The Journal of
Physical Chemistry
0 Copyright 1994 by the American Chemical Society
VOLUME 98, NUMBER 9, MARCH 3,1994
LETTERS Line Width Study in Fluorescence Excitation Spectra of Single Pentacene Molecules Introduced as Impurities in pTerphenyl Crystal V. Palm, K. K. Rebane, and A. Suisalu’ Institute of Physics, Estonian Academy of Sciences, 142 Riia Str., EE2400 Tartu, Estonia Received: October 8, 1993; In Final Form: December 27, 1993’
We report results on line widths in the fluorescenceexcitation spectra of single pentacene molecules inp-terphenyl crystal flakes for the 0 1 and 02sites at 1.85 K. A setup for the measurement of fluorescence excitation spectra of single impurity molecules with an overall quantum efficiency of about 0.1% has been designed. Spectra obtained with a signal-to-noise ratio up to 1O:l were recorded at different laser excitation intensities in the range from 1 to 5 mW/cm2. The line widths measured for different pentacene impurity molecules and extrapolated to zero probing intensity show remarkable variation in the range from 5 to 15 MHz. A reason why some lines are narrower than the lifetime-limited one (8 MHz) is discussed.
Introduction The purely electronic zero-phonon line (ZPL)-the optical analog of the M6ssbauer y-resonance line-is a remarkable feature of impurity molecules (atoms, ions) introduced in lowtemperature solid matrices (ref 1 and references therein). ZPL is the cornerstone of high-resolution spectroscopy of solids and molecules (refs 1 and 2 and references therein), including chlorophyll and its relatives? as well as fluorescence line narrowing: persistent spectral hole and, the latest achievement, single impurity molecule spectroscopy (SIMS).8-” The decisive point of SIMS, which actually is spectroscopy of ZPL, is the peak value of the ZPL absorption cross section u. A simple and theoretical consideration shows that it is less than that for a free atom (aato, = X2/2?r) by a factor which comprises the product of the DebyeWaller factor FDW( 0,the ratio of the radiative line width to the homogeneous width AbmT), ( and the squared ratio of the excitation wavelength in the solid to that in vacuum:l2J3
* To whom correspondence should be addressed. *Abstract published in Advance ACS Abstracrs, February 1, 1994. 0022-3654/94/2098-2219$04.50/0
Here & = A/n, where n is the index of refraction of the host solid. For quite a number of impurity molecules in molecular matrices we can take as an estimate, for a temperature T = 2 K,F ~ w ( 2 K) = 0.1, Brad/bhom(2 K) = 0.5, and n2 = 2.5. Then we get
uZp,(2 K) = 0.02X2/277 = 4 X lo-” cm2
(2)
This rather large value agrees well with that evaluated from experiments for the current most popular SIMS system-a pentacene impurity moleculein a p-terphenyl singlecrystaP-and constitutes the unique basis for SIMS of ZPL. SIMS becomes very difficult at higher temperatures because u . ~ p7‘) ~ ( rapidly decreases with the increase of T. Experimental Section
The measurement of fluorescence excitation spectra-a zerobackground method-has proven its efficiency in SIMS.9 A single-modedye laser has been used for tuning the laser frequency across the molecular ZPL absorption, and the Stokes-shifted fluorescence intensity has been monitored as a function of 0 1994 American Chemical Society
2220 The Journal of Physical Chemistry, Vol. 98, No. 9, 1994 1~IuwC\LL'IICC
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Figure 1. Scheme of the experimental setup for fluorescence detection
of a single impurity molecule in solid. The excitation laser beam enters from the right, and fluorescenceis recorded behind the transparent sample (S)by a photomultiplier (PMT). (For more details see text.)
