J . Phys. Chem. 1991, 95, 10004-10009
10004
OH groups of Si02, followed by chemical treatments with aqueous ammonia and water vapors and by treatments at given temperatures. 2. On Si02surface niobic acids grew in a two-dimensional mode, forming islands in the range of N b loadings from 0 to 8.0 wt %. 3. The local structure around the N b atom in the one atomic layer showed direct bonds of Nb-Si, Nb-Nb, and Nb-Nb at 0.328, 0.335, and 0.392 nm, respectively. 4. The catalytic activity of the A-Nb205/Si02catalyst for the
esterification from ethanol and acetic acid was 20 times higher than that of a niobic acid bulk catalyst. The activity per surface Nb of the A-Nb205/Si02catalyst at 573 K was also 6 times higher than that of the bulk catalyst. 5 . The esterification proceeded on the Lewis acid sites which were stable even at 873 K, suggesting the application of this surface layer catalyst in wide reaction conditions. Registry No. Nb(OC,H,),, 3236-82-6; SO2, 7631-86-9; Nb205, 13 13-96-8; C2HSOH,64-17-5; CH,COOH, 64- 19-7; pyridine, 1 10-86- I .
Ultraviolet-Laser-Induced Ablation of Poly(ethylene terephthalate) Peter E. Dyer,+ Geoffrey A. Oldemhaw,*?*and Jagjit Sidbut*o Department of Applied Physics and School of Chemistry, University of Hull, Hull, HU6 7RX England (Received: April I O , 1991; In Final Form: July 17, 1991)
The ablation of poly(ethy1ene terephthalate) (PET) by 308-nm XeCl laser irradiation and by 193-nm ArF laser irradiation has been studied. In this process etching of the polymer occurs for laser fluences above a threshold value FT.Stress waves exhibited by thin films of PET during XeCl laser irradiation at fluences below FThave a form characteristic of thermoelastic stress and demonstrate that relaxation of the absorbed energy to heat is rapid on the time scale of the laser pulse. In films irradiated with the XeCl laser at fluences above FT, the initial part of the thermoelastic stress wave is succeeded by a compressive stress due to ablation, which starts during the laser pulse. The major gaseous products of both XeCl laser ablation and ArF laser ablation are CO, COz,CH,, C2H2,CzH4, C4H2, C4H4,C6H6, and CH,CHO, which are also found in the cozlaser ablation of PET. Yields of gaseous products of the XeCl laser ablation are low for fluences below 2FT, but in the case of ArF laser irradiation greater fragmentation of the ablated PET occurs and yields of gaseous products in the fluence range FTto 2FT are appreciable. The mechanism of ablation is discussed and it is concluded that XeCl laser ablation of PET occurs by rapid relaxation of the initial electronic excitation to heat, resulting in thermolysis of the polymer. Thermolysis is probably also an important factor in ArF laser ablation, although a contribution from direct photolysis cannot be excluded. The greater fragmentation of ablation products observed in ArF laser ablation is attributed to secondary photolysis in the ablation plume.
The ablative photodecomposition of an organic polymer by pulsed ultraviolet radiation from an excimer laser was first demonstrated in 1982.'*2 In this process material is forcibly ejected from the polymer surface and etching occurs at the site of irradiation when the laser fluence exceeds a threshold value. The threshold fluence, FT, is characteristic of the polymer and the wavelength of irradiation, and the depth of the etched hole 1 is approximately a linear function of the logarithm of the laser fluence F for a limited range above FT3 1 = k;I In (F/FT) (1) where k, is the effective absorption coefficient for the laser radiation. Etching at fluences below threshold, while sometimes detectable, is very small. Photoablation produces etching directly without the need for chemical development and has potentially important applicationsc6 in microlithography and micromachining as well as in surgery. Since the original observation there has been increasing interest in, and work on, this phenomenon, involving studies of several different polymer^.^-^ The time scale of the ablative process has been established by using poly(viny1idene fluoride) (PVDF) film piezoelectric transducers to study the acoustic response of UVlaser-irradiated polymers.'** These experiments show that excimer-laser-induced ablation occurs rapidly, starting during the lifetime (- 20 ns) of a typical laser pulse. One of the interesting mechanistic questions about excimer laser ablation is whether decomposition occurs by direct photolysis or by thermolysis of the polymer following rapid degradation of the
'Department of Applied Physics.
