J. Phys. Chem. 1995, 99, 750-757
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Time-Resolved Spectroscopic and Photographic Studies on Laser Ablation of Poly(methy1 methacrylate) Film Doped with Biphenyl Hiroshi Fukumura,*yt Ei-ichi Takahashi: and Hiroshi Masuhara*J Department of Applied Physics, Osaka University, Suita 565, Japan, and Department of Polymer Science and Engineering, Kyoto Institute of Technology, Matsugasaki, Kyoto 606, Japan Received: July 8, 1994; In Final Form: October 13, 1994@
Nanosecond time-resolved spectroscopic and photographic measurements have been conducted for studying the mechanism of laser ablation of polymer films doped with biphenyl. Comparisons between the two different measurements made it possible to assign transient species observed during the ablation. The triplet state of the dopant was detected prior to the polymer ejection, while the excited singlet state was confirmed with its fluorescence. Besides these, biradicals such as C2, CH, and CN were identified and plasma emission was observed, but only when the fluence was increased highly above the laser ablation threshold. The excited singlet and triplet states were concluded to be main transient species leading to ablation. Temporal behavior of the excited singlet state was simulated with a model involving the ground, excited singlet, and triplet states, and the ablation mechanism was discussed.
Introduction UV laser ablation of polymer film was fiist reported for poly(methyl methacrylate) (PMMA) by Namba et al.’ and for poly(ethylene terephthalate)by Srinivasan et al.? which is an etching of the film by pulse laser excitation. Photon energy of excimer lasers often exceeds the bond energy of polymers, possibly leading to highly dense bond scissions. As a result, the film is fragmented and ejected, leaving a hole on the surface. Compared to the excitation by C02 and Nd3+:YAGlasers, the etched surface is smooth and spatial resolution is excellent. This phenomenon was interpreted due to the so-called “photochemical mechanism”.2 On the other hand, morphological changes of solid surface irradiated by IR lasers had been well-known, and their mechanism was eventually ascribed to a high temperature at the surface. The excitation energy is converteq to thermal energy through vibrational relaxation, and the film temperature is raised quickly. When the attained temperature is higher than the decomposition one, dissociation, melting, vaporization, and so on are induced. This etching behavior has been called the “photothermal me~hanism”.~-~ In the early stage of laser ablation studies, these two mechanisms were considered as competitive ones; however, both mechanisms are closely related to each other and cannot be separated distinctly.6-* It is more essential to understand such energy relaxation and reaction processes from the viewpoint of molecular photophysics and photochemistry. It should be noted that the ablation mechanism has been discussed in the literature mostly based on the results by respective one methodology. Observation of ablated surface by SEM?s10 mass spectrometry of ejected fragments,11,12laserinduced fluorescence measurement: photoacoustic measurement,13J4fast ph~tography,’~-~ and so on are frequently applied to ablation studies. These provide, of course, very important and indispensable information; however, molecular and electronic mechanisms are not directly revealed enough on the basis of these measurements. We considered that time-resolved emission and absorption spectroscopy of the irradiated film (not
* To whom correspondence should be addressed.
+ Osaka University.
* Kyoto Institute of Technology. @
Abstract published in Advanced ACS Abstracrs, December 1, 1994.
0022-365419512099-0750$09.00/0
fragmented materials) would be very fruitful in analyzing dynamics and mechanism of laser ablation and have applied the spectroscopy to polymers bearing or containing aromatic molecules as chromophore.21-26 Photophysics and photochemistry of aromatic molecules are well-known, so that the ablation mechanism of the polymers can be considered in detail. Doping a polymer with aromatic molecules makes it possible to vary the absorptivity of the film without changing the polymer matrix, and therefore the method has been used extensively.7~8~23~25-30 The ablation mechanism for doped polymers may be different from that for neat polymers which intrisically bear chromophores, since light energy absorbed by the chromophores can be directly transferred to the main chain and the chromophores are never free from the chian unless chemical bonds are broken. Although these differences restrict the scope gained from studies of doped polymer ablation, we consider that the spectroscopic detection of photoexcited states during laser ablation of doped polymers is the first step to understanding how the absorbed light energy is converted into that bringing morphological changes of the polymers. In the present paper, PMMA film doped with biphenyl was investigated by transient absorption as well as emission spectroscopy and nanosecond dynamic photography just upon ablation. The excited species involved in ablation are identified, and their dynamic behaviors have been revealed with nanosecond time resolution. The morphological change is correlated to spectral information and considered in terms of molecular photophysics and photochemistry.
