J. Phys. Chem. 1993,97, 13761-13766
13761
Mass Spectrometric Studies on Laser Ablation of Polystyrene Sensitized with Anthracene Hiroshi Fukumura,*i+Nobuko Mibuka? Shigeru Eura,* and Hiroshi Masuhara*if Department of Polymer Science and Engineering, Kyoto Institute of Technology, Matsugasaki, Kyoto 606, Japan
Nobuyuki Nishil Institute for Molecular Science, Myodaiji, Okazaki 444, Japan Received: July 6, 1993; In Final Form: September 17. 1993” Polystyrene films doped with anthracene or anthracene-dlo were ablated by 35 1-nm laser irradiation in a high-vacuum apparatus. Velocity distribution of ejected species were measured with a quadrupole mass spectrometer. Repetitively irradiated surfaces were found to differ from fresh surfaces in giving different velocity distributions. Hence, only the velocity distributions of the ejected species from the fresh surfaces were analyzed in detail. Below the ablation threshold, only intact anthracene molecules were detected, and their velocity distribution was analyzed by assuming a composite Maxwell-Boltzmann (MB) distribution with hightemperature MB (550-600K) and low-temperature MB (25&300 K) components. When the etching of the surface took place at several hundred mJ/cm2, the detected species were mainly anthracene and styrene monomer. The translational energy distribution changed depending on the concentration of anthracene and the laser intensity. Deuteration of anthracene reduced the translational energy of both anthracene and styrene monomers. It was suggested that photon energy absorbed by anthracene is converted into thermal energy via rapid internal conversion in a polystyrene matrix, resulting in thermal decomposition of the matrix.
Introduction Irradiation of aromatic molecules dispersed in polymer films with an intense laser pulse can cause a b l a t i ~ n l and - ~ irreversible swelling7 of the surfaces. Using laser absorbing molecules as sensitizersand changing the concentration,it is possible to examine various combinations of polymers and sensitizers under different conditions. Since we can choose the sensitizer whose photochemistry and photophysics are well characterized, it is promising tostudy sensitizerinduced ablation for understanding the ablation mechanism from a molecular viewpoint. Benzoin,’ acridine: and 2-(2’-hydroxy-3’,5’-diisopentylphenyl)benzotriazole (Tinuvine)3 have been used as sensitizers to cause the ablation of poly(methy1 methacrylate) (PMMA). The ablation mechanism in those works has been ascribed to multiphotonic decomposition of the dopant molecules. It has also been reported that ben~ophenone,~(4-aminobenzoy1)hydrazine,5 and pyreneM are effective sensitizers for PMMA and other polymers, but the ablation is most likely due to the transient heating. Similar results supporting photothermal mechanism were obtained for ablation and swelling of the polymers doped with porphyrin deri~atives.~ Recently, transient absorption measurement has been applied to study laser ablation of PMMA doped with 1,1,3,3-tetraphen~lacetone~ and p-ter~henyl.~ However, no novel reaction intermediate was observed above the ablation threshold. Thus, the mechanism of sensitized laser ablation still remains unclear as well as that of laser ablation without sensitizer.I0 Mass spectroscopic analysis has been employed extensively to study the mechanism of laser ablation of neat polymer films: PMMA by 193-nm irradiation;” PMMA by 240-341 nm;12 PMMA and polystyrene by 248 nm;I3 polystyrene by 193 nm;I4 f Present address: Department of Applied Physics, Osaka University,Suita 565, Japan. t Present address: Central Research Institute, Toyobo Co., Katada, Otsu 520-02, Japan. 8 Present address: System Division, Hamamatsu Photonics, 325-6 Sunayama-cho, Hamamatsu 430, Japan. 1 Present address: Department of Chemistry, Faculty of Science, Kyushu University, Fukuoka 812, Japan. Abstract published in Adoance ACS Abstracts, December 1, 1993.