*School of Chemistry. I R a e n t address: Sowerby Research Centre, British Aerospace plc, Filton, Bristol, U.K.
initial electronic excitation to heat.24*9-13 In the case of poly(ethylene terephthalate) (PET) irradiated by the XeCI, KrF, and ArF lasers, it has been shown,14by measurements of the thermal loading of thin films, that at fluences below threshold essentially all the absorbed energy is converted to heat. This implies a substantial temperature rise near the polymer surface owing to the small absorption depth. Above threshold the thermal loading remains approximately constant as the excess energy is carried off by the ablated material. It has also been foundI5 that the threshold fluence for ablation of PET by the XeCl laser is substantially reduced by preheating the polymer with COz laser radiation. (1) Srinivasan, R.; Mayna-Banton, V. Appl. Phys. Lerr. 1982, 41, 576. (2) Srinivasan, R.; Leigh, W. J. J . Am. Chem. Soc. 1982, 104, 6784. (3) Andrew, J. E.; Dyer, P. E.; Forster, D.; Key, P. H. Appl. Phys. Lerr. 1983, 43, 717. (4) Srinivasan, R. Science 1986. 23, 559. (5) Ych. J. T. C . J . Vac. Sei. Technol. 1986, A4, 653. ( 6 ) Dyer, P. E.; Sidhu, J. Opr. Loser Eng. 1985, 6, 67. (7) Dyer, P. E.; Srinivasan, R. Appl. Phys. Leu. 1986, 48, 445. (8) Dyer, P. E. In Phoroacousric and Phororhermal Phenomena; Hess, P., Petzl, J., Eds.; Springer Series in Optical Sciences; Springer-Verlag: Heidelberg, 1988; Vol 58, p 164. (9) Srinivasan, R.; Braren, B.; Dreyfus, R. W.; Hadel, L.; Seeger, D. E. J. Opr. Soc. Am. 1986, 83, 785. (10) Brannon, J. H.; Lankard, J. R.;Baise, A. 1.; Burns, F.; Kaufman, J. J . Appl. Phys. 1985, 58, 2036. (11) Dijkkamp, D.; Gozdz, A. S.;Venkatesan, T.; Wu, X . D. Phys. Reo. k r r . 1987, 58, 2142. (12) Srinivasan, R. Phys. Reu. Letr. 1988, 60, 381. (13) Venkatesan, T.; Gozdz, A. S.;Wu X.D.; Djikkamp, D. Phys. Reu. Lerr. 1988, 60, 382. (14) Dyer, P. E.; Sidhu, J. J. Appl. Phys. 1985, 57, 1420. ( I 5) AI-Dhahir, R.K.; Dyer, P. E.: Sidhu, J.; Foulkes-Williams, C.; 01dershaw, G. A. Appl. Phys. 1989, B49, 435.
0022-3654/91/2095-10004%02.50/0 0 1991 American Chemical Society
Ablation of Poly(ethy1ene terephthalate)
The Journal of Physical Chemistry, Vol. 95, No. 24, 1991 10005 100.
TABLE I: Relative Yields# of Volatile Ablation Products
laser F/Fr* co co2 XeCl (308 nm) 2.6 2.3 1 2.7 2.8 1 ArF (193 nm) C 0 2 (9.2 pm) 3.2 1.6 1 OMoles of product/moles of C02. CI), 0.028 (ArF), 0.77 (CO,).
CH4 C2H2 C2H4 C6H6 0.05 0.21 0.27 0.09 0.06 0.37 0.35 0.09 0.05 0.08 0.26 0.12
FT/(Jcm-*) = 0.17 (Xe-
Ablative etching of polymers can be carried out with pulsed infrared as well as ultraviolet lasers, as has been demonstrated in the cases of polyimideI6 and PET.17 Time-resolved photoacoustic measurements show that ablation of C02-laser-irradiated PET occurs relatively quickly, beginning within the initial spike (-220 ns) of a TEA laser pulse.17 We report here more work on PET, including photoacoustic studies of the XeCl laser ablation and measurements of the volatile products of the XeCl laser and ArF laser ablation. The results provide further evidence of the importance of thermal factors in the mechanism of excimer laser ablation.