Experimental Section Materials. PMMA (Kuraray) was three times reprecipitated from benzene-methanol and dried under low temperature. Biphenyl (zone refined, Tokyo Kasei) was used as received. Chlorobenzene (special grade, Wako) was treated with H2S04 to remove impurities, dried with CaC12, and distilled. Benzene (chromatograph grade, Wako) was used without further purification. A thin PMMA film (-2.5 pm) was prepared by coating chlorobenzene solution of 15 wt % PMMA and an appropriate amount of biphenyl on a quartz plate with a spinner (Mikasa, 1H-2D). The film was dried under vacuum for 3 h. A thick film (-30 pm) was prepared by casting benzene solution 0 1995 American Chemical Society
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PMMA Film Doped with Biphenyl
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Figure 1. Schematic diagram of nanosecond photography.
of 10 wt % PMMA and an appropriate amount of biphenyl and was dried under vacuum for 5 h. Ablation Experiments. A 248 nm pulse of excimer laser [Lambda Physik, EMG 101 MSC (fwhm 18 ns) or EMG 201 MSC (fwhm 30 ns)] was used as an excitation light source. Its beam profile was adjusted by an aperture and focused perpendicularly on the film. The intensity was attenuated by combining five reflecting mirrors with transmittance of 100, 68, 50.5, 17.5, and 8.5%. A power meter (Gentec, ED-200) was used to measure the laser intensity. Etch depth of ablated films was measured by a depth profiler (Sloan, DEKTAK-3030). Transient Emission and Absorption Spectroscopy. A streak camera (Hamamatsu, C2830) was attached to a polychromator (Jobin-Yvon, HR-320) and used as a detector. Streak images were measured by a CCD camera (Hamamatsu, C3 140) and transferred to a computer. The timing between excimer laser and streak camera was adjusted with a digital delay/pulse generator (Stanford Research System, DG535). The jitter and drift of the timing were monitored by a storage scope. Since both time-resolved spectra and rise/decay curves at each wavelength were obtained by analyzing a single streak image and not averaged over some measurements, information on ablation dynamics of the fresh surface was available. In the case of emission spectroscopy, emission just upon ablation was simply led to a polychromator. A pulsed 150 or 300 W Xe lamp (Wacom, KXL 150F or KXL 300F) was used as a monitoring light for transient absorption spectral measurements. In both spectroscopic measurements, only ablated area was monitored, which is very critical for precise analysis. Nanosecond Dynamic Photography. A schematic diagram for photography is shown in Figure 1, where two lasers are used to induce laser ablation and to light a dye solution for observation. The laser pulse for ablation is introduced to an edge of the film in the horizontal direction, while the dye fluorescence illuminates it upward. Since morphological changes at the edge are limited to a small region, the observation was done under a microscope (Olympus, BHS or Nikon, Labophot2). The second harmonic of Nd3+:YAG laser was used to excite a methanol solution of Rhodamine 101. Fluorescence of the dye solution has a pulse width of 17 ns, so that it is a suitable flash lamp for nanosecond photography. For avoiding the exposure due to a scattering of excitation laser pulse and luminescence from the film itself, a filter (Toshiba, 0-57) was set in front of the camera. The camera film (Kodak, TMAX P3200) was developed with a Kodak D-76 developer. The delay time between two lasers was adjusted by the digital delay/pulse generator mentioned above.
