Interplay between Chemical, Thermal, and Mechanical Processes

J. Phys. Chem. C 2009, 113, 11491–11506

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Interplay between Chemical, Thermal, and Mechanical Processes Occurring upon Laser Excitation of Poly(methyl methacrylate) and Its Role in Ablation† Manish Prasad, Patrick F. Conforti, and Barbara J. Garrison* Department of Chemistry, The PennsylVania State UniVersity, 104 Chemistry Building, UniVersity Park, PennsylVania 16802 ReceiVed: August 15, 2008; ReVised Manuscript ReceiVed: October 10, 2008

Experiments and modeling studies have identified thermal, chemical, and mechanical processes as likely sources of laser ablation in polymeric materials. In our earlier study, molecular dynamics simulations with an embedded Monte Carlo based reaction scheme have been used to investigate the role of each of these processes separately, in a poly(methyl methacrylate) substrate irradiated by ultraviolet lasers. In the present study, using the same substrate, we allow for interactions among these processes, to better model the real ablation process, and investigate how it affects the system evolution during ablation. In the purely thermal case, chemical reactions are allowed to occur via the thermo-mechanical bond break and radical formation leading to subsequent gas formation. Similarly, in purely photochemical cases, additional bond breaks and reactions occur due to high temperatures and stresses. In all cases, it is observed that the ablation process is extended over longer time scales (. laser pulse width) resulting in higher yields. The thermo-mechanical bond breaks and ensuing reactions ensure that the substrate remains hotter for a long time, causing more fragmentation and ejection of the substrate. The mechanism of ejection for all thermal and chemical pathways is found to be both the thermo-mechanical in nature, driven by critical fraction of broken bonds, as well as chemical in nature, governed by near complete disintegration of the polymer matrix into monomers, small polymer fragments, and gas molecules. The formation of volatile gases, just underneath the surface assists in the ejection of the fragmented substrate. In all cases, secondary damage due to thermo-mechanical bond break process stimulated by high temperature and stresses, and by chemistry of the polymeric substrate, is found to contribute significantly more than the direct laser photochemical or photothermal damage, to the substrate and plume evolution as well as total ablation yield. 1. Introduction Laser ablation is a widely used tool in a number of applications, ranging from electronics, photonics, fabrication of materials based on nanotechnology, medical diagnostics and surgery, to spectrometric analysis.1-9 The rate, characteristic features, and quality of laser ablation in these applications depend on laser processing parameters such as, wavelengths, fluences, and pulse widths, along with electronic, thermal, mechanical, and chemical properties of the material. The ablation process also occurs over multiple length and time scales further complicating its study using a particular set of experimental and modeling tools. Numerous scientific studies have tried to unravel the underlying physical mechanism governing the ablation process, but have instead broken it down into simpler and disparate set of modes of action based on one dominant process or other, owing to the complex interplay among these processes as well as their concurrent nature.7,10-13 A better understanding of the dynamics of the laser-material interaction and the relationship among the different processes occurring during ablation will aid in better control and optimization of the existing processes as well as development of the newer ones. In the case of polymeric materials, the absorption of laser photon leads to electronic excitation of the chromophore which can be followed by intra- and intermolecular conversion of †

Part of the “Hiroshi Masuhara Festschrift”. * To whom correspondence should be addressed. E-mail: [email protected].

energy to heat or bond scission. The polymer ablation studies, using both experiments and modeling,4,13-20 have suggested two principal classes of ablation mechanisms based on these energy consumption pathways: photothermal and photochemical. In photothermal ablation, laser energy absorption is followed by its conversion into heat, which causes high temperature, thermal degradation, charring, melting, and sublimation of the polymer, coupled with massive ejection of material.21-24 In photochemical ablation, the photon energy is utilized to directly break bonds at absorption sites. This photofragmentation creates radicals and new molecular entities by chemical degradation and reactions which leads to ablation.14,25-28 The photochemical ablation process is often characterized by a clean cut, negligible thermal damage and ejection of gaseous fragments formed upon photolytic scission and chemical reactions.14,19,24,29-32 In both cases, high pressure/mechanical stresses may also play a major role, especially for shorter pulse widths.12,33-36 The source of this stress can be physical, i.e., thermoelastic, due to rapid heating and expansion, or chemical, due to rapid decomposition of polymer matrix and formation of volatile products within the matrix.37 The buildup of mechanical forces can lead to violent phase explosion, development of shock waves, void nucleation, and spallation all of which can be directly associated with the ablation process.12,35,37-39 The exact dynamics of the ablation process is not well understood as it appears to be a complex combination of these different processes and also a function of the material properties which may change during the course of ablation.

