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Ablation of liquid benzene by pulsed ultraviolet (248 or 308 nm) laser radiation
Volume 143, number 6
CHEMICAL PHYSICS LETTERS
5 February I988
ABLATION OF LIQUID BENZENE BY PULSED ULTRAVIOLET (248 OR 308 nm) LASER RADIATION R. SRINIVASAN and A.P. GHOSH IBM T.J. Watson Research Center, Yorktown Heights, NY 10598, USA
Received 10 August 1987; in final form 24 November 1987
Irradiation of liquid benzene with laser pulses (248 nm, z 20 ns fwhm) at fluences > 0.2 J/cm2 caused ablation of the liquid surface. The shock wave which was transmitted through the liquid when the ablated material left the surface was detected by a piezoelectric transducer. When the liquid surface was constrained by a quartz plate, the threshold for ablation increased ( > 0.4 J/cm’), the intensity of the shock wave was amplified by an order of magnitude, and carbon was formed abundantly as a product. Ablation was observed with 308 nm laser pulses also even though there is no corresponding one-photon absorption. Ablation of benzene is attributed to the absorption of mo;e than 2 photons per molecule.
1, Introduction
We wish to report that in the condensed phase, even a simple molecule (as opposed td a polymer) such as benzene can undergo ablative photodecomposition. Ablation of polymer surfaces by ultraviolet laser radiation (193 to 308 nm) has been shown to result from the successive absorption of two or more photons by single chromophores followed by extensive decomposition of the polymer bonds in a few nanoseconds to give a variety of gaseous and solid products which are explosively ejected from the surface at supersonic velocities [ 1 ] ‘I. The choice of liquid benzene was suggested by the extensive work that exists on its excitation by multiphoton absorption in the condensed phase [ 3-51 and its decomposition following multiphoton absorption of ultriaviolet laser radiation in the gas phase [ 6,7].
2. Experimental Two different experimental arrangements were used. In the first one (fig. 1)) benzene was contained in a glass vessel 4 cm in diameter and 0.5 cm deep. The laser beam was incident vertically on the liquid x’For a recent review, see ref. [ 21.
546
TO OSCILLOSCOPE
248 nm LASER
0P
Fig. 1. Schematic of apparatus used to study the effect of confinement of ablation.
surface. A piezoelectric transducer in the form of poled polyvinylidene film with evaporated metal electrodes on both faces was placed at the bottom of the vessel in contact with the liquid. A second impedance matching, thick PVDF film (unpoled) was attached to one side of the transducer to delay the reflected acoustic wave from the primary one in order to avoid possible interference between the two. It was possible to support a quartz plate which was essentially transparent to the UV radiation on top of the liquid surface in order to confine the ablating material. The second experimental arrangement which was
0 009-2614/88/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
Volume 143, number 6
5 February I988
CHEMICAL PHYSICS LETTERS
suitable for the collection and identification of the products consisted of a quartz test tube 1.5 cm in diameter and 6 cm deep which was irradiated from the side. The test tube was rotated at 120 rpm around a vertical axis in order to expose a fresh portion of the liquid to each pulse. The benzene was outgassed by bubbling nitrogen through it and the volume above the liquid was swept with nitrogen during irradiation. It was important to direct the laser beam at a point just below the liquid surface. Otherwise twodimensional confinement of the force of the ablation by the wall and the hydrostatic pressure of the liquid caused the test tube to rupture. The Lambda-Physik model 201 E excimer laser that was used had a pulse width of 20 ns (fwhm), which was invariant over this study. Therefore, fluences rather than power densities are quoted. HPLC grade benzene was used without further purification.