10
'
F l u e n c e (mJ/cm2) Figure 2. Etch depth of poly(methy1 methacrylate) films doped with biphenyl as a function of laser fluence. Biphenyl concentration is 2.0 (0),4.0 (0), and 8.0 wt % (A).
Results and Discussion Etching Behavior. Biphenyl has an appreciable absorption coefficient at 248 nm compared to that of PMMA itself; therefore, it is clear that most of light energy is absorbed by biphenyl. Etch depth, which is the depth of a hole dug by oneshot irradiation, was plotted against laser fluence in Figure 2. The etch profile depends upon dopant concentration, and the ablation threshold became lower as the concentration of the dopant was increased. The present threshold values for 4.013.3 wt % biphenyl are less than 100 mJ/cm2 and are clearly lower than that of the neat PMMA film (>650 mJ/cm2). This means that the morphological changes of PMMA are induced by excitation of biphenyl. Nanosecond Photographic Study. Since the film is etched efficiently, the decomposed materials, molecules, atoms, and so on should be ejected in the air. Under the present conditions, etch depth produced by one shot excitation at a few hundred mJ/cm2 is about 1 pm, so it is difficult to observe directly the preparation process of an etched hole under the microscope. Instead, we report the observation of ejected materials. In Figure 3 is given the behavior of PMMA film doped with 2.0 wt % biphenyl excited at 0.6 J/cm2. The materials were ejected in a spokelike pattern at the initial stage, and then their explosion developed normal to the film. At 5 ,us after excitation, the ejection still continued. As the reference, a neat PMMA film was examined at the same fluence of 0.6 J/cm2. Nothing was observed at any delay time, which is quite reasonable since the present fluence, 0.6 J/cm2, is lower than the threshold. By increasing the laser fluence, the neat PMMA film was also ablated, and the result at 2.5 J/cm2 excitation is shown in Figure 4. At 50 ns after the peak of the excitation pulse, ejection of materials was detected a little. Its fragmentation was faster at the edge part compared to that in the center of the ablated area, giving a crownlike pattern. This was observed up to 1 ,us, and then the ejection in the central part was accelerated. The ejection of materials was confirmed until 10 ps after excitation. Similar behavior was observed in the above-doped film at 1.3 J/cm2. Within 50 ns after excitation, no appreciable ejection was detected, indicating optical conditions in this time range are good for spectroscopic measurement in the visible region. The crownlike pattern of the ejected materials and the succeeding behavior were analogous with those of the doped film at 0.6 J/cm2. Similar nanosecond photography was applied to PMMA and polyimide films by Srinivasan et al., reporting that a shock wave was observed at 60 ns and followed by an explosion of fragmented debris.17 The shock wave was not clearly confirmed in present work, which may be ascribed to differences in experimental conditions.
Fukumura et al.
752 J. Phys. Chem., Vol. 99, No. 2, I995
1 C
Figure 3. Nanosecond photographs of poly(methy1 methacrylate) film doped with 2.0 wt % biphenyl upon laser ablation at 0.6 J/cm2.
The average speed of the ejected materials can be estimated from these time-resolved photographs. The front position of the materials was plotted against the delay time as in Figure 5 , where PMMA film doped with 2.0 wt % biphenyl was ablated.
Figure 4. Nanosecond photographs of neat poly(methy1 methacrylate) film upon laser ablation at 2.5 J/cm2.
A linear relation was confirmed until 2 ,us for ablation at 0.6 and 1.3 J/cm2. It is worth noting that the ejection speed depends on the laser fluence. and it becomes faster as the fluence .
~~
~
J. Phys. Chem., Vol. 99, No. 2, 1995 753
PMMA Film Doped with Biphenyl
3001
o 1.3 Jlc:' O.BJ/Cm
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Figure 5. Distance between the front of ejected fragment particles
and the film surface as a function of the delay time. Laser fluence is given in the figure.
