J. Phys. Chem. 1995,99, 11844- 11853
11844
Laser Ablation of a Pyrene-Doped Poly(methy1 methacrylate) Film: Dynamics of Pyrene Transient Species by Spectroscopic Measurements Hisashi Fujiwara; Hiroshi Fukumura," and Hiroshi Masuhara" Department of Applied Physics, Osaka University, Suita 565, Japan Received: December 20, 1994: In Final Form: May 22, 1995'
The 248 nm ablation of pyrene-doped poly(methy1 methacrylate) (PMMA) has been investigated by various spectroscopic and etch depth measurements with much attention to the dynamics of pyrene transient species in PMMA under intense excitation. It is shown that pyrene is rather stable even in the case that it absorbs more than several 248 nm photons during the laser pulse. It suggests that the photodecomposition of a dopant has no or little importance in laser ablation. The lowest excited singlet (SI) and triplet (TI) states and the cation of pyrene were detected within and after the laser pulse by time-resolved emission and absorption spectroscopy. The broadening and red shift of the fluorescence of pyrene at 440 rdlcm2 suggest that not only pyrene but also the matrix PMMA becomes hot, which supports the thermally induced ablation of pyrenedoped PMMA. It is suggested that the TI state and cation of pyrene are produced from a highly excited state of pyrene which is produced by successive photoabsorption by the SI state of pyrene. It is also suggested that the temperature increase due to the photoabsorption facilitates the S I-SI and TI-TIannihilation and the recombination of the cation and electron or anion. A detailed analysis of transient absorbance at the excitation wavelength (248 nm) at 150 mJlcm2, a little below the ablation threshold, suggests that there should be a further unknown or undetected species which contributes to the observed multiphoton absorption in addition to the detected ones.
1. Introduction In recent years, we have extensively investigated laser ablation of doped polymer films by using various techniques such as time-resolved spectroscopy, photography, and interferometry.'-8 Doped polymer films have the following advantages for laser ablation: (1) we can sensitize, by using an appropriate dopant, a film which has no or little absorption coefficient at the excitation wavelength; (2) the careful choice of a dopant whose photochemical and photophysical properties are well-known enables us to consider the mechanism of polymer ablation from molecular viewpoints. In addition to these advantages, one important characteristic of doped polymer films is their quite low chromophore concentration compared to representative neat polymer films. This characteristic and a high extinction coefficient of a dopant at the laser wavelength should easily result in dense formation of dopant transient states such as the lowest excited singlet (SI)and triplet (TI) states, cation, and anion accompanied with considerable depletion of the ground state. Thus transient states of a chromophore (dopant) should have a crucial role in doped polymer ablation compared with neat polymer ablation. Quite recently, we demonstrated that pyrene in poly(methy1 methacrylate) (PMMA) absorbs more than ten 248 nm photons at the ablation threshold and suggested that transient states of pyrene should contribute to the multiphotonic absorptiomS However, it was not much clear what mechanism can achieve such efficient absorption of photons during one-shot excitation. The first purpose of this work is to elucidate the mechanism by comprehensive investigation of the dynamics of the transient states by time-resolved spectroscopy. On the other hand, doped polymer ablation can be regarded as a photochemical and photophysical phenomenon generally
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Present address: Department of Information Machines and Interfaces, Hiroshima City University, 150-1 Ozuka, Numata-cho, Asa-minami-ku, Hiroshima 731-31, Japan. Abstract published in Advance ACS Abstracts, July 1, 1995. @
0022-365419512099-11844$09.00/0
observed in the case of a rather high dopant concentration under high-intensity excitation, and it should contain many processes related to high-density excitation. For example, simultaneous and stepwise multiphoton absorption,4-6ss mutual interactions between excited states?.l0 ionization of a dopant," and vibrational energy transfer from a dopant to the matrix p01yme#-63s.12313 should occur simultaneously. The second purpose of this work is to investigate these processes by spectroscopic and other experimental techniques before, during, and after the ablation. For both purposes of this work, pyrene is quite suitable in contrast to other molecules, because there are a lot of spectroscopic data of its ground and transient states. Furthermore, the fluorescence and absorption of the characteristic pyrene transients (SI, T I , cation, and anion) can be monitored simultaneously in the wavelength range of 350-550 nm; this wavelength range (200 nm) is the limit which our streak camera system can cover for a measurement.
2. Experimental Section Pyrene (Special Guarantee, Wako) was twice chromatographed on a column of silica gel by using n-hexane as eluting solution, and then recrystallized from ethanol solution. In some cases pyrene (Guaranteed Reagent, Nacalai Tesque) was used as purchased, but no difference was observed in the results irrespective of the type of the pyrene used. Chlorobenzene (99%, Wako)was purified by shaking repeatedly with portions of sulfuric acid. It was washed with water and with dilute sodium hydroxide solution, dried with calcium chloride, and fractionally distilled. l 4 Poly(methy1 methacrylate) (PMMA) (polymerization degree x 1000, supplied by Kuraray Co. Ltd.) was purified by reprecipitating it from benzene solution with methanol for several times. Sample films of pyrene-doped PMMA were prepared by spin-coating on a quartz plate from the solution of PMMA and pyrene in chlorobenzene. In most cases the films were dried under vacuum at 110 "C in an oven for more than 30 min, but some of the films were dried under
0 1995 American Chemical Society
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J. Phys. Chem., Vol. 99, No. 31, I995 11845
Laser Ablation of a Pyrene-Doped PMMA Film
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vacuum at room temperature for several hours. The methods of drying have no substantial influence on the results.I5 The typical values of the thickness, concentration of pyrene, and absorption coefficient at 248 nm of the film were 2.2 f 0.2 pm, 0.11 f 0.02 m o m , and 0.35 f 0.03pm-I, respectively. A KrF excimer laser (248nm, Lambda Physik, EMG201MSC) was used for the irradiation of the films. The laser pulse width was about 30 ns. The laser pulse was trimmed through an aperture and focused normally onto a film surface by using a quartz lens (f = 20 cm). The laser fluence was adjusted by placing partially transmitting laser mirrors in front of the laser output. We always irradiated a fresh film surface to avoid complexity arising from the interactions between the irradiated surface and the successive laser pulses. All the data shown in this paper are those obtained by single-shot irradiation, even though some of them were averaged over several measurements. The surface profiles of the irradiated films were measured with a depth profiler (Sloan, Dektak). The details of the spectroscopic measurements were described e l ~ e w h e r e ,and ~ . ~we only summarize them here. The transient absorbance at the laser wavelength (248nm) was investigated by monitoring the incident and transmitted excitation laser pulses with two photodiodes. The time-resolved absorbance was calculated from the time-resolved signals from both diodes, while the time-integrated absorbance was calculated from the integrals of both signals. The time-resolved emission and absorption spectra of the films were measured by using the combination of a polychromator (Jovin Yvon, HR-320) and a streak camera system (Hamamatsu, C2830 and C3140). The angle between the detection direction and the film surface was 45'. A pulsed 150 or 300 W xenon lamp (Wakom, KXL-15OF or KXL-300F) was used as a monitoring light source for the transient absorption measurement, and the monitoring light was incident at 45" on the film. The streak images were averaged over more than three measurements, from which the spectra and rise-and-decay curves were obtained. The emission spectra were not corrected and accordingly contain some deviations arising from the wavelength-dependent sensitivity of the detector. Thus, only a qualitative discussion is valid on the emission spectral shape. The origin of the time axis, 0 ns, is defined as the time when the intensity of the excitation laser pulse at the film surface became maximum. All the measurements were done in air at room temperature.
