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Optical studies of single terrylene molecules in polyethylene
LUMINESCENCE
Journal of Luminescence 56 (1993) 1—14
JOURNALOF
Optical studies of single terrylene molecules in polyethylene Paul Tchénio’, Anne B. Myers2 and W.E. Moerner IBM Research Division, Almaden Research Center, 650 Harry Road, San Jose, CA 95120-6099, USA Received 12 March 1993 Accepted 17 May 1993
Fluorescence excitation techniques have been used to study the optical spectroscopy and dynamics of single impurity molecules ofterrylene in a poly(ethylene) matrix at 1.5 K. We observe a variety of spectral diffusion effects, including spontaneous resonance frequency changes on the 10—100 MHz scale which lead to apparent fluctuations in the absorption line width and shape, discontinuous jumps in the resonance frequency of 100—2000 MHz on a time scale longer than 2.5 s, and long-lived light-induced changes in resonance frequency of more than a few GHz (single-molecule spectral hole-burning). We also observe for some molecules the unusual effect that the spectral diffusion rate and the frequency range increase for higher probing light intensity. For completeness, we also show the vibrationally resolved dispersed fluorescence spectrum in the ensemble-averaged (large N) limit, since such spectra have not been reported previously to our knowledge.
1. Introduction The recent achievement of optical detection and spectroscopy of single impurity molecules in solids (single molecule detection or SMD) by both absorption [1] and fluorescence excitation methods [2,3] has stimulated a variety of new measurements of the properties of single absorbers where ensemble averaging has been removed [4]. In the important model crystalline system pentacene in p-terphenyl, studies of power broadening and temperature-dependent dephasing [5], spectral diffusion [6], Stark effects [7], photon bunching [8], quantum antibunching [9], and dispersed fluorescence spectra [10] have been reported. Recently, the extension of these techniques to polymeric host materials was accomplished with the system perylene in poly(ethylene) (PE), which
Correspondence to: Dr. WE. Moerner, IBM Research Division, Almaden Research Center, 650 Harry Road, San Jose, CA 951206099, USA. . Permanent address: Laboratoire A. Cotton, CNRS II, Universite Paris XI, Bat. 405, 91405 Orsay, Cedex, France. 2 Permanent address: Department of Chemistry, University of Rochester, Rochester, NY 14627-0216, USA. 0022-2313/93/506.00 © 1993 SSDI 0022-2313(93)E0032-S
—
also exhibited spectral diffusion as well as singlemolecule hole-burning effects [11]. It was even possible in this case to measure the stochastic kinetics of a single molecule interacting with a single degree of freedom of the matrix behaving as a twolevel system (TLS) [12]. In the related system of terrylene (a higher homolog of perylene) in PE, a study of local fields by linear Stark effect measurements was reported [13]. Such studies provide complementary information to that from more conventional hole-burning studies [14—16] and may help to unravel the complex physical effects that arise from the nonequilibrium nature of these amorphous materials and the multiple “ground state” configurations of the chromophore-matrix system (i.e., the so-called two-level systems) [17,18]. In this paper we report fluorescence excitation spectra and kinetics of terrylene in PE at 1.5 K. Due to the likelihood that terrylene will become a useful new model system by virtue of its solubility in nonpolar polymers, we first describe the fluorescence emission spectrum for terrylene and cornpare the observed vibrational frequencies with the .
predictions of a standard force field calculation. Proceeding to the single-molecule regime, we first present detailed studies ofthe effect of probing laser
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studies of single terrylene molecules in polyethylene
power. We observe both triplet-state saturation and a new effect for some molecules, light-induced spectral diffusion, in which the spectral range accessible for diffusion increases at high probing intensity. We then describe our observations of spontaneous resonance frequency changes which are slow on the 2.5 s time scale of our measurements. We conclude with a summary of our measurements of light-induced hole-burning behavior where the absorption frequency of the molecule changes by more than several GHz for longer than 10 s as a result of the laser excitation. The broad array of physical effects that we have observed for different single molecules makes it clear that such studies open up a new and complex menagerie of physical effects for investigation,
photomultiplier tube and processed with photon counting electronics. The focused beam diameter at the sample, determined by monitoring the intensity of a single-molecule fluorescence feature while finely translating the laser spot across the sample [5], was 9.5 l.tm FWHM. Measured incident laser powers were converted to intensities by assuming that the molecule was located at the center of a Gaussian beam, and no local field corrections were applied. Dispersed fluorescence spectra of the bulk sample were obtained by scanning a Spex 1702 single monochromator equipped with a 1200 groove/mm grating blazed at 500 nm. The detector was a Hamamatsu R943-02 GaAs PMT with photon counting electronics. The excitation laser line and the 632.8 nm line from a He—Ne laser were used to calibrate the spectra in frequency.
2. Experimental 3. Fluorescence spectrum and analysis A small sample ofsolid terrylene was dissolved in methylene chloride and its concentration determined by absorption spectroscopy assuming a maximum molar extinction coefficient of 79400 M~cm’ [19]. A small quantity of this solution was added to a slurry of low density polyethylene powder (Microthene FN 500-00, Quantum Chemical Corp., ~ 25% crystallinity) in methylene chloride to give a final concentration of either 10—6 or iO~ by mass. After removing the solvent by drying under vacuum at 50°Covernight, samples were prepared by melting a small amount of the powder between two glass plates at 150°C,pressing to form a thin film, and quenching by plunging into liquid nitrogen. The resulting good optical quality films were spot-glued onto a LiF substrate and covered with a LiF cover slip coated with perylene in poly(vinylbutyral) as an optical alignment aid as described previously [10]. The optical set-up and electronics for excitation and detection of total fluorescence from single molecules have been described in detail elsewhere [5]. All experiments were carried out in superfluid hehum at 1.5 ±0.1 K. Terrylene fluorescence excited with a single-frequency tunable laser covering the 564—580 nm range was detected, typically through one or more Schott 00590 and/or RG61O longpass filters, by a cooled RCA C31034A-02 GaAs
Fig. 1 shows the dispersed fluorescence spectrum of terrylene excited near the center of the inhomogeneously broadened origin band. To our
500
cm1 from laser frequency 1000 1500 2000 2500
8
I
I
3000
I
I
000
.-~.
0 0 0 0 0 (x4)
0 I
600 .
.
I
625 650 Wavelength (nm)
I
675
Fig. 1. Dispersed fluorescence spectrum of terrylene in poly(ethylene) (PE) at )~= 570.4 nm and an incident intensity of 1 W/cm2. Slit width is 200 .im, providing a spectral resolution of 0.22 nm (~6 cm’).
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Optical studies of single terrylene molecules in polyethylene
knowledge this is the first vibrational spectrum of any kind reported for terrylene. Each vibronic band consists of a sharp line plus a broad phonon wing at longer wavelengths. The dominant feature in the spectrum is the intense low-frequency line at 242 cm which is not resolved in the much broader room temperature absorption spectrum [19]. Our emission spectrum has not been correc~,
ted for the wavelength dependence of the detection efficiency, which drops off slowly to longer wavelengths. Nevertheless, it is clear that the 242 cm ‘ line is very strong compared with the other vibrational features. The fluorescence linenarrowed spectrum of perylene in glassy ethanol [20] or pohy(methylmethacrylate) [21] also exhibits a strong band near 355 cm~that is presumably the analog of the 242 cm terrylene line, but the relative intensity of this band appears to be greater in terrylene than in perylene. Approximate integration of the band areas indicates that the 242 cm ‘line and its associated phonon wing account for about 42% of the total emission to the red of the origin. This demonstrates that the long-pass filters which we use in the single-molecule detection experiments unfortunately reject a significant fraction of the total fluorescence. A vibrational normal mode calculation for the in-plane modes of terrylene was carried out using the semi-empirical force field developed by Ohno for polycychic aromatics [22]. This force field consists of empirical parameters for CH stretching, CCH and CCC bending, and interactions between CC stretches and CCH and CCC bends, and cornputes the CC stretching diagonal and off-diagonal force constants from the bond orders and bond polarizabilities obtainedthere from simple interacHUckel calculation. In general areanonzero tion force constants between all pairs of CC stretches, but for our calculations of the vibrational frequencies we retained only those having absolute values greater than 0.1 mdyne A-’ (a total of 56 independent force constants not related by symmetry). Table 1 compares the experimental and calculated vibrational frequencies. Only the 23 totally symmetric modes are included, as only these are expected to have significant intensity in the emission spectrum of this strongly allowed transition. The agreement between experimental
3
Table I Experimental and calculated vibrational frequencies for terrylene Experimental Intensityb Assignment Calculated frequency frequency [cm~]’ [cm_h]c 242
vs
266 48~7
s
v
23 h v~+ P onon 2v23
534 584 736 780 830 1037
s m w w w m
—
—
V16,
1269 1280 1297 1311 1355
S
V13
s m m m
v12 v13 + phonon v12 + phonon v10, V11 v9, v9 v7, (v12, v13) + v23 v6 v6 + phonon, v5 v6+v23 2(v12, v13) v6 + (v12, v13)
—
1529 1556 1580
m s m m w w
2553 2834
225 —
523 581 3v23 v19
—
v17 v15, v14
783 832 1051 1080,1140,1186 1285 1327 — —
1349, 1361 1429, 1440 1562 1569 1591 — — —
‘Estimated ±6cm’. Qualitativeuncertainties intensities based on peak heights. ‘Calculated from the force field of Ohno [22]. All totally symmetric modes except for CH stretches (v1 through v4) are listed. b
and calculated frequencies is remarkably good considering the empirical nature of the potential. The normal mode for the tovibration calculated 1, corresponding the observed line at 225 cm 242 cm has a large contribution from stretching of the pen-CC bonds (those connecting the naphthalene units). In fact, the pen-CC stretching diagonal force constant makes a larger contribution to the potential energy distribution of the 225 cm mode than to any other. The other vibrations haying a large contribution from this force constant are those calculated at 1285, 1327 and 1569 cm~, which probably correspond to the strong bands observed at 1269, 1280 and 1556 cm respectively. These results strongly suggest that the greatest geometry change accompanying electronic — ~,
-
~,
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P. Tchénio et a!.
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Optical studies of single terrylene molecules in polyethylene
excitation is in the lengths of the bonds connecting the naphthalene units, and the contribution of this coordinate to the nominally “CCC bending” mode at 242 cm accounts for its high intensity in emission.