10.1021/jp807305r CCC: $40.75  2009 American Chemical Society Published on Web 01/06/2009

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Molecular dynamics (MD) simulation has been used successfully to model ablation processes in metals, semiconductors, molecular solids, polymers and biological matrices.10,38,40-45 In our earlier MD studies of polymer ablation, we reported how ablation in a poly (methyl methacrylate) (PMMA) substrate is affected by laser fluence,36,46-48 pulse width,36 penetration depth,49 and wavelength vs number of photons.48 We also reported how the ablation threshold and mechanism depend on the way laser energy is utilized under purely photothermal vs purely photochemical conditions.46,47,49 In these simulations the two suggested pathways were modeled separately, to steer clear of the various uncertainties arising out of different experimental studies proposing different dominant ablation pathways,14,15,17,27,29-32,50,51 and to outline their indiVidual mechanisms of achieving ablation as a guide to future experiments. It was found that pressure and mechanical stresses play a dominant role only for short laser pulse widths in the stress confinement regime and they can set off ablation by mechanical failure (void generation, and spallation) of the polymer matrix in the pure heating case.36,46,47 For the longer pulse width studied, however, the ejection process is predominantly thermally activated, driven by the bond break process with ablation occurring when a critical bond break fraction is reached.36 The purely thermal mechanism was also found to be inefficient in creating small gaseous molecules. In the photochemical pathway, bond break, radical generation and subsequent reactions alter the composition of the substrate by causing rapid and near complete decomposition of the polymer matrix into volatile gases and smaller polymer fragments. This change destroys the cohesive polymer matrix and causes small molecules and polymer fragments to eject from the substrate. The hollowed out substrate facilitates ejection of larger clusters also via a thermally activated process.36 Furthermore, the longer wavelength photons ablate more material for a given fluence, with everything else being the same (i.e., penetration depth, absorption coefficient, and quantum efficiency), because of higher number of photons causing more fragmentation and chemical transformation deeper within the substrate.48 Likewise, a deeper penetration depth shifted the ablation threshold to a higher fluence as expected from Beer’s Law. The deposition of a critical amount of energy was found necessary to initiate ablation. The simulations showed this critical energy value depended on the laser energy excitation path with the highest amount necessary for purely photothermal pathways and subsequently lower amount needed as more photochemistry was introduced in the system.49 As summarized above, the ablation studies and modeling efforts have largely focused on studying the photothermal and photochemical pathways as two different ways which can cause polymer ablation. Few, if any, models fully incorporate the effects of simultaneous thermal and chemical damage (during ablation), thermally induced damage leading to creation of radicals and possible chemical damage, or exothermic chemical reactions following generation of photoradicals leading to high temperature and likely thermal damage to the material. In refs 20 and 52, the damage from two pathways were added to yield total ablation damage, whereas in refs 23 and 53-55, the thermal and chemical modifications are considered as thermally activated (first order rate) processes. Both of these types of models fail to account for subsequent chemical reactivity of the disintegrated polymer substrates. This reactivity has been shown to play an important role in ablation of many polymers.14,18,27,32 A model by Kalontarov56 considers ablation as a coupled two step process with chemical damage followed by thermally activated ejection. However, the thermal destruction