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Figure 7. Absorption rise and decay curves of poly(methy1 methacrylate) film with 2.0 wt % biphenyl upon laser ablation. The observation wavelength is 360 nm. Laser fluence is given in the figure.
8
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(-11-26 ns), this was replaced at the late stages by a very flat absorption descending to the long wavelength. No other transient absorption band was detected. Figure 6. Time-resolvedabsorption spectra of poly(methy1methacrySince the flat band was observed only when excited at 360 late) film doped with 2.0 wt % biphenyl upon laser ablation. Laser mJ/cm2 above the threshold, this is characteristic of laser fluence is (a) 30 and (b) 360 mJ/cm2. Gate time is given in the figure. ablation. One interpretation for the flat band is based on the scattering of the monitoring light by ejected particles and debris. increases. The speed obtained for excitation at 0.6 and 1.3 J/cm2 This is quite reasonable, since the light in the shorter wavelength m/s, respectively. These values are consistent was 106 and 160 is more efficiently scattered compared to in the longer one. Such with the reported value for PMMA and p01yimide.l~ The shock a continuum-like flat band was confirmed also for the present wave in the latter case has a speed of 600 m / s , and the speed of neat PMMA film above the ablation threshold. Namely, the ejected Cz radical reported to be IO4 m/s.' It is reasonable that spectral shape has nothing to do with biphenyl and can be the lighter species have a higher ejection speed. The fragmented ascribed to the formed fragments. materials observed under the microscope may have a dimension Absorption rise and decay curves were measured at 360 nm, of a few micrometers; hence, the present speed is rather slow. where the absorption peak of the triplet biphenyl is located, Transient Absorption Measurement upon Ablation. The and summarized in Figure 7. At low laser fluence (10 mJ/cm2) fluorescence and intersystem crossing yields of biphenyl are in the absorption did not decay in the present time region, while the ranges 0.13-0.1831,32 and 0.51-0.84,33-35 respectively. a fast decaying component was additionally observed at 84 mJ/ From the high yields of the two relaxation pathways, we cm2 excitation. The relative contribution of the new component consider that fluorescence and triplet absorption spectroscopic increased, and its decay became faster as the laser fluence was analyses of biphenyl upon laser ablation are very important and increased. The behavior can be explained in terms of T1-TI meaningful. In Figure 6 are shown transient absorption spectra annihilation. In the laser fluence range over the ablation of PMMA films at excitation intensity of 30 and 360 mJ/cm2, threshold, the T,-T1 absorption of the biphenyl was measured which are below and above the threshold, respectively. The like a spike and followed by a new intense absorption. This origin of the time axis is defined as the time when excitation means that there is a time lag between the T1-T1 annihilation pulse reaches the maximum intensity. The absorption spectrum decay and a large absorption increase. The absorbance at the obtained at 30 mJ/cm2 is broad and has a peak at 360 nm. Since plateau part was increased as the fluence was high. This the spectral shape and the peak position are similar to those of absorption corresponds to the flat band described above, which the T,-T1 transition of biphenyl in s o l ~ t i o n , ~the~ ,main ~ ~ , ~ ~ is directly confirmed by correlation between the absorption rise transient species in this time range is the triplet biphenyl. The curve and the dynamic photography. spectrum did not show any time-dependent change. Since the In Figure 8, the result obtained at 600-670 mJ/cm2 are fluorescence lifetime is short (1.8 ns), an effective concentration shown. Until a few tens of nanoseconds, no appreciable ejection of the excited singlet state is low, giving no absorption of fragmented debris was observed and the T,-T1 absorption band. A similar result was observed at the excitation of 82 was clearly measured. The quite rapid T1-T1 annihilation was and 140 mJ/cm2. When excitation fluence was increased up to completed within the time scale of 50 ns, while the intense 360 mJ/cm2, a time-dependent spectral change was observed. absorbance of the flat band increased as a number of fragments Although the T,-T1 band was obtained during excitation expanded in space. We consider that this is the direct 350
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754 J. Phys. Chem., Vol. 99, No. 2, 1995
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Time (ns) Figure 8. Comparisons between transient absorption at 360 nm and nanosecond photograph of poly(methy1 methacrylate) film doped with 2.0 wt % biphenyl upon laser ablation. Laser fluence for absorption measurement and photograph is 670 and 600 mJ/cm2, respectively.