Experimental Section Studies of the etching caused by excimer laser ablation were carried out on films of PET (Melinex S,IC1 pic) of thickness 55 pm. The experiments employed a laboratory-built XeCl laser delivering up to 70 mJ at 308 nm in an -8-ns (fwhm) pulse and an ArF laser (Lambda Physik EMGSOE), which produced up to 30 mJ at 193 nm in an 1 I-ns (fwhm) pulse. A small aperture was used to select a region of uniform fluence from the output beam, and an image of the aperture was projected onto the polymer film using a lens of focal length 250 mm. For measurements of the depth of etching,I4 films were irradiated in air a t a repetition rate of less than 3 Hz. Experiments on the identification and estimation of gaseous ablation products were carried out by enclosing films in evacuated cells of known volume between 64 and 76 cm3. The film was mounted at one end of the cell and radiation from the XeCl or ArF laser focused onto it through a Spectrosil window at the other end. The cell was fitted with a side arm which was cooled in liquid nitrogen to freeze down gaseous products during irradiation. Products from 200-5000 laser pulses were collected, allowed to vaporize by removing the liquid nitrogen, and analyzed by gas chromatography. Two columns were used, a 2 m X 4 mm i.d. carbon seive S column operated at 130 OC and a 2 m X 6 mm i.d. column of Porapak QS operated with a temperature program between 140 and 200 "C. A few analyses were carried out using GC/MS to aid product identification. The pressure of each volatile product was used to calculate the number of molecules generated per pulse, and quantum yields of products were obtained by division by the number of photons per pulse. Time-resolved photoacoustic measurements of stress waves were made with a transducer consisting of a 9-pm-thick poly(viny1idene fluoride) piezoelectric film with 0.04-pm AI surface electrodes bonded to a 4-mm-thick lucite acoustic impedance matching stubS7s8 A 2.5-pm-thick film of PET was mounted on the transducer surface allowing stress waves generated by laser irradiation to be detected. The XeCl laser used for irradiation generated a pulse of width approximately 20 ns (fwhm), and the overall response time of the transducerdisplay system was about 3.6 ns. Results Etch Depths. Results of etch depth measurements for excimer laser irradiation of PET have been reported previously;14however, in Table I of that paper the value of k, for ArF laser ablation is listed incorrectly as 1.2 X IO5 cm-I. Etch depths for ArF laser irradiation in air are shown in Figure 1, and are described by eq 1 with FT = 0.028 J cm-2 and k, = 2.0 X lo3 cm-l. The etch depths are in reasonably good agreement with other values de(16) (17)
Brannon, J. H.; Lankard, J. K. Appl. Phys. Let?. 1986, 18, 1226. Dyer, P. E.;Oldershaw,G. A.; Sidhu, J. Appl. Phys. 1989. B18,435.
E
50.
000
20
10
I
1
I
50
100
200
FlmJ cm-?
Figure 1. Etch depth per pulse I as a function of laser flucnce F for ArF laser irradiation of PET.
Loser p u l s e
0
I
10
20
T h er m o el o sti c Stress
Figure 2. Calculated thermoelastic stress response for PET. The XeCl
laser irradiance is shown (top) together with the resulting thermoelastic stress wave (bottom) calculated from the equations given in ref 8 with a = 5 ns-I. The relative stress u is plotted against 1' = t - u / l , where u is the acoustic velocity in the film and I the film thickness. termined using a microbalance,18 for which a threshold of 0.017 J cm-2 has been given. This is consistent with the fact that, as can be seen in Figure 1, very small etch depths are observable below the effective threshold of 0.028 J cm-2. The values of FT and k, for XeCl laser ablation are 0.17 J cm-2 and 2.2 X 104 cm-I, respectively. Time-Resolved Acoustic Measurements. Time-resolved measurements of stress waves generated in thin films of PET by XeCl laser irradiation at fluences both below and above the ablation threshold were made. The stress waves arise either from the thermoelastic effect, resulting from the inability of the material to readjust instantaneously by thermal expansion to the new equilibrium position, or from ablation of material from the polymer surface. Consider the case in which no ablation occurs. Thermoelastic stress can be modeled* by assuming that absorption of U V radiation produces electronic excitation which is instantaneously transformed to heat. An example of the thermoelastic stress wave calculated using the equations given in ref 8 for a time-dependent irradiance chosen to model the XeCl laser pulse is shown in Figure 2. The characteristic rate for stress relaxation used in this calculation, a = ko = 5 ns-I, was obtained using an (18)
Lazare, S.; Granier, V. Luser Chem. 1989, IO, 25.
The Journal of Physical Chemistry, Vol. 95, No. 24, 1991
Dyer et al.
U 20 11s
0.2
0.5
1
F/J cm-z
Figure 4. PET removal and CO and C02 production as a function of fluence in the XeCl laser ablation of PET. PET removal is calculated from the etch curve.