0.001 ’ 0
’
‘= ‘= ’= ’= ’ 0.2 0.3 0.1
Fluence
’
’ 0.4
/
’ 0.5
J/cm2
3. Results Irradiation of liquid benzene in a test tube with pulsed 248 nm laser radiation gave three sets of products depending upon the fluence. At fluences -Z0.1 J/cm2, very little conversion was observed, the only detectable product being fulvene, the formation of which has been described previously [ 81. From 0.1 to %0.4 J/cm’, biphenyl and as many as six dimerit products were formed. The phenyl 1,3- and 1,Ccyclohexadienes of isomeric structures which were reported to be formed by the exposure of liquid benzene to high-energy positive ions of nitrogen, CO, or argon [ 91 were observed in the product mixture. When the fluence exceeded x0.3 J/cm2, the nature of the irradiation process changed dramatically. The passage of each laser pulse through the liquid produced a loud audible report. The liquid was violently agitated. After less than ten pulses, the entire liquid started to turn black with the separation of a precipitate which was identified as elemental carbon. The inside surface of the quartz tube was etched at the point at which the light beam entered the quartz/ benzene interface. When the tluence exceeded 0.5 JIcm2, the reaction vessel was invariably shattered after a few pulses. The fluences used were not adequate to cause dielectric breakdown of the liquid. The ablative decomposition observed in liquid benzene
Fig. 2. Yield of product/1000 J laser energy versus laser fluence at cell surface (248 nm). The uncertainties are indicated in the values at 0.2 and 0.5 J/cm’.
at 0.4 J/cm2 was not duplicated in liquid acetone so that the phenomenon can be plausibly attributed to a photochemistry that is specific to benzene. In fig. 2, the ordinate represents a quantity that is proportional to the quantum yield of each product. These values are plotted on a logarithmic scale against the fluence. The increase in the quantum yield of biphenyl is linear, which is to be expected for a product that can be formed by the combination of two free radicals. The formation of neither biphenyl nor the dimers seem to have a threshold fluence. In contrast, ablation and carbon production are observed only at fluences >0.3 J/cm2. The quantum yield of carbon is 0.18 at 0.5 J/cm2. Gas chromatographic analysis of the benzene irradiated under ablative conditions showed that there were small amounts of many products (other than those mentioned earlier) of MW r 78 of which the most important was naphthalene. It was verified that the yield of naphthalene was at least loo-fold in excess of the upper limit for any naphthalene that could have been present as an impurity in the benzene itself. Irradiation of liquid benzene with 308 nm laser 547
Volume 143, number 6
CHEMICAL PHYSICS LETTERS
5 February 1988
IO -
3
.c-
s
a-
c
s
i 60
6-
4’
5
iiJ
Z-
0’
0.0
I 40
z4
20
I
1
I
I
0.2
0.4
0.6
0.6
I
1.0
I
1.2
I 1.4
FLUENCE (J/cm2)
Fig. 4. Amplitudes of transducer signals as a function of fluence (248 nm). 0, confining plate removed; 0, confining plate present.
Fig. 3. Transducer signals in absence and presence of confining plate (248 nm).
pulses showed that ablation can be brought about when the fluence exceeded 1.0 J/cm*. The yield of carbon per 1000 J of incident energy was 3.5 mg and the ratio of carbon to naphthalene was similar to the value at 248 nm. The yield of biphenyl was only a tenth of its yield in relation to carbon at 248 nm while the formation of dimers was not detectable. It should be noted that at 308 nm liquid benzene has no measurable cross section for one-photon absorption. In order to sort out the role played by the wall of the test tube in the experiments described above, a second set of experiments were conducted in the apparatus shown in lig. 1. The electrical signals generated by the transducer on the impact of a laser pulse on the liquid surface, both in the absence and presence of the confining quartz plate are shown in fig. 3. The absolute magnitudes of these signals were difficult to calibrate but there was no question that the signal in the presence of the quartz plate was an order of magnitude greater than in its absence. The amplitude of this signal is indicative of the magnitude of the shock wave due to ablation. A plot of am548
plitude versus fluence (248 nm) is shown in fig. 4. The sudden jump in the intensity of the signal, which is indicative of the onset of significant ablation, is at a greater fluence in the presence of the plate and its absolute magnitude is also several-fold greater as already mentioned above. Carbon formation in the presence of the plate is also much more abundant than in its absence.