0022-3654/93/2097-13761$04.00/0
polyimide and graphite by 266 nm;15 polycarbonate, polyimide, poly(ethy1ene terephthalate), and poly(a-methylstyrene) by 266 nm;I6 polystyrene and polyimide by 245-260 and 308 nm;” polyimide by 308 nm;I8 polytetrafluoroethylene by 193,248, and 308 nm.I9 Assignment of decompmed products, as well as analysis of their translational energy and rotational energy distribution, supported results that showed that the thermal process is involved in the laser ablation of PMMA,” polystyrene,13poly(a-methyland polytetraflu~roethylene.~~ Positive and negative ionic species were also detected in laser ablation p r o c e s ~ . ’ ~ J ~ J ~ On the basis of the observed phenomena, supersonic gas expansion,I1,17 adiabatic expansion,14 and Coulomb explosion1* have been proposed for the ablation mechanism. Time-of-flight mass spectroscopythus provides a velocity distribution of ejected species upon laser ablation. Despite the usefulness of the spectroscopy,no mass spectroscopicstudy has been reported for sensitized laser ablation of polymer films to our knowledge. In this paper, we present studies of the ejected species from anthracene-doped polystyrene films upon 35 1-nm excimer laser irradiation. We employed conventional electron impact mass spectrometry to detect the ejected neutral species. Although the electron impact ionizer may cause fragmentation of ejected compounds,I the calibration for the drift time in the spectrometer and that for the ionization efficiency have been studied very w e l P Z 1and the reliability is high. Around the threshold of laser ablation, photochemicalprimary processes of sensitizer molecules in a polymer film have been studied with time-resolved emission and absorption spectroscopy.9s22923While the film surface was turned to be etched at the threshold, no marked change of excited-statedynamicswas found. Connecting the results with mass spectrometric analysis to elucidate the ablation mechanism, here we focus our attention to the phenomena that occur around the ablation threshold. This condition is also favorableto the present mass spectrometricstudy, since deep etching increases markedly the pressure in a mass spectrometer resulting in an increase of background noise. Under high-intensity laser irradiation, sensitizer molecules may be excited to the higher excited singlet and triplet states from which reactions and relaxation processes occur. TOdiscuss the mechanism of sensitized laser ablation from a molecular point 0 1993 American Chemical Society
Fukumura et al.
13762 The Journal of Physical Chemistry, Vol. 97, No. 51, 1993
-
Turbo Pump 110
l / S
sta
’
Mass
L -Turbo
r“” v
Spectrometer
u
e
r
T
t
z
Pump 400 ) / a
Window
3
5a &
I 30
Anthracene 96mM . * * * * * Anthracene [48mM{ -a*Anthracene-d,,, (97mM)
20 -
1 0 -
5 E
-
1
00
-... ...
..... “%
f=bOOmm L e n s
Exc i m e r
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Figure 1. Schematic diagram of apparatus for the mass spectrometric study of laser ablation.
ofview, we have toget full information about relaxation pathways of each excited state of the sensitizer. Deuterium substitution is well-known to affect nonradiative transitions and reactions of the bond break. This is helpful in elucidating the ablation mechanism in relation to inter- and intramolecular relaxation processes of electronic excited energy. Since the dynamics of higher excited states of anthracene derivativesand the deuterated compounds has been studied exten~ively,~~ we used anthracene as sensitizer in this experiment. Experimental Section Samples. Polystyrene with aromatic additives less than 200 ppm was kindly supplied by Kuraray Co. and was purified by several reprecipitations from benzene solution with methanol. Anthracene (scintillation grade, E. Merck) and anthracene-dlo (>98% deuterium, Aldrich Chemical Co.) were used as received. The polymer and the sensitizer were dissolved in chlorobenzene and spin-coatedon quartz disks of 1OO-mmdiameter. The polymer films were dried again under vacuum for several hours. A typical thickness of the films was 4 pm. All the solvents were purified by distillation before use. When anthraceneconcentrations in polymer were heigher than 500 mM, sedimentation of anthracene microcrystalline was observed under a fluorescencemicroscope. In this study, therefore, the concentrations were set to be lower than 100 mM. The uniformity of the films were checked with the microscope. The absorptivity of a film with 96 mM of anthracene was 0.054 pm-l at 351 nm. Apparatus. The setup for mass spectrometric measurement is outlined in Figure 1. The details of theultrahigh-vacuum chamber and the detection system have been described elsewhere.2+21A rotatable sample holder was situated in the reaction chamber for the irradiation of fresh surfaces of a polymer film. This is very important since the time-of-flight signal is considered to vary with the number of laser shot^.^^,^^ To clarify the ablation mechanism from molecular point of view, we have to specify the molecules which absorb laser light, and therefore we should avoid the repetitive excitation causing light absorption by reaction intermediates or products. The film surface was hit at an incidence angle of 70’ by light beam from a XeF excimer laser (351 nm, 20-11s fwhm, Lumonics EX600) after passing through a focusinglens and a quartz window. Particles which were ejected by laser irradiation at an angle of 30° with the surface normal were ionized by an electron impact ionizer and were extracted by ion optics into a quadrupole mass spectrometer (Extra Nuclear Laboratory Model 4-270-9 with High-Q-head 15) equipped with a Galileo 48 16 type channeltron. Electron bombardment energy was set at a voltage of 20 eV in most cases to avoid further fragmentation of the ejected particles. The distance from the irradiated surface to the center of the