U
50 ns
Figure 3. (A) Transducer response in XeCl laser irradiation of PET. Fluence = 0.06 J cm-2. (e) Transducer response in XeCl laser irradiation of PET. Fluence = 0.24 J cm-*. acoustic velocity u of 2.25 X IO5 cm s-l and absorption coefficient k of 2.2 X IO4 cm-l for PET. This modeling predicts a bipolar stress wave which is initially positive (compressive), rises to a maximum, and then falls to become negative and exhibit a shallower minimum as shown in Figure 2. After allowing for the acoustic transit time through the film (l/u), the time of the maximum corresponds approximately to the start of the laser pulse and the stress wave changes sign at a time corresponding closely to the maximum of the laser pulse. Although this treatment oversimplifies the situation, as it does not account for the precise shape of the laser pulse used experimentally and there is uncertainty over the exact absorption coefficient that applies just below the ablation threshold, it serves to illustrate the characteristic stress response expected. The transducer signal recorded in the XeCl laser irradiation of a thin film of PET at a fluence of 0.06 J cm-2, well below the ablation threshold, is shown in Figure 3A. This bipolar signal is of the form predicted by eq 2 (Figure 2) and is attributed to thermoelastic stress. The interval between the maximum of the stress wave and the point at which it changes sign is about 6 ns, approximately the same as the time for the XeCl laser pulse to reach its maximum irradiance. Thus the time scale of the stress wave is that expected from the model, which assumes rapid relaxation of electronic energy to heat. Furthermore, when appropriate synchronization of the laser and acoustic pulses with allowance for the acoustic transit time is carried out, it is found that the maximum of the laser pulse coincides within experimental uncertainty (-2 ns) with the time at which the stress wave changes sign. The stress wave measurements therefore show that relaxation of the initial electronic excitation to heat occurs rapidly, in a time not resolvable by this nanosecond technique. Irradiation at higher fluences causes ablation with the rapid ejection of material from the polymer surface. As a result of recoil momentum, a positive (compressive) stress wave is generated in the polymer. Figure 3B displays the signal generated by XeCl laser irradiation of PET at a fluence (0.24 J cm-2) above the ablation threshold (0.17 J cm-2). This shows the initial part of a thermoelastic stress wave which is followed by a strong positive signal as ablated material leaves the polymer surface. The later part of the thermoelastic stress
wave is obscured by the compression due to ablation. Thus relaxation of the absorbed energy to heat is seen to precede ablation. The ablation starts during the laser pulse and the compressive stress lasts for about 100 ns. Stress waves generated by ArF laser irradiation of PET have been described p r e v i o ~ s l y .In ~ ~this ~ case compressive transients are observed at fluences both above and well below the threshold for significant etching of 28 mJ cm-2. The observation of this weak "subthreshold" ablation is consistent with the detection of minor etching in this fluence region as shown in Figure 1. As with XeCl irradiation, ablation starts during the laser pulse but the compressive stresses tend to decay faster, persisting typically for about 40 ns. The absence of bipolar thermoelastic signals at fluences below threshold is attributed* to the very strong absorption of 193-nm radiation by PET, which leads to a reduction of the thermoelastic stress. Under these circumstances any thennoelastic response is probably outweighed by weak subthreshold ablation and a purely compressive signal is observed. Ablation Products. The volatile products observed both in the XeCl-laser-induced ablation and in the ArF-laser-induced ablation of PET were similar to those generated by C 0 2 laser ab1ation.l' Those detected in significant quantity were carbon monoxide, carbon dioxide, methane, ethyne, ethene, butadiyne (C4H2), but-1-en-3-yne (C4H4), benzene, and ethanal. Some of these products, together with others of higher molecular weight such as toluene, benzaldehyde, and ethylbenzene, have been observed previously in the ArF laser ablatiom2 Ions of masses corresponding tO C6H6+, CIOH~',C I O H ~ +C12Hlo' , . C~HSCO', and C6H5+, among others, have been detected by photoionization of the neutral species ejected during the 266-nm laser ablation of PET.19 In the present case the product composition was dependent on the fluence used for ablation and a comparison between the products generated by the different lasers is best carried out by examining results corresponding to similar values of the ratio of the fluence to the threshold FT. Relative yields of the more abundant products are shown in Table I, where results obtained in the C 0 2 laser ablation" are also given. Variation of the fluence produced qualitatively similar trends in the product composition for all three lasers; for example, the ratios CO/CO2 and C2H2/C2H4both increased with increasing fluence. The general similarity of the volatile products of ablation induced by the XeCI, ArF, and C 0 2 lasers is evident from the table. As has been pointed out in a discussion of the C02 laser ablation,17 the products are also qualitatively similar to those observed in the pyrolysis of PET.20 Variation of the fluence used for ablation strongly affected the yields of volatile products. Figure 4 shows the yields of CO and C 0 2 produced in the XeCl laser ablation of PET as a function (19) Hansen, S. G. J . Appl. Phys. 1989, 66, 141 I .