4. Discussion The phenomenon of ablation caused by lasers of any wavelength, whether of an organic polymer or an inorganic compound or metal is usually ascribed to a volume explosion that is created by a sudden rise in pressure and temperature in a confined volume. The exact mechanism by which the pressure rise is accomplished (vaporization or thermal/photochemical decomposition) may vary from system to system. In discussing the ablation of liquid benzene, we will focus on the processes that will lead to fragmentation and a consequent increase in pressure and leave out processes such as addition which can yield products of molecular weight greater than benzene itself (e.g. naphthalene). According to the energy diagram proposed by Scott,
Volume143,number6
CHEMICAL PHYSICSLETTERS
Braun, and Albrecht [3,5] for liquid benzene, two photons at 248 or 308 nm possess sufficient energy to excite benzene above its ionization potential ( x 7.1 eV), The benzene ion in the gas phase is generally believed to absorb another photon before it undergoes fragmentation [ 61. Cl is known to be an important product [ 7 1. From an analysis of the dynamics of the ablation of polymers by UV laser pulses, Sutcliffe and Srinivasan [ lo] have proposed a model in which ablation is postulated to result from the photochemical decomposition of upper excited states of the chromophores in a polymer. Excitation to higer states is considered to be a sequential, many-photon process and therefore has to compete with the deactivation process from the first excited state. The concentration (p) of “useful” photons in a volume element of thickness x at a time t during a pulse is given by
dt,x)=
s
[Z7(t',x)-Z7,]
dt’ ,
(1)
0
where the flux (n) of absorbed photons is corrected by the term Z7,, which is the flux threshold which has to be exceeded before the absorbed photons are effective in the ablation of a polymer. The critical value PT at which ablation occurs is 4 X lo*’ photons/cm’ ( + 25W) for a variety of polymers which includes both weak absorbers (polyethylene) as well as strong ones (polyimide). It is surprising that eq. (1 ), in which the dependence is on the first power of intensity, successfully predicts the ablation of polymers by UV laser pulses. It may be explained by the following scheme in which M represents a single chromophore in a polymer chain and the decomposition step (5) is always followed by ablation: Mthv+M*,
(2)
M*+M ,
(3)
M*+ hv+M** ,
(4)
M**+products ,
(5)
From (2), (3), (4) and (5), it can be derived that the rate of decomposition is given by Z2a*a**[M]lk,tZcu**, where CY*is the absorption
5 February1988
coefficient from the ground to the first excited state and cy** is the absorption coefficient from the first to the second state. This will simplify to Za*[M] if and only if Zcu**=sk3. Since a small value for k3 would be consistent with a weak absorber, the model will work best for a polymer with weakly absorbing chromophores (e.g. PMMA) and this is actually found to be the case. In applying this analysis to benzene, one notes that the absorptivity of liquid benzene at 248 nm, which is only 2100 cm-‘, is of the same order as that of polystyrene. The correction term Z7.rfor polystyrene is determined from experiment to be only 2O/bof Z7(t’, x), If this is assumed to be equally true for benzene, it can be ignored. For a 20 ns pulse, taking the threshold for ablation as 0.3 J/cm* from fig. 4, PT can be calculated to be 0.65 x lo*’ photons/cm3. This value is only about i of the threshold for ablation of polymers, which is not unexpected in a condensed material that does not possess an extended structure that is made of covalent bonds. Comeal tissue, which is an example of a solid material which is made of small molecules to the extent of 85%, has an ablation threshold of 0.64~ 102’photons/cm3 [ 111. LIF detection of C2 in the ablation of solid benzene with 193 nm laser pulses [ 121 shows that the production of C, begins at a fluence that is tenfold smaller than the fluences at which Cz can be detected from polymers under pulsed laser irradiation. The phenomenon of the confinement of the ablation by a quartz plate has been demonstrated previously by Carome, Clark, and Moeller [ 131 in the irradiation of aqueous solutions by a ruby laser. They also reported the replacement of the sinusoidal signal from the transducer in the absence of the confining plate by a unidirectional signal in the presence of the plate and presented a theory to explain it. In the case of benzene, since the phenomenon is not one of transient heating but a photochemcial decomposition that would lead to a permanent change in the number of molecules in the irradiated volume, confinement would lead to an increase in the threshold for decomposition. The observation that there is more carbon produced in the presence of a confining boundary than in its absence deserves comment. The response of the transducer (figs. 3 and 4) shows that the shock is greatly amplified in the confined system. It is probable that the more intense shock wave de549
Volume 143, number 6
CHEMICAL PHYSICS LETTERS
composes an intermediate species to yield carbon as a product. This intermediate is not likely to be the ground electronic state of benzene as the decomposition of liquid benzene in a shock wave is known [ 141 to give only polymeric material, and occurs at pressures which are loo-fold greater than those produced by laser ablation. A more likely precursor is an excited state of benzene, perhaps the benzene ion, which is the main product of two-photon excitation. This speculation can be verified by comparing the temporal profiles of the shock wave and the carbon production to see if the two are matched. Such studies are now in progress.