experimental foundation for assigning the flat band to decomposed particles. Now coming back to the result on etch depth in Figure 2, etching behavior can be discussed on the basis of dynamic photography and transient absorption spectroscopy. The relationship between the etch depth and the logarithm of the fluence is nonlinear, which seems to contradict a simple expectation based on the Lambert-Beer equation. Sutcliffe and Srinivasan once proposed a dynamic mechanism that employs the flux threshold defining the concentration of photons absorbed per unit time required to overcome relaxation paths which would not lead to ph~tofragmentation.~~ They also considered that a
shielding of the incoming laser pulse by the ejected fragments would be important. The nonlinearity of the etch curves, however, can be simply explained in terms of a transient change in absorbance of the polymer film. In the present study, the triplet state of biphenyl was found to be the main species during laser pulse. If the molar absorption coefficient of the triplet state is different from that of the ground state at the laser wavelength, the generation of the triplet state would result in a change of the laser light penetration. We have also found out an apparent transient increase of absorbance by decomposed particles; however, the band due to light scattering emerged after the laser pulse below 600 mJ/cm2. Thus, we consider that a
J. Phys. Chem., Vol. 99, No. 2, 1995 755
PMMA Film Doped with Biphenyl
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Wavelength [nm] Figure 9. Time-resolved emission spectra of poly(methy1methacrylate) film doped with 2.0 wt % biphenyl upon laser ablation. Laser fluence is given in the figure. Gate time for the upper two and the lower two spectra is -23-193 and -23-222 ns, respectively.
change in absorbance owing to the generation of excited states is somewhat responsible for the etching behavior, but the shielding by fragmentation is less important around the ablation threshold. Transient Emission Measurement upon Ablation. We tried to find new transient, emissive species which have a key role in laser ablation, since only the triplet biphenyl was identified in transient absorption spectroscopy. Total emission spectra of 2 wt % biphenyl in PMMA are shown in Figure 9. Although the S/N value of the spectra was not good because of one shot measurement, two peaks at 305 and 315 nm were barely identified at low laser fluence. This can be assigned to the fluorescence spectrum of biphenyl because of the similarity to the literature fluorescence in c y ~ l o h e x a n e .At ~ ~340 mJ/cmZ excitation the emission spectrum became slightly broad, and a further increase in laser fluence (1.8 J/cm2) resulted in an appearance of the tail in the long wavelength region. At 3.7 J/cmZ,some sharp peaks were superimposed, indicating different transient species are generated during laser ablation. To identify clearly the structured emission bands, timeresolved emission spectra were measured at 3.7 J/cm2 as shown in Figure 10. At the initial delay time (-16.2 to 5.9 ns), the emission spectrum is similar to biphenyl fluorescence, while the broad emission in the range 260-430 nm and the emission bands of CN (358 and 382 nm), CH (431 nm), and C2 (433, 467, and 5 11 nm) were observed at late stages. The latter line spectra were generally observed during laser ablation of organic compounds in air,22,z4.39-42 which means that a part of the polymer is decomposed to small molecular fragments by photothermal or photochemical reactions. The former band (260-430 nm) is much broader than the biphenyl fluorescence and cannot be assigned at the moment; however, its contribution to the total emission is extremely low. The behavior was common to PMMA film doped with 4.0 and 8.0 wt % biphenyl. Furthermore, similar spectra and behavior were observed for poly(N-vinylcarbazole) film.22 The broad emission in the range 260-430 nm at 1.8 J/cm2may be due to plasma emission, which is confirmed by Koren et al.39 and us.21,22 The emission problem, however, was observed only when excited at the fluence considerably higher than the threshold. On the basis of the present emission spectral examinations, we consider that the main emissive species in the primary processes is the excited singlet state of biphenyl around the threshold fluence.
