by using resonanceenhanced two-photon ionization mass ~pectroscopy.~*~ Bbrnsen et al.' clearly resolved an exciton splitting of the SIorigin of the dimer arising from the resonance interaction between two benzene molecules. The lifetimes of several SIvibronic levels of the dimer have been reported by Hopkins et ale5and by Law et aL3 Excimer formation has been inferred for the SI-statedimer to account for the shortening of SI lifetime with respect to the monomer. However, this nonradiative decay channel has been questioned recently by Bornsen et a1.6 on the basis of their measurements on the ionization potential of the dimer. Unlike most electronically excited aromatic dimers that usually decay by dominant relaxation processes to their ground state, the fluorene dimer undergoes a large structural change to form excimer upon excitation into the SI origin.' The discrete nature of the excitation spectra of the excimer fluorescence indicates that the initial excitation is mainly localized on either half of the dimer. The dimer then rearranges the initial configuration into a deeper excimer potential well, resulting in the broad and red-shifted fluorescence. The initially prepared fluorene vdW dimer has been assumed to have a displaced sandwich structure, while the excimer is expected to be less displaced and more strongly bound than the dimer. The emission spectrum of the excimer exhibits no wellresolved structure because of its repulsive potential in the ground state. The excimer, which is stable only in the excited electronic state, is known for many aromatic molecules in solutions.' The important observation in the fluorene dimer is that its fluorescence excitation spectrum is congested with irregular low-frequency structures. Many other aromatic dimers and clusters display strong evidence of several different conformational isomers, suggesting the possibility of multiple conformers as an inhomogeneous source of the spectral congestion. In addition, homogeneous sources, such as low-frequency intermolecular vibrations and/or exciton splittings, can contribute to the spectrum of the fluorene dimer. In this study, two pumpprobe experiments have been employed to monitor the excimer formation-dissociation dynamics which is initiated by exciting the fluorene dimer into the SIstate. One especially useful spectroscopic probe is hole-burning spectroscopy, which has been widely used in condensed phase to obtain the homogeneous line width of a molecular transition in an inhomogeneously broadened absorption band.* The essence of the spectral hole-burning is that irradiation of the broad absorption band with a narrow-band laser excites a set of molecules that are in resonance with the laser frequency. When these molecules form photoproducts that absorb at different frequencies, the absorption spectrum of the molecule displays a hole burnt at the frequency of the hole-burning laser. This technique, when combined with the supersonic jet spectroscopy, will enable us to select a particular dimer configuration of fluorene undergoing excimer formation. A persistent hole may be produced in the dimer transition and then probed to resolve spectral features of the dimer from spectral congestion due to other conformational isomers. This hole-burning technique has been demonstrated to be powerful in investigating photophysical relaxation processes of excited states of vdW clusters formed in supersonic jets.e11 The excimer formation-dissociation dynamics can also be probed by tuning the probe laser frequency to various transitions of a set of the dissociation products which are a pair of monomer fragments. In solution phase, excimer formation is believed to (3)Law, K. S.;Schauer, M.;Bernstein, E. R. J. Chem. Phys. 1984, 81, 4871. (4) BBrnsen, K.0.; Selzle, H.L.; Schlag, E.W. J. Chem. fhys. 1986.85, 1126. -.
(5) Hopkins, J. 8.; Powers,D. E.: Smalley, R. E. J . fhys. Chem. 1981,85, 3739. (6) BBmsen. K.0.; Lin, S.H.; Sclzle, H. L.; Schlag. E. W. J . Chem. Phys. 1989, 90. 1299. (7)Sainusa, H.:Itoh, M.J . fhvs. Chem. 1985,89, 5436. (8) See, for example: Viilker, S Annu. Rev. fhys. Chem. 1989, 40,499. (9)Lipert. R. J.; Colson, S.D. J . Phys. Chem. 1989, 93, 3894. (IO) Wittmeyer, S. A.; Topp, M.R. Chem. fhys. Len. 1989, 163, 261. (1 I) Knouchenmuss, R.;Leutwyler, S.J . Chem. Phys. 1990, 92, 4686.
be due to a diffusion-controlled process by which excited-state molecule A* encounters its ground-state partner A to form excimer, Le., A* A (A-A)*. The excimer that is thus formed dissociates in the ground state, leading to the formation of fragments A. In condensed phase, these monomer fragments cannot be distinguished from the original monomers which were not subjected to the excimer formation-dissociation cycle since solute-solvent interactions smear out the initial internal-state distribution of the monomer fragment. Under the rotationally and vibrationally cold conditions attainable in a supersonic jet, a subensemble of the monomer fragments produced by the excimer dissociation can be selectively detected by means of the p u m p probe technique, allowing us to probe the internal-state distribution of the fragments. Upon dissociation, the excess energy available from the excimer formation-dissociation process will be distributed over various motions of the monomer fragment. Therefore, the fragment is expected to be rotationally and vibrationally hot with respect to the thermal distribution of the monomer in the supersonic expansion, leading to spectral broadening in the fragment transitions (or antiholes). In this paper, we demonstrate the utility of pumpprobe techniques to isolate the spectra of the fluorene homodimer and probe the dynamics of the excimer formation-dissociation process. A detailed analysis of the dimer spectra pertaining to the intermolecular electronic (exciton) and vibrational interactions will be published elsewhere.I2
+
1
-
Experimental Section The fluorene vdW dimers were formed in a supersonic jet. Zone-refined fluorene (Tokyo Kasei) was heated to 100-105 O C and seeded into He carrier gas a t 3 atm. The pulsed molecular beam system employed in this experiment has been previously described.I3 The molecular beam was crossed with the mildly focused output (-4 mm2 at the jet) of a YAG pumped dye laser system (Quanta-Ray DCR-2A/PDL-1/WEX-1). The pulse energy of the pump laser was 0.05-0.1 mJ/pulse. The probe laser was the frequency-doubled output of a second YAG-pumped dye laser (Quanta-Ray DCR-l/Lambda Physik FL2002), which was delayed by