Acknowledgement The author thanks Professor K. Eisenthal for many useful discussions and Ms. B. Braren for technical assistance.
References [ I] R. Srinivasan and V. Mayne-Banton, Appl. Phys. Letters 4 1 (1982) 576; T. Deutsch and M.W. Geis, J. Appl. Phys. 54 (1983) 201: J.E. Andrew, P.E. Dyer, D. Forster and P.H. Key, Appl. Phys. Letters 43 (1983) 717;
550.
5 February 1988
R. Srinivasan and W.J. Leigh, J. Am. Chem. Sot. 104 (1982) 6784. [2] R. Srinivasan, Science 234 (1986) 559. [3] A.C. Albrecht, in: Photochemistry and photophysics above 6 eV, ed. F. Lahmani (Elsevier, Amsterdam, 1985) p. 227. [4] T.W. Scott and AC. Albrecht, J. Chem. Phys. 74 (1981) 3807. [5] T.W. Scott, C.L. Braun and A.C. Albrecht, J. Chem. Phys. 76 (1982) 5195. [6] B.D. Koplitz and J. McVey, J. Phys. Chem. 89 (1985) 4196. [7] L. Zandee and R.B. Bernstein, J. Chem. Phys. 70 (1979) 2574; 71 (1979) 1359; U. Boesl, H.J. Neusser and H.J. Schlag, J. Chem. Phys. 72 (1980) 4327; J.T. Meek, R. Jones and P. Reilly, J. Chem. Phys. 73 (1980) 3503; W.M. Hetherington, G. Korenowski and K.B. Eisenthal, Chem. Phys. Letters 77 ( 1981) 275. 18 D. BryceSmith, Pure Appl. Chem. 16 (1968) 60. 19 0. Puglisi, S. Pignataro, G. Foti, P. Baeri and E. Rimini, Chem. Phys. Letters 70 (1980) 392; 0. Puglisi, G. Marletta, S. Pignataro, G. Foti, A. Trovato and E. Rimini, Chem. Phys. Letters 78 (198 1) 207. [ lo] E. Sutcliffeand R. Srinivasan, J. Appl. Phys. 60 (1986) 3315. [ 111 E. Sutcliffe and R. Srinivasan, Lasers Ophthalm. 1 (1987), to be published. [ 121 R.W. Dreyfus and R. Srinivasan, to be published. [ 131 E.F. Carome, N.A. Clark and C.E. Moeller, Appl. Phys. Letters 4 (1964) 95. [ 141 L.V. Babare, A.N. Dremin, S.V. Pershin and V.V. Yaklovlev, Combust. Explosion Shock Waves 5 (1969) 364.
CHEMICAL PHYSICS LETTERS
5 February I988
ABLATION OF LIQUID BENZENE BY PULSED ULTRAVIOLET (248 OR 308 nm) LASER RADIATION R. SRINIVASAN and A.P. GHOSH IBM T.J. Watson Research Center, Yorktown Heights, NY 10598, USA
Received 10 August 1987; in final form 24 November 1987
Irradiation of liquid benzene with laser pulses (248 nm, z 20 ns fwhm) at fluences > 0.2 J/cm2 caused ablation of the liquid surface. The shock wave which was transmitted through the liquid when the ablated material left the surface was detected by a piezoelectric transducer. When the liquid surface was constrained by a quartz plate, the threshold for ablation increased ( > 0.4 J/cm’), the intensity of the shock wave was amplified by an order of magnitude, and carbon was formed abundantly as a product. Ablation was observed with 308 nm laser pulses also even though there is no corresponding one-photon absorption. Ablation of benzene is attributed to the absorption of mo;e than 2 photons per molecule.