I
I I
Molecule A(m)
÷64
20~32~ ~÷16
4. Single molecule spectra: power dependence and photoinduced spectral diffusion Since a variety of spectral changes occur as a function of time and laser irradiation in the terrylene in PE system, we qualitatively divide up the observed phenomena into three regimes. We describe in this section the effects of changes in the resonance frequency of single molecules which are fast on the time scale required to obtain a (time) average of the spectrum, ~ 40 s. As in prior studies [12], we will use the term “spectral diffusion” to refer to changes in the resonance frequency of an individual absorber that are less than several GHz in magnitude. In this paper, we will where possible distinguish between spontaneous spectral diffusion (driven by phonons) and light-induced spectral diffusion (driven by photon absorption), a new effect we have observed for some molecules. In the second regime, spectral changes that occur on a time scale longer than 40s are described in Section 5. In the third regime, light-induced spectral shifts that persist for times longer than tens of seconds are formally equivalent to nonphotochemical hole burning [15] and are described in Section 6. Figure 2 shows a typical series of fluorescence excitation spectra of a single molecule of terrylene in PE (molecule “A” at 0 GHz) at 1.5 K as a function of laser intensity. At the lowest powers, the spectra clearly exhibit a narrow structure that is characteristic of the single molecule, as well as several smaller structures from other, out-of-focus molecules. As with perylene in PE [12], the line widths measured at low intensities greatly exceed the lifetime-limited value. Integration of the roomtemperature absorption spectrum of terrylene in CH yieldsradiative an oscillator strength and2C12 a natural lifetime of aboutof 10f=ns.0.55 As the fluorescence quantum yield is high (probably higher than 0.7), the estimated lifetime-limited line width is about 20 MHz, compared to the measured
:2 0
~
05
—1
0 1 Detuning (GHz)
2
Fig. 2. Fluorescence excitation spectra of a single molecule of terrylene in PE at a center wavelength of A = 577.45 nm (molecule “A”). Each trace is an average of 16 scans of 2.5 s duration. The incident laser intensities are 6, 12, 24,48,96, 190, 380 and 770 2 from bottom (a and b) to top (m), respectively. mW/cm
low-intensity widths of 110—200 MHz. Therefore, the line widths result from pure dephasing and/or spectral diffusion during the 40 s needed to record each spectrum. Evidence that spectral diffusion makes some contribution to the width comes from the line shape and line width fluctuations from one recording to another, which cannot be explained solely by the detection noise. To be more precise, the line width and the peak amplitude and frequency for molecule A differ for successive acquisitions of the spectrum at the same intensity (compare traces (d) and (e), (f) and (g), and (h) and (i)). These fluctuations in apparent line width and shape are clear evidence for fast spectral diffusion within a frequency range of 100—200 MHz during the laser scans. Because no significant increase 2 of the line observed to 100 mW/cm (traces (h),width (i)), iswe believeup that light-induced broadening is negligible at the lowest powers. The line widths measured in our experiment are nevertheless about 50 MHz higher than those reported
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by Orrit et al. [13]. This is not particularly surprising in light of the different sources of PE used and the well-known variations in crystallinity that can occur for PE [23]. The different spectra of Fig. 2 are normalized by a displayed at the right each profile which is factor inversely proportional to theofintensity (except for the highest power scan). In the absence of power saturation, spectral diffusion, or hole burning, the different profiles should be identical. The decrease of the peak amplitude with increasing laser intensity is evidence of power saturation. In addition, some broadening of the line is observed at the highest powers, although the variations in line width at intensities below 200 mW/cm2 appear to be essentially uncorrelated with intensity and reflect the random fluctuations described above. To quantify this behavior, the peak detected emission rate above background and the average observed line width were extracted from data like those of Fig. 2 for five different molecules as shown in Figs. 3 and 4. For all the molecules studied, the
I
I
I
40 .Molecule C
‘-.°
200
0
~
C
0 ~~ 40 Molecule B_~,,,9Z 0
fl
0 200
0
CD
a.
~ N
05
80 MoIecuI~
200
_______________________
0~ 1
10 100 IntensIty (mW/cm2)
5
Optical studies of single terrylene molecules in polyethylene
1000
0
Fig. 3. Saturation behavior of single molecules of terrylene in PE. Plots for molecule A summarize the data of Fig. 2. Molecules B and C were observed in two other similar data sets at center wavelengths of 579.62 and 564.19 nm, respectively. Experimental peak signal levels and line widths are shown as open squares and filled dots, respectively. Dashed and solid curves are theoretical fits to the count rate and line width as calculated from Eqs. (1) and (2)with saturation intensities I~of 100, 550 and 125 mW/cm2 and corresponding limiting line widths AVFWHM(O) of 110, 180 and 165 MHz for molecules A, B and C, respectively,
~ (12
4
Molecule
E
. ~
~-‘. .
CD
0 0 ‘~D
200 r-
a
.~
~
~
~ 40
0 ~
~
200 ~
05 .
0
o;
~
1
0 0
10
Intensity
100 2)
1000
0
(mW/cm
Fig. 4. Same as Fig. 3, for molecules at A = 575.05 nm (molecule “D”) and at 574.40 nm (molecule “E”). Fitting parameters are Is = 70 and 7.5 mW/cm2 and ESVFWHM(O) = 180 and 175 MHz for molecules D and E, respectively. In each case the anomalous point at the highest intensity was omitted when fitting the data to Eqs. (1) and (2).
peak count rate saturates at high intensities. The most striking initial observation is that the detected fluorescence count rate in the saturation regime is at least 6—10 times higher than the maximum emission rate observed for the pentacene in p-terphenyl system [5]. This implies that the fluorescence quantum yield 4~is high, and, assuming that the triplet state is the bottleneck producing saturation, the ratio YI~/YT of the intersystem crossing rate to the triplet decay rate is relatively small compared to pentacene. The former point is supported by a measurement of 4~ = 0.7 for a tetra-t-butyl-substituted terrylene in 1,4-dioxane at room temper-
ature [24]. It is clear that direct measurement of the usual ensemble-averaged photophysical parameters (fluorescence yield and/or lifetime, intersystern crossing yield, and triplet state lifetime) for terrylene would be most helpful. The smooth curves in Fig. 3 were obtained from the standard three-level model of power broadening in the presence of a bottleneck state [5], which presumably would be the lowest triplet state in this system. 1~VFWHM(I)= 1\VFWHM(O)[l +
J/J~]~I2,
(1)
I/I R(I)
=
Rv, 1 + I/Is
(2)
where I is the laser intensity, R~is the saturated emission rate, I~is the saturation intensity, and
6
P. Tchènio et a!. / Optical studies of single zerrylene molecules in polyethylene
the low-intensity line width. In each case, the count rate data were fit with Eq. (2) and the resulting value of 1s was used in the fit to the line width. For the three molecules in Fig. 3, the broadening and rate-saturation behavior are consistent with the three-level model, except that there is a distribution of values for 1s and the low-power line width. Some caution is required in interpreting the dispersion in 1s values, since in these experiments neither the position of the focused laser spot on the sample nor the laser polarization were optimized for each molecule. Therefore the actual intensity at the molecule may be less than that calculated by assuming it is located at the center of the Gaussian beam. Rescaling the intensities to obtain the same low-field count rate (cps/(mW/ cm2)) for each molecule reduces the ratio of saturation intensities between molecules A and B from 5.5 to 2.6, but a fairly large difference remains. The variations in I~values are apparently not correlated to line width variations and may reflect differences in the intersystem crossing rates and triplet lifetimes as observed for the pentacene in p-terphenyl system [5,8], which have been attributed to sitedependent out-of-plane distortions [25]. Fig. 4 shows similar results for two additional single molecules (D and E), which do not exhibit simple three-level saturation. Here as in Fig. 3, the saturation intensity was determined by fitting to the count rate data, and the equations predict appreciable broadening of the lines at intensity levels where essentially no broadening is observed (cornpare the measured line widths and the solid curve). Clearly another process is involved in the count rate saturation. We propose that this new rnechanism is photoinduced spectral diffusion of the molecular absorption line. Direct evidence of such a process is shown for molecules D and E in Fig. 5 where satellite lines appear at high intensity. These satellite lines also allow us to explain the drops in the peak count rate observed for molecules D and E at the highest intensity. In the “spectrally jumping molecule” picture, a drop in the peak count rate at one laser frequency corresponds to more time spent in other spectral positions. In the case of molecule E, the intensity-dependent repartitioning of time spent at different spectral positions apparently did not result from an irreversible trans-
Molecule E
L~VFWHM(O)is
C 0)
~
_________________________________
~
Molecule D
z —2
—1
0
Detuning (GHz)
1
2
Fig. 5. Spectral jumps in the resonance frequencies of single terrylene molecules. D and E are the same molecules for which the saturation data are displayed in Fig. 4. Traces (1) and (3) each show two scans at low intensity (1 and 2 mW/cm2). Trace (2) shows the appearance of an additional spectral feature presumably due Scans to molecule the laser intensity is raised to 16 mW/cm2. (1) andD(2)when are normalized to the same height for the main peak, making the weak features due to out-of-focus molecules at — 1.3 and 1.5 GHz appear relatively stronger. Trace (4) shows molecule E appearing at two different spectral positions on successive scans at higher intensity (32 mW/cm2).
formation of the molecular environment, since the original spectrum was recovered when the laser intensity was lowered from its maximum value. However, in many other molecules attempts to study the highest intensities produced “permanent” hole burning as described in Section 6, while in other cases (e.g. molecule D) we did not examine whether the original spectrum was recovered upon lowering the intensity. The light-induced spectral diffusion effect and the appearance of satellite lines at high intensity could result from at least two physical mechanisms. In the standard picture [12], TLSs close to the impurity produce large spectral shifts that can be as large as the inhomogeneous line width (~100 cm ~‘) [26], TLSs farther away produce smaller spectral shifts, and TLSs very far away give rise to the very small shifts which manifest themselves as the broadening of the line width above the lifetime-limited value [15]. One possibility is that excitation of the chromophore is accompanied by some probability of flipping TLSs that are far enough from the molecule to shift the resonance by only a few hundred MHz. These shifts are more likely to be observed at high powers when the
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7
studies of single terrylene molecules in polyethylene
return rate of the TLS cannot compete with the photon absorption rate. This mechanism cannot easily be distinguished from a second possibility in which a nearby strongly coupled TLS is reversibly flipped by the optical excitation, accompanied by rearrangements of distant TLSs. Finally, we note that molecules which seem to exhibit reasonably normal three-level saturation
behavior were observed both on the red edge of the absorption band (molecules A and B) and on the blue side (molecule C), while molecules D and E, which clearly do not follow simple three-level saturation kinetics, absorb on the red side but closer to the band center. At present, however, we have insufficient statistics to determine whether there is a real correlation between the photophysical behavior and the molecule’s position within the inhomogeneous band, as the majority of molecules we examined absorbed far to the red (beyond 577 nm).