E, = m/2(1/t12 It is, however, necessary to take into account the ionization efficiency of a neutral particle in the ionizer and the drift time (td)of the formed ion in themass Theionization efficiency is in inverse proportion to the speed of the neutral particle; therefore we use the following relationship:
6E, = -(ml/t2)6t
(2) to transform a distribution function of energy space into that of time space. The drift time of an ion in the mass spectrometer is a function of ionizer potential and is approximatelyproportional to m 1 / 2 .Since we selected an ion energy of 15 eV, the measured drift time was expressed by 4.975m1l2(ps), and this value should be subtracted from the measured flight time (r’) to get the real one. Consequently, when the energy of particles is given by the Maxwell-Boltzmann (MB) distribution, the time-of-flightsignal as a function of the measured flight time is described by the equation:
At’) dt = q l / ( t ’ - td)I3 exp{-m12/2kT(t’ - td)’)dt (3) where C is a constant, k is the Boltzmann factor, and T is a translational temperature. RCSultS
Etch Depth. The etch depth was measured as a difference of averaged levels between non-irradiated and irradiated areas and plotted against the laser intensity in Figure 2. The negativevalues observed for anthracene-dlo represent the irreversible swelling of the surface. The ablation threshold of the low-concentration(48
Laser Ablation of Polystyrene with Anthracene
The Journal of Physical Chemistry, Vol. 97, No. 51, 1993 13763
3001 n
300
200
200 ffl 100
4
c 3
0
0
0 500 400
'",*,
2001 I 100 Ob'0
'
.--.., 500 5 00 -
1000
1500
Time ( p s )
d2000 o0
Figure 3. Comparison between time-of-flightsignals from fresh surfaces (thick solid lines) and those from incubated surfaces (thin broken lines) for anthracene (a) and styrene monomer (b). Sampleswere polystyrene films doped with 96 mM anthracene. Laser intensity was 610 mJ/cm2, and the signals are accumulated over 128 times for all signals. mM) film is higher than that of the high-concentration (96/97 mM) one. The dependenceof the etch depth on the laser intensity and the sensitizer concentration is quite similar to that observed for the other s e n ~ i t i z e r s The . ~ ~ ~film ~ with the high anthracene concentration undergoes morphological changes at laser intensity of 300-400 mJ/cm2. Note that the surface swelling takes place in the case of anthracene-d,o, although the concentration of both films is almost the same. As the electronic structureof anthracene is the same as that of the deuterated compound, the present data suggest that energy relaxation modes have a key role in the ablation. Time-of-Flight Signals. When a certain spot on the surface was irradiated repetitively by laser pulses without rotation of the sample, both the intensity and the distribution of the time-offlight signal were confirmed to vary with the number of laser pulses. The signal intensity due to anthracene molecule ( m / e = 178), accumulated over 128 shots from the fresh surface, was stronger than that repetitively irradiated, as shown in Figure 3a. Since the signal intensity a t the same spot was reduced by the single shot irradiation, it is considered that anthracene molecules were ejected preferentially from the polymer film or decomposed in the etched film particularly by the first several pulses. On the contrary, the signal intensity of styrene monomer ( m / e = 104), as shown in Figure 3b, was weak at the fresh surface and became considerably stronger after repetitive irradiation. The signals of the other small fragments ( m / e = 26, 39, 51, 78, 91) were also observed to rise markedly after irradiation with the several shots, although those were not detected from the fresh surfaces. The observed phenomena described above seem to correspond to of PMMA neat films irradiated with 193 or 248 nm. Namely, during the incubation process, the decomposition of the polymer to produce oligomers and volatile products may occur, changing the absorptivity of the polymer in the laserirradiated area. In this process, it is difficult to identify the molecules absorbing light energy and to clarify the kinetics of excited states from molecular point of view. Hence, we focus our attention on the results for the fresh surfaces in this report. We also tried to detect ionic species by turning off the ionizer and with high sensitivity. Ionic species was, however, hardly detected and negligible in this experimental condition. From the fresh surfaces, detected species were mainly anthracene (Figure 4) and styrene (Figure S), even though the laser intensity was high enough to etch the surfaces. In the case of high dopant concentration (96 mM), the detectable species at 300 mJ/cm2was only anthracene ( m / e = 178). Further increase in laser intensity enhanced the signal of anthracene, and, in addition, styrene monomer ( m / e = 104) was detected around
50 '0
500
1000
1500
2000
Time (ps) Figure 4. Time-of-flightsignals (thin lines) of anthracene ejected from polystyrene films doped with 96 mM anthracene (a), with 97 mM anthracene-dlo(b), and 48 mM anthracene (c). The laser intensity was 610 (solid line) and 300 mJ/cm2 (broken line) for a; 610 mJ/cm2 for b and c. The thick smooth curves are best fit by the calculation: see text.
