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R. MCNEIL, J. T.RICHARDS, AND J. K. THOMAS
The Laser Flash Photolysis of Solutions of Naphthalene and 1,2-Benzanthracene1& by R. McNeil, J. T. Richards, and J. K. Thomas Chemistry Divieion, Argonne National Laboratory, Argonne, Illinois 60439
(Received September 92, 1969)
Solutions of naphthalene and l,2-benzanthracene have been flash photolyzed with nanosecond pulses of 2650- and 3472-A light from a quadrupled neodymium and doubled ruby laser, respectively. Short-lived nanosecond spectra are observed which by lifetime and by heavy atom spin-orbit coupling studies are assigned to escited singlet states. Long-lived microsecond spectra are also observed and are attributed to the triplet state of these molecules. The absorption spectra of the singlet states decay, giving rise to the absorption spectra of the triplet state. The excited singlet and triplet spectra are quite similar, and this detail is discussed with regard to previous pulse-radiolysis data on these systems.
Introduction The initial development of flash photolysis2 provided a direct method of observing species with lifetimes of only a few microseconds. The time region has been extended to nanoseconds by observing fluorescence from excited species produced either photochemically3or by ionizing r a d i a t i ~ n . ~Recently the development of powerful sources of excitation, such as lasers and high-current accelerators, has enabled the chemist to observe short-lived species by absorption spectroscopy. Sever?l aromatic molecules have been excited by the 3472-A line from a doubled ruby laser5t6 and quadrupIed neodymium laser,’ and the spectra of excited singlet states have been recorded. The pulse radiolysis of aromatic molecules also produces excited singIet and triplet ~ t a t e s the , ~ ~data ~ indicating that the predominant mode of formation is via ion recombination. The accepted mechanism of decay of an excited singlet state is fluorescence, radiationless transitions, and intersystem crossing to the triplet state. For many molecules the total rate of decay of the singlet by the above three processes has been measured by fluorescence techniques, and it remains to consolidate the measurements by direct observations on the triplet state by absorption spectroscopy. Initial data in the pulse radiolysis of anthracene, 1,2benzanthracene, and naphthalenes-I0 in cyclohexane and benzene illustrate that the absorption spectrum of the triplet state shows only a small increase over the time period of decay of the excited singlet state. For naphthalene it was shown that this is due primarily to the fact that the spectra of the excited singlet and triplet state are very similar, leading to only a small spectral change associated with the intersystem crossing process. This effect is compounded in aromatic solvents due to a significant solvent effect on the excited singlet and triplet spectra which tends to broaden the spectra and produce even more similarity between the spectra. This aromatic solvent effect has also been noted in the pulse radiolysis of naphthalene in polystyrene and polymethyl methacrylate. l 1 T h e Journal of Physical Chemistry, Vol. 74,N o . 11, 1970
It is a little disturbing that the above effects have been noted in systems excited by ionizing radiation in a situation which is more complicated than direct photochemical excitation. However, it has been recently shown that there is a close similarity between the absorption spectra of the excited singlet and triplet states of anthracene,12 in agreement with the trend shown in the radiolysis data. The present study is devoted to an examination of the photolysis of solutions of naphthalene and 1,2-benzanthracene by short pulses from a &-switched laser, to compare the photochemical data with the pulse-radiolysis data. In addition, the experimental technique, which is particularly useful for observing absorption and fluorescence spectra of short-lived species produced by &-switched lasers, is described in detail.
Experimental Section Cyclohexane and benzene were purified by passing these solvents through a 3-ft column of activated alumina. Naphthalene was supplied by J. T. Baker and was twice recrystallized from ethanol before use; 1,2-benzanthracene (1,2-BA) was supplied by the Aldrich Chemical Co; xenon, helium, and argon were supplied by the Matheson Scientific Co. Solutions were prepared and deaerated by the syringe technique which has already been described.la (1) Work performed under the auspices of the U. 6. Atomic Energy Commission. (2) G. Porter, Proc. Roy. Soc., A200, 284 (1950). (3) I. Berlrnan, “Handbook of Fluorescence Spectra of Aromatic ,Molecules,” Academic Press, New York, N. Y., 1965. (4) M. Burton and H. Dreeskamp, Phys. Reu. Lett., 2, 45 (1959). (5) J. R. Novak and M. ’IV. Windsor, Science, 161, 1342 (1968). (6) G. Porter and M. R. Topp, Nature, 220, 1228 (1968). (7) R. Bonneau, J. Faune, and J. Joussot-Dubien, Chem. Phys. Lett., 2, 65 (1968).