(20) Mnas, M.E.;Day,M.;Ho,R. K.; Sander, R.; Wiles, D. M.J . Appl. Polymer Sci. 1981, 26, 271.
The Journal of Physical Chemistry, Vol. 95, No. 24, 1991 10007
Ablation of Poly(ethylene terephthalate)
0
a 0
0
0
0 0
0
e
e
e 0
0
e 0
0
o
Oe
O 0
e
0.05
0.15
t
0 0
-
I
Figure 5. Quantum yield of CO production in the excimer laser ablation of PET as a function of the ratio of the fluence F to the threshold fluence FT: 0, XeCl laser (Ft= 0.17 J cm-2);0 , ArF laser (FT= 0.028 J cm-2).
0
0.5
00
0.
1
O ,'
5
2
Figure 7. Quantum yield of C2H2production in the excimer laser ablation of PET as a function of F/F,: 0,XeCL laser; 0, ArF laser.
,',r
0
0
0
0 . 0
0
o
m
o
0
0 0
F/F,
Figure 6. Quantum yield of C02production in the excimer laser ablation of PET as a function of F/FT: 0, XeCl laser; 0 , ArF laser.
of the fluence. Data for the removal of PET, expressed in terms of repeat units (-OCOC6H4COOC2H4-) and calculated from the etch curve determined in air,I4 are given for comparison. Appreciable production of CO and C 0 2 does not occur until the fluence is raised to more than twice the threshold value for PET removal. Thus for fluences around and considerably above the threshold the ablation products are predominantly involatile species, but at the highest fluences employed substantial fragmentation of the polymers to small molecules occurs. Figure 4 bears a striking resemblance to the corresponding plot for C02 laser ablation," where appreciable production of CO and COz is also observed only at relatively high fluence. Quantum yields of CO production in XeCl laser irradiation are shown as the unfilled points in Figure 5 , where the ordinate is the ratio of the fluence to the threshold fluence. At the lower fluences employed the quantum yields are very small. For a fluence of 0. I4 J cm-2, below but comparable to the threshold of 0.17 J cm-2, 4(CO) < 9 X I@, which is consistent with quantum yields of 6 X IO4 (A = 313 nm)2' and 6.1 X IO4 (A = 300-420 nm)22 measured in the conventional low-intensity photolysis of PET. (21) Marcotte, F. B.;Campbell, D.; Cteaveland, J . A.; Turner, D. T. J . Polym. Sei. Polym. Chem. Ed. 1967, 5, 481. (22) Day. M.; Wiles, P. M. J . Appl. Polym. Sei. 1972, 16, 203.
~
~___________
(23) Allan, R. J. P.; Forman, R. L.;Ritchie, P. D. J . Chem. Soc. 1955, 2717.
Dyer et al.
10008 The Journal of Physical Chemistry, Vol. 95, No. 24, 1991
comparable to those shown in Figure 8, with 4(C6H6) 0.7FT. rising to 0.02 at 1 . 9 F ~and above.
-
0 at
Discussion XeCl Laser Ablation. The similarity of the nature and distribution of the volatile products of the XeCl laser ablation to those of the C 0 2 laser ablation (Table I) and to those observed in the pyrolysis of PETmis evidence that extensive thermal decomposition of PET occurs in the XeCl laser ablation at fluences well-above threshold. This is not surprising in view of the results of thermal loading experiments,14which indicate that substantial temperature rises occur in the surface layers of samples of PET irradiated at or above the threshold fluence. However, this evidence does not in itself answer the question of whether the initial bond breaking which causes the ablation of material at fluences close to the threshold is a result of direct photolysis or of thermal decomposition following rapid relaxation of the initial electronic excitation to ground-state vibrational energy. In the low-intensity photolysis of PET at room temperature and wavelengths around 300 nm,21-22 quantum yields of photochemical processes, including production of CO, C02, -COOH, free radicals, and cross-linking, are low, amounting altogether to 3 X and demonstrating the rapidity of competing relaxation processes. If such low quantum yields persisted at fluences near the threshold, direct photolysis would not lead to the substantial bond fission required for ablation. Moreover, the results of the photoacoustic measurements demonstrate that relaxation of the absorbed energy to heat is rapid on the time scale of the laser pulse, and this leads to a substantial rise in temperature in the surface layer of the polymer. The initial step in the thermolysis of PETm*24is believed to be the production of carboxylic acid and vinyl ester groups via a six-membered transition state, process 2, and the Arrhenius
parameters for this step are a p p r o ~ i m a t e l y ’A ~ -= ~ ~1.4 X 10l2 s-I and EA = 196 kJ mol-’. The question then arises as to whether thermal decomposition, following rapid relaxation of absorbed energy to heat, can occur sufficiently rapidly to account for ablation which starts during the XeCl laser pulse. Work on the C02 laser ablation of PET” demonstrates that thermolysis can lead to relatively rapid ablation, but for decomposition during the shorter XeCI laser pulse, a higher rate and therefore a greater temperature rise would be required. Such a requirement is in keeping with the values of the surface energy density a t threshold, k,FT, which are approximately 2.4 kJ cm-3 for C 0 2 laser ablation and 3.7 kJ mol-’ for XeCl laser ablation. A more detailed calculation, allowing for conduction of heat from the polymer surface,25indicates that the threshold of 0.17 J cm-2 for the XeCl laser is consistent with ablation by thermolysis with a reaction rate based on the Arrhenius parameters given above. Thus, while some direct photolysis of PET is expected to occur during XeCI laser irradiation, it is probable that ablation is a result of rapid relaxation