much lower than that o f a neat PMMA film (1100 ml/cm2).8.i6 This apparent decrease of the threshold indicates that the doping of pyrene sensitizes the ablation of PMMA at 248 nm. Above the ablation threshold the etch depth increased with an increase in the laser fluence, but a saturation tendency was observed above 500 ml/cm2. Such a saturation has been frequently observed in the ablation of various polymer~,5.'~.'* and it has been attributed to the scattering of the laser pulse by small bubbles or fragments produced during a b l a t i ~ n . ~ - " . ' ~ After the laser irradiation, we examined not only the morphological change of the film but also the absorption spectral change of it. Figure 2 shows the absorption spectra of pyrenedoped PMMA before and after one-shot irradiation of 150 mll cm2. The spectrum after the irradiation became a little broader than the original one. The absorbance around a peak slightly decreased while that in a tail slightly increased. However, the characteristics of the absorption spectrum showed no change; both spectra had the same vibrational structures and the same absorption peaks at the same wavelengths. It suggests that the change of the spectrum arises not from a considerable change in pyrene such as decomposition but from a slight change in pyrene such as aggregation or in the surrounding matrix PMMA such as the orientation of the carbonyl group in the side chain. Figure 2 allows us, at least, to conclude that most of pyrene do not decompose even after 150 ml/cm2 irradiation a little below the ablation threshold. In addition, as a preliminary experiment, we investigated the 308 nm ablation of pyrene-doped PMMA in a vacuum chamber equipped with a quadrupole. mass spectrometer and detected intact pyrene. Furthermore we previously investigated the 35 1 nm ablation of anthracene-doped polystyrene films by time-offlight mass spectrometry, and the main detected species around the ablation threshold was also confirmed to be intact neutral anthracene: Those experiments and the present one indicate that aromatic dopants are somewhat stable even in the case of the intense excitation for laser ablation. They also suggest that the decomposition of a dopant has no or at least little effect on laser ablation, which is quite consistent with the conclusion obtained by the investigation of the 248 nm ablation of PMMA doped with an efficient photodecomposable molecule of 5-diazo2,2-dimetyl-l,3-dioxane-4,6-dione (5-diazo Meldrum's acid).8 Transient Absorbance at the Laser Wavelength (248 nm). Intense excitation of a sample film should result in dense formation of excited states with depletion of the ground state. Thus absorbance at the excitation wavelength is expected to change during the excitation, and the present work provides detailed information on the absorbance change of pyrene-doped PMMA. Figure 3 shows the time-resolved absorbance of
3. Results and Discussion Characterization after Irradiation. Figure I shows the fluence dependence of the etch depth of pyrene-doped PMMA. The threshold of the ablation was about 200 mJ/cm2, and it is
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Figure 3. Time-resolved absorbance of pyrene-doped poly(methy1 methacrylate) at 248 nm during the laser pulse. The black solid lines represent the absorbance at (a) 0.2 mJ/cm2, (b) 80 mJ/cm2, (c) 270 mJ/cm2,(d) 590 mJ/cm2,and (e) 1000 mJ/cm2.The broken lines in the
figure represent the absorbance before irradiation obtained on a conventional spectrophotometer. The thin gray line in (a) represents the time profile of the laser pulse obtained with the photodiode. pyrene-doped PMMA at 248 nm. At 0.2 mJ/cm2, the absorbance remained constant almost during the laser pulse and was quite similar to that obtained on a conventional spectrophotometer. At 80 mJ/cm2, however, it gradually increased during the laser pulse. When we increased the fluence to be 270 mJ/ cm2, a little higher than the ablation threshold, a similar but faster absorbance increase was observed. At 590 mJ/cm2highly above the ablation threshold, the absorbance increased similarly in the earlier part of the laser pulse, but a new increasing component was observed in the late part. This new increasing component is attributed to the scattering of the laser pulse, and this scattering causes the saturation tendency of the etch depth mentioned a b ~ v e . Further ~ ~ ~ ~increase ~ ’ ~ in the laser fluence resulted in the earlier onset of the new increasing component or the scattering (see Figure 3, d and e). It indicates that the morphological changes of the film become rapid with an increase in the laser fluence. We measured the transient absorbance change at various fluences including those in Figure 3, and the results indicate that the scattering of the laser pulse becomes apparent above 300 mJ/cm2. Figure 4 shows the fluence dependence of the time-integrated absorbance at 248 nm below 250 mJ/cm2 where we can neglect the influence of the scattering. The absorbance increased with the laser fluence up to 150 mJ/cm2 and showed a saturation tendency above several tens of mJ/cm2. From the timeintegrated absorbance, the pyrene concentration, the film thickness, and the fluence, we can calculate the number of the photons absorbed per pyrene on an average over the film. Its fluence dependence is also shown in Figure 4. At the ablation threshold of 200 mJ/cm2, a pyrene molecule absorbs about 12 photons. Note that a pyrene absorbs about 9 photons at 150 mJ/cm2, while the absorption spectrum of the film after the irradiation (Figure 2) indicates that most of the doped pyrene do not decompose even after such multiphotonic excitation. Thus we conclude that the absorbed photon energy is efficiently transferred to the matrix polymer, and we recently proposed an
Figure 4. Quantitative analysis of the fluence dependent photoabsorption of pyrene-doped poly(methy1methacrylate).The closed circles
represent the time-integrated absorbance. The open circles represent the number of the photons absorbed per pyrene which is calculated
from the time-integrated absorbance.