1, Introduction
We wish to report that in the condensed phase, even a simple molecule (as opposed td a polymer) such as benzene can undergo ablative photodecomposition. Ablation of polymer surfaces by ultraviolet laser radiation (193 to 308 nm) has been shown to result from the successive absorption of two or more photons by single chromophores followed by extensive decomposition of the polymer bonds in a few nanoseconds to give a variety of gaseous and solid products which are explosively ejected from the surface at supersonic velocities [ 1 ] ‘I. The choice of liquid benzene was suggested by the extensive work that exists on its excitation by multiphoton absorption in the condensed phase [ 3-51 and its decomposition following multiphoton absorption of ultriaviolet laser radiation in the gas phase [ 6,7].
2. Experimental Two different experimental arrangements were used. In the first one (fig. 1)) benzene was contained in a glass vessel 4 cm in diameter and 0.5 cm deep. The laser beam was incident vertically on the liquid x’For a recent review, see ref. [ 21.
546
TO OSCILLOSCOPE
248 nm LASER
0P
Fig. 1. Schematic of apparatus used to study the effect of confinement of ablation.
surface. A piezoelectric transducer in the form of poled polyvinylidene film with evaporated metal electrodes on both faces was placed at the bottom of the vessel in contact with the liquid. A second impedance matching, thick PVDF film (unpoled) was attached to one side of the transducer to delay the reflected acoustic wave from the primary one in order to avoid possible interference between the two. It was possible to support a quartz plate which was essentially transparent to the UV radiation on top of the liquid surface in order to confine the ablating material. The second experimental arrangement which was
0 009-2614/88/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
Volume 143, number 6
5 February I988
CHEMICAL PHYSICS LETTERS
suitable for the collection and identification of the products consisted of a quartz test tube 1.5 cm in diameter and 6 cm deep which was irradiated from the side. The test tube was rotated at 120 rpm around a vertical axis in order to expose a fresh portion of the liquid to each pulse. The benzene was outgassed by bubbling nitrogen through it and the volume above the liquid was swept with nitrogen during irradiation. It was important to direct the laser beam at a point just below the liquid surface. Otherwise twodimensional confinement of the force of the ablation by the wall and the hydrostatic pressure of the liquid caused the test tube to rupture. The Lambda-Physik model 201 E excimer laser that was used had a pulse width of 20 ns (fwhm), which was invariant over this study. Therefore, fluences rather than power densities are quoted. HPLC grade benzene was used without further purification.
0.001 ’ 0
’
‘= ‘= ’= ’= ’ 0.2 0.3 0.1
Fluence
’
’ 0.4
/
’ 0.5
J/cm2
3. Results Irradiation of liquid benzene in a test tube with pulsed 248 nm laser radiation gave three sets of products depending upon the fluence. At fluences -Z0.1 J/cm2, very little conversion was observed, the only detectable product being fulvene, the formation of which has been described previously [ 81. From 0.1 to %0.4 J/cm’, biphenyl and as many as six dimerit products were formed. The phenyl 1,3- and 1,Ccyclohexadienes of isomeric structures which were reported to be formed by the exposure of liquid benzene to high-energy positive ions of nitrogen, CO, or argon [ 91 were observed in the product mixture. When the fluence exceeded x0.3 J/cm2, the nature of the irradiation process changed dramatically. The passage of each laser pulse through the liquid produced a loud audible report. The liquid was violently agitated. After less than ten pulses, the entire liquid started to turn black with the separation of a precipitate which was identified as elemental carbon. The inside surface of the quartz tube was etched at the point at which the light beam entered the quartz/ benzene interface. When the tluence exceeded 0.5 JIcm2, the reaction vessel was invariably shattered after a few pulses. The fluences used were not adequate to cause dielectric breakdown of the liquid. The ablative decomposition observed in liquid benzene
Fig. 2. Yield of product/1000 J laser energy versus laser fluence at cell surface (248 nm). The uncertainties are indicated in the values at 0.2 and 0.5 J/cm’.