Prasad et al. of the polymers cannot be ignored especially at the high fluences where high temperatures and stresses may prevail.26,29,32,37,57-62 Similarly, the influence of mechanical stresses on thermal and chemical processes and their interaction have been largely ignored in modeling efforts despite direct evidence of their presence in the sample.12,33,63 In earlier works investigating the destruction of polymeric materials subjected to thermo-mechanically generated stress-strain profiles, it was suggested that the polymer can fail via excitation of any of the two types of bonding networks: one involving direct breakage of intramolecular covalent bonds and the other by breaking of intermolecular bonds causing slipping and rearrangement of polymer chains.64-67 Simulations of linear polymer chain subject to Lennard-Jones potential indicated that irreversible breaking of polymeric bonds occurs when the bond length gets stretched far greater than the equilibrium bond length or the length at which its restoring force is maximized.68 The attempt frequency of the bond break was also noted to be much smaller than dominant phonon frequencies highlighting the possible role of interchain forces in stabilizing a stretched bond in its unbroken state.68 The breakage of bonds under stress can lead to formation of radicals via the thermo-mechanical process, followed by generation of volatile products, microvoids, cracks, and melting.67,69-72 These byproducts have been detected via chemical methods by introducing low molecular weight reactants73 and by physical means using mass spectrometry and electron paramagnetic resonance (EPR).69,70 The concentration of ruptured polymer chains and radicals obtained via EPR studies has generally been smaller than what is expected to account for other observed physical changes such as decrease in tensile strength and average molecular weight of the polymer,67,69,74 suggesting a role of fast radical recombination reactions. The radical generation also promotes further damage to the polymer matrix with propagation reactions leading to a large number of broken polymer fragments and radicals.67 The kinetics of thermal degradation has also been modeled with reaction schemes based on random chain scission, propagation and termination reactions studying the impact of reaction parameters on the change of molecular weight distribution, weight loss, polydispersity with time.75,76 For the PMMA system, these modeling studies coupled with experimental data have suggested multiple reaction pathways based on main chain as well as side chain scission to explain the evolution of the aforementioned characteristics of polymers during thermal degradation.70,77,78 Here, it is worth noting that the rate and magnitude of total thermal and mechanical loading in these experimental studies were several order of magnitudes lower than that encountered during laser ablation, but they nevertheless provide insight into many degradation pathways for the polymeric matrix in addition to the purely photochemical ones. In addition to creating radicals, the thermo-physical and chemical stress also alters energy and kinetic parameters associated with radical formation, propagation, recombination, and hydrogen abstraction reactions.74,79 The presence of stresses is known to alter the rate of direct photochemical degradation of polymers with tensile and shear stress generally expected to accelerate the breakdown process, while compressive stresses cause it to slow down, although the overall dynamics is much more complex.74,79,80 Zhurkov has suggested that the rate of reaction governed by activation barrier ∆E, gets modified under stress to ∆E - γσ, where γ is a constant and σ the stress.67 In contrast, the computation of energy barriers and kinetic coefficients either experimentally67 or using electronic structure calculations81 do not account for thermally, mechanically, or