ablation mechanism which is consistent with the present c~nsideration.~-~~* The mechanism termed “cyclic multiphotonic absorption” is summarized as follows.4,6 Intense excitation of a doped polymer film should result in the dense formation of the transient states such as S I , TI,cation, and anion. The higher excited states (Sn, T,,,and excited states of ions), which are produced by the successive absorption of the excitation photons, generally have a lifetime extremely shorter than the duration of a nanosecond laser pulse. The quite rapid relaxation of those states to the original transient states allows the cyclic absorption of excitation photons. In addition, this relaxation should consist of the internal conversion in the dopant and the subsequent intermolecular vibrational energy transfer from the dopant to the surrounding matrix polymer. Thus the mechanism explains not only the multiphotonic absorption but also the effective energy transfer from the dopant. The matrix polymer is heated up repeatedly during the multiphotonic absorption, which should result in the thermal decomposition of the polymer, Le., photothermal mechanism of laser ablation. When we assume that all of the absorbed photon energy converts into thermal energy through the processes mentioned above, the film temperature after the laser pulse is estimated to be 530 K on an average over the film at the ablation threshold.20 This value is similar to that (550 K) estimated in the case of a 0.1 M anthracene-doped polystyrene film surface irradiated with a 35 1 nm pulse of 300 mJ/cm2 a little below the t h r e ~ h o l d . ~ , ~ Our proposed multiphotonic absorption was originally not a simultaneous but a stepwise one. This characteristic of the mechanism is consistent with the present results, which is shown by a further analysis of the time-resolved absorbance change. In Figure 3 the absorbance is plotted against time, but from this plot we can make the plot of the absorbance at a certain time vs the total fluence sent until that time only by a simple calculation from the time profile of the laser pulse. Figure 5 is an example of it obtained from Figure 3 b, d, and e; Figure 5a shows the plots in a low-fluence region, while Figure 5b shows up to a high-fluence region. It is surprising that the three different lines calculated from the data at 80, 590, and 1000 mJ/cm2 are in good agreement with each other. It indicates that the transient absorbance is governed by the total fluence integrated until the time. In other words, the kinetics of the species which contribute to the absorbance change, is governed by the amount of the absorbed photons; the species should be produced by the photoabsorption and the subsequent processes which are rapid enough to give the characteristics of the absorbance change. Figure 5 also indicates that simultaneous
J. Phys. Chem., Vol. 99, No. 31, I995 11847
Laser Ablation of a Pyrene-Doped PMMA Film
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Fluence (mJ/cm2) Figure 5. Plots of the transient absorbance of pyrene-doped poly(methyl methacrylate) at a certain time during the laser pulse vs the fluence sent until the time. The upper part (a) shows the plots in a low-fluence region, while the lower part (b) shows the plots up to a high-fluence region. In both parts, the black, medium gray, and light gray lines represent the plots obtained from the time-resolved absorbance (Figure 3) at 80, 590, and 1000 mJ/cm2, respectively.
multiphoton absorption is quite minor process in this experiment, because such absorption is not fluence dependent but intensity dependent. In Figure 5b the two gray lines above 300 mJ/cm2, which correspond to the scattering at 590 and 1000 mJ/cm2, are in good agreement with each other. It indicates that the morphological changes are also governed by the fluence. The little or no contribution of simultaneous multiphoton absorption to the pyrene-doped PMMA ablation should arise from the original sufficient absorption coefficient at the excitation wavelength. When we irradiate, however, a film with an extremely small absorption coefficient with a high-intensity laser pulse, simultaneous multiphoton absorption may occur. If this multiphoton absorption initially produces an excited state with a sufficient one-photon absorption coefficient, the cyclic multiphotonic absorption by the state can occur. This extended mechanism of cyclic multiphotonic absorption is not applicable to the present results but may be applicable to femtosecond or picosecond pulse ablation of a film with no or little absorption coefficient at the laser wavelength.21 Time-Resolved Emission and Absorption Spectra. In Figure 6, the black lines represent the time-resolved emission spectra of pyrene-doped PMMA at two fluences with two different gate times, while the gray line represents the timeintegrated spectrum obtained at 0.09 mJ/cm2. This reference spectrum is similar to that obtained on a conventional spectrofluorophotometer and assigned to the pyrene monomer fluorescence. The spectra represented by the black lines also should be assigned to the fluorescence or the S I state of pyrene, although their characteristics in shape changed with the fluence and the gate time. The excimer emission around 480 nm was not observed in the present study. At 40 mJ/cm2 (Figure 6, a and b), the spectra at two gate times were similar to the reference one except for the slight broadening and the difference in the ratio between the peak intensities of the vibrational bands. At 440 mT/cm2, there were appreciable changes in the spectra compared with the reference. The spectrum at -20-0 ns was rather broad and lost the vibrational structure (Figure 6c). At 0-20 ns, both broadening and loss of the vibrational structure became more considerable and the spectrum was clearly redshifted. These three characteristics are the same as those of a “hot spectrum” which has been frequently observed in the studies of molecular spectroscopy.22 The hot spectrum arises
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Wavelength (nm) Figure 6. Time-resolved emission spectra of pyrene-doped poly(methy1 methacrylate). The black lines represent the spectra obtained with the following fluences and gate times: (a) 40 mJ/cm2 and -20-0 ns, (b) 40 mJ/cm2 and 0-20 ns, (c) 440 mJ/cm2 and -20-0 ns, and (d) 440 mJ/cm2 and 0-20 ns. The gray line represents the time-integrated spectrum at 0.09 mJ/cm2 as the reference.