(8) J. W. Hunt and J. K. Thomas, J . Chem. Phys., 46, 2954 (1967). (9) J. K. Thomas, ibid., 51, 770 (1969). (10) R. Cooper and J. K. Thomas, ibid., 48, 5097 (1968). (11) 9. K. Ho and 9.Siegel, ibid., 50, 1142 (1969). (12) D. S. Kliger and A. C. Albrecht, ibid., 50, 4109 (1969).
LASERFLASH PHOTOLYSIS
OF
NAPHTHALENE AND I&BENZANTHRACENE
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ioon's
POWER SUPPLY
MONOCHROMATOR SHUTTER ELECTRONICS
PHOTO MULTlPLlER
.
4175 R'
SCOPE B SCOPE A for TRANSIENT M E I S U R E M E N T SCOPE 8 IO, Io MLASURLMENT
Figure 1. Diagrhmmatic representation of laser flash photolysis apparrttus.
A diagrammatic representation of the laser flash photolysis lay-out using a Korad KIQP laser is shown in Figure 1. A 30-nsec, 2 . 5 4 pulse of light of wavelength X = 10,6000 iis produced in the laser cavity, and about 50/, of this light is frequency doubled to 5300 if, by the first potassium dihydrogen phosphate (KDP) crystal doubler. The 5300-i light exits the laser cavity (while the red light is retained within the cavity by the exit mirror which is 100% reflecting a t 10,600 i) and passes through the second external KDP doubler giving a mixed output beam of 5300- and 2650-if light. The final ultraviolet output of the laser system was measured by means of a malachite green leucocyanide actinometer developed by Johns, et uZ.,'~ and was found to be about J. The flash duration of the 2650-if light was 15 nsec (half-width) as measured by observation of the fluorescence from a solution of p-terphenyl in cyclohexane. The beam cross section was roughly circular, with a radius of 2 mm, and impinged onto a cylindrical quartz irradiation cell of 5-mm length and 5-mm diameter. A light beam from a 450-W xenon lamp (Osram XB0450) was focused through the cell and onto a diaphragm at the cell window farther away from the lamp. The light is refocused onto the entry slit of a Bausch and Lomb monochromator, which is operated with a band width of 13 k,while the intensity of the light emerging from the exit slit of the monochromator was monitored by an RCA IP28 photomultiplier. The output of the photomultiplier was displayed on a Tektrouix 585A oscilloscope (A), where the signals were photographed on Polaroid 410 film. The risetime of the optical and electronic system was faster than 5 nsec. To achieve large output signals from the photomultiplier, the 450-W xenon lamp was pulsed to 50 times the normal power for a few milliseconds,'6 thus enabling a current of 20 mA to be drawn from the photomultiplier.'6 This light pulse was recorded
4400'A Figure 2. Laser photolysis of 5 naphthalene in GHn.
x 10-4 M
on oscilloscope B and measured the I , of the light passing through the irradiation cell. The oscilloscope (A) records the emission of light from the cell and the transitory changes in the level of the light passing through the cell, due to absorption by transitory species. A typical picture of the oscilloscope trace recording such an event is shown in Figure 2.
Results and Discussion Naphthalene in C6HI2. Figure 2 shows typic+ pictures of oscilloscope traces taken at X = 4175 A and h = 4400 A in the laser photolysis of deaerated solutions of 5 X M naphthalene in cyclohexane. When the laser pulse occura, transitory species are produced which absorb some of the analyzing light passing through the cell, causing a decrease in the light intensity, Io,which in turn is recorded as a deflection of the oscilloscope trace toward the top of the picture. The optical density of the solution a t any time may be calculated from the percentage decrease in Io. Figure 2 illustrates that following the laser pulse the intensity of the analyzing light may continue to decrease as shown in the top picture of Figure 2, or it (13) J. K.Thomas, 8. Gorddn, and E. J. Hart, J . Phy& Chen.,68, 15a5 (ISM). (14) G. J. Fisher, J. C. LeBlsno, and H. E. Johns. Photoehem. PhotobbL, 6 , 757 (1967). (15) B. Michael. unmblished work. (16)J. W. Hunt and J. K. Thomas, Raddc&~ion Res., 32, 149 (1967).
The Journal of Phyadcal Chemistry, Vol. 74, No. 11, 1970
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R. MCNEIL,J. T. RICHARDS, AND J. I