Laser Excitation of Poly(methyl methacrylate) chemically generated stress during the polymer degradation process. Its role in ablation needs to be further explored.12,13 In this article, we extend our earlier MD studies of PMMA ablation by introducing a degree of coupling to link the thermal, mechanical and chemical processes. We continue to maintain our original partition of the photothermal and photochemical ablation pathways, i.e. one of these two pathways is assumed to be the dominant and direct mode of laser energy absorption in a simulation, but we also allow other likely processes to contribute as the polymer matrix evolves during and after the laser pulse. The terms “purely (photo-)thermal” and “purely (photo-)chemical” used later in the text refer to this separation of the initial laser energy utilization pathway and not to the fundamental mechanism of ablation. Thus, purely thermal energy absorption is coupled with stretching of covalent bonds by high temperature, high pressure, and stress which can subsequently produce radicals and initiate chemical reactions in the matrix. Likewise, in the purely photochemical mode of laser energy absorption, secondary thermo-mechanical bond breaks and reactions are allowed. The contributions made by the interplay among these processes are the focus of this article. The introduction of these interactions enables us to mimic the ablation physics more realistically. In our previous studies,36,46-49 the thermo-mechanical bond breaks were not labeled as radicals in the simulation scheme and no subsequent reaction occurred. However, a considerable fraction of bonds were broken by high temperature and stress in the polymer matrix, e.g., in the pure heating case, about 10% of the covalent bonds were broken by the time ablation took place, i.e., close to 20% of all of the beads were theoretically changed into radicals.36 Ablation in this case occurred by ejection of one or more of large aggregates of polymer, which simply drifted away from the substrate with no further evolution. The addition of radical formation and subsequent reactions leads to a substantial number of new chemical events, both in the substrate and ejected plume, likely leading to rapid breakup of ejected clusters and more late ejections from the substrate. Similarly, additional reactions by thermo-mechanically formed radicals in the photochemical case may lead to more violent ejections earlier during ablation. Given all of the complexity associated with stress and high temperature related damage in polymeric systems, the aim here is not to predict or model all the events that can occur when thermal and stress driven breakdown come into the picture, but to demonstrate how the these degradation processes may play an important role in laser ablation of polymer matrices. The organization of this article is as follows. The simulation setup is described in section 2, along with details related to implementation of thermo-mechanical bond breaks and radical formation. Both these events are concurrent in real systems, but they are implemented in two steps in our setup, i.e., breaking of an overstretched bond by thermal and mechanical stresses do not automatically lead to two free radicals. In section 3, the results for current set of simulations are presented and compared with the previous data set which modeled pure thermal and chemical ablation processes. Finally, the results are analyzed to identify the mechanisms leading to ablation and conditions that determine the ablation condition in terms of breakdown of covalent bonds, transformation of the original matrix to polymer fragments and gaseous species and the temperature of the polymeric substrate. 2. Simulation Setup The sample preparation and MD-MC simulation methodology is described in detail in ref 82 and only outlined in brief here.

J. Phys. Chem. C, Vol. 113, No. 27, 2009 11493 The amorphous PMMA matrix used in MD simulation was made up of 951 polymeric chains of 19 monomeric units each, built using coarse grained beads, with each bead representing a functional group.44,82 For PMMA system, these beads are CHx (0 e x e 3), CO, and O, along with radicals like, •CH3 and •O, created by bond cleavage and other species such as, HCO, CH4, and OH, produced by chemical reactions in the substrate. The bonding and angular potentials are of the hybrid form, i.e., harmonic near equilibrium and of Morse-type away from it to facilitate bond fracture due to high temperature and stress.44,82,83 The angular potentials also incorporate a distance based screening function to smoothly diminish to zero as bonds are stretched and broken. The nonbonding inter- and intramolecular interactions are described using another Morse potential fitted to match PMMA physical properties such as cohesive energy and glass transition temperature of the polymer.44,82 Each ablation simulation was started with an initial temperature of 300 K and at the density of 1.2 g/cm3 (51 × 51 × 936 Å3). Periodic boundary conditions were applied on the sides and a pressure absorbing boundary condition was applied at the bottom of the sample to prevent the downward traveling pressure wave from reflecting back into the sample.82,84 These boundary conditions together mimic a deep substrate (i.e., no reflected pressure wave is formed at the back end of the sample) at the center of laser pulse where the mass and energy transfers are expected only along the depth of the sample. The simulations were followed over a time period of up to 2-2.5 ns, to ensure that we capture all possible large cluster ejection events (read further discussion in section 3.5). A single laser pulse of wavelength λ ) 157 or 248 nm (7.9 and 5 eV per photon respectively), pulse width τ ) 5, 150 ps (ps), and penetration depth 100 Å, was applied to the sample from t ) 0 to τ. The fluence range was 8-20 mJ/cm2. The photons are absorbed by the chromophores (-C- or -COfor PMMA) in accordance with the Beer’s Law. Each chromophore absorbs at most 1 photon. The laser wavelengths, short pulse widths, small penetration depth and other simulation conditions specified below were chosen here with the aim to maximize incoming laser energy density in the substrate along with the ablation yield while keeping the MD-MC simulation time manageable. The photon energy is directed to only one excitation path in a simulation, as mentioned above, in order to simulate one dominant mode of absorption. This selection leads to one pure thermal case where the laser energy goes directly into heating the polymer. The absorption process is accomplished by converting the laser energy into kinetic energy of the beads of the absorbing monomeric unit. The two other cases involve direct photochemical bond breaks, where photon energy is utilized to break two different types of bonds: the C-CO bond on a PMMA side chain (Norrish type I reaction) or the C-CH2 bond on the main chain (Norrish type II reaction). The leftover photon energy goes into the kinetic energy of the beads involved in the bond cleavage. Upon direct bond break, species with a broken bond are tagged as radicals and tracked for subsequent reactions. In addition to these, there are indirect bond breaks (of overstretched covalent bonds) which can happen throughout the simulation because of the high temperature and pressure. For this purpose, the critical interbead distance was set at 5 Å, i.e., the covalently bonded beads separated by more than 5 Å were considered noninteracting as their bonding potential energies go below the available thermal energy.36 The interbead separation is measured during the force calculation and all such covalently bonded beads are tagged as “un-bonded”. If there are multiple bonds in the overstretched state for a bead,