I
8
-~
4
0 C.,
___________________________________
-~ C .~
C’)
8
0
5 Relative Frequency (GHz)
~ 8
(a)
10
,~.
(c)
0
:~6 T (b)
o
5. Slow spontaneous spectral jumping and wandering behavior
I
~ 4
??
1’
ta) 0.
C’~ 2
In addition to the line width and line shape fluctuations symptomatic of “fast” spectral diffusion, we also observed slower discontinuous jumps in resonance frequency between or during the laser scans even at low power, similar to the behavior reported earlier for perylene in PE [12]. The fluorescence excitation spectra in the upper half of Fig. 6 illustrate this effect for molecule “F”. Each spectrum is an average of 16 scans of 2.5 s each. The spectral region chosen is far enough into the wings of the inhomogeneous line that only one singlemolecule feature is present in the frequency range scanned, as evidenced by the fact that when the absorption line appears at a new spectral position, it simultaneously disappears from the previous one. The irregular shape of profile (c) results from a jump of the absorption frequency during the 16-scan averaging time, as we demonstrate below, We observed that such molecules tend to repeatedly sample a small number of well-defined resonance frequencies. About 10 molecules were observed to exhibit this discontinuous jumping behavior, with jump sizes in the range of a few hun-
I
0
500
1000 1500 Time (s)
2000
2500
Fig. 6. Multi-state spectral jumping of a single molecule of terrylene in PE (molecule “F”). The bottom trace shows the relative peak2.5 frequency (central wavelengthexcitation 580.77 nm) recorded on repeated s scans of thefluorescence spectrum at 2. Arrows show points at an incident intensity of 25 mW/cm which the molecule was not found within the 11 GHz scanning range (see the text). The upper trace shows the excitation spectra (averages of 16 scans) recorded during time intervals when the molecule was at the spectral positions indicated by the labels in the lower trace.
dred MHz to a few GHz. Over one order of magnitude of intensity near 10 mW/cm2, no clear increase of the jumping rates with probing intensity was observed, which is why we tentatively conclude that this form of spectral diffusion is spontaneous. Similar effects were reported for pentacene in p-terphenyl crystal [6], for the amorphous system perylene in PE [11], and recently for terrylene in PE [13,27]. It should be noted that unless there is
8
P. Tchènio et a!.
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studies of single terrylene molecules in polyethylene
a strong correlation between the jump sizes for many different molecules in an ensemble, the observation of such spectral jumps is specific to single molecule experiments. For example, when many molecules are probed as in standard hole-burning experiments, the effect of the ensemble averaging is to produce an overall broadening of the observed linewidth. This effect was further explored by determining the peak position after each individual 2.5 s laser scan. The trajectory or trend of peak frequencies can then be plotted as a function of time with 2.5 s time resolution [5,6]. The bottom trace of Fig. 6 is such a frequency trend. At the points marked by arrows in the figure, the molecule disappeared from the detection range. The number of photons detected when the laser was in resonance during the count interval of 60 ms was 100 counts above background on average. The probability that, due to shot noise alone, the number of photons above background could drop to 30 counts (the level at which the molecule cannot be distinguished from other small spectral features in the scan range) is negligible (106). Therefore, we believe that the arrows represent times during which the molecule’s resonance frequency was either outside the 11 GHz scanning range or happened to be in the process of jumping between two positions during the laser scan. Between 0 and 980 s, the molecule appears mainly as a 2-state jumper, visiting only two spectral positions separated by 450 MHz. Between 980 s and 2600 s, the molecule almost exclusively visits two different spectral positions separated by the same spectral interval of 450 MHz. The spectral offset of these two new positions from the two previous positions is 2.5 0Hz. In the two-level system (TLS) picture for dynamics in glasses, this might suggest that the two jump sizes result from the coupling of the molecule to two different TLS, where flipping of one TLS shifts the molecular resonance by 450 MHz while the other one shifts it by 2.5 0Hz. The relatively slow (seconds) jumping rates we observe suggest that these jumps could be attributed to so-called “extrinsic” two-level systems (those associated with the physical interface between the impurity and the glass) [15,28], although the conclusion that the slower dynamics are due to extrinsic TLS is derived from data on hydrogen-
bonded glasses and may not be appropriate to PE. There is some suggestion that the flipping rate of the TLS inducing the 450 MHz jump is affected by the flipping of the TLS inducing the 2.5 GHz jump (the 450 MHz jumps are somewhat more frequent during the 0 to 980 s interval than during the 980—2600 s interval), but our data are far too limited to allow any definite conclusion to be drawn in this regard. Pure two-state jumpers (molecules that visit only two spectral positions before eventually undergoing permanent hole burning) were originally reported for perylene in PE [12]. Recently a fluorescence autocorrelation study of terrylene in PE reported many two-state jumpers, but on a much shorter time scale than ours (flipping rates of few ms compared to few tens of seconds in our experiment [27]). We seldom observed pure two-state jumping behavior for our terrylene in PE samples. The molecule displayed in Fig. 6 is clearly a multistate jumper that visits five spectral positions within the detection range and, possibly, an undetermined number of spectral positions outside it. Nevertheless, on a time scale of a few hundred seconds, the molecule effectively behaves as a twostater. Some spectral positions are visited for only a short time period (see location (c) in the trend which corresponds to the excitation spectrum (c) in the upper part of the figure). Such events can be missed when the observation time is short. Figure 7 illustrates a qualitatively different type of spontaneous spectral diffusion that was studied in detail for only one molecule, molecule “0”. First, the excitation spectra (a) and (b) in the upper panel were recorded at laser intensities of 8 and 2 mW/cm2, respectively. Note that the band shapes in the two scans are considerably different and the peak frequency has shifted by about 130 MHz. These variations from scan to scan are similar to, but larger than, those observed for molecules A through E. Also, the greater breadth of the spectrum at higher intensity suggested that these fluctuations might be partially light-induced. We therefore recorded the peak frequency of molecule G as function of time in the manner described above for molecule F. The results are displayed in the bottom panel of Fig. 7 as traces (c) and (d), obtained with intensities of 2 and 8 mW/cm2,
P. Tchènio et a!. / Optical studies of single terrylene molecules in polyethylene
8 I
~ C.)
I
I
~a),3 4
9
occurs on time scales intermediate between the ~ 200 ms needed to scan across the absorption line and the 2.5 s between successive scans, but the uncertainty in the peak positions prevents us from determining whether this motion represents jumping among a discrete set of closely spaced (< 100 MHz) frequencies or continuous spectral diffusion
Q
0
_____________________________________ 2 3 Relative Frequency (GHz)
2 I
(c)
:~ a)
o.1.5
C/)
(d)
~ FI ~
_____________________________________ 0 500 1000 Time (s)
Fig. 7. Spectral “wandering” of a single molecule of terrylene in PE (molecule “G”). The upper panel shows excitation spectra (averages of s each) for recorded at intensities of (a) 2 16 andscans (b) of 2 2.5 mW/cm2 a molecule at a center 8 mW/cm wavelength of 574.60 nm. The lower panel shows the relative peak frequency found in successive 2.5 s scans at (c) 2 mW/cm2 and (d) 8 mW/cm2. Arrows denote points at which the molecule was not found within the scanned range.
respectively. The lack of synchronization between the 60 ms photon counting interval and the start of each laser scan causes an uncertainty in the detected peak position of ±50 MHz; that is, even a perfectly stable absorption line would appear to undergo fluctuations of ±50 MHz. However, the spectral range of ~ 400 MHz visited by this molecule greatly exceeds this uncertainty. The probability that the peak frequency changes by at least 100 MHz between two successive 2.5 s scans is 58% and 53% in traces (c) and (d), respectively. This demonstrates that considerable spectral evolution
within a 400 MHz frequency interval. Within the small (factor of four) variation in laser intensity between traces (c) and (d), we observe no significant intensity dependence of either the overall range of frequencies visited or the probability of a significant (>100 MHz) jump between successive scans. This suggests that the differences in band shape between traces (a) and (b) are accidental and probably arise from line shape fluctuations similar to tral range. The greater number of occasions when the molecule was not 2, found scanning those described in Fig. albeitwithin over athe wider specrange at the lower intensity probably reflects the ifthe is shifting during the time it being lowerfrequency increases signal-to-noise the likelihood ratio that the of these peak will scans, beismissed which scanned. The behavior of molecule 0 is similar to that of molecules A through E, but 0 diffuses through a wider spectral range. It is clear that the measurement of the center frequency trend at 2.5 s intervals gives more information.than simple averaging of the line shape on a long time scale. 6. Single-molecule spectral hole burning and hole filling Spectral hole burning of a single molecule was previously studied in detail for the system perylene in PE [12]. We have observed similar, though somewhat more varied, effects in the terrylene in PE system. Figure 8 shows a relatively simple holeburning chronology observed for one molecule (molecule “H”). We first scanned the fluorescence excitation spectrum until a stable single-molecule feature was found, shown in trace (a) of the upper panel. This procedure already tends to bias our observations in favor of those molecules having higher photostability, as those that hole-burn too quickly simply are not detected as strong features in the initial fluorescence excitation scan. Having
10
P. Tchénio et a!. / Optical studies of single terrylene molecules in polyethylene
12 I
(c)
Detuning (GHz)
0
100 Time (s)
200
Fig. 8. “Permanent” spectral hole burning ofa single molecule of terrylene in PE (molecule “H”). Trace (a) in the upper panel shows the 2. Trace (b) below shows the excitation spectrum at a central wavelength of 578.64 nm recorded with a incident intensity of 5 mW/cm fluorescence intensity as a function of time as the laser is tuned into resonance (rising signal during first 20 s) and then held at a fixed frequency. At 105 s, the arrowdenotes an interruption to start a new acquisition. At -~ 180 s, the emission intensity abruptly drops to its background value. (c) A subsequent scan of the excitation spectrum shows that the molecule is no longer within the scanned range.
identified a strong and well isolated spectral feature, we then manually tuned the laser frequency to the peak of the molecular resonance (first 20 s of trace (b)) and then held it fixed at that frequency while monitoring the fluorescence intensity. The photon counting time was set to 200 ms for these recordings, corresponding to approximately 300 counts above background in each counting interval. After ~ 180 s of observation, the count rate suddenly dropped to and remained at the background level observed at the beginning of trace (b), presumably signaling irreversible burning of the molecule. A new excitation spectrum (trace (c)) confirmed that the molecule no longer absorbed within
the 4 0Hz region scanned. The resonance was still missing from a second excitation spectrum recorded 5 mm later. Trace (b) exhibits numerous “dropouts” of the emission rate to the background level before irreversible burning occurs. If we set a threshold of one-third of the mean count rate above background (which prevents interference from shot noise and dark noise) as a criterion for deciding whether the molecule is in resonance, we conclude that the molecule moved out of resonance 26 times before the final burning event. The durations of these “dark” periods range from 0.2 to 3 s and the average return time is 0.65 s. These dropouts can
P. Tchènio et al.
/
Optical studies of single terrylene molecules in polyethylene
11
Time (s) 2) is tuned into Fig. 9. Spectral hole burning with spontaneous reversal (molecule “I”). The laser (A = 579.14 nm, intensity 5 mW/cm resonance with a single-molecule excitation feature during the first 20 s. At ‘~ 40 s the fluorescence signal abruptly drops to the background level in a hole-burning event, recovers spontaneously at 80 s, and disappears again at 110 s.