excitation wavelength. For this purposea new version of an optical setup with a higher numerical aperture of 0.6 compared to the design used in ref 10 has been developed (Figure 1). The sample holder (SH), which is placed into an optical immersion helium cryostat (CR), consists of a specimen under study (S) on a 5-pm pinhole (PH), condenser lenses (CL) (focal length of 15 mm), and a laser beam stop (T). Up to 10% of the fluorescence is collected by the CL and directed to a photomultiplier tube (a selected exemplar of FEU-79 with quantum yield of 12%at 633 nm) by the external lenses (L). A red-pass filter (F) (RG630 and KS-11) for spectral separation of the Stokesshifted fluorescence aod an aperture (A) (diameter = 5 mm) for spatial filtering of scattered light are used. Fluorescence excitation spectra of approximately 5-pm-thick flakes of a p-terphenyl single crystal doped with pentacene at a concentrationof lWmol/molarerecordedat 1.85Kona CR69929 Autoscan single-frequency dye laser spectrometer with frequency jitter of about 1 MHz. The amplitude of the laser output is stabilized and controlled by an electrooptical modulator CA307 (Coherent) in the range of sample illumination of 1-5 mW/cm2. Before the laser beam is focused to the pinhole with the sample under study, the linearly polarized laser radiation was converted into circularly polarized one by means of a quaterwave retarder (optimized for 632.8 nm). A weak fluorescence signal detected in the photon counting mode is accumulated into a multichannel analyzer LP4900 synchronously with the 1-MHz steps of the laser scan. The accumulation time (0.2 s per 1 MHz) for a data point was sufficiently short to avoid any distortion caused in the measured spectra by long-term drift of the laser frequency (100 MHz per hour). The accuracy of the frequency scale was provided by the CR699-29Autoscan construction comprisinga built-in wavemeter and adopted in our laboratory by introducing a computercontrolled line between the analyzer and laser. To guarantee reproducibility of the measured spectra, three successive laser scans were made over the same spectral region. The ZPL parameters (amplitude and width) were obtained by a nonlinear least-squares fit of experimental data to the superposition of Lorentzians and a linear background. RWultS
Figure 2 shows a fluorescence excitation spectrum consisting of absut 20 absorption ZPLs of single pentacene impurity molecules in a spectral range of 3000 MHz (0.09 cm-1). This spectrum was obtained from a single scan of the laser frequency near the maximum of the 02 site (laser sweep started at wavenumber of 16 886.083 cm-l). The intensities of ZPLs differ because of the orientational distribution of pentacene molecules relative to the polarization of the excitation light and also because of the uneven spatial distributionof excitationpower in the probing volume selected by the pinhole. The fluorescence collection efficiencies all excited pentacene molecules are practically equivalent. The average signal of 400 counts/s at a laser power of 3.6 mW/cm2 corresponds to an overall quantum yield of 0.1% for our SIMS system. For the most intense ZPL a SNR of 1O:l has been reached.
Figure 2. Fluorescence excitation spectrum of single pentacenemolecules in ap-terphenyl single crystal at 1.85 K. (This spectrum is a single scan of the laser frequency of YO.) The inset shows ZPL for two pentacene impuritymoleculesAandB. (Thisspectrumisthesumofthreesuccessive
laser scans.) The solid curve represents two-Lorentzian fitting.
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Fgrre3. Plot of the measured ZPL parameters (line widths vs amplitude) for single pentacene molecules in the 0 1 (crosses) and 02 (circles) sites
of a p-terphenyl crystal. The inset of Figure 2 represents a fit to the fluorescence excitation spectrum with two Lorentzians in an expanded scale for two pentacene molecules A and B. To determine the real homogeneous line widths &,om, the presence of the metastable triplet state has to be taken into account:14about one out of 100 excited molecules undergoes a SI-TI transition, after which the molecule is not absorbing during the triplet lifetime of some tenth of milliseconds. The number of the "lost acts of excitation" during a given photon counting time (0.2 s) is larger for higher counting rates and c a u m a deformation of the measured spectral line comparedto the real ZPL shape. This effect of "triplet saturation" reveals excitation rates of 104-105 s-I, which are still below that of power broadening of the S,,-S, transition. Several ZFLs of single molecules were measured at different excitation densities in the range of 1-5 mW/cm2, and the corresponding line widths dhom were determined. The "zerointensity" homogeneous width & , was obtained by extrapolation of ahom according to the formula (3) where Zand Z, are excitation and three-level saturation intensities, respectively.* The extrapolated line widths vs the measured ZPL intensitiesare depicted in Figure 3. The distribution of line widths around thelifetime-limitedvalueof 8 M H Z ~shows ~ J ~remarkable dispersion (from 5,to 15 MHz), which exceeds our measurement accuracy (1 MHz). Discussion We are inclined to conclude that the ZPL homogeneous width can be different for different individual molecules. If so, it opens
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Letters novel possibilities to check experimentally various theoretical models for impurity and its local environment, including impurityimpurity and imurity-two-level system interactions. 1. The line widths considerably brodaer than the average lifetime determined one may be caused by spectral diffusion, Le., the widths are actually inhomogeneous.16J7 In samples cooled fast to low temperatures nonequilibriumdeformationsare created, someof which may havevery long relaxation times. On the other hand, ZPL are extremely sensitive: a pressure change of 0.01 bar shifts its frequency by about 1 MHz.l*Jg If so, the intensity of spectral diffusion should be dependent on the time delay between placing the sample in liquid helium and time of measurement. 2. The ZPL widths which are considerably narrower than the width determined by the (average) lifetime look peculiar. We hope that the artifacts caused by the experimental equipment and the method of measurement have been eliminated. A possibility to explain this effect is to suppose that individual pentacene impurity molecules havedifferent rates of fluorescence quenching. The established quantum yield of fluorescence is 0.78.*O The nonradiative processes (together with spontaneous radiation and some temperature-initiated dephasing) give the above-mentioned line width around 8 MHz. The nonradiative processes in pentacene molecules in p-terphenyl crystals show high sensitivity to a local site: the fluorescenceis very weak when pentacene is located in 03 and 0 4 sites. It seems possible that some pentacene can occupy sites at which their fluorescenceyield is higher, up to 1, and this can bring about lifetime-limited lines up to 20%narrower than theaverageone. Theoscillatorystrength of the transition also depends to some extent on the inhomogeneous structure of the matrix, and longer purely optical lifetimes and narrower line widths than the average ones are possible. But the oscillatorystrengths of allowed transitions should be considerably less sensitive to the environment than the probabilities of the nonradiative electronic transitions and dephasing.