from S 1 with high vibrational excess energy, and in general such a spectrum is not observed from a molecule in a matrix because of rapid vibrational cooling. However, if not only the molecule but also the matrix becomes hot, the SI state of the molecule can have high vibrational excess energy and thus the hot spectrum can be observed. The spectra in Figure 6, c and d, indicate that the matrix PMMA becomes hot and support a thermal mechanism of doped polymer ablation.2. Figure 7 shows the transient absorption spectra of pyrenedoped PMMA at two fluences with two different gate times. At 40 mJ/cm2, the spectrum at 10-30 ns (Figure 7a) had three peaks at 365, 415, and 450 nm. According to previous work,23-26those peaks are assigned to the SI,TI, and cation of pyrene, respectively, and the broad spectrum should arise from the overlap of the three transients’ spectra. The SI state of pyrene has another absorption peak at 468 nm,23+24*26 but there was no peak at 468 nm in the spectrum in Figure 7a. This should be also because the overlap between the spectra of the SI state and the cation. The anion of pyrene was not observed in the spectrum, although it has a strong absorption peak at 492 nm.24 Its extinction coefficient, 5 x lo4 M-I cm-I, is higher than that of the cation at 450 nm (4 x lo4 M-I ~ m - l ) . ~26~If. the amount of the produced anion is similar to that of the cation, a strong absorption peak should be observed at 492 nm, but that is quite different from the case in Figure 7. It suggests that an electron released from pyrene is trapped not by other pyrene molecules but by the matrix PMMA. On the basis of the stability of the cation of N,N,N‘,N‘-tetramethyl-p-phenylenediamine (TMPD) and N,N,N‘,N‘-tetramethylbenzidine (TMB) in PMMA, Tsuchida et al. concluded that PMMA has electronaccepting ability,” which supports our consideration. At a later time region (Figure 7b), the existence of the TI and the cation became clearer; the SI decayed out and accordingly the sharp absorption peaks of the TI and the cation were observed. At 440 mJ/cm2, an absorption spectrum similar to that in Figure 7a was observed at an early time stage (-20-0 ns); the three 4-8~12713
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11848 J. Phys. Chem., Vol. 99, No. 31, 1995
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Time (ns) Figure 8. Fluence dependence of the rise-and-decay curves of the emission (350-450 nm) of pyrene-doped poly(methy1 methacrylate) at relatively low fluences. The black lines represent the curves at (a) 0.09 mJ/cm2, (b) 6 mJ/cm2,and (c) 20 mJ/cm2. The gray line in (a) represents the time profile of the laser pulse obtained under the same experimental condition. 400
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Wavelength (nm) Figure 7. Transient absorption spectra of pyrene-doped poly(methy1 methacrylate) obtained with the following fluences and gate times: (a) 40 mJ/cm2and 10-30 ns, (b) 40 mJ/cm2 and 40-60 ns, (c) 440 mJ/ cmz and -20-0 ns, and (d) 440 mJ/cm2 and 10-30 ns. The vertical
dotted lines represent the wavelengths of the characteristic absorption peaks of the transient states of pyrene. transient states are already produced in this time region. At 10-30 ns, however, a quite different spectrum was observed (Figure 7d). The spectrum is characterized by a monotonous absorption increase with a decrease in the wavelength. We previously observed similar spectra both with aromatic-doped PMMA and with neat PMMA when ablation o ~ c u r r e d .The ~ characteristics of those spectra were independent of the presence of the dopant and related to ablation of a matrix polymer, and thus we assigned the spectra to the scattering of the monitoring light by the small bubbles or fragments produced during ablation. In the present case, the transient absorbance measurement at 248 nm indicated that the scattering of the laser pulse becomes apparent above 300 d / c m 2 , and thus the spectrum in Figure 7d is also assigned to the scattering of the monitoring light. Dynamics of the Transient States of Pyrene in PMMA. The three transient states have been detected by the analysis of the emission and absorption spectra, and their formation dynamics is now discussed by the analysis of the rise-and-decay curves shown in Figures 8-12. ( i ) Dynamics of the SIState. Figures 8 and 9 show the riseand-decay curves of the fluorescence (350-450 nm) or the SI in the case of low- and high-fluence irradiation re~pectively.~~ At 0.09 mJ/cm2 (Figure 8a), the obtained curve is successfully analyzed as a exponential one with a lifetime of 360 ns. When the laser fluence was increased, the decay of the SIstate became fast and the peak of the decay curve shifted to earlier time (Figure 8, b and c). At 20 mJ/cm2, a considerable amount of the produced SIstate decayed out within the laser pulse. One possible cause of this rapid decay is SI-SI annihilation which process was proposed to explain the deactivation of fluorescence with dense excitation.*s Nakashima et al. confirmed, in the case of the fluorescence decay of pyrene in rigid solvents at 77 K,
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Time (ns) Figure 9. Fluence dependence of the rise-and-decay curves of the emission (350-450 nm) of pyrene-doped poly(methy1 methacrylate) at relatively high fluences. The black lines represent the curves at (a) 50 mJ/cm2,(b) 160 mJ/cm2,and (c) 290 mJ/cm2.The gray line in (a) represents the time profile of the laser pulse obtained under the same experimental condition.
that the annihilation is due to the excitation energy transfer from one SIto another through Forster's dipole-dipole coupling.29 This type of SI-SI annihilation is expressed by the equation SI4- SI SO -t S,. If this annihilation competes with usual monomolecular decay processes and can be regarded as a diffusion controlled bimolecular process, the plot of the inverse of the concentration of SIat time t vs exp(t/z) gives a straight line, where t is a lifetime without the a n n i h i l a t i ~ n . ~ ~ In the present experiment, such plots made from the fluorescence decay curves (310-500 nm, not shown as a figure) at 0.3, 0.6, and 1.3 mJ/cm2 actually gave straight lines.3' This is an interesting result because at those low fluences the temperature increase due to the photoabsorption is quite small and accordingly PMMA remains rigid; the migration of the SI resulting in the annihilation may occur through the excitation energy transfer among the doped pyrene molecules. On the other hand, the plot made from the decay curve at 6 d / c m 2
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Figure 10. Fluence dependence of the rise-and-decay curves of the absorption (410-420 nm) of pyrene-doped poly(methy1 methacrylate). The black lines represent the curves at (a) 50 mJ/cm2, (b) 120 mJ/cm2, (c) 150 mJ/cm2, and (d) 290 mJ/cm2. The gray line in (a) represents the time profile of the laser pulse obtained under the same experimental condition.
(Figure 8b) showed slight deviation from a straight line, and the plot made from the decay curve at 20 mJ/cm2 (Figure 8c) showed a large deviation. Thus the simple SI-SI annihilation mechanism cannot explain the rapid decay in Figure 8, b and C.