originate from spontaneous spectral diffusion and/dr photoinduced spectral diffusion. The time scales involved are relevant to our observations of power saturation and intensity-dependent spectral broadening discussed in Section 4. In particular, return times greater than 200 ms are compatible with saturation of the emission rate without significant line broadening if the saturation mechanism is photoinduced jumping to a completely distinct spectral region. Of the 12 molecules studied in the fashion shown in Fig. 8(b), nine showed dropouts before well-defined burning and three did not. Another issue in hole-burning studies is the reversibility of the effect. For perylene in PE, Basché et al. showed that molecules often returned to their initial spectral position within seconds to minutes after burning at low laser powers [12]. For terrylene in PE, we (somewhat arbitrarily) distinguish between returns on a time scale shorter than 10 s and returns that take longer, because longer return times allow us to scan the spectrum to determine whether the molecule has in fact moved entirely outside the original spectral range. We have generally used the term “hole burning” to refer to unambiguous jumps to an entirely new spectral region from which the molecule returns either slowly or not at all. However, unlike the perylene/PE case where clear reversibility and long return times were observed, here we cannot easily prove that the jumps are indeed photoinduced, since most mdi-
vidual molecules underwent “irreversible” modification of their resonance frequencies too quickly for the intensity dependence of the disappearance and return rates to be explored. Among the 31 molecules that we tried to hole-burn, 22 disappeared and did not return to their initial spectral position within a 3—15 mm observation interval, eight disappeared and then returned, and we did not succeed in burning one molecule because it was spectrally diffusing too much to stay in resonance with the laser. Of the eight that did return, seven returned only once and the eighth returned only twice. Figure 9 shows one example (molecule “I”) for which the absorption line did return (once) to its initial spectral position. After the second burning event, it did not return within 15 mm. For the eight molecules that did return, the return times ranged from tens of seconds to 11 mm. (As discussed above, molecules that required less than ~ 10 s to return were not counted as having burned.) Taken as a whole, these holeburning data illustrate that irreversible phenomena are difficult to characterize for single quantum systems. Fig. 10 shows a fascinating effect that is reported here for the first time to our knowledge. Trace (a) shows the initial fluorescence excitation spectrum of molecule “J”. In trace (b) the laser was tuned into resonance with the absorption line and held at a fixed frequency. The very “noisy”
12
P. Tchènio et a!.
/ Optical
studies of single terrylene molecules in polyethylene I
(a)
~
50 100 time(s)
(e)
0
o4
LiJ~ ~~b)
Ce
a) C’)
(f)
4
4(g).
Relative laser frequency (GHz) Fig. 10. Spectral hole burning with slow recovery (molecule “J”). Traces (a) and (c) are the fluorescence excitation spectra 2, average of 16 scans of 2.5 seach) taken immediately before the hole-burning traces in (b) and (d) which (A = 580.29 nm, I = 5 mW/cm show the time dependence of the fluorescence emission at a fixed laser frequency. Traces (e) and (f) are the averages of the first 16 and second 16 2.5 s excitation scans after the final hole burning event. Trace (g) was obtained after a further 10 mm in the dark. See the text for details.
trace and large number of dropouts (similar to molecule H in Fig. 8 and different from molecule I in Fig. 9) suggest that molecule J was undergoing large-amplitude spontaneous and/or light-induced spectral diffusion. After 100 s it appeared to burn cleanly away, but a subsequent excitation spectrum (trace c) revealed that the resonance had merely shifted by 120 MHz. The laser was tuned to the peak of this new resonance and another attempt was made to hole-burn it (trace d). Again the fluorescence signal appeared to vanish, but on repeating the excitation scan the molecule had returned to the same frequency as in trace (c) (data not shown). Once again the laser was tuned into resonance, and this time the emission vanished after a few seconds of irradiation (data not shown). Subsequent scans of the excitation spectrum
showed the original resonance returning not all at once, but gradually! Traces (e) and (f) show the first two excitation spectra recorded “immediately” after the last hole-burning event (averages of the first 16 and second 16 scans of 2.5 s each), while trace (g) was obtained after a further 10 mm in the dark. The original resonance returns gradually over the course of the first 80 s, and has completely recovered its original shape, position, and count rate after “annealing” in the dark. This suggests that the molecule’s return was accompanied by a slow relaxation of the environment that resulted in either an increase in the time spent at the original resonance frequency or a change in the photophysical parameters controlling the emission rate. At this time, one can only speculate about the reasons for these changes.
P. Tchènio et a!.
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Optical studies of single terrylene molecules in polyethylene
13
7. Conclusions
Acknowledgements
In this work we have described a variety of spectral diffusion and hole-burning effects for single molecules of terrylene in poly(ethylene) at 1.5 K. Spontaneous spectral diffusion events on a small frequency scale lead to apparent fluctuations in the absorption line width and shape. At high probing intensities, a new light-induced spectral diffusion effect was observed for some molecules which causes decreases in the emission rate without an appreciable increase in the line width, suggesting that the molecule is spending an increased amount of time at spectral positions away from the laser wavelength. Discontinuousjumps in resonance frequency of 100—2000 MHz were also observed for several molecules which led to frequency trends with both discrete multi-state jumping and wandering character. Light-induced frequency changes lasting longer than 10 s in duration were attributed to nonphotochemical hole-burning processes. In contrast to the perylene/PE system reported earlier [12], little reversible burning was observed which prevented a detailed power-dependent and time-dependent study of the spectral hole-burning process. One particularly intriguing effect, slow reversal of the hole burning, was observed in one case. Taking all of these observations into account, it is quite clear that the properties of single molecules of terrylene in PE are complex and highly varied. This might have been expected, since each molecule is exquisitely sensitive to the immediate local environment, and the host material is an amorphous polymer with a broad distribution of ground state configurations. Single-molecule spectroscopy provides a detailed picture of the behavior of each member of the ensemble which contributes to the properties of the material as a whole. Many more such observations must be completed in order to provide better statistics about the occurrence of the previously unreported light-induced spectral diffusion effect and the slow reversal of hole-burning effect. In all cases, single-molecule spectroscopy clearly provides a wealth of new information about the properties of impurity centers in condensed matter which may be used as a test of detailed microscopic theories in the future.
We thank Dr. John Fetzer for the generous gift of a sample of terrylene, IBM for a Flory Sabbatical Award to A.B.M., and CNRS for support for P.T. We also thank M. Orrit for providing a copy of Ref. [27] prior to publication.
References [1] W.E. Moerner and L. Kador, Phys. Rev. Lett. 62 (1989) 2535. [2] M. Orrit and J. Bernard, Phys. Rev. Lett. 65 (1990) 2716. [3] WE. Moerner and W.P. Ambrose, Phys. Rev. Lett. 66 (1991) 1376. [4] WE. Moerner and Th. Basché, Angew. Chemie Int. Ed. EngI. 32 (1993) 457. [5] W.P. Ambrose, Th. Basché and WE. Moerner, J. Chem. Phys. 95 (1991) 7150. [6] W.P. Ambrose and WE. Moerner, Nature 349 (1991) 225. [7] UP. Wild, F. Güttler, M. Pirotta and A. Renn, Chem. Phys. Lett. 193 (1992) 451. [8] J. Bernard, L. Fleury, H. Talon and M. Orrit, J. Chem. Phys. 98 (1993) 850. [9] Th. Basché, WE. Moerner, M. Orrit and H. Talon, Phys. Rev. Lett. 69 (1992) 1516. [10] P. Tchénio, A.B. Myers and WE. Moerner, J. Phys. Chem. 97 (1993) 2491. [11] Th. Basché and WE. Moerner, Nature 355 (1992) 335. [12] Th. Basché, W.P. Ambrose and WE. Moerner, J. Opt. Soc. Am. B 9 (1992) 829. [13] M. Orrit, J. Bernard, A. Zumbusch and RI. Personov, Chem. Phys. Lett. 196 (1992) 595. [14] J. Friedrich and D. Haarer, in: Optical Spectroscopy of Glasses, ed. I. Zschokke (D. Reidel, 1986) pp. 149—198. [15] R. Jankowiak and G.J. Small, Science 237 (1987) 618. [16] See: Persistent Spectral Hole-Burning: Applications, ed. WE. Moerner, Topics in Science Current and Physics, Vol. 44 (Springer, Berlin, Heidelberg, 1988). [17] P.W. Anderson, B.I. Halperin and CM. Varma, Philos. Mag. 25 (1972) 1—9.. [18] See: Amorphous Solids: Low Temperature Properties, ed. WA. Phillips (Springer, Berlin, 1981). [19] E. Clar, 1964). Polycyclic Hydrocarbons (Academic Press, London, [20] 1.1. Abram, R.A. Auerbach, R.R. Birge, BE. Kohler and J.M. Stevenson, J. Chem. Phys. 63(1975) 2473. [21] GB. Hurst and iC. Wright, J. Chem. Phys. 95(1991)1479. [22] K. Ohno, J. Chem. Phys. 95 (1991) 5524. [23] H.P.H. Thijssen and S. VOlker, J. Chem. Phys. 85 (1986) 785. [24] A. Bohnen, K-H. Koch, W. Lüttke and K. Mullen, Angew. Chem. mt. Ed. Eng. 29 (1990) 525.
14
P. Tchènio et a!. / Optical studies of single terrylene molecules in polyethylene
[25] C. Kryschi, B. Wagner, W. Gorgas and D. Schmid, J. Lumin. 53 (1992) 468. [26] U. Bogner, T. Attenberger and R. Bauer, in: Spectral HoleBurning and Luminescence Line-Narrowing: Science and Applications Technical Digest, 1992, Vol. 22 (Optical
Society of America, Washington, DC, 1992) pp. 22—25. [27] A. Zumbusch, L. Fleury, R. Brown, J. Bernard and M. Orrit, Phys. Rev. Lett. 70 (1993) 3584. [28] J.M. Hayes, R.P. Stout and G.J. Small, J. Chem. Phys. 74 (1981) 4266.