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Figure 1. MMA monomer represented by coarse-grained beads and the covalent bonds. All of the bonds, shown by a solid line, can be broken by stretching.

the most stretched one is tagged for breaking. This “un-bonded” state is allowed to persist as is for a period of 1 ps. This ensures that the bond stays broken and does not return to the bonded state. It is worth noting that the bond can continue to exist for much longer than this time, due to the constraints on energy change, as discussed further in section 3.1. If another bond stretches further (for a particular bead) during this interval, it is tagged and the countdown clock reset to 1 ps. Finally, an attempt to break the bond is made by changing particle types, tagging them as radicals and hence changing the interaction potentials to measure the net change in energy. The acceptance criteria for this attempt are same as the one we have used in our chemical reaction scheme.36,82 If the new state has lower energy, the change is accepted with the resulting energy difference going into kinetic energy of the two beads which form new radicals, otherwise the new state is accepted only if the energy difference between the two energy states is within 0.1 eV, the desired energy change for this reaction being zero. In such cases, the bond break occurs if the required energy is within 5% of the kinetic energy of the beads.82 If the bond break attempt fails due to adverse energy change, it is then reattempted after a lapse of 1 ps. This energy check ensures that the bond break does not lead to sudden rise in repulsive forces and introduce unphysical instability in the polymer matrix. The possible bond breaks include all the bonds in the original PMMA matrix as shown in Figure 1. Likewise, the bonds in the newly formed species can also become cleaved, e.g., CH3-OH can break into •CH3 and •OH. The carbon-carbon double bonds and the carbonyl group are not included, due to their bond strengths, and so are the carbon-hydrogen and oxygen-hydrogen bonds, maintaining the united atom representation used in our simulation. Also, simultaneous or sequential cleavage of multiple bonds to form diradicals is not included (they have extremely short lifetimes). Once a bond is broken and the bead tagged as a radical, it can only take part in one of the reactions to lose the radical tag before it will be allowed to break another stretched bond. For the radicals, a reaction is only possible if reacting species are identified within an interaction radius of 2 Å. However, a bead (radical or otherwise) is always free to move away from other beads it is bonded to, if it possesses enough energy to overcome the potential energy of interaction. The choice of primary chemical reactions, for the PMMA system, included in the simulation scheme has already been discussed in earlier publications.36,48,82,85 A representative set of chemical reactions is chosen, based on the experimentally observed ablation products, such as CO, CO2, CH4, HCHO, HCOOCH3, CH3OH, and MMA monomer, and the likely chemical reactions suggested for their formation.14,15,17,27,29-32,50,51 In principle, there is a much larger number of possible reactions that can take place during ablation due to high temperatures and stresses. However, the aim in our simulations is not to predict likely reactions under the high temperatures and stresses prevailing during ablation, but to see how these reactions impact the polymer substrate and ablation process. The reactions36,48,82,85