300
350
400
450
500
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Wavelength [nm]
Figure 10. Time-resolvedemission spectra of poly(methy1 methacrylate) fiim doped with 2.0 wt % biphenyl upon laser ablation at 3.7 J/cm*. Gate time is given in the figure.
Time (ns) Figure 11. Comparisons between T-T absorption (360 nm, -) and fluorescence (325 nm, 0)rise and decay curves of 2.0 wt % biphenyl doped in poly(methy1methacrylate) film laser ablation. Laser fluence is (a) 30, (b) 140, and (c) 360 mJ/cm2. The rise component after 40 ns in (c) is due to the materials ejection (see Figure 7). Fluorescence time profiles of PMMA films doped with 2.0 wt % biphenyl are shown in Figure 11. Since the fluorescence lifetime is 1.8 ns, the profile is a little wider than that of the excitation pulse even at 5 mJ/cm2. As the laser fluence was increased, the decay became fast and the apparent peak shifted to the early stage during the laser pulse. One of the crucial factors is S1-S1 annihilation, which is quite general for polymer fluorescence under high-intensity excitati0n.4~The behavior was also confirmed for the films doped with 4 and 8 wt % biphenyl. The total fluorescence intensity was examined as a function of laser fluence. For films with different biphenyl concentrations, fluorescence intensity increased with the fluence, although the relation is not linear but showed a saturation tendency. Above the threshold, the total fluorescence intensity did not increase.
156 J. Phys. Chem., Vol. 99,No. 2, 1995 0.3
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Fluence/Pulse (mJ/cm2)
Figure 12. (A) Dependence of Tn-T1 absorbance at 360 nm upon laser fluence. The concentration of biphenyl is given in the figure. The arrows indicates the respective ablation threshold.
This indicates that a new channel for deactivation of the excited states is additionally opened around the threshold, leading to ablation. In Figure 11 the fluorescence profiles are compared with the rise dynamics of the triplet absorption which were measured under the same conditions. The triplet rise corresponds to the fluorescence decay, indicating that the triplet state is produced through the intersystem crossing both below and above the laser ablation threshold. The peak absorbance of the T,-T1 transition was plotted against the laser fluence in Figure 12. In the case of films doped with 2 and 4 wt % biphenyl, the triplet absorbance increased gradually with the fluence showing a strong saturation tendency, and a h i c k point was barely observed around the threshold. For a highly doped film (8 wt %), the triplet absorbance is almost constant far below the threshold. However, the estimation of triplet concentration at the polymer surface from the absorbance data is not simple in highly doped films, because the distribution of excited states in the film is inhomogeneous along the depth from the surface. Assuming that the quantum efficiency yielding the triplet state is constant and the Lambert-Beer law holds for the excitation and monitor lights, we can presume the concentration of the triplet state at the surface ([TI,) from the concentration obtained from the experiment ([TIex)by the following equation. [TI, = 2.303~[G]d(l- 10-EIG1d)-l[T],, Here, E and [GI denote the molar absorption coefficient and the concentration of the ground state, respectively, and d denotes the thickness of the film. The factor multiplying [TIex to estimate the value of [TI, reaches 5.7 in the case of the highly doped film. This means that the estimation of the real concentration of excited states at the surface is very difficult in this experiment. Even under this situation, however, we can safely state that the main transient species during the ablation is the triplet state, and its concentration does not vary around the threshold. Simulation of the Excited Singlet State Dynamics. As discussed above, the flat band in absorption was identified due to scattering of the monitoring light by the fragmented particles. No transient species has been confirmed by transient emission and absorption spectroscopy besides the excited singlet and triplet states. Furthermore, the plasma emission and biradicals such as CN, C2, and CH were observed only when excited with high fluence above the threshold. Namely, ablation is possibly induced with the excited singlet and triplet states as transient species, while the generation of those states would not directly lead to the ablation of the polymer matrix. Here we tried to