Acknowledgment. The authors express their gratitude to Professors Urs Wild and Frank Giittler, Laboratory of Physical
The Journal of Physical Chemistry, Vol. 98, No. 9, 1994 2221
Chemistry, Swiss Federal Institute of Technology, Zurich, for providing the samples, the know-how, and numerous valuable discussions. This work was supported, in part, by a Soros Foundation Grant awarded by the American Physical Society and by the International Center for Scientific Culture-World Laboratory. References and Notes (1) Rebane, K. K. Impurity Spectra ofSolids: Plenum Press: New York, 1970 (translation of the Russian edition, Nauka Publisher, Moscow. 1968). (2) Sild, O.,Haller, K. Eds. ZerctPhonon LinesandSpectralHole Burning in Spectroscopy and Photochemistry; Springer-Verlag: Berlin, 1988. (3) Avarmaa, R. A.; Rebane, K. K. Sou. Phys.-Usp. 1988, 31, 225 (translation of Usp. Fir. Nauk USSR 1988, 154, 433). (4) Personov, R. I.; Alshits, E. I.; Bykovskaya, L. A.; Kharlamov, B. M. Zh. Eksp. Teor. Fir. 1973, 65, 1825 (in Russian). (5) Gorokhovski, A. A.; Kaarli, R. K.; Rebane, L. A. Pis’ma Zh. Eksp. Teor. Fir. 1974, 29, 474 (in Russian). (6) Kharlamov, B. M.; Personov, R. I.; Bykovskaja, L. A. Opt. Commun. 1974, 12, 191. (7) Moerner, W. E. Ed. Persistant Spectral-Hole-Burning. Science and Applications; Springer-Verlag: Berlin, 1988. (8) Moerner, W. E.; Kador, L. Phys. Reu.Lert. 1989,62,2535. Ambrose, W. P.; Moemer, W. P.; Nature 1991,349,225. Moerner, W. E.; Bashe’, T. Angew. Chem., Int. Ed. Engl. 1993, 32, 457. (9) Orrit, M.; Bernard, J. Phys. Rev. Lett. 1990, 65, 2716. (10) Wild, U.; Giittler, F.; Pirotta, M.; Renn, A. Chem. Phys. Lett. 1992, 193, 451. (1 1) Palm, V.; Rebane, S.; Suisalu, A. In 4th Nordic-Baltic Workshop on Photochemistry; August 22-24, Program and Abstracts, Tartu, 1993. (12) Rebane, K. K.; Rebane, I. J . Lumin., in press. (13) Rebane, K. K.; Rebane, I. To be published. (14) Ambrose, W. P.; Basche’, T.; Moerner, W. E. J. Chem. Phys. 1991, 95, 7150. (15) Pirotta, M.;Giittler, F.; Gygax, H.; Renn,A.; Wild, U. Chem. Phys. Lett. 1993, 208, 379. (16) Basche’, T.; Ambrose, W. P.; Moerner, W. E. J. Opt. SOC.Am. B 1992, 9, 829. (17) Bernard, J.; Fleury, L.; Talon, H.; Orrit, M. J . Chem. Phys. 1993, 98, 850. (18) Ellervee, A.; Kikas, J.; Laisaar, A.; Shcherbakov, V.; Suisalu, A. J . Opt. SOC.Am. B 1992, 9, 972. (19) Croci, M.; Miischenborn, H.-J.; Giittler, F.; Renn, A.; Wild, U. Chem. Phys. Lett. 1993, 212, 71. (20) de Vries, H.; Wiersma, D. A. J . Chem. Phys. 1979, 70, 5807.