at 0.4 J/cm2 was not duplicated in liquid acetone so that the phenomenon can be plausibly attributed to a photochemistry that is specific to benzene. In fig. 2, the ordinate represents a quantity that is proportional to the quantum yield of each product. These values are plotted on a logarithmic scale against the fluence. The increase in the quantum yield of biphenyl is linear, which is to be expected for a product that can be formed by the combination of two free radicals. The formation of neither biphenyl nor the dimers seem to have a threshold fluence. In contrast, ablation and carbon production are observed only at fluences >0.3 J/cm2. The quantum yield of carbon is 0.18 at 0.5 J/cm2. Gas chromatographic analysis of the benzene irradiated under ablative conditions showed that there were small amounts of many products (other than those mentioned earlier) of MW r 78 of which the most important was naphthalene. It was verified that the yield of naphthalene was at least loo-fold in excess of the upper limit for any naphthalene that could have been present as an impurity in the benzene itself. Irradiation of liquid benzene with 308 nm laser 547
Volume 143, number 6
CHEMICAL PHYSICS LETTERS
5 February 1988
IO -
3
.c-
s
a-
c
s
i 60
6-
4’
5
iiJ
Z-
0’
0.0
I 40
z4
20
I
1
I
I
0.2
0.4
0.6
0.6
I
1.0
I
1.2
I 1.4
FLUENCE (J/cm2)
Fig. 4. Amplitudes of transducer signals as a function of fluence (248 nm). 0, confining plate removed; 0, confining plate present.
Fig. 3. Transducer signals in absence and presence of confining plate (248 nm).
pulses showed that ablation can be brought about when the fluence exceeded 1.0 J/cm*. The yield of carbon per 1000 J of incident energy was 3.5 mg and the ratio of carbon to naphthalene was similar to the value at 248 nm. The yield of biphenyl was only a tenth of its yield in relation to carbon at 248 nm while the formation of dimers was not detectable. It should be noted that at 308 nm liquid benzene has no measurable cross section for one-photon absorption. In order to sort out the role played by the wall of the test tube in the experiments described above, a second set of experiments were conducted in the apparatus shown in lig. 1. The electrical signals generated by the transducer on the impact of a laser pulse on the liquid surface, both in the absence and presence of the confining quartz plate are shown in fig. 3. The absolute magnitudes of these signals were difficult to calibrate but there was no question that the signal in the presence of the quartz plate was an order of magnitude greater than in its absence. The amplitude of this signal is indicative of the magnitude of the shock wave due to ablation. A plot of am548
plitude versus fluence (248 nm) is shown in fig. 4. The sudden jump in the intensity of the signal, which is indicative of the onset of significant ablation, is at a greater fluence in the presence of the plate and its absolute magnitude is also several-fold greater as already mentioned above. Carbon formation in the presence of the plate is also much more abundant than in its absence.
4. Discussion The phenomenon of ablation caused by lasers of any wavelength, whether of an organic polymer or an inorganic compound or metal is usually ascribed to a volume explosion that is created by a sudden rise in pressure and temperature in a confined volume. The exact mechanism by which the pressure rise is accomplished (vaporization or thermal/photochemical decomposition) may vary from system to system. In discussing the ablation of liquid benzene, we will focus on the processes that will lead to fragmentation and a consequent increase in pressure and leave out processes such as addition which can yield products of molecular weight greater than benzene itself (e.g. naphthalene). According to the energy diagram proposed by Scott,
Volume143,number6
CHEMICAL PHYSICSLETTERS
Braun, and Albrecht [3,5] for liquid benzene, two photons at 248 or 308 nm possess sufficient energy to excite benzene above its ionization potential ( x 7.1 eV), The benzene ion in the gas phase is generally believed to absorb another photon before it undergoes fragmentation [ 61. Cl is known to be an important product [ 7 1. From an analysis of the dynamics of the ablation of polymers by UV laser pulses, Sutcliffe and Srinivasan [ lo] have proposed a model in which ablation is postulated to result from the photochemical decomposition of upper excited states of the chromophores in a polymer. Excitation to higer states is considered to be a sequential, many-photon process and therefore has to compete with the deactivation process from the first excited state. The concentration (p) of “useful” photons in a volume element of thickness x at a time t during a pulse is given by