In the case of higher fluence irradiation (Figure 9), the decay of the S I state almost completed within the laser pulse, and its fluence dependence was not remarkable compared with that shown in Figure 8. Unfortunately it is somewhat difficult to discuss the kinetics of the S I state within the laser pulse. This is because there are several processes relating to it as indicated in the rest of the discussion below: SI-SI annihilation, TI-TI annihilation, and the conversion of SIinto both TI and cation following its photoabsorption. Then instead of comprehensive elucidation of the kinetics of the S I state, we will indicate in the following discussion that so many photochemical and photophysical processes concern the kinetics simultaneously within an intense laser pulse. ( i i ) Dynamics of the TI Stare. Figure 10 shows the rise-anddecay curves of the transient absorption (410-420 nm) where the T I state should be a major species. The fluence dependence of the curves was similar to that of the SIstate; the decay of the TI state became rapid with an increase in the laser fluence. At 290 mJ/cm2, a new increasing component was observed after 20 ns and it is assigned to the scattering of the monitoring light. This scattering began at almost the end of the laser pulse, and it is consistent with the conclusion that the scattering of the laser pulse becomes apparent above 300 mJ/cm2. The observed rapid formation of the TI state within the laser pulse and the subsequent rapid decay are quite interesting; the formation of the T I state through the intersystem crossing is a process in the time scale of a few hundred nanoseconds, while the TI state has a lifetime of a few microseconds under low e ~ c i t a t i o n . ~ ~ The rapid formation of the TI state of aromatics has been studied extensively both by pulse radiolysis and by laser p h o t o l y ~ i s . * ~ , ~In ~-~ the ' present work, the following three
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Figure 11. Analysis of the mechanism of the rapid formation of the T I state of pyrene in poly(methy1 methacrylate). (a) The gray line represents the same but normalized rise-and-decay curve of the absorption (410-420 nm) at 50 mJ/cm2 as in Figure loa, while the black dotted and solid lines represent the simulated TI rise curves based on the processes S I SI 2TI and S I hv - T I , respectively. Both curves were simulated from the fluorescence rise-and-decay curve and the time profile of the laser pulse at 50 mJ/cm2 in Figure 9a. (b) The black and gray lines represent the fluorescence (330-500 nm) riseand-decay curve and the time profile of the laser pulse at 30 mJ/cm2, respectively. (c) The gray line represents the rise-and-decay curve of the absorption (410-420 nm) at 30 mJ/cm2, while the black dotted and solid lines represent the simulated TI rise curves based on the processes S I SI 2Tl and SI hv - T I , respectively. Both curves were simulated from the curves in (b).
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processes may be responsible for the rapid formation of the TI: (1) recombination of the cation and electron or anion, (2) S ISIannihilation expressed by the equation S I S I 2T1, and (3) formation from a highly excited state of pyrene. Process 1 has been proposed to explain T I formation in aliphatic hydrocarbon solution in the picosecond to nanosecond time region in radiation experiment^.^^.^^,^' Process 2 was proposed to explain the fluorescence decay of the photosynthetic unit in vivo.38 Process 3 was proposed to explain both magnetic field and excitation wavelength dependence of the fluorescence of anthracene and in the present experiment a highly excited state of pyrene can be produced by the photoabsorption by the S I state. Process 1 is easily excluded by the analysis of the rise-anddecay curve of the cation (Figure 12). For example, the curve at 50 mJ/cm2 (Figure 12a) shows that an appreciable amount of the cation exists even after the laser pulse. If process 1 substantially contributes to the T I formation, the rate of the T I formation should correlate with the cation concentration, and accordingly the TI should be formed after the laser pulse as within the laser pulse. However, it is quite inconsistent with the rise-and-decay curve of the TI at 50 mJ/cm2 (Figure loa). On the other hand, to examine which of the remaining two processes is more plausible, we did a simple simulation whose results are presented in Figure 11. The simulation is based on the assumption that the rate of the formation through process 2 is proportional to the square of the S I concentration while that through the process 3 is proportional to the product of the SI concentration and the laser pulse intensity. In practice, the former rate was estimated from the square of the observed fluorescence intensity, while the latter rate was estimated from
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11850 J. Phys. Chem., Vol. 99, No. 31, 1995
the product of the observed fluorescence intensity and the pulse intensity. The solid and dotted black lines in Figure l l a represent the TI rise curves simulated from the fluorescence rise-and-decay curve and the time profile of the laser pulse in Figure 9a (50 mJ/cm2), and they are normalized so that their values finally reach to 1. The gray line in the Figure represents the normalized TI rise-and-decay curve obtained from the curve in Figure loa. It is clear that the simulated curve based on process 3 (solid line) is in better agreement with the experimentally obtained curve. A similar result was obtained in the case of 30 mJ/cm2 irradiation, which is shown in Figure 11, b and c. The black and gray lines in Figure l l b represent the experimentally obtained fluorescence rise-and-decay curve and time profile of the laser pulse, respectively. The black dotted and solid lines in Figure 1ICrepresent the simulated curve based on process 2 and that on process 3 respectively, while the gray line represents the experimentally obtained rise-and-decay curve of T I . The simulated curve based on process 3 is again in better agreement with the experimentally obtained curve, which suggests that in the present experiment the T I state is rapidly produced from a highly excited state of pyrene which state is produced by the photoabsorption by the S I . The simulation mentioned above is too simple to give the final conclusion because it neglects the inhomogeneities of the S I concentration and the laser intensity in the film from the surface to the bottom. However, the characteristics of the simulation at 30 mJ/cm2, namely that the formation through process 3 completes within the laser pulse while that through process 2 lasts even after the laser pulse, will remain unchanged even if we consider the neglected factors. This is because the lasting nature of the simulated formation through process 2 will appear owing to the lasting nature of the fluorescence or the SI irrespective of the approximation used in the simulation. This consideration also supports the rapid T I formation through process 3. On the other hand, as discussed below, the rapid decay of the T I is ascribed to a facilitated TI-TI annihilation as a result of the temperature increase of the matrix PMMA. El-Sayed et al. indicated, by examining the phosphorescence decays of several aromatic molecules in PMMA, that a temperature increase from 77 to about 300 K results in the appearance of effective TI-TI annihilation due to an increased diffusion of the solute