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(17) D. J. Bradley, U S . Patent 3761 614 (1973). (18) D. J. Bradley and W. Sibbett, Appl. Phys. Lett., 27, 382 (1975). (19) D. J. Bradley, A. G. Roddie, W. Sibbet, M. H. Key, M. J. Lamb, C. L. S.Lewis, and P. Sachsenmaier, Opt. Commun., 15, 231 (1975). (20) J. Rentjes, C. Y. She, R. C. Eckhardt, R. A. Andrews, and R. C. Elton, Appl. Phys. Lett., 30, 300 (1977). (21) D. J. Bradley, M. H. R. Hutchinson, and C. C. Ling in "Springer Series in Optical Sciences", Vol. 3, "Tunable Lasers and Applications", A. Mooradian, T. Jaeger, and P. Stokseth, Ed., Springer, West Berlin, 1976, p 41. (22) See for example, S. Tolansky, "High Resolution Spectroscopy", Methuen, London, 1947, p 1. (23) A. H. Sommer, "Photoemissive Materials", Wiley, New York, N.Y., 1968, p 76. (24) D. J. Bradley, S.F. Bryant, J. R. Taylor, and W. Sibbett, Rev. Sci. Instrum., 49, 215 (1978). (25) W. Sibbet, D. J. Bradley, and S. F. Bryant, Opt. Commun., 18, 107 (1976). (26) R. S.Adrain, D.Phil Thesis, University of London, 1974. (27) J. Nuckolls, L. Wood, A. Thiessen, and G. Zimmerman, Nature (London), 239, 139 (1972). (28) M. H. R. Hutchinson, C. C. Ling, and D. J. Bradley, Opt. Commun., 18, 203 (1976). (29) J. Reintzes, C. Y. Shea, R. C. Eckhardt, R. A. Andrews, and R. C. Elton, Appl. Phys. Lett., 30, 380 (1977). (30) B. L. Henke, J. A. Smith, and D. T. Attwood, Appl. Phys. Lett., 29, 539 (1976). (31) K. Helbrough, M. C. Adams, W. Sibbett, and T. H. Williams, Proceedlngs 12th International Congress on High Speed Photography, SPIE, Washington, 1977 p 54; D. J. Bradley, US. Patent 3 973 117 (1976). (32) D. von der Linde, I€€€ J . Quantum Electron., QE-8, 328 (1972). (33) D. J. Bradley and W. Sibbett, Opt. Commun., 9, 17 (1973). (34) M. C. Richardson, I€€€ J. Quantum Nectron., QE-9, 768 (1973). (35) F. De Martini, C. H. Townes, T. K. Gustafson, and P. L. Kelley, Phys. Rev., 164, 312 (1967). (36) D. Milam and M. J. Weber, I€€€J. Quantum Electron., QE-12, 512 (1976).
Damschen et al. (37) H. Kuroda, H. Masuko, and S. Maekawa, Opt. Commun., 18, 169 (1976). (38) J. R. Taylor, W. Sibbett, and A. J. Cormier, Appl. Phys. Left., 31, 184 (1977). (39) G. H. C. New, I€€€ J . Quantum Nectron., QE-10, 115 (1974). (40) E. Lill, S. Schneider, and F. Dorr, Opt. Commun., 22, 107 (1977). (41) E. Lill, S. Schneider, F. Dorr, S. F. Bryant, and J. R. Taylor, private communication (to be published). (42) E. G. Arthurs, D. J. Bradley, and A. G. Roddie. Chem. Phys. Lett., 22, 230 (1973); Opt. Commun., 8, 118 (1975). (43) L. S.Goldberg and C. A. Moore, Appl. Phys. Left., 27, 217 (1975). (44) C. V. Shank and E. P. Ippen, Appl. Phys. Lett., 24, 373 (1974). (45) I . S. Ruddock, Ph.D. Thesis, University of London, 1976. (46) H. A. Haus, I€€€ J . Quant. Nectron., QE-11, 736 (1975); Opt. Commun., 15, 29 (1975). (47) Born and Wolf, "Principles of Optics", 2nd ed, Pergamon Press, Oxford, 1964, p 496. (48) W. H. Glenn, M. J. Brienza, and A. J. De Maria, Appl. Phys. Lett., 12, 54 (1968). (49) D. J. Bradley, A. J. F. Durrant, F. O'Neill, and B. Sutherland, Phys. Lett. A , 30, 535 (1969). (50) D. J. Bradley, J. P. Ryan, and W. Sibbett, to be published. (51) K. B. Eisenthal, Top. Appl. Phys., 18, 275 (1977). (52) A. J. Campillo and S.L. Shapiro, Top. Appl. Phys., 18, 317 (1977). (53) J. J. Ewing and C. A. Brau, "Sprlnger Serles in Optical Sciences", Vol. 3, "Tunable Lasers and Applications", A. Mooradan, T. Jaeger, and P. Stokseth, Ed., Springer, West Berlin, 1976, p 21. (54) The recorded pulse duration (At,) has three contributionsarising from (i) the flnite spatial resolution of the image tube (At8);(ii) the spread in transit times of the photoelectrons (At,) through the tube due to the initial photoelectron velocity distribution; and (iii) the laser pulse duration (At ).10,'3 To a good approximation the relation At, = [(At,? (At,)' (Atp)z]1'2can be used in deconvolving. A t , = ((mV,/2e)1'2€-')where eV, is the initial photoelec!ron energy spread, €is the electric field strength close to the photocathode, and e and m a r e the electronic charge and mass, respectively. The value of eV, depends on the wavelength of the light and is smallest close to the cathode sensitivity wavelength