Journal of Luminescence 56 (1993) 1—14
JOURNALOF
Optical studies of single terrylene molecules in polyethylene Paul Tchénio’, Anne B. Myers2 and W.E. Moerner IBM Research Division, Almaden Research Center, 650 Harry Road, San Jose, CA 95120-6099, USA Received 12 March 1993 Accepted 17 May 1993
Fluorescence excitation techniques have been used to study the optical spectroscopy and dynamics of single impurity molecules ofterrylene in a poly(ethylene) matrix at 1.5 K. We observe a variety of spectral diffusion effects, including spontaneous resonance frequency changes on the 10—100 MHz scale which lead to apparent fluctuations in the absorption line width and shape, discontinuous jumps in the resonance frequency of 100—2000 MHz on a time scale longer than 2.5 s, and long-lived light-induced changes in resonance frequency of more than a few GHz (single-molecule spectral hole-burning). We also observe for some molecules the unusual effect that the spectral diffusion rate and the frequency range increase for higher probing light intensity. For completeness, we also show the vibrationally resolved dispersed fluorescence spectrum in the ensemble-averaged (large N) limit, since such spectra have not been reported previously to our knowledge.
1. Introduction The recent achievement of optical detection and spectroscopy of single impurity molecules in solids (single molecule detection or SMD) by both absorption [1] and fluorescence excitation methods [2,3] has stimulated a variety of new measurements of the properties of single absorbers where ensemble averaging has been removed [4]. In the important model crystalline system pentacene in p-terphenyl, studies of power broadening and temperature-dependent dephasing [5], spectral diffusion [6], Stark effects [7], photon bunching [8], quantum antibunching [9], and dispersed fluorescence spectra [10] have been reported. Recently, the extension of these techniques to polymeric host materials was accomplished with the system perylene in poly(ethylene) (PE), which
Correspondence to: Dr. WE. Moerner, IBM Research Division, Almaden Research Center, 650 Harry Road, San Jose, CA 951206099, USA. . Permanent address: Laboratoire A. Cotton, CNRS II, Universite Paris XI, Bat. 405, 91405 Orsay, Cedex, France. 2 Permanent address: Department of Chemistry, University of Rochester, Rochester, NY 14627-0216, USA. 0022-2313/93/506.00 © 1993 SSDI 0022-2313(93)E0032-S
—
also exhibited spectral diffusion as well as singlemolecule hole-burning effects [11]. It was even possible in this case to measure the stochastic kinetics of a single molecule interacting with a single degree of freedom of the matrix behaving as a twolevel system (TLS) [12]. In the related system of terrylene (a higher homolog of perylene) in PE, a study of local fields by linear Stark effect measurements was reported [13]. Such studies provide complementary information to that from more conventional hole-burning studies [14—16] and may help to unravel the complex physical effects that arise from the nonequilibrium nature of these amorphous materials and the multiple “ground state” configurations of the chromophore-matrix system (i.e., the so-called two-level systems) [17,18]. In this paper we report fluorescence excitation spectra and kinetics of terrylene in PE at 1.5 K. Due to the likelihood that terrylene will become a useful new model system by virtue of its solubility in nonpolar polymers, we first describe the fluorescence emission spectrum for terrylene and cornpare the observed vibrational frequencies with the .
predictions of a standard force field calculation. Proceeding to the single-molecule regime, we first present detailed studies ofthe effect of probing laser
Elsevier Science Publishers B.V. All rights reserved
2
P. Tchènio et a!.
/ Optical
studies of single terrylene molecules in polyethylene
power. We observe both triplet-state saturation and a new effect for some molecules, light-induced spectral diffusion, in which the spectral range accessible for diffusion increases at high probing intensity. We then describe our observations of spontaneous resonance frequency changes which are slow on the 2.5 s time scale of our measurements. We conclude with a summary of our measurements of light-induced hole-burning behavior where the absorption frequency of the molecule changes by more than several GHz for longer than 10 s as a result of the laser excitation. The broad array of physical effects that we have observed for different single molecules makes it clear that such studies open up a new and complex menagerie of physical effects for investigation,
photomultiplier tube and processed with photon counting electronics. The focused beam diameter at the sample, determined by monitoring the intensity of a single-molecule fluorescence feature while finely translating the laser spot across the sample [5], was 9.5 l.tm FWHM. Measured incident laser powers were converted to intensities by assuming that the molecule was located at the center of a Gaussian beam, and no local field corrections were applied. Dispersed fluorescence spectra of the bulk sample were obtained by scanning a Spex 1702 single monochromator equipped with a 1200 groove/mm grating blazed at 500 nm. The detector was a Hamamatsu R943-02 GaAs PMT with photon counting electronics. The excitation laser line and the 632.8 nm line from a He—Ne laser were used to calibrate the spectra in frequency.
2. Experimental 3. Fluorescence spectrum and analysis A small sample ofsolid terrylene was dissolved in methylene chloride and its concentration determined by absorption spectroscopy assuming a maximum molar extinction coefficient of 79400 M~cm’ [19]. A small quantity of this solution was added to a slurry of low density polyethylene powder (Microthene FN 500-00, Quantum Chemical Corp., ~ 25% crystallinity) in methylene chloride to give a final concentration of either 10—6 or iO~ by mass. After removing the solvent by drying under vacuum at 50°Covernight, samples were prepared by melting a small amount of the powder between two glass plates at 150°C,pressing to form a thin film, and quenching by plunging into liquid nitrogen. The resulting good optical quality films were spot-glued onto a LiF substrate and covered with a LiF cover slip coated with perylene in poly(vinylbutyral) as an optical alignment aid as described previously [10]. The optical set-up and electronics for excitation and detection of total fluorescence from single molecules have been described in detail elsewhere [5]. All experiments were carried out in superfluid hehum at 1.5 ±0.1 K. Terrylene fluorescence excited with a single-frequency tunable laser covering the 564—580 nm range was detected, typically through one or more Schott 00590 and/or RG61O longpass filters, by a cooled RCA C31034A-02 GaAs
Fig. 1 shows the dispersed fluorescence spectrum of terrylene excited near the center of the inhomogeneously broadened origin band. To our
500
cm1 from laser frequency 1000 1500 2000 2500
8
I
I
3000
I
I
000
.-~.
0 0 0 0 0 (x4)
0 I
600 .
.
I
625 650 Wavelength (nm)
I
675
Fig. 1. Dispersed fluorescence spectrum of terrylene in poly(ethylene) (PE) at )~= 570.4 nm and an incident intensity of 1 W/cm2. Slit width is 200 .im, providing a spectral resolution of 0.22 nm (~6 cm’).
P. Tchénio et a!.
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Optical studies of single terrylene molecules in polyethylene
knowledge this is the first vibrational spectrum of any kind reported for terrylene. Each vibronic band consists of a sharp line plus a broad phonon wing at longer wavelengths. The dominant feature in the spectrum is the intense low-frequency line at 242 cm which is not resolved in the much broader room temperature absorption spectrum [19]. Our emission spectrum has not been correc~,
ted for the wavelength dependence of the detection efficiency, which drops off slowly to longer wavelengths. Nevertheless, it is clear that the 242 cm ‘ line is very strong compared with the other vibrational features. The fluorescence linenarrowed spectrum of perylene in glassy ethanol [20] or pohy(methylmethacrylate) [21] also exhibits a strong band near 355 cm~that is presumably the analog of the 242 cm terrylene line, but the relative intensity of this band appears to be greater in terrylene than in perylene. Approximate integration of the band areas indicates that the 242 cm ‘line and its associated phonon wing account for about 42% of the total emission to the red of the origin. This demonstrates that the long-pass filters which we use in the single-molecule detection experiments unfortunately reject a significant fraction of the total fluorescence. A vibrational normal mode calculation for the in-plane modes of terrylene was carried out using the semi-empirical force field developed by Ohno for polycychic aromatics [22]. This force field consists of empirical parameters for CH stretching, CCH and CCC bending, and interactions between CC stretches and CCH and CCC bends, and cornputes the CC stretching diagonal and off-diagonal force constants from the bond orders and bond polarizabilities obtainedthere from simple interacHUckel calculation. In general areanonzero tion force constants between all pairs of CC stretches, but for our calculations of the vibrational frequencies we retained only those having absolute values greater than 0.1 mdyne A-’ (a total of 56 independent force constants not related by symmetry). Table 1 compares the experimental and calculated vibrational frequencies. Only the 23 totally symmetric modes are included, as only these are expected to have significant intensity in the emission spectrum of this strongly allowed transition. The agreement between experimental
3
Table I Experimental and calculated vibrational frequencies for terrylene Experimental Intensityb Assignment Calculated frequency frequency [cm~]’ [cm_h]c 242
vs
266 48~7
s
v
23 h v~+ P onon 2v23
534 584 736 780 830 1037
s m w w w m
—
—
V16,
1269 1280 1297 1311 1355
S
V13
s m m m
v12 v13 + phonon v12 + phonon v10, V11 v9, v9 v7, (v12, v13) + v23 v6 v6 + phonon, v5 v6+v23 2(v12, v13) v6 + (v12, v13)
—
1529 1556 1580
m s m m w w
2553 2834
225 —
523 581 3v23 v19
—
v17 v15, v14
783 832 1051 1080,1140,1186 1285 1327 — —
1349, 1361 1429, 1440 1562 1569 1591 — — —
‘Estimated ±6cm’. Qualitativeuncertainties intensities based on peak heights. ‘Calculated from the force field of Ohno [22]. All totally symmetric modes except for CH stretches (v1 through v4) are listed. b
and calculated frequencies is remarkably good considering the empirical nature of the potential. The normal mode for the tovibration calculated 1, corresponding the observed line at 225 cm 242 cm has a large contribution from stretching of the pen-CC bonds (those connecting the naphthalene units). In fact, the pen-CC stretching diagonal force constant makes a larger contribution to the potential energy distribution of the 225 cm mode than to any other. The other vibrations haying a large contribution from this force constant are those calculated at 1285, 1327 and 1569 cm~, which probably correspond to the strong bands observed at 1269, 1280 and 1556 cm respectively. These results strongly suggest that the greatest geometry change accompanying electronic — ~,
-
~,
4
P. Tchénio et a!.
/
Optical studies of single terrylene molecules in polyethylene
excitation is in the lengths of the bonds connecting the naphthalene units, and the contribution of this coordinate to the nominally “CCC bending” mode at 242 cm accounts for its high intensity in emission.