of electronic energy to heat followed by thermolysis. Strong support for this mechanism is provided by the observation that the threshold fluence for ablation is substantially reduced by preheating the polymer with pulsed infrared radiation.I5 A graphical illustration is available in Figure 3B, where the thermoelastic stress due to relaxation of the absorbed energy to heat precedes the compressive stress due to ablation. The possibility that the quantum yield for direct photolysis might increase at the high temperatures caused by laser irradiation should be considered. If such an increase in quantum yield occurred it would have little effect on the temperature rise in the polymer, and rapid thermolysis leading to ablation would accompany photolysis. Since the postulate of increased quantum yield is not necessary to explain the phenomenon of ablation, it is not adopted here. (24) Buxbaum, L. H. Angew. Chem., Int. Ed. Engl. 1968, 7 , 182. (25) Oldershaw, G. A . Chem. Phys. Left., submitted for publication.
ArF Laser Ablation. The volatile products of the ArF laser ablation shown in Table I are similar to those of ablation by the XeCl and C 0 2 lasers and to those of the pyrolysis of PET. This suggests that substantial thermal decomposition accompanies the ArF laser ablation at fluences well-above threshold. A major difference in the experimental results for ArF and XeCl laser ablation lies in the quantum yields of volatile products shown in Figures 5-8. While these are similar for high fluences, near threshold and in the range FT to ~ F they T are much larger for ArF laser irradiation than for XeCl laser irradiation. This implies a greater fragmentation of the products in the former case. The validity of this observation depends of course on the correct choice of thresholds as the basis of comparison. The values used, derived from Figure 1 and a similar etch curve for XeCl laser a b l a t i ~ n , are ’ ~ 0.028 J cm-* for ArF and 0.17 J cm-2 for XeCl laser ablation. As discussed earlier, some etching occurs at fluences below this effective threshold for ArF laser ablation. This is evident in Figure 1 and is supported by the photoacoustic experiments8 Lazare and Granier18 have given a threshold of 0.017 J cm-2, which if adopted would reduce substantially the differences between the two lasers shown in Figures 5-8. However, the etch curve for XeCl laser ablation shows “tailing” similar to that in Figure 1 which could also be used for downward adjustment of the threshold. The values used here represent in each case the threshold for significant etching as derived by linear extrapolation of the higher fluence points to zero I according to eq I . They should therefore form a valid basis for the comparisons shown in Figures 5-8. The greater fragmentation of the ablation products at fluences near threshold observed for ArF laser irradiation may possibly point to a somewhat greater contribution of photolysis to the ablation mechanism than is the case for XeCl laser ablation. This might occur by direct photolysis, by singletsinglet annihilation,26 or by absorption, of a second photon by the initially populated state.27 The resultant high level of excitation could lead to substantial fragmentation. Thermal decomposition is, however, expected to be significant. Although there is no direct evidence from the photoacoustic measurements of rapid relaxation of absorbed energy to heat, the results of the thermal loading experi m e n t ~ imply ’ ~ a large temperature rise in the surface layer of PET irradiated at 193 nm at fluences close to the threshold. Owing to the very small absorption depth for 193-nm radiation, the heated surface layer of PET cools more quickly by conduction than in the case of 308-nm irradiation. To achieve appreciable thermal decomposition on the time scale of the laser pulse, a higher absorbed energy density a t threshold, k& is expected for ArF laser irradiation and this is observed with kJT = 5.6 kJ mol-’ compared with 3.7 kJ mol-’ for XeCl laser ablation. This qualitative observation is supported by calculations of thermolysis rates allowing for heat transfer by conducti0n.2~The calculations show that the observed threshold for ArF laser ablation is similar to that expected on the basis of thermal decomposition with the same criterion for ablation as that employed in the case of XeCl laser irradiation. Thermal decomposition is also consistent with the observation that the compressive stress due to ablation persists after the laser pulse for a shorter time for the ArF laser than for the XeCl laser, in keeping with more rapid surface cooling by conduction in the former case. It is unlikely that the greater extent of fragmentation of products in the ArF laser ablation is due to the occurrence of substantially higher temperatures during ArF laser irradiation. calculation^^^ indicate that differences between the temperatures attained in ArF laser and XeCl laser ablation at threshold are quite small. A more probable explanation is that relatively large gaseous molecules generated in the ablation undergo secondary photolysis in the ablation “plume” which leaves the polymer surface. Since these molecules are much more likely to absorb strongly at 193 nm than (26) Rabek, J. F. Mechanisms of Photophysical Processes and Photochemical Reactions in Polymers; John Wiley: New York, 1987; p 67. (27) Srinivasan, R.; Sutcliffe, E.;Braren, B. Appl. Phys. Lett. 1987, SI, 1285.