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Intersystem Crossing Kinetics of Aromatic Ketones in the Condensed Phaset Donald E. Damschen, Charles D. Merritt, David L. Perry, Gary W. Scott," and Larry D. Talley Department of Chemistiy, University of California, Riverside, Riverside, California 9252 1 (Received December 76, 7977; Revised Manuscript Received April 24, 1978)
We compare the kinetics of intersystem crossing at room temperature in different solvents for a family of aromatic ketones: benzophenone, anthrone, xanthone, and para-substituted benzophenones. We report our recent observations: (1)fast buildup (k1-l 5 10 ps) of anthrone transient absorption in dioxane followed by a long (h-' 175 ps) decay; (2) fast buildup (kl5 10 ps) of anthrone transient absorption in benzene; (3) fast buildup (kl= 8 f 2 ps) of benzophenone transient absorption in ethanol; (4)fast buildup (/--I = 6.5 f 2 ps) of 4,4'-dimethoxybenzophenone transient absorption in benzene. We conclude that the rates of intersystem crossing for this family of aromatic ketones are all approximately the same.
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1. Introduction
The purpose of the present paper is to summarize our current research on the kinetics of intersystem crossing for aromatic ketones in room temperature solutions measured by picosecond absorption spectroscopy. We will compare our present results with those from the literature. Specifically, we will discuss the buildup of T-T absorption subsequent to ultraviolet excitation for the aromatic ketones benzophenone, anthrone, xanthone, and some para-substituted benzophenones. We will show that these aromatic ketones undergo rapid intersystem crossing as 'This research was supported b y the Research Corporation, the National Science Foundation, and the Committee on Research of the University of California, Riverside. Based, in part, on a paper presented a t the 32nd Symposium o n Molecular Spectroscopy, Ohio State University, Columbus, Ohio, June 1977.
0022-3654/78/2082-2268$01 .OO/O
indicated by previous work.l-1° We will also show that for these ketones, which have lowest singlet n7r* states in the solvents investigated, the intersystem crossing rates are all approximately the same (within a factor of -1.5). There are some slight solvent dependences of these rates which are best documented for the case of benzophenone. The exact source of the solvent dependences has not been fully explained. One of the cases explored in this paper involves the planar ketone, anthrone. Recently, the reportg of a 70-ps risetime of T-T absorption in anthrone a t 400 nm subsequent to 347-nm excitation led to speculation that the intersystem crossing rate was slower than in benzophenone.z,5 we investigate the possibility of rapid intersyshm Crossing in anthrone followed by slow vibrational relaxation and/or internal conversion as the rate-limiting step in the observed buildup. For this purpose we measure 0 1978 Amerlcan Chemical Society
Intersystem Crossing Kinetics of Aromatic Ketones
The Journal of Physical Chemistry, Vol. 82, No. 27, 1978
MI SA
c
lltl
0 - -0.05 d
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Figure 1. Experimental arrangement: mode-locked Nd3+: glass laser cavity consisting of a 99.9% reflector (Ml), saturable absorber dye cell (SA), Nd3+: glass rod (NDl) and a 50% reflector (M2); single pulse extractor consisting of polarizers (P1 and P2), Pockels cell (PC),and laser triggered spark gap (LTSG); amplifier (ND2); second (SHG) and third (THG) harmonic generating crystals; dichroic mirrors (DM); variable delay line (VD) for the 533-nm pulse; sample cell (S) of 1 mm length; photodiodes (PD); beam splitters (BS); diffuser (D); lenses (L); filters (F); and Tektronix oscilloscope (7904).