aromatics? Quite recently, we investigated both temperature and laser-fluence dependence of the decay of the T I of biphenyl and p-terphenyl in PMMA and obtained experimental evidence that most of the absorbed photon energy converts into thermal one and the resulting temperature increase facilitates TI-TI annihilation.4O This observation together with the fluence dependence of the photoabsorption of pyrene-doped PMMA (Figure 4) explains the fluence dependence of the TI decay; namely, an increase in the laser fluence results directly in an increase in the film temperature and then in an increase in the dopant diffusion rate, which should be the reason why the decay of the TI becomes rapid with an increase in the laser fluence. It is worth noting that this type of facilitation is also possible in the case of SI-SI annihilation. This is because the annihilation of pyrene is governed by the distance between two S I which is a factor of Forster's dipole-dipole coupling and then also facilitated by an increase in the diffusion rate. ( i i i ) Dynamics of the Cation. Figure 12 shows the rise-anddecay curves of the transient absorption (445-455 nm) where the cation is a major species especially in the time range after 40 ns. The fluence dependence of the curves was quite similar to that of the T I state; the decay of the cation became rapid
I
"
"
"
"
1
'
"I
Figure 12. Fluence dependence of the rise-and-decay curves of the absorption (445-455 nm) of pyrene-doped poly(methy1methacrylate). The black lines represent the curves at (a) 50 mJ/cm2,(b) 120 mJ/cm2, (c) 150 mJ/cm2, and (d) 290 mJ/cm2.The gray line in (a) represents the time profile of the laser pulse obtained under the same experimental
condition. with an increase in the laser fluence and the scattering of the monitoring light was again observed at 290 mJ/cm2. In general, the cation of aromatic molecules can be formed by a successive photon absorption either by S I or by T1.I1*41 The ionization potential of pyrene is about 7.5 eV in gas and the value in PMMA should be smaller than it, because the electron affiiity of the ester group in PMMA is about 1.9 eV.'1b,43The energy levels of S I and T I are about 3.6 and 2.1 eV from the ground state, respectively,44and the energy of a 248 nm photon is 5.0 eV. All these values allow us to conclude that both two-photon processes via S I and via TI can energetically produce the cation. However, the simple simulation shown above suggests that the T I state is formed through the two-photon process via the S I , and thus it cannot be a precursor of the cation at least in an early part of the laser pulse. Thus in an early time region the cation is produced from a highly excited state of pyrene which state is produced by the photoabsorption by the S I state. In contrast, an appreciable amount of the T I state exists in the latter part of the laser pulse, and accordingly the cation formation via the TI cannot be excluded. The rise of the cation cannot be reproduced by the same simulation as that of the TI formation through process 3, which is possibly due to the tail of the S I absorption band whose peak should be at 468 nm. The decay of the cation under the present excitation condition is another interesting subject. The cation in PMMA should be somewhat stable because of the electron-accepting ability of PMMA. A possible explanation for the present rapid decay is a facilitated recombination of the cation and electron or anion as a result of the temperature increase of the matrix PMMA. This explanation is consistent with the enhanced TI-TI annihilation discussed above. Furthermore, in the case of TMPD or TMB in PMMA, Tsuchida et al. observed a thermally facilitated recombination of the cation and electron,' which also supports our explanation. (iv) Quantitative Analysis of the Transient States Formation. Figure 13a shows the fluence dependence of the maximum
Laser Ablation of a Pyrene-Doped PMMA Film
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0. L&
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,
0 A
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u
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100
200
J. Phys. Chem., Vol. 99, No. 31, 1995 11851
300
400
B
I
500
Fluence (mJ/cm2) I
' Y
-
I
I
I
. .
0
Fluence (mJ/cm*)
Figure 14. Comparison of'the transient absorbance change at 248 nm with the temporal changes of the concentrations of the transient states of pyrene in poly(methy1 methacrylate) film. The laser fluence is 150
50
Time (ns)
Figure 13. (a) Fluence dependence of the maximum concentrations of the transient states of pyrene. The open circles, the closed circles, and the open triangles represent the concentrations of the SI,TI,and cation of pyrene, respectively. (b) The closed squares represent the total amount of the species produced from a highly excited state of pyrene which state is produced by the successive photoabsorption by the S I .
concentrations of the transient states of pyrene produced by the laser irradiation. In the case of the T I and the cation, the concentrations were calculated from the peaks of the absorption rise-and-decay curves at 410-420 and 445-455 nm, respectively, and in the calculation we used the film thickness of 2.2 pm and the following extinction coefficients: 3 x 104 M-' cm-I at 415 nm for T I and 4 x lo4 M-' cm-I for the ati ion.^^-^^,^^ The obtained values should be a little overestimated, because our calculation neglected the overlap of the spectra of the transients. In the case of S I , the concentration was obtained by multiplying the peak intensity of the fluorescence (310500 nm) rise-and-decay curve by the calibration factor obtained as follows: first we obtained the peak intensity of the fluorescence rise-and-decay curve at 0.09 mJ/cm2 where we can neglect a nonlinear effect; second we calculated the concentration of the SIstate at the peak at the fluence on the assumption that the amount of the S I state at the peak equals to that of the absorbed photons within the laser pulse; third we calculated the ratio of the concentration to the measured fluorescence peak intensity as the calibration factor. The obtained three concentrations showed similar fluence dependence; the concentrations increased with the laser fluence, but saturation tendencies were observed above a few tens of mJ/cm2. Those tendencies should arise from the deactivation processes such as SI-SI and TI-TI annihilation, and in the case of the TI state and cation this is supported by the estimation of the amount of the transients produced during the laser pulse. We have suggested in the discussion above that the T I state and the cation are formed through a two-photon process via S I . Thus the amount can be estimated roughly from the fluorescence rise-and-decay curve and the time profile of the laser pulse in the same way as taken to simulate the rise of T I shown in Figure
mJ/cm2except for the case of S I (160 mJ/cm2).In (a), the black solid, the black broken, and the gray lines represent the time-resolved absorbance at 248 nm, the absorbance before irradiation obtained on a conventional spectrophotometer, and the time profile of the laser pulse obtained with the photodiode, respectively. The solid lines in (b)-(d) represent the temporal changes of the concentrations of the S I , the TI, and the cation calculated from the rise-and-decay curves in Figure 9b, Figure lOc, and Figure 12c, respectively. The solid line in (e) represents the sum of the concentrations of the three transients.