I
I I
Molecule A(m)
÷64
20~32~ ~÷16
4. Single molecule spectra: power dependence and photoinduced spectral diffusion Since a variety of spectral changes occur as a function of time and laser irradiation in the terrylene in PE system, we qualitatively divide up the observed phenomena into three regimes. We describe in this section the effects of changes in the resonance frequency of single molecules which are fast on the time scale required to obtain a (time) average of the spectrum, ~ 40 s. As in prior studies [12], we will use the term “spectral diffusion” to refer to changes in the resonance frequency of an individual absorber that are less than several GHz in magnitude. In this paper, we will where possible distinguish between spontaneous spectral diffusion (driven by phonons) and light-induced spectral diffusion (driven by photon absorption), a new effect we have observed for some molecules. In the second regime, spectral changes that occur on a time scale longer than 40s are described in Section 5. In the third regime, light-induced spectral shifts that persist for times longer than tens of seconds are formally equivalent to nonphotochemical hole burning [15] and are described in Section 6. Figure 2 shows a typical series of fluorescence excitation spectra of a single molecule of terrylene in PE (molecule “A” at 0 GHz) at 1.5 K as a function of laser intensity. At the lowest powers, the spectra clearly exhibit a narrow structure that is characteristic of the single molecule, as well as several smaller structures from other, out-of-focus molecules. As with perylene in PE [12], the line widths measured at low intensities greatly exceed the lifetime-limited value. Integration of the roomtemperature absorption spectrum of terrylene in CH yieldsradiative an oscillator strength and2C12 a natural lifetime of aboutof 10f=ns.0.55 As the fluorescence quantum yield is high (probably higher than 0.7), the estimated lifetime-limited line width is about 20 MHz, compared to the measured
:2 0
~
05
—1
0 1 Detuning (GHz)
2
Fig. 2. Fluorescence excitation spectra of a single molecule of terrylene in PE at a center wavelength of A = 577.45 nm (molecule “A”). Each trace is an average of 16 scans of 2.5 s duration. The incident laser intensities are 6, 12, 24,48,96, 190, 380 and 770 2 from bottom (a and b) to top (m), respectively. mW/cm
low-intensity widths of 110—200 MHz. Therefore, the line widths result from pure dephasing and/or spectral diffusion during the 40 s needed to record each spectrum. Evidence that spectral diffusion makes some contribution to the width comes from the line shape and line width fluctuations from one recording to another, which cannot be explained solely by the detection noise. To be more precise, the line width and the peak amplitude and frequency for molecule A differ for successive acquisitions of the spectrum at the same intensity (compare traces (d) and (e), (f) and (g), and (h) and (i)). These fluctuations in apparent line width and shape are clear evidence for fast spectral diffusion within a frequency range of 100—200 MHz during the laser scans. Because no significant increase 2 of the line observed to 100 mW/cm (traces (h),width (i)), iswe believeup that light-induced broadening is negligible at the lowest powers. The line widths measured in our experiment are nevertheless about 50 MHz higher than those reported
P. Tchènio et al.
/
by Orrit et al. [13]. This is not particularly surprising in light of the different sources of PE used and the well-known variations in crystallinity that can occur for PE [23]. The different spectra of Fig. 2 are normalized by a displayed at the right each profile which is factor inversely proportional to theofintensity (except for the highest power scan). In the absence of power saturation, spectral diffusion, or hole burning, the different profiles should be identical. The decrease of the peak amplitude with increasing laser intensity is evidence of power saturation. In addition, some broadening of the line is observed at the highest powers, although the variations in line width at intensities below 200 mW/cm2 appear to be essentially uncorrelated with intensity and reflect the random fluctuations described above. To quantify this behavior, the peak detected emission rate above background and the average observed line width were extracted from data like those of Fig. 2 for five different molecules as shown in Figs. 3 and 4. For all the molecules studied, the
I
I
I
40 .Molecule C
‘-.°
200
0
~
C
0 ~~ 40 Molecule B_~,,,9Z 0
fl
0 200
0
CD
a.
~ N
05
80 MoIecuI~
200
_______________________
0~ 1
10 100 IntensIty (mW/cm2)
5
Optical studies of single terrylene molecules in polyethylene
1000
0
Fig. 3. Saturation behavior of single molecules of terrylene in PE. Plots for molecule A summarize the data of Fig. 2. Molecules B and C were observed in two other similar data sets at center wavelengths of 579.62 and 564.19 nm, respectively. Experimental peak signal levels and line widths are shown as open squares and filled dots, respectively. Dashed and solid curves are theoretical fits to the count rate and line width as calculated from Eqs. (1) and (2)with saturation intensities I~of 100, 550 and 125 mW/cm2 and corresponding limiting line widths AVFWHM(O) of 110, 180 and 165 MHz for molecules A, B and C, respectively,
~ (12
4
Molecule
E
. ~
~-‘. .
CD
0 0 ‘~D
200 r-
a
.~
~
~
~ 40
0 ~
~
200 ~
05 .
0
o;
~
1
0 0
10
Intensity
100 2)
1000
0
(mW/cm
Fig. 4. Same as Fig. 3, for molecules at A = 575.05 nm (molecule “D”) and at 574.40 nm (molecule “E”). Fitting parameters are Is = 70 and 7.5 mW/cm2 and ESVFWHM(O) = 180 and 175 MHz for molecules D and E, respectively. In each case the anomalous point at the highest intensity was omitted when fitting the data to Eqs. (1) and (2).
peak count rate saturates at high intensities. The most striking initial observation is that the detected fluorescence count rate in the saturation regime is at least 6—10 times higher than the maximum emission rate observed for the pentacene in p-terphenyl system [5]. This implies that the fluorescence quantum yield 4~is high, and, assuming that the triplet state is the bottleneck producing saturation, the ratio YI~/YT of the intersystem crossing rate to the triplet decay rate is relatively small compared to pentacene. The former point is supported by a measurement of 4~ = 0.7 for a tetra-t-butyl-substituted terrylene in 1,4-dioxane at room temper-
ature [24]. It is clear that direct measurement of the usual ensemble-averaged photophysical parameters (fluorescence yield and/or lifetime, intersystern crossing yield, and triplet state lifetime) for terrylene would be most helpful. The smooth curves in Fig. 3 were obtained from the standard three-level model of power broadening in the presence of a bottleneck state [5], which presumably would be the lowest triplet state in this system. 1~VFWHM(I)= 1\VFWHM(O)[l +
J/J~]~I2,
(1)
I/I R(I)
=
Rv, 1 + I/Is
(2)
where I is the laser intensity, R~is the saturated emission rate, I~is the saturation intensity, and
6
P. Tchènio et a!. / Optical studies of single zerrylene molecules in polyethylene
the low-intensity line width. In each case, the count rate data were fit with Eq. (2) and the resulting value of 1s was used in the fit to the line width. For the three molecules in Fig. 3, the broadening and rate-saturation behavior are consistent with the three-level model, except that there is a distribution of values for 1s and the low-power line width. Some caution is required in interpreting the dispersion in 1s values, since in these experiments neither the position of the focused laser spot on the sample nor the laser polarization were optimized for each molecule. Therefore the actual intensity at the molecule may be less than that calculated by assuming it is located at the center of the Gaussian beam. Rescaling the intensities to obtain the same low-field count rate (cps/(mW/ cm2)) for each molecule reduces the ratio of saturation intensities between molecules A and B from 5.5 to 2.6, but a fairly large difference remains. The variations in I~values are apparently not correlated to line width variations and may reflect differences in the intersystem crossing rates and triplet lifetimes as observed for the pentacene in p-terphenyl system [5,8], which have been attributed to sitedependent out-of-plane distortions [25]. Fig. 4 shows similar results for two additional single molecules (D and E), which do not exhibit simple three-level saturation. Here as in Fig. 3, the saturation intensity was determined by fitting to the count rate data, and the equations predict appreciable broadening of the lines at intensity levels where essentially no broadening is observed (cornpare the measured line widths and the solid curve). Clearly another process is involved in the count rate saturation. We propose that this new rnechanism is photoinduced spectral diffusion of the molecular absorption line. Direct evidence of such a process is shown for molecules D and E in Fig. 5 where satellite lines appear at high intensity. These satellite lines also allow us to explain the drops in the peak count rate observed for molecules D and E at the highest intensity. In the “spectrally jumping molecule” picture, a drop in the peak count rate at one laser frequency corresponds to more time spent in other spectral positions. In the case of molecule E, the intensity-dependent repartitioning of time spent at different spectral positions apparently did not result from an irreversible trans-
Molecule E
L~VFWHM(O)is
C 0)
~
_________________________________
~
Molecule D
z —2
—1
0
Detuning (GHz)
1
2
Fig. 5. Spectral jumps in the resonance frequencies of single terrylene molecules. D and E are the same molecules for which the saturation data are displayed in Fig. 4. Traces (1) and (3) each show two scans at low intensity (1 and 2 mW/cm2). Trace (2) shows the appearance of an additional spectral feature presumably due Scans to molecule the laser intensity is raised to 16 mW/cm2. (1) andD(2)when are normalized to the same height for the main peak, making the weak features due to out-of-focus molecules at — 1.3 and 1.5 GHz appear relatively stronger. Trace (4) shows molecule E appearing at two different spectral positions on successive scans at higher intensity (32 mW/cm2).
formation of the molecular environment, since the original spectrum was recovered when the laser intensity was lowered from its maximum value. However, in many other molecules attempts to study the highest intensities produced “permanent” hole burning as described in Section 6, while in other cases (e.g. molecule D) we did not examine whether the original spectrum was recovered upon lowering the intensity. The light-induced spectral diffusion effect and the appearance of satellite lines at high intensity could result from at least two physical mechanisms. In the standard picture [12], TLSs close to the impurity produce large spectral shifts that can be as large as the inhomogeneous line width (~100 cm ~‘) [26], TLSs farther away produce smaller spectral shifts, and TLSs very far away give rise to the very small shifts which manifest themselves as the broadening of the line width above the lifetime-limited value [15]. One possibility is that excitation of the chromophore is accompanied by some probability of flipping TLSs that are far enough from the molecule to shift the resonance by only a few hundred MHz. These shifts are more likely to be observed at high powers when the
P. Tchènio et al.
/ Optical
7
studies of single terrylene molecules in polyethylene
return rate of the TLS cannot compete with the photon absorption rate. This mechanism cannot easily be distinguished from a second possibility in which a nearby strongly coupled TLS is reversibly flipped by the optical excitation, accompanied by rearrangements of distant TLSs. Finally, we note that molecules which seem to exhibit reasonably normal three-level saturation
behavior were observed both on the red edge of the absorption band (molecules A and B) and on the blue side (molecule C), while molecules D and E, which clearly do not follow simple three-level saturation kinetics, absorb on the red side but closer to the band center. At present, however, we have insufficient statistics to determine whether there is a real correlation between the photophysical behavior and the molecule’s position within the inhomogeneous band, as the majority of molecules we examined absorbed far to the red (beyond 577 nm).