the buildup kinetics of absorption for anthrone in benzene solution at 533 nm, a region of weak T-T absorption, and at 422 nm, the red shoulder of a stronger T-T absorption peak. In order to see if there is an S, S1interfering absorption, we also investigate the excited state absorption kinetics in a triplet quenching solvent. Finally, we report on an interesting decay of 533-nm absorption for excited state anthrone in the solvent dioxane. We speculate on the source of this decay which was not observed for either benzophenone or xanthone in that solvent. A preliminary account of some of these results has been prepared and published elsewhere.1°
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2. Experimental Section 2.1. Materials. Benzophenone (Aldrich) and 4,4'-dimethoxybenzophenone (Aldrich) were extensively zonerefined before use. Anthrone (Aldrich) was used without further purification; the anthrone solutions used in the experiments did not show, from absorption spectra taken a t 400 nm, interfering concentrations of the common impurity 9-anthrano1,ll 1-Methylphenanthrene (Aldrich) and coronene (Aldrich) were also used without further purification. The solvents used were benzene (Mallinckrodt, spectrophotometric), dioxane (Mallinckrodt, AR grade), cis-1,3-pentadiene (Chemical Samples Co.), and toluene (Mallinckrodt, spectrophotometric). 2.2. Apparatus and Methods. The apparatus used in the experiments which measure picosecond absorption kinetics at 533 nm is shown in Figure 1. An Nd3+: glass laser was passively mode-locked with a 1-cm Brewsterangled cell of Eastman Kodak 9860 dye. The 6-in. Owens-Illinois ED-2 glass rod was pumped by a helical flashlamp in a Korad K-1 laser head. A single pulse was switched from near the beginning of the train by a laser triggered spark gap which activated a KD*P Pockels cell. This single pulse was double passed through an 8411. long Nd3+: glass amplifier rod pumped with a flashlamp in an Apollo Model 5 laser head. The second (533 nm) and third (355 nm) harmonics of the amplified pulse were produced in angle-tuned type I and type I1 KDP crystals, respectively. The 355- and 533-nm pulses were separated at the dichroic mirror, with the 533-nm pulse then experiencing a
11
I
l
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-10
l
I
I
1 A, I
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I
I
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0 IO 20 30'V300 400 500 600 T I M E (psec)
Figure 2. Optical densities at 422 nm due to excited state absorption of coronene in toluene resulting from singlet state excitation at 355 nm. The are normalized optical densities as described in the text, and the smooth curve is the calculated D ( t ) also described in the text. Note: A change in the timescale occurs between 30 and 300 ps.
variable delay and depolarization, before being recombined with the 355-nm pulse and focused on the sample solution in a 1-mm quartz cell. The single pulse energy at 355 nm was in the range of 0.5-1.0 mJ. The position of t = 0, which is defined as that delay which gives maximum overlap of excitation and probe pulses at the sample, was determined by measuring the S1 absorption in a solution of 1buildup of S, methylphenanthrene. These measurements consisted of making several determinations of the optical density change at 533 nm for a normalized excitation pulse intensity at each of several values of the variable delay. Deconvolution and curve fitting established the t = 0 position as well as the pulse duration. Assuming the 355and 533-nm pulses had equal duration, the pulse width was found to be 10 ps (fwhm). This technique was previously described in more detail.4 The experimental apparatus used for measuring picosecond absorption kinetics at 422 nm was slightly different. The Nd3+: glass laser which was used had a generalized confocal cavity arrangement.12 The rear reflector, 99.9 % R, had a 0.5-m radius of curvature, and an AR coated negative lens (f = -241 mm) was empirically placed in the cavity to compensate for thermal lensing in the laser rod and to provide collimation. Mode-locking was accomplished with the EK9860 saturable dye in a 50-pm cell contacted to the front reflector (40% R) of the cavity. A single pulse was extracted from the 1064-nm pulse train, and amplified in a single pass. The 355-nm single pulse was produced as above, but a fraction of this pulse (-25%) was sent through a variable length optical delay line and used to excite a short cavity dye laser13314containing a 3.5 X M solution of bis-MSB in dioxane. This short cavity dye laser yielded a single blue pulse at 422 nm with -9-nm bandwidth and a pulse duration of 5 4 ps. (See below.) This pulse was used as the probe pulse. The 422-nm probe pulse was polarized parallel to the 355-nm pulse whereas the 533-nm probe pulse was depolarized. Timing and pulse duration experiments at 422 nm were accomplished using a coronene in toluene solution. The 355-nm pulse initially excites the S2state of coronene in this experiment, but the S, SI absorption buildup has been determined5i6J5to be quite S 2 ps). The coronene excited state absorption rapid (k1 buildup curve at 422 nm is shown in Figure 2. The smooth curve corresponds to a theoretical optical density (D(t)4) assuming Gaussian pulses at 355 and 422 nm of an equal width of 4 ps, fwhm. Actually, the 422-nm pulses are
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The Journal of Physical Chemistry, Vol. 82, No. 21, 1978
Damschen et al.
TABLE I: Summary of Aromatic Ketone Excited State Picosecond Absorption Kinetics wavelength, nm molecule benzophenone
xanthone
anthrone
4,4'-dimethoxybenzophenone 4-phenylbenzophenone 44 l-naphthylmethy1)benzophenone
time (h-I), ps
solvent
excitation
probe
benzene benzene benzene benzene n-heptane p-dioxane ethanol cis- 1,3-pentadiene trans-1,3-pentadiene 1-methylnaphthalene benzene p-dioxane ethanol glycerol benzene
347 385 355 355 355 355 355 355 355 355 355 355 355 355 347
694,976 694 533 630 533 533 533 533 533 533 533 533 533 conta 400
benzene benzene dioxane cis-1,3-pentadiene benzene
355 355 355 355 355
533 422 533 533 533
12i 2 7c2 8i: 2 lot 2 8t 2 7 t 3 7 t 3 510 8c 1 8t 2 812