dt,x)=
s
[Z7(t',x)-Z7,]
dt’ ,
(1)
0
where the flux (n) of absorbed photons is corrected by the term Z7,, which is the flux threshold which has to be exceeded before the absorbed photons are effective in the ablation of a polymer. The critical value PT at which ablation occurs is 4 X lo*’ photons/cm’ ( + 25W) for a variety of polymers which includes both weak absorbers (polyethylene) as well as strong ones (polyimide). It is surprising that eq. (1 ), in which the dependence is on the first power of intensity, successfully predicts the ablation of polymers by UV laser pulses. It may be explained by the following scheme in which M represents a single chromophore in a polymer chain and the decomposition step (5) is always followed by ablation: Mthv+M*,
(2)
M*+M ,
(3)
M*+ hv+M** ,
(4)
M**+products ,
(5)
From (2), (3), (4) and (5), it can be derived that the rate of decomposition is given by Z2a*a**[M]lk,tZcu**, where CY*is the absorption
5 February1988
coefficient from the ground to the first excited state and cy** is the absorption coefficient from the first to the second state. This will simplify to Za*[M] if and only if Zcu**=sk3. Since a small value for k3 would be consistent with a weak absorber, the model will work best for a polymer with weakly absorbing chromophores (e.g. PMMA) and this is actually found to be the case. In applying this analysis to benzene, one notes that the absorptivity of liquid benzene at 248 nm, which is only 2100 cm-‘, is of the same order as that of polystyrene. The correction term Z7.rfor polystyrene is determined from experiment to be only 2O/bof Z7(t’, x), If this is assumed to be equally true for benzene, it can be ignored. For a 20 ns pulse, taking the threshold for ablation as 0.3 J/cm* from fig. 4, PT can be calculated to be 0.65 x lo*’ photons/cm3. This value is only about i of the threshold for ablation of polymers, which is not unexpected in a condensed material that does not possess an extended structure that is made of covalent bonds. Comeal tissue, which is an example of a solid material which is made of small molecules to the extent of 85%, has an ablation threshold of 0.64~ 102’photons/cm3 [ 111. LIF detection of C2 in the ablation of solid benzene with 193 nm laser pulses [ 121 shows that the production of C, begins at a fluence that is tenfold smaller than the fluences at which Cz can be detected from polymers under pulsed laser irradiation. The phenomenon of the confinement of the ablation by a quartz plate has been demonstrated previously by Carome, Clark, and Moeller [ 131 in the irradiation of aqueous solutions by a ruby laser. They also reported the replacement of the sinusoidal signal from the transducer in the absence of the confining plate by a unidirectional signal in the presence of the plate and presented a theory to explain it. In the case of benzene, since the phenomenon is not one of transient heating but a photochemcial decomposition that would lead to a permanent change in the number of molecules in the irradiated volume, confinement would lead to an increase in the threshold for decomposition. The observation that there is more carbon produced in the presence of a confining boundary than in its absence deserves comment. The response of the transducer (figs. 3 and 4) shows that the shock is greatly amplified in the confined system. It is probable that the more intense shock wave de549
Volume 143, number 6
CHEMICAL PHYSICS LETTERS
composes an intermediate species to yield carbon as a product. This intermediate is not likely to be the ground electronic state of benzene as the decomposition of liquid benzene in a shock wave is known [ 141 to give only polymeric material, and occurs at pressures which are loo-fold greater than those produced by laser ablation. A more likely precursor is an excited state of benzene, perhaps the benzene ion, which is the main product of two-photon excitation. This speculation can be verified by comparing the temporal profiles of the shock wave and the carbon production to see if the two are matched. Such studies are now in progress.
Acknowledgement The author thanks Professor K. Eisenthal for many useful discussions and Ms. B. Braren for technical assistance.
References [ I] R. Srinivasan and V. Mayne-Banton, Appl. Phys. Letters 4 1 (1982) 576; T. Deutsch and M.W. Geis, J. Appl. Phys. 54 (1983) 201: J.E. Andrew, P.E. Dyer, D. Forster and P.H. Key, Appl. Phys. Letters 43 (1983) 717;
550.
5 February 1988
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