I
8
-~
4
0 C.,
___________________________________
-~ C .~
C’)
8
0
5 Relative Frequency (GHz)
~ 8
(a)
10
,~.
(c)
0
:~6 T (b)
o
5. Slow spontaneous spectral jumping and wandering behavior
I
~ 4
??
1’
ta) 0.
C’~ 2
In addition to the line width and line shape fluctuations symptomatic of “fast” spectral diffusion, we also observed slower discontinuous jumps in resonance frequency between or during the laser scans even at low power, similar to the behavior reported earlier for perylene in PE [12]. The fluorescence excitation spectra in the upper half of Fig. 6 illustrate this effect for molecule “F”. Each spectrum is an average of 16 scans of 2.5 s each. The spectral region chosen is far enough into the wings of the inhomogeneous line that only one singlemolecule feature is present in the frequency range scanned, as evidenced by the fact that when the absorption line appears at a new spectral position, it simultaneously disappears from the previous one. The irregular shape of profile (c) results from a jump of the absorption frequency during the 16-scan averaging time, as we demonstrate below, We observed that such molecules tend to repeatedly sample a small number of well-defined resonance frequencies. About 10 molecules were observed to exhibit this discontinuous jumping behavior, with jump sizes in the range of a few hun-
I
0
500
1000 1500 Time (s)
2000
2500
Fig. 6. Multi-state spectral jumping of a single molecule of terrylene in PE (molecule “F”). The bottom trace shows the relative peak2.5 frequency (central wavelengthexcitation 580.77 nm) recorded on repeated s scans of thefluorescence spectrum at 2. Arrows show points at an incident intensity of 25 mW/cm which the molecule was not found within the 11 GHz scanning range (see the text). The upper trace shows the excitation spectra (averages of 16 scans) recorded during time intervals when the molecule was at the spectral positions indicated by the labels in the lower trace.
dred MHz to a few GHz. Over one order of magnitude of intensity near 10 mW/cm2, no clear increase of the jumping rates with probing intensity was observed, which is why we tentatively conclude that this form of spectral diffusion is spontaneous. Similar effects were reported for pentacene in p-terphenyl crystal [6], for the amorphous system perylene in PE [11], and recently for terrylene in PE [13,27]. It should be noted that unless there is
8
P. Tchènio et a!.
/ Optical
studies of single terrylene molecules in polyethylene
a strong correlation between the jump sizes for many different molecules in an ensemble, the observation of such spectral jumps is specific to single molecule experiments. For example, when many molecules are probed as in standard hole-burning experiments, the effect of the ensemble averaging is to produce an overall broadening of the observed linewidth. This effect was further explored by determining the peak position after each individual 2.5 s laser scan. The trajectory or trend of peak frequencies can then be plotted as a function of time with 2.5 s time resolution [5,6]. The bottom trace of Fig. 6 is such a frequency trend. At the points marked by arrows in the figure, the molecule disappeared from the detection range. The number of photons detected when the laser was in resonance during the count interval of 60 ms was 100 counts above background on average. The probability that, due to shot noise alone, the number of photons above background could drop to 30 counts (the level at which the molecule cannot be distinguished from other small spectral features in the scan range) is negligible (106). Therefore, we believe that the arrows represent times during which the molecule’s resonance frequency was either outside the 11 GHz scanning range or happened to be in the process of jumping between two positions during the laser scan. Between 0 and 980 s, the molecule appears mainly as a 2-state jumper, visiting only two spectral positions separated by 450 MHz. Between 980 s and 2600 s, the molecule almost exclusively visits two different spectral positions separated by the same spectral interval of 450 MHz. The spectral offset of these two new positions from the two previous positions is 2.5 0Hz. In the two-level system (TLS) picture for dynamics in glasses, this might suggest that the two jump sizes result from the coupling of the molecule to two different TLS, where flipping of one TLS shifts the molecular resonance by 450 MHz while the other one shifts it by 2.5 0Hz. The relatively slow (seconds) jumping rates we observe suggest that these jumps could be attributed to so-called “extrinsic” two-level systems (those associated with the physical interface between the impurity and the glass) [15,28], although the conclusion that the slower dynamics are due to extrinsic TLS is derived from data on hydrogen-
bonded glasses and may not be appropriate to PE. There is some suggestion that the flipping rate of the TLS inducing the 450 MHz jump is affected by the flipping of the TLS inducing the 2.5 GHz jump (the 450 MHz jumps are somewhat more frequent during the 0 to 980 s interval than during the 980—2600 s interval), but our data are far too limited to allow any definite conclusion to be drawn in this regard. Pure two-state jumpers (molecules that visit only two spectral positions before eventually undergoing permanent hole burning) were originally reported for perylene in PE [12]. Recently a fluorescence autocorrelation study of terrylene in PE reported many two-state jumpers, but on a much shorter time scale than ours (flipping rates of few ms compared to few tens of seconds in our experiment [27]). We seldom observed pure two-state jumping behavior for our terrylene in PE samples. The molecule displayed in Fig. 6 is clearly a multistate jumper that visits five spectral positions within the detection range and, possibly, an undetermined number of spectral positions outside it. Nevertheless, on a time scale of a few hundred seconds, the molecule effectively behaves as a twostater. Some spectral positions are visited for only a short time period (see location (c) in the trend which corresponds to the excitation spectrum (c) in the upper part of the figure). Such events can be missed when the observation time is short. Figure 7 illustrates a qualitatively different type of spontaneous spectral diffusion that was studied in detail for only one molecule, molecule “0”. First, the excitation spectra (a) and (b) in the upper panel were recorded at laser intensities of 8 and 2 mW/cm2, respectively. Note that the band shapes in the two scans are considerably different and the peak frequency has shifted by about 130 MHz. These variations from scan to scan are similar to, but larger than, those observed for molecules A through E. Also, the greater breadth of the spectrum at higher intensity suggested that these fluctuations might be partially light-induced. We therefore recorded the peak frequency of molecule G as function of time in the manner described above for molecule F. The results are displayed in the bottom panel of Fig. 7 as traces (c) and (d), obtained with intensities of 2 and 8 mW/cm2,
P. Tchènio et a!. / Optical studies of single terrylene molecules in polyethylene
8 I
~ C.)
I
I
~a),3 4
9
occurs on time scales intermediate between the ~ 200 ms needed to scan across the absorption line and the 2.5 s between successive scans, but the uncertainty in the peak positions prevents us from determining whether this motion represents jumping among a discrete set of closely spaced (< 100 MHz) frequencies or continuous spectral diffusion
Q
0
_____________________________________ 2 3 Relative Frequency (GHz)
2 I
(c)
:~ a)
o.1.5
C/)
(d)
~ FI ~
_____________________________________ 0 500 1000 Time (s)
Fig. 7. Spectral “wandering” of a single molecule of terrylene in PE (molecule “G”). The upper panel shows excitation spectra (averages of s each) for recorded at intensities of (a) 2 16 andscans (b) of 2 2.5 mW/cm2 a molecule at a center 8 mW/cm wavelength of 574.60 nm. The lower panel shows the relative peak frequency found in successive 2.5 s scans at (c) 2 mW/cm2 and (d) 8 mW/cm2. Arrows denote points at which the molecule was not found within the scanned range.
respectively. The lack of synchronization between the 60 ms photon counting interval and the start of each laser scan causes an uncertainty in the detected peak position of ±50 MHz; that is, even a perfectly stable absorption line would appear to undergo fluctuations of ±50 MHz. However, the spectral range of ~ 400 MHz visited by this molecule greatly exceeds this uncertainty. The probability that the peak frequency changes by at least 100 MHz between two successive 2.5 s scans is 58% and 53% in traces (c) and (d), respectively. This demonstrates that considerable spectral evolution
within a 400 MHz frequency interval. Within the small (factor of four) variation in laser intensity between traces (c) and (d), we observe no significant intensity dependence of either the overall range of frequencies visited or the probability of a significant (>100 MHz) jump between successive scans. This suggests that the differences in band shape between traces (a) and (b) are accidental and probably arise from line shape fluctuations similar to tral range. The greater number of occasions when the molecule was not 2, found scanning those described in Fig. albeitwithin over athe wider specrange at the lower intensity probably reflects the ifthe is shifting during the time it being lowerfrequency increases signal-to-noise the likelihood ratio that the of these peak will scans, beismissed which scanned. The behavior of molecule 0 is similar to that of molecules A through E, but 0 diffuses through a wider spectral range. It is clear that the measurement of the center frequency trend at 2.5 s intervals gives more information.than simple averaging of the line shape on a long time scale. 6. Single-molecule spectral hole burning and hole filling Spectral hole burning of a single molecule was previously studied in detail for the system perylene in PE [12]. We have observed similar, though somewhat more varied, effects in the terrylene in PE system. Figure 8 shows a relatively simple holeburning chronology observed for one molecule (molecule “H”). We first scanned the fluorescence excitation spectrum until a stable single-molecule feature was found, shown in trace (a) of the upper panel. This procedure already tends to bias our observations in favor of those molecules having higher photostability, as those that hole-burn too quickly simply are not detected as strong features in the initial fluorescence excitation scan. Having
10
P. Tchénio et a!. / Optical studies of single terrylene molecules in polyethylene
12 I
(c)
Detuning (GHz)
0
100 Time (s)
200
Fig. 8. “Permanent” spectral hole burning ofa single molecule of terrylene in PE (molecule “H”). Trace (a) in the upper panel shows the 2. Trace (b) below shows the excitation spectrum at a central wavelength of 578.64 nm recorded with a incident intensity of 5 mW/cm fluorescence intensity as a function of time as the laser is tuned into resonance (rising signal during first 20 s) and then held at a fixed frequency. At 105 s, the arrowdenotes an interruption to start a new acquisition. At -~ 180 s, the emission intensity abruptly drops to its background value. (c) A subsequent scan of the excitation spectrum shows that the molecule is no longer within the scanned range.