11, a and c. Figure 13b shows the results of the estimation, and it suggests that the total amount of the transient state produced from a highly excited state via SI increases almost linearly with the laser fluence. This estimation allows us to exclude the saturation of the generation of those transients, and thus the observed saturation tendencies are attributed to the deactivation processes such as TI-TI annihilation and the charge recombination. (v) Summary of the Dynamics of the Transient States. We have indicated that intense excitation of pyrene-doped PMMA causes a series of interesting photochemical and photophysical processes within and after the laser pulse. It is worth noting that in those processes the temperature elevation due to the photoabsorption has great importance and thus cannot be neglected, which is quite different from conventional photochemistry. To elucidate the dynamics of the transient states comprehensively, more detailed examination is requested. However, we have practically obtained the temporal changes in the amounts of the transients, which will be a basis to discuss which transient is responsible for the cyclic multiphotonic absorption. Contribution of the Transient States of Pyrene to the Cyclic Multiphotonic Absorption. To discuss which transient mainly contributes to the multiphotonic absorption, we compared the time-resolved absorbance at 150 mJ/cm2 with the temporal changes in the concentrations of the transients at the same or similar fluence (Figure 14). The concentration curves of S I , T I , and the cation (Figure 14b-d) were obtained from the riseand-decay curves shown in Figures 9b, lOc, and 12c in the same way as taken to obtain the concentrations in Figure 13. In
11852 J. Phys. Chem., Vol. 99,No. 31, 1995
addition, Figure 14e shows the sum of the concentrations of the transients in Figure 14b-d. In principle, we have to calculate the transient absorption at 248 nm of each transient from the obtained concentrations and their extinction coefficients at 248 nm and then discuss the contribution of them to the photoabsorption. However, such a calculation is quite difficult; the extinction coefficients are hard to obtain because of the existence of the ground state absorption. In practice, even a simple comparison between curves in Figure 14a-d allows us to come to an important conclusion. The measured transient absorbance at 248 nm increased up to the delay of 30 ns, while the decay of each transient began before that time. It indicates that the absorbance increase cannot be reproduced by the consideration of the detected species only, which is also supported by Figure 14e. The sum of the concentrations of the three transients was at least less than 0.05 m o m during the laser pulse, and thus the rest of pyrene should be in the different states including the ground state. There are at least two species which may be responsible for the transient absorbance change but were not detected in 350550 nm by the time-resolved spectroscopy used in this work. One possible species is a vibrationally excited ground electronic state because the temperature elevation is indicated by the present results. Another possible species is a slightly changed ground state of pyrene which was detected with a conventional absorption spectrum after the laser irradiation (Figure 2). Although further investigation is required with considering the two species mentioned above, we can point out the following: there is a transient species which is not conventional one but really contributes to the observed absorbance increase, and then to the confirmed multiphoton absorption.
4. Conclusion We have indicated the dynamics of pyrene transient species in PMMA under intense excitation below and above the ablation threshold. Under ablation conditions, S I ,TI, and the cation of pyrene are already produced within the laser pulse. Their deactivation processes include SI-SI as well as TI-TI annihilation and the charge recombination which processes can be much affected by the temperature increase due to the photoabsorption. It is surprising that most of the pyrene do not decompose after such intense excitation and subsequent relaxation processes, which ensures or supports the ability of pyrene to absorb more than several 248 nm photons during the laser pulse.
Acknowledgment. This work was partly supported by a Grant-in-aid from the Japanese Ministry of Education, Science and Culture (06239101). We are grateful to the following company and scientists for their help in the experiments: ANELVA Corp. for the use of the excimer laser, Dr. M. Takagi of the Institute of Laser Engineering in Osaka University for the use of the depth profiler, and Dr. S. Kawanishi of Osaka Laboratory for Radiation Chemistry of Japan Atomic Energy Research Institute for mass spectroscopic measurement. We are also grateful to Prof. S. Tagawa (Osaka University), Dr. A. Tsuchida (Kyoto University), and Dr. H. Miyasaka (Kyoto Institute of Technology) for their useful discussions. References and Notes (1) (a) Masuhara, H.; Itaya, A.; Fukumura, H. In Polymers in Microfithography; ACS Symposium Series 4 12; Reichmanis, E., MacDonald, S. A., Iwayanagi, T., Eds.; American Chemical Society: Washington, DC, 1989; Chapter 24. (b) Masuhara, H.; Fukumura, H. Polym. News 1991, 17, 5-10.