Time (pus) Figure 5. Time-of-flight signals (thin lines) of styrene ejected from polystyrene films doped with 96 mM anthracene (a) and 97 mM anthracene-dlo (b). The laser intensity was 610 mJ/cm2 for both. The thick smooth curves are best fit by the calculation: see text. 430 mJ/cm2. The signal of styrene also increased with laser intensity. For the sample with low anthracene concentration (48 mM), the dopant signal was very weak a t 300 mJ/cm2 but increased with laser intensity. For an anthracene-& doped film, the signal a t 300 mJ/cm2 was extremely weak, although its concentration was high (97 mM). In the latter two samples, the signal of styrene monomer was weak even at 610 mJ/cm2. The dependence of the ejected species on laser intensity was in good correspondence with the results of the etch depth measurement. It should be noted that only the signal of m/e = 188was observed from anthracene-& doped films. This indicates that the exchanging reaction of deuterium by hydrogen did not take place under the experimental condition. Data Analysis. The time-of-flight spectra were first analyzed with a simple Maxwell-Boltzmann distribution function as described above. A deviation of the anthracene signal from the eq 3 for the high-concentration sample was smaller at 610 mJ/ cm2 than that at 300 mJ/cm2. This is much clearer as depicted when the signals were transformed to the energy distribution, as shown in Figure 6. The curves by a Maxwell-Boltzmann distribution describe the experimental results very well for highenergy regions. Then the subtraction of the calculated curves
13764 The Journal of Physical Chemistry, Vol. 97, No. 51, 1993
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1
1 -zooY
k‘
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-400
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E n e r g y (eV)
0.1
0.2
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E n e r g y (eV)
Figure 6. Translational energy distribution of anthracene molecules (closed circles): data taken from Figure 4a. The laser intensity was 300 mJ/cm2for a and 610 mJ/cm2for b. The solid lines represent MaxwellBoltzmann distributions, and closed triangles are the difference between the experimentalresultsand thecalculated distributions. The long-dashed lines represent negative Maxwell-Boltzmann distributions.