identified a strong and well isolated spectral feature, we then manually tuned the laser frequency to the peak of the molecular resonance (first 20 s of trace (b)) and then held it fixed at that frequency while monitoring the fluorescence intensity. The photon counting time was set to 200 ms for these recordings, corresponding to approximately 300 counts above background in each counting interval. After ~ 180 s of observation, the count rate suddenly dropped to and remained at the background level observed at the beginning of trace (b), presumably signaling irreversible burning of the molecule. A new excitation spectrum (trace (c)) confirmed that the molecule no longer absorbed within
the 4 0Hz region scanned. The resonance was still missing from a second excitation spectrum recorded 5 mm later. Trace (b) exhibits numerous “dropouts” of the emission rate to the background level before irreversible burning occurs. If we set a threshold of one-third of the mean count rate above background (which prevents interference from shot noise and dark noise) as a criterion for deciding whether the molecule is in resonance, we conclude that the molecule moved out of resonance 26 times before the final burning event. The durations of these “dark” periods range from 0.2 to 3 s and the average return time is 0.65 s. These dropouts can
P. Tchènio et al.
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Optical studies of single terrylene molecules in polyethylene
11
Time (s) 2) is tuned into Fig. 9. Spectral hole burning with spontaneous reversal (molecule “I”). The laser (A = 579.14 nm, intensity 5 mW/cm resonance with a single-molecule excitation feature during the first 20 s. At ‘~ 40 s the fluorescence signal abruptly drops to the background level in a hole-burning event, recovers spontaneously at 80 s, and disappears again at 110 s.
originate from spontaneous spectral diffusion and/dr photoinduced spectral diffusion. The time scales involved are relevant to our observations of power saturation and intensity-dependent spectral broadening discussed in Section 4. In particular, return times greater than 200 ms are compatible with saturation of the emission rate without significant line broadening if the saturation mechanism is photoinduced jumping to a completely distinct spectral region. Of the 12 molecules studied in the fashion shown in Fig. 8(b), nine showed dropouts before well-defined burning and three did not. Another issue in hole-burning studies is the reversibility of the effect. For perylene in PE, Basché et al. showed that molecules often returned to their initial spectral position within seconds to minutes after burning at low laser powers [12]. For terrylene in PE, we (somewhat arbitrarily) distinguish between returns on a time scale shorter than 10 s and returns that take longer, because longer return times allow us to scan the spectrum to determine whether the molecule has in fact moved entirely outside the original spectral range. We have generally used the term “hole burning” to refer to unambiguous jumps to an entirely new spectral region from which the molecule returns either slowly or not at all. However, unlike the perylene/PE case where clear reversibility and long return times were observed, here we cannot easily prove that the jumps are indeed photoinduced, since most mdi-
vidual molecules underwent “irreversible” modification of their resonance frequencies too quickly for the intensity dependence of the disappearance and return rates to be explored. Among the 31 molecules that we tried to hole-burn, 22 disappeared and did not return to their initial spectral position within a 3—15 mm observation interval, eight disappeared and then returned, and we did not succeed in burning one molecule because it was spectrally diffusing too much to stay in resonance with the laser. Of the eight that did return, seven returned only once and the eighth returned only twice. Figure 9 shows one example (molecule “I”) for which the absorption line did return (once) to its initial spectral position. After the second burning event, it did not return within 15 mm. For the eight molecules that did return, the return times ranged from tens of seconds to 11 mm. (As discussed above, molecules that required less than ~ 10 s to return were not counted as having burned.) Taken as a whole, these holeburning data illustrate that irreversible phenomena are difficult to characterize for single quantum systems. Fig. 10 shows a fascinating effect that is reported here for the first time to our knowledge. Trace (a) shows the initial fluorescence excitation spectrum of molecule “J”. In trace (b) the laser was tuned into resonance with the absorption line and held at a fixed frequency. The very “noisy”
12
P. Tchènio et a!.
/ Optical
studies of single terrylene molecules in polyethylene I
(a)
~
50 100 time(s)
(e)
0
o4
LiJ~ ~~b)
Ce
a) C’)
(f)
4
4(g).
Relative laser frequency (GHz) Fig. 10. Spectral hole burning with slow recovery (molecule “J”). Traces (a) and (c) are the fluorescence excitation spectra 2, average of 16 scans of 2.5 seach) taken immediately before the hole-burning traces in (b) and (d) which (A = 580.29 nm, I = 5 mW/cm show the time dependence of the fluorescence emission at a fixed laser frequency. Traces (e) and (f) are the averages of the first 16 and second 16 2.5 s excitation scans after the final hole burning event. Trace (g) was obtained after a further 10 mm in the dark. See the text for details.
trace and large number of dropouts (similar to molecule H in Fig. 8 and different from molecule I in Fig. 9) suggest that molecule J was undergoing large-amplitude spontaneous and/or light-induced spectral diffusion. After 100 s it appeared to burn cleanly away, but a subsequent excitation spectrum (trace c) revealed that the resonance had merely shifted by 120 MHz. The laser was tuned to the peak of this new resonance and another attempt was made to hole-burn it (trace d). Again the fluorescence signal appeared to vanish, but on repeating the excitation scan the molecule had returned to the same frequency as in trace (c) (data not shown). Once again the laser was tuned into resonance, and this time the emission vanished after a few seconds of irradiation (data not shown). Subsequent scans of the excitation spectrum
showed the original resonance returning not all at once, but gradually! Traces (e) and (f) show the first two excitation spectra recorded “immediately” after the last hole-burning event (averages of the first 16 and second 16 scans of 2.5 s each), while trace (g) was obtained after a further 10 mm in the dark. The original resonance returns gradually over the course of the first 80 s, and has completely recovered its original shape, position, and count rate after “annealing” in the dark. This suggests that the molecule’s return was accompanied by a slow relaxation of the environment that resulted in either an increase in the time spent at the original resonance frequency or a change in the photophysical parameters controlling the emission rate. At this time, one can only speculate about the reasons for these changes.
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Optical studies of single terrylene molecules in polyethylene
13
7. Conclusions
Acknowledgements
In this work we have described a variety of spectral diffusion and hole-burning effects for single molecules of terrylene in poly(ethylene) at 1.5 K. Spontaneous spectral diffusion events on a small frequency scale lead to apparent fluctuations in the absorption line width and shape. At high probing intensities, a new light-induced spectral diffusion effect was observed for some molecules which causes decreases in the emission rate without an appreciable increase in the line width, suggesting that the molecule is spending an increased amount of time at spectral positions away from the laser wavelength. Discontinuousjumps in resonance frequency of 100—2000 MHz were also observed for several molecules which led to frequency trends with both discrete multi-state jumping and wandering character. Light-induced frequency changes lasting longer than 10 s in duration were attributed to nonphotochemical hole-burning processes. In contrast to the perylene/PE system reported earlier [12], little reversible burning was observed which prevented a detailed power-dependent and time-dependent study of the spectral hole-burning process. One particularly intriguing effect, slow reversal of the hole burning, was observed in one case. Taking all of these observations into account, it is quite clear that the properties of single molecules of terrylene in PE are complex and highly varied. This might have been expected, since each molecule is exquisitely sensitive to the immediate local environment, and the host material is an amorphous polymer with a broad distribution of ground state configurations. Single-molecule spectroscopy provides a detailed picture of the behavior of each member of the ensemble which contributes to the properties of the material as a whole. Many more such observations must be completed in order to provide better statistics about the occurrence of the previously unreported light-induced spectral diffusion effect and the slow reversal of hole-burning effect. In all cases, single-molecule spectroscopy clearly provides a wealth of new information about the properties of impurity centers in condensed matter which may be used as a test of detailed microscopic theories in the future.
We thank Dr. John Fetzer for the generous gift of a sample of terrylene, IBM for a Flory Sabbatical Award to A.B.M., and CNRS for support for P.T. We also thank M. Orrit for providing a copy of Ref. [27] prior to publication.
References [1] W.E. Moerner and L. Kador, Phys. Rev. Lett. 62 (1989) 2535. [2] M. Orrit and J. Bernard, Phys. Rev. Lett. 65 (1990) 2716. [3] WE. Moerner and W.P. Ambrose, Phys. Rev. Lett. 66 (1991) 1376. [4] WE. Moerner and Th. Basché, Angew. Chemie Int. Ed. EngI. 32 (1993) 457. [5] W.P. Ambrose, Th. Basché and WE. Moerner, J. Chem. Phys. 95 (1991) 7150. [6] W.P. Ambrose and WE. Moerner, Nature 349 (1991) 225. [7] UP. Wild, F. Güttler, M. Pirotta and A. Renn, Chem. Phys. Lett. 193 (1992) 451. [8] J. Bernard, L. Fleury, H. Talon and M. Orrit, J. Chem. Phys. 98 (1993) 850. [9] Th. Basché, WE. Moerner, M. Orrit and H. Talon, Phys. Rev. Lett. 69 (1992) 1516. [10] P. Tchénio, A.B. Myers and WE. Moerner, J. Phys. Chem. 97 (1993) 2491. [11] Th. Basché and WE. Moerner, Nature 355 (1992) 335. [12] Th. Basché, W.P. Ambrose and WE. Moerner, J. Opt. Soc. Am. B 9 (1992) 829. [13] M. Orrit, J. Bernard, A. Zumbusch and RI. Personov, Chem. Phys. Lett. 196 (1992) 595. [14] J. Friedrich and D. Haarer, in: Optical Spectroscopy of Glasses, ed. I. Zschokke (D. Reidel, 1986) pp. 149—198. [15] R. Jankowiak and G.J. Small, Science 237 (1987) 618. [16] See: Persistent Spectral Hole-Burning: Applications, ed. WE. Moerner, Topics in Science Current and Physics, Vol. 44 (Springer, Berlin, Heidelberg, 1988). [17] P.W. Anderson, B.I. Halperin and CM. Varma, Philos. Mag. 25 (1972) 1—9.. [18] See: Amorphous Solids: Low Temperature Properties, ed. WA. Phillips (Springer, Berlin, 1981). [19] E. Clar, 1964). Polycyclic Hydrocarbons (Academic Press, London, [20] 1.1. Abram, R.A. Auerbach, R.R. Birge, BE. Kohler and J.M. Stevenson, J. Chem. Phys. 63(1975) 2473. [21] GB. Hurst and iC. Wright, J. Chem. Phys. 95(1991)1479. [22] K. Ohno, J. Chem. Phys. 95 (1991) 5524. [23] H.P.H. Thijssen and S. VOlker, J. Chem. Phys. 85 (1986) 785. [24] A. Bohnen, K-H. Koch, W. Lüttke and K. Mullen, Angew. Chem. mt. Ed. Eng. 29 (1990) 525.
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[25] C. Kryschi, B. Wagner, W. Gorgas and D. Schmid, J. Lumin. 53 (1992) 468. [26] U. Bogner, T. Attenberger and R. Bauer, in: Spectral HoleBurning and Luminescence Line-Narrowing: Science and Applications Technical Digest, 1992, Vol. 22 (Optical
Society of America, Washington, DC, 1992) pp. 22—25. [27] A. Zumbusch, L. Fleury, R. Brown, J. Bernard and M. Orrit, Phys. Rev. Lett. 70 (1993) 3584. [28] J.M. Hayes, R.P. Stout and G.J. Small, J. Chem. Phys. 74 (1981) 4266.