IO009
J. Phys. Chem. 1991, 95, 10009-10018 at 308 nm, the increased fragmentation of the products of ArF laser ablation is understandable. Whether the distribution of products in ArF laser ablation near threshold is due to such secondary photolysis or to a significant photolytic contribution to the ablation mechanism as discussed above is an unresolved question. In general a number of competing processes may occur during the excimer laser ablation of any polymer. These include direct photolysis, multiple photon processes? and thermolysis following the relaxation of electronic energy to heat. It should be observed that the conversion of electronic to vibrational energy may occur partly by a process involving cage recombination of radicals initially generated by photolysis. The ablation of PET by XeCl laser irradiation occurs by rapid relaxation of the initial electronic excitation to heat, resulting in thermal decomposition. This
conclusion is reached on the basis of the observed ablation products, the lowering of the ablation threshold by preheating, the evidence of photoacoustic measurements, and the results of model calculations involving thermolysis rates. The available evidence also supports thermolysis as an important factor in the ablation of PET by ArF laser irradiation, although in this case the possibility of a contribution from photolysis cannot be excluded.
Acknowledgment. We are grateful for the skilful assistance of B. L. Tait. We thank Laser Applications Ltd. and the S.E.R.C. for the award of a C.A.S.E. studentship to J.S.and the S.E.R.C. for support by research Grants GR/D/28065 and GR/D/97740. Registry No. PET, 25038-59-9; XeCI, 55130-03-5; ArF, 56617-31-3; COZ, 124-38-9;CO,630-08-0; CH+74-82-8;CZHZ,74-86-2; CZHI,7485-1;C6H6,71-43-2.
Spectral Differences between Enantiomeric and Racemic Ru(bpy):’ Probable Causes
on Layered Clays:
Prashant V. Kamat; K. R. Copidas; Tulsi Mukherjee,*Vishwas Joshi,j Diiip Kotkar,s Vinit S. Pathak,g and Pushpito K. Chosh*.* Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana 46556, Chemistry Division, Bhabha Atomic Research Centre, Trombay, Bombay 400 085, India, and Alchemie Research Centre, Thane- Belapur Road, Thane 400 601, India (Received: April 15, 1991; In Final Form: July I S , 1991)
The preferential self-annihilation (static and dynamic) of A,A-Ru(bpy)?+* over A- or A-Ru(bpy)?+* is reported for aqueous dispersions of sodium hectorite lightly loaded with the Ru(I1) chelate and subjected to pulsed laser excitation. By varying the loading level over a factor of ca.60, it is also shown that racemate emission falls off sharply with increased loading whereas emission from the enantiomeric adsorbate remains more nearly constant. The decrease in luminescence yield of racemate with increased loading is mainly associated with an attenuation in the peak emission intensity, I(O), as found from time-resolved measurements. It is proposed, based on these studies, that clays offer both quenching and nonquenching sites for sorption and that A,A-Ru(bpy),” prefers the latter at low loadings, the ions being clustered within such regions. Enantiomeric Ru(bpy),”, on the other hand, is more randomly distributed over the sites. The above model also permits rationalization of (i) observed changes in emission intensity with time, (ii) anomalies in the relative emission yields of Ru(bpy)?+* and Ru(phen):+*, and (iii) the effect of Zn(phen)?+ on emission. Finally, differences in binding modes of enantiomeric and racemic chelate forms also induce differences in the flocculation trends of dispersed clays, the effects being most prominent for freshly prepared ruthenium(l1) montmorillonite.