Fujiwara et al. (2) Fukumura, H.; Mibuka, N.; Eura, S.; Masuhara, H. Appl. Phys. A 1991,53, 255-259. (3) (a) Fukumura, H.; Hamano, H.; Masuhara, H. Chem. Lett. 1993, 245-248. (b) Fukumura, H.; Takahashi, E.; Masuhara, H. J. Phys. Chem. 1995, 99, 750-757. (4) Fukumura, H.; Mibuka, N.; Eura, S.; Masuhara, H.; Nishi, N. J. Phys. Chem. 1993, 97, 13761-13766. ( 5 ) Fujiwara, H.; Hayashi, T.; Fukumura, H.; Masuhara, H. Appl. Phys. Lett. 1994, 64, 2451-2453. (6) Fukumura, H.; Masuhara, H. Chem. Phys. Lett. 1994, 221, 373378. (7) Furutani, H.; Fukumura, H.; Masuhara, H. Appl. Phys. Lett. 1994, 65, 3413-3415. (8) Fujiwara, H.; Nakajima, Y.; Fukumura, H.; Masuhara, H. J. Phys. Chem. 1995, 99, 11481- 11488. (9) El-Sayed, F. E.;MacCallum, J. R.; Pomery, P. J.; Shepherd, T. M. J. Chem. SOC., Faraday Trans. 2 1979, 75, 79-87. (10) Masuhara, H.; Tamai, N.; Ikeda, N.; Mataga, N.; Itaya, A,; Okamoto, K.; Kusabayashi, S. Chem. Phys. Lett. 1982, 91, 113- 116. (1 1) (a) Tsuchida, A.; Nakano, M.; Yoshida, M.; Yamamoto, M.; Wada, Y. Polym. Bull. 1988, 20, 297-304. (b) Yamamoto, M.; Tsuchida, A,; Nakano, M. MRS Int. Meeting Adv. Mater. 1989, 12,243-248. (c) Tsuchida, A.; Nakano, M.; Yamamoto, M. In Polymers f o r Microelectronics; Tabata, Y., Mita, I., Nonogami, S., Horie, K., Tagawa, S., Eds.; Kodansha: Tokyo, 1990; pp 541-548. (12) Chuang, T. J.; Hiraoka, H.; Mod1 A. Appl. Phys. A 1988,45,277288. (13) (a) Lee, 1.-Y. S.; Wen, X.; Tolbert, W. A,; Dlott, D. D.; Doxtader, M.; Amold, D. R. J. Appl. Phys. 1992, 72, 2440-2448. (b) Chen, S.; Lee, LY.; Tolbert, W. A.; Wen, X.; Dlott, D. D. J. Phys. Chem. 1992,96,71787 186. (14) Riddick, J. A.; Bunger, W. B.; Sakano, T. K. Organic Solvents, 4th ed.; John Wiley & Sons: New York, 1986. (15) The high-temperature drying resulted in a slight decrease in the pyrene concentration, but the decrease was carefully checked by conventional spectrophotometry before laser irradiation. (16) Fukumoto, H.; Fujiwara, H.; Fukumura, H.; Masuhara, H. To be submitted for publication. (17) Srinivasan, R.; Sutcliffe, E.; Braren, B. Laser Chem. 1988,9, 147154. (18) Srinivasan, R.; Braren, B. Chem. Rev. 1989, 89, 1303-1316. (19) Koren, G. Appl. Phys. Lett. 1987, 50, 1030-1032. (20) The time-integrated absorbance (Figure 4) indicates that pyrenedoped PMMA absorbs the energy per volume of 600 J/cm3 on an average over the film at the threshold. The temperature is estimated from the absorbed energy and the following physical properties of PMMA: density 1.19 g/cm3; specific heat capacity 1.42, 1.72, 2.05, 2.38, 2.35, and 2.50 J&K) at 25, 100, 120, 180, 240, and 300 "C, respectively. Those physical properties are available in the following reference: Brandrup, J.; Immergut, E. H. Polymer Handbook, 3rd ed.; John Wiley & Sons: New York, 1989; Chapter V. (21) Srinivasan, R.; Sutcliffe, E.: Braren, B. Appl. Phys. Lett. 1987, 51, 1285-1287. (22) (a) Stevens, B.; Hutton, E. Mol. Phys. 1960, 3, 71-78. (b) Borisevich, N. A.; Gruzinskii, V. V. Dokl. Akad. Nauk Belorussk. SSR 1963, 7, 309. (c) Freed, K. F.; Nitzan, A. J. Chem. Phys. 1980, 73, 4765-4778. (d) Smally, R. E. Annu. Rev. Phys. Chem. 1983, 34, 129-153. (e) Shan, K.; Yan, Y. J.; Mukamel, S. J. Chem. Phys. 1987, 87, 2021-2035. (23) (a) Nakato, Y.; Yamamoto, N.; Tsubomura, H. Chem. Phys. Lett. 1968, 2, 57-58. (b) Post, M. F. M.; Langelaar, J.; Van Voorst, J. D. W. Chem. Phys. Lett. 1971, 10, 468-472. (24) Schomburg, H. Ph.D. Thesis, Gottingen, 1975. (25) Bensasson, R.; Land, E. J. Trans. Faraday SOC.1971, 67, 19041915. (26) Miyasaka, H.; Masuhara, H.; Mataga, N. J. Phys. Chem. 1990, 94, 3577-3582. (27) The rise-and-decay curve of fluorescence was obtained by integrating its intensity in the streak image between certain wavelengths, and the wavelength range of the integration was sometimes slightly changed depending on the experimental condition. However, it has no substantial influence on the results or discussion as can be seen in the text. (28) Tolstoi, N. A.; Abramov, A. P. Sov. Phys. Solid State 1%7,9,255257. (29) Nakashima, N.; Kume, Y.; Mataga, N. J. Phys. Chem. 1975, 79, 1788-1793. (30) (a) Bergman, A.; Levine, M.; Jortner, J. Phys. Rev. Lett. 1967, 18, 593-596. (b) Masuhara, H.; Mataga, N. Chem. Phys. Lett. 1970, 7, 417419. (31) In making this plot, we reset the time axis, 0 ns, at a certain time after the laser pulse to exclude the influence of photoexcitation from the plot. (32) Carmichel, I.; Hug, G. L. J. Phys. Chem. Ret Data 1986, 15, 1-250. (33) Masuhara, H.; Tanaka, J.; Mataga, N.; Sisido, M.; Egusa, S.; Imanishi, Y. J. Phys. Chem. 1986, 90, 2791-2796.
Laser Ablation of a Pyrene-Doped PMMA Film (34) Thomas, J. K.; Johnson, K.; Klippert, T.; Lowers, R. J. Chem. Phys. 1968,48, 1608-1612. (35) Daiton, F. S.; Ledger, M. B.; May, R.; Salmon, G. A. J. Phys. Chem. 1973, 77, 45-49. (36) Tagawa, S.; Washio, M.; Tabata, Y.; Kobayashi, H. Radiat. Phys. Chem. 1982, 19, 277-281. (37) Jonah, C. D.; Sauer Jr., M. C. Chem. Phys. Lett. 1982, 90, 402406. (38) Beddard, G. S.; Porter, G. Biochim. Biophys. Acta 1977,462,6372. (39) Klein, G.; Voltz, R.; Schott, M. Chem. Phys. Lett. 1972,16, 340344. (40) Hayashi, T.; Fujiwara, H.; Fukumura, H.; Masuhara, H. To be submitted for publication.
J. Phys. Chem., Vol. 99, No. 31, 1995 11853 (41) Taniguchi, Y.; Nishina, Y.; Mataga, N. Bull. Chem. SOC.Jpn. 1972, 2923-2924. (42) Meot-Ner, M. J. Phys. Chem. 1980, 84, 2716-2723. (43) Janousek, B. K.; Brauman, J. I. In Gas Phase Zon Chemistty;Bower, M. T., Ed.; Academic: New York, 1979; Vol. 2, Chapter 10. (44) Birks, J. B. Photophysics ofAromatic Molecules; John Wiley & Sons: London, 1970. (45) The path length of the monitoring light should be not J 2 but 1.14 times longer than the film thickness (6) because of the refraction. We used 1.14d as the path length in the calculation of the concentration. JP943364K