Prasad et al. are hydrogen abstraction, decarboxylation, carbon monoxide elimination, unzipping, •CH3 elimination with a CHx-CHx double bond formation, and •CHx radical-radical recombination to form a double bond. The activation and reaction energies for these reactions were obtained from electronic structure calculations85,86 providing a physically accurate description of the chemical processes in the substrate. Some of the species taking part in the reactions under the current setup are formed following the bond breaks occurring in the products produced in the primary reactions and were not present in the previous setup. A few examples of these bond breaks and subsequent reactions are shown in Figure 2. There will be many of these secondary reactions added to the list of possible reaction at any given step, with many not playing significant role in ablation events, and slowing down the MD time integration of the process. But no attempts have been made to restrict these reactions, as it would require identifying some of these reactions as irrelevant. Also, the primary aim of this article is to the investigate role of thermo-mechanical stress driven bond breaking in ablation process. The energetics for these newly added reactions are set to the same values as those of their close representatives among our primary set of reactions, e.g., R2sCdCH2, RsCHdCH2, and CH2dCH2 all have the same energy of formation for the double bond in our setup. 3. Results and Discussion 3.1. Bond Breaks and Radical Generation. We start the discussion by highlighting the primary changes observed in evolution of the polymer matrix due to addition of the bond break and radical generation process. In the following, we discuss these changes and results mostly in terms of the simulation performed with 15 mJ/cm2 fluence, 150 ps pulse width and 157 nm laser light (unless stated otherwise). Also, all the discussion here and later involve frequent comparison of the new/current set of runs, which allow thermo-mechanical bond breaks, radical generation and subsequent chemical reactions, and old/earlier/preVious set of runs, where this process was not included. The change first visible is in the total number of radicals present in the system, at a given time, as shown in Figure 3. For the runs from the previous data set, the radicals are only generated during the pulse and only for photochemical Norrish type I & II cases by direct photoscission. For the old Norrish type I case, the total number of radicals can be seen rising during the laser pulse, and thereafter going in a steady decline, as the radicals are lost due to reactions (black dashed-dot-dot line). With the current runs, the thermomechanical radical generation process starts at about 50-100 ps, contributing to increased number of radicals. This time lag is attributed to the slow process of bonds overstretching and breaking after the buildup of temperature and/or pressure. This delay was also observed in pure heating case in the previous data set, where no radical creation was allowed.36 The radical generation for the Norrish type I pathway results in almost twice the total number of radicals present at the end of the pulse in the current (orange dashed line) vs the previous data set. This trend continues throughout the run and more radicals are formed, as shown by the solid line indicating cumulative number of the radical created over time for Norrish type I case. The radical generation process appear to slow during the 200-800 ps time range, but afterward it increases again and continues over the entire simulation time scale. Similar trends are observed for the Norrish type II case (not shown). The total number of radicals for the current heating runs (1) is also comparable to both the Norrish reaction cases, whereas

Laser Excitation of Poly(methyl methacrylate)

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Figure 2. Sample set of new reactions introduced by thermo-mechanical bond breaks in the current set of simulations. In (a) and (b), R1 and R2 are any two leftover groups following scission of one of the side chain bonds on the CH bead. This bond break is followed by hydrogen abstraction by the radical or a double bond formation. Similarly in (c) and (d) C and CH bonds are cleaved, with the radical later abstracting hydrogen from any hydrogen rich species nearby.

Figure 3. Cumulative number of broken bonds tagged as radicals (magenta solid line), and the total number of radicals present in the simulation (orange dashed line) for the Norrish type I pathway shown as a function of time. The total number of radicals, also for Norrish type I pathway, obtained from the previous data set is shown by a black dashed-dot-dot line. The symbols show the results for the heating pathway from the current data set: Cumulative number of broken bonds tagged as radicals ((), total number of radicals present in the sample (1), followed by the distribution of radicals in the leftover substrate (9), and in the plume (2). The jumps in the last two quantities happen when a part of the substrate ejects and is classified as plume. All these simulations were performed with 157 nm laser, 150 ps pulse width and 15 mJ/cm2 fluence.

in the previous set of heating runs no radicals were labeled. There is a minor difference however. For the heating case, the cumulative number of radicals generated via bond-breaks (() is greater not smaller than total number of radicals present at any instant (1), as there is no direct photochemical radical generation in this case. Also shown in Figure 3 is the distribution of radicals in the plume (2) and the