We have recently reported results of our studies on the absorption and emission spectral behavior of optically active Ru(bpy),2+ (bpy = 2,2’-bipyridine) and Ru(phen);+ (phen = 1,IO-phenanthroline) chelates adsorbed on naturally occurring layered c1ays.l These studies-which followed Yamagishi and Soma’s original observations concerning the degree of exchange of such chelate types on sodium montmorillonite2-indicated that the binding states of enantiomeric and racemic complexes of Ru(l1) are different on clay, in marked contrast to their behavior on other supports. Although some form of spontaneous chiral interaction is implicit in the studies on lightly loaded clays, no conclusive evidence could be obtained so far to suggest that the spectral differences reflect genuine interactions between optical antipode~.’~J Spectral variations could also arise from differences in distribution patterns of the chelates over environmentally distinguishable exchange sites, influenced as such patterns may be by interactions during the sorption process. Firm evidence on the origin of the observed effects would clearly be necessary to develop models of chiral interactions on clay^.^-^ To this end, we have compared the effect of excitation intensity on the time-resolved luminescence behavior of enantiomeric and racemic Ru(bpy)t+* lightly loaded on sodium ‘University of Notre Dame. *Bhabha Atomic Research Centre. Alchemie Research Centre.
hectorite.l*b* By varying the loading level over a factor of ca. 60, the site selectivity of the chelate forms could also be probed. (1) (a) Joshi, V.; Kotkar, D.; Ghosh, P. K. J . Am. Chem. Soc. 1986.108, 4650. (b) Kotkar, D.; Joshi, V.; Ghosh, P. K. Proc. I d . Narl. Sci. Acad. A 1986,52,736.(c) Joshi, V.; Ghosh, P. K.J . Chem. Soc., Chem. Commun. 1987,789. (d) Joshi, V.; Kotkar, D.; Ghosh, P. K. Curr. Sci. 1988,57,567. (e) Joshi, V.; Ghosh, P. K. J . Am. Chem. SOC.1989, 111, 5604. (f) Joshi, V.; Kotkar, D.; Ghosh, P. K. Proc. Ind. Acad. Sci. (Chem. Sci.) 1990, 102, 203. (2)(a) Yamagishi. A.; Soma,M. J . Am. Chem. Soc. 1981,103,4640. (b) Yamagishi, A. J. Phys. Chem. 1982,86,2472. (c) Yamagishi, A.; Fujita, N. J. Colloid Interface Sci. 1984,100, 1778. (d) Yamagishi, A. J. Coord. Chem. 1987,6, 131 and references therein. (3) Villemure, G.; Bard, A. J. J . Electroanal. Chem. 1990, 283, 403. (4) (a) Pirkle, W. H.; Finn, J. M.; Hamper, B. C.; Schreiner, J.; Pribish, J. R. In Asymmetric Reactions and Processes in Chemistry; E M , E., Otsuka, S., Eds.; ACS Symposium Series No. 185; American Chemical Society: Washington, DC, 1982;p 256. (b) Pirkle, W.H.; Hyun, M. H.;Banks, B. J . Chromarogr. 1984, 316, 585. (c) Salem, L.; Chapuisat, X.;Segal, G.; Hilberty, P. C.; Minot. C.; Leforestier, C.; Sautet, P.J . Am. Chem. Soc. 1987, 109, 2887. (d) Lipkowitz, K. B.; Demeter, D. A.; Zegarra, R.; Larter, R.; Darden, T. J . Am. Chem. Soc. 1988, 110, 3446. (e) Metcalf, D. H.; Snyder, S.W.; Demas, J. N.; Richardson, F. S . J . Am. Chem. SOC.1990,112,5681 and references therein. ( 5 ) (a) Arnett, E. M.; Harvey, N. G.; Rose, P. L. Ace. Chem. Res. 1989, 22,13 1 and references therein. (b) Andelman, D. J . Am. Chem. SOC.1989, 111,6536. ( 6 ) Turro, N. J.; Kumar, C. V.; Grauer, 2.; Barton, J. K. Lungmuir 1987, 3, 1056.
(7)Kuykendall, V. G.; Thomas, J. K. J. Phys. Chem. 1990, 94, 4224.
0022-3654191 12095-10009%02.50/0 0 1991 American Chemical Society