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Time (ns) Figure 13. Observed (325 nm, *) and simulated (-) fluorescence rise and decay curves of poly(methy1methacrylate) film doped with 2.0 wt % biphenyl upon laser ablation. Details of the simulation are given in the text. Laser fluence is given in the figure. simulate the dynamics of the excited singlet state by the usual photochemical processes by the following equation. d[S,lldt = - (kf + kist + ~,,[SlI)[S,l + d G 1 where k,, is the rate constant of the S1-S1 annihilation process. kf and kist are the rate constants of fluorescence decay and intersystem crossing, respectively. I is the laser fluence, [SI] is the concentration of the excited singlet state, and E and [GI are defined above. The time profile of [SI] was numerically obtained by using the equation [Sl(t)] = [SI(?- At)]
+ At(d(t - At)[G(t-At)]
- (kf
+
kist + k,,[s,(t-At)l)[s,(t-At)l) Assuming a Gaussian pulse with 18 ns of fwhm and using E = 16 000 M-l cm-', kf = 6.25 x lo7 s-l, ki,, = 4.06 x lo7 s-l, and k,, = 2.0 x lo9 s-l, the time profile of [SI] was obtained as a function of laser fluence. As shown in Figure 13, a good agreement was obtained between simulated and measured fluorescence profiles in the cases of 5.1 and 53 mJ/cm2. Above the threshold, a difference between them became noticeable; the experimental profile is a narrower than the simulated curve, and in particular, the difference in the late stage is large. Thus, we now consider that a new deactivation channel of the excited singlet state is opened in the late stage of the excitation pulse. One possible explanation is that a temperature rise of a polymer matrix takes place when the matrix is ablated by laser irradiation. Indeed, this is very recently supported by spectroscopic If the temperature of a system changes, rate constants in the system are no more constant, which is not taken into accunt in this simulation. Therefore, the deviation from the simulation curve may indicate a marked rise of the matrix temperature. The other explanation is based on the inner filter effect by the lowest triplet state. The high concentration of the state attained at the late part of the excitation pulse may result in its competitive absorption of the laser pulse with the ground state.
Conclusion Spectroscopic and morphological aspects of laser ablation have been revealed on the basis of comparable measurements of fluorescence, absorption, and photography. Doping of
J. Phys. Chem., Vol. 99, No. 2, 1995 757
PMMA Film Doped with Biphenyl biphenyl in PMMA induces laser ablation at 248 nm with less fluence compared to a neat film, so that photophysics and photochemistry of biphenyl are important in view of primary processes of polymer ablation. The ejection of materials was observed at a later stage than 50 ns after excitation, giving the flat transient absorption band. Luminescence spectroscopic identification was made for plasma emission and biradicals such as Cz, CH, and CN, the latter two of which were observed highly above the threshold. By transient absorption spectroscopy, the triplet state was clearly detected, and its decay became fast as the laser fluence increased. It is worth noting that no other transient molecular species besides the excited singlet and triplet states was confirmed spectroscopically at the fluence range around the threshold. On the basis of these results, we showed that the dynamics of the excited singlet state characteristic of the 5-50 d / c m 2 range can be simulated by normal photochemical processes. It was concluded that additional processes are involved in photochemical primary processes for simulating the dynamics of excited states upon laser ablation. The present results demonstrate that an approach taken in this study to laser ablation of polymer is essential to bridge morphological changes and molecular level understanding, and further systematic and detailed experiments are indispensable and now in progress in our laboratory.
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