TABLE 1: T ~ P I I S ~ ~Energies ~ O M ~of Ejected Molecules under Various Conditions
translational energies,‘ eV
sample (concn, mM)
laser intensity, mJ/cm2
anthracene (96)
610
anthracene (96)
300
anthracene (48)
610
anthracene-dlo (97)
610
detected species anthracene styrene anthracene styrene anthracene styrene anthracene410 styrene
fast slow positive negative components components
0.13 (995) 0.081 (626) 0.072 (549) b 0.076(591) b 0.096 (738) 0.070(541)
0.032(249) 0.030 (232) 0.033 (251) b 0.038(293) b 0.042(324) 0.025 (193)
Temperatures (K) in parentheses are calculated from E = 3kT/2. Not detected. from the results gives negative distributions, and, surprisingly, the distributions can be fit by a negative Maxwell-Boltzmann distribution. Assuming thesignals were thesumof a main positive distribution and a small negative distribution, we can describe the experimental results quite well, as shown in Figure 4 (thick smooth lines). Alternatively, one may analyze the results by using an offset velocity or a nbzzle expansion for the ejected species as described in a few papers.l4JSbJ7The effect of angular distribution for ejected speciesmay be involved in this phenomena. However, if the nozzle expansion or the angular-anisotropic ejection is responsible for the deviation from a simple MaxwellBoltzmann distribution, the deviation should be increased with laser intensity. This is in contrast to the experimental results. The signals for styrene monomers were also fit similarly with positive and negative components. As shown in Figure 5 (thick smooth line), a small deviation from perfect fitting was obtained, suggestingthat theejection processof styrenemonomerisdifferent from that of anthracene. The obtained fitting parameters are summarized in Table 1. As the concentration of anthracene is decreased from 96 to 48 mM, the translational energy of anthracene was decreased. The result for the high-concentration system at low excitation intensity is similar to that for the lowconcentration system at high intensity. It is of great interest that the translational energies of sensitizer and of styrene monomer were reduced by the deuteration of the sensitizer, whereas the light absorptivity is considered to be the same. This result is also in accord with the result of the etch depth measurement. Discussion
The above results on the ejected species suggest that thermal effect is dominant in the present sensitized ablation process. Ionic species and other photodecomposed fragments were not ejected
Fukumura et al. from the surface, while anthracene and styrene monomer were observed without further decomposition. The translational energy distributions of the ejected species are well explained with the thermal mechanism. If electrostatic repulsion causes the ejection of molecules from a surface, the translational energy distribution of the molecules are expected to be constant regardless of the laser intensity.21 In contrast, the translational energy was found to increase with laser intensity. The energy also increased with sensitizer concentration in polymer films. These results indicate that in the ejection process the total energy absorbed near the surface of a polymer film plays an important role rather than the energy of an excited electronic state or the absorbed photon energy. It is, therefore, suggested that the photon energy adsorbed by anthracene converts rapidly to thermal energy of polymer matrix through inter- and intramolecular vibrational relaxation in the time scale of laser irradiation. The sensitizer may behave as a “molecular heater” surrounded by polymer chains, leading to thermal decomposition of the chains and/or taking off from the surface by its own high kinetic energy. A similar interpretation that the electronic energy dissipates rapidly to create local heating has been given for the laser induced photodesorption of CH212 adsorbed on a Ag surface at high surface coverage and with high laser intensitySz8 Suppose anthracene molecules only work to heat up the surrounding and then thermal equilibrium is completely achieved in the irradiated area of the polymer film, it would be difficult to explain why the negative Maxwell-Boltzmann distribution is involved and also why the translational energy of styrene monomers is lower than that of anthracene. Here, we assume that molecular ejection with a high kinetic energy competes with thermalization of the system. The melting point of polystyrene is estimated to be 5 13-523 K,29and styrene monomer was detected by thermal decomposition above 533 K.30At 673-773 K, 46.556.4% of the polystyrene thermally decomposed to styrene monomer, while 36.546.9% of the polymer remained asa waxlike fraction under vacuum.31 This means that the main volatile product of the thermal decomposition is styrene monomer in this temperature region. Since thermal decomposition rate generally increases exponentiallywith temperature, the decomposition yield would be very low at temperatures around 550 K, which were estimated for the 96 mM samples at 300 mJ/cm2 and the 48 mM samplesat 6 10mJ/cm2. Transient melting of thesurface probably occurred, but the thermal decomposition scarcely did. Under these conditions, anthracene can rapidly evaporate from the melt liquid surface with energy of a simple thermal distribution. However, it takes a certain lag time to yield styrene monomer from the melting polymer via thermal decomposition and the thermal energy would be dissipated in the meantime. In addition, bond cleavage leading to the production of the monomer requires energy, which lowers the temperature of the system. This results in less efficient ejection of the styrene monomer from the polymer surface. In the case of the high-concentration samples at 610 mJ/cm2, the input energy is high enough to generate styrene monomer but its translational energy is lower than that of anthracene because of the decomposition time lag and the energy requirement described above. Thus, the temperature estimated for styrene monomer is lower than that for anthracene as shown in Table I. The negative Maxwell-Boltzmann distribution means that molecules which have low translational temperature around 250 K is statistically lost by repetitive collisions and cannot reach the detector. The average translational energy of the lost components falls in the range of 0.025-0.042 eV in this experiment (Table I) and corresponds to the magnitude of van der Waals attractive interaction. An adiabatic expansion model might be able to explain the phenomena, since the molecules with high kinetic energy may kick out partially decomposed styrene oligomers or
Laser Ablation of Polystyrene with Anthracene
The Journal of Physical Chemistry, Vol. 97, No. 51, 1993 13765
large molecular clusters from the surface, and those ejected species with low internal energy may trap low translational energy components by collision. This possibility depends on the density of the gas which is composed of ejected materials just above the surface. Thus, as the laser intensity increases further, the probability of adiabatic expansion would be higher. This is, however, inconsistent with the obtained results and therefore the explanation does not hold for our experiment. Another explanationis to assume that small bubble-like cavities are produced by an incomplete and inhomogeneous melting of the original surface and the molecules collide repetitively with walls of the cavities. Actually, many holes were observed at the swollen surface for porphyrin-sensitized PMMA ablation, suggesting the generation of gas bubbles beneath the surface when the laser intensity was lower than the ablation threshold but strong enough to change the surface m~rphology.~ As the laser intensity was increased, the irradiated surface was etched deeply and the surface roughness due to bubble formation was relatively reduced. Then, the relative contribution of the negative distribution would be reduced. Assuming the ablation mechanism is completely thermal, we consider an energy balance for light absorption and temperature rise in a polystyrene matrix containing anthracene. When the concentration of anthracene is 96 mM, a polystyrene cube of 17 nm3 contains one anthracene molecule on average. At 610 mJ/ cm2, the temperature rise for anthracene is 697 K and that for styrene is 332 K, as estimated by the analysis of time-of-flight signals. We can estimate the number of absorbed photons assuming that heat capacity is constant to be 0.14 eV/K.29 If we use the temperature rise for styrene as the cube temperature, 14 photons are necessary to heat up the cube. This value is the minimum because further energy is necessary as phase transition and depolymerization are involved during temperature elevation. Heat of polymerization and heat of fusion for polystyrene are reported to be-0.69 and +0.087 eV/monomer unit, respecti~ely.~~ Since the cube involves 103 styrene units, 30 photons should be absorbed to prepare a melt polymer cube at 995 K, which is the translational temperature of anthracene obtained in the experiment with 610 mJ/cm2. Then, anthracene can leave the cube with this translational energy, and subsequentlydepolymerization may take place consuming 71 eV, lowering the temperature of the cube to 487 K. The estimated temperature is lower than the observed value, since we have assumed that the yield of thermal depolymerization is unity. The rate constant of depolymerization, however, is a function of temperature and may be reduced while generated styrene monomers detach from the cube. When 27% of polystyrene remains unreacted, the temperature of the cube is consistent with the observed value. The above consideration leads us to pose the followingquestion. How does one anthracene molecule absorb 30 photons and transfer theenergy to the surrounding without thedecomposition of itself? One possible process is “cyclicmultiphotonicabsorption”,as shown in Figure 7. As the lifetime of the lowest excited singlet state (SI) is reported to be 5.5 ns?“ the ground state cannot absorb 30photons in the duration of the laser pulse. However,the lifetime of higher excited states is generally considered to be in picosecond region due to fast internal conversion. This leads to the redistribution of vibrational energy in the laser-absorbing molecule, followed by the intermolecularenergy transfer of vibrational excess energy to the surrounding molecules within several tens of picoseconds.32 If the laser-absorbing molecule relaxes to the state which can absorb photons again, the molecule would be excited repeatedly during the laser pulse. As a result, the molecule would behave as a “molecular converter” of absorbed photon energy into the surrounding heat energy. To specify the excited state which contributes the cyclic multiphotonic absorption, further studies monitoring the dynamics of excited states are necessary and will be published elsewhere.33
hv (cyclic muliiphoionlc exciiailon)
/
hv
\
A-A*
A**
intermolecular vlbratlonai energy
Polymer
- a partially melted polymer
Monomer
Sensitizer Eject ion
Monomer Eject ion
n
u
m e 7. Mechanism for the sensitized laser ablation. Note that a sensitizer is excited and relaxes repeatedly in excited states.
Deuteration of the sensitizer resulted in a lowering of the translational energy of the ejected species. Sensitizer molecules would behave as hot spots in polymer, but the hotness of the spots might be less in the deuterated compound than in anthracene. The term “internal t e m p e r a t ~ r e ”or~ ~“q~asitemperature”3~ is used to describe the measure of such hotness, which represents the energy distribution of transiently hot molecules. Assuming one photon (3.5 eV) is absorbed by anthracene and converted to the vibrational energy completely, we can estimate the internal temperature (T) by using the following equation: 66
Here the vibrational wave numbers (vi) were taken from the l i t e r a t ~ r e .The ~ ~ estimated internal temperature for anthracene and the deuterated compound is 1282 and 1212 K, respectively. This difference is too small to explain the experimental results. Another possible explanationis the differencein intramolecular nonradiative decay and intermolecular energy transfer. If we assume that the cyclic multiphotonic absorption takes place in triplet manifold, the rate controlling process would be the decay of the second excited triplet state (T2) because of a relatively large energy gap between T2 and the lowest triplet state (TI) in the triplet m a n i f ~ l d . ~ ~The ” . ~lifetimes of TZwere estimated to be 8.8 and 10.4 ps for anthracene and the deuterated compound, respectively.*“ The both values are short enough for repetitive excitation during the laser pulse of 15 ns, and this difference cannot explain the low efficiency of the deuterated compound. Recently, it has been proposed that the most of the intermolecular energy transfer in a polymer matrix occurs through the lower frequency molecular vibrations, termed “doorway modes”.36 However, the low-frequency modes around 200 cm-l, which are considered to be dominating doorway modes,34 are scarcely affected by the d e u t e r a t i ~ n .Thus, ~ ~ it seems unlikely that the rate of intra- or intermolecular energy migration is responsible for the inefficient sensitization by the deuterated compound. The other possibility is simply the difference in the absorption coefficients of the excited states which are involved in the cyclic multiphotonic absorption. The number of absorbed photons by the deuterated compound is estimated to be 20 with the same way described above. Therefore, it requires that the absorption coefficients of light-absorbingstates of the deuterated compounds is 60-704 of those of anthracene. Since the transient spectra during laser ablation has not been measured yet, it is difficult to ascribe the deuteration effect to one of the reasons described above by the present investigation.
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The Journal of Physical Chemistry, Vol. 97, No. 51, 1993
Conclusions Polystyrene films doped with anthracene or anthracene-& were ablated by 351-nm laser irradiation. Velocity distributions of ejected species have been studied by time-of-flight mass spectrometry. Below the ablation threshold, nonreacted anthracene molecules were only detected and their velocity distribution was analyzed by assuming a composite Maxwell-Boltzmann distribution with high-temperature MB (550-600 K) and low-temperature MB (25&300 K) components. When the etching of the surface took place at several hundred mJ/cm2,the detected species were mainly anthracene and styrene monomer. The velocity distribution changed depending on the concentration of anthracene and laser intensity. Deuteration of anthracene reduced the translational energy of both anthracene and styrene monomer. The obtained results suggest that photon energy absorbed by anthracene is converted into thermal energy via rapid internal conversion in polystyrene matrix, resulting in thermal decomposition of the matrix. Acknowledgment. The authors wish to express their sincere thanks to Professor I. Yamada and Dr. H. Usui of Kyoto University for the use of the depth profiler. This work was supported by the Joint Studies Program (1989-1990) of the Institute for Molecular Science. References and Notes (1) Kawamura, Y.; Toyoda, K.; Namba, S. Rev. Laser Eng. (Laser Kenkyu) 1980,8, 941. (2) Srinivasan, R.; Braren, B.; Dreyfus, R. W.; Hadel, L.; Seeger, D. E. J . Opt. SOC.Am. B 1986, 3, 785. (3) Srinivasan, R.; Braren, B. Appl. Phys. A 1988,45, 289. (4) Masuhara, H.; Hiraoka, H.; Domen, K. Macromolecules 1987, 20, 450. ( 5 ) Chuang, T. J.; Hiraoka, H.; Mbdl, A. Appl. Phys. A 1988,45,277. (6) Hiraoka, H.; Chuang, T. J.; Masuhara, H. J . Vac. Sci. Technol. B 1988. - - - - , 6. -,463. -(7) Fukumura, H.; Mibuka, N.; Eura, S.;Masuhara, H. Appl. Phys. A 1991, 53, 255. ( 8 ) Arnold, B. R.; Scaiano, J. C. Macromolecules 1992, 25, 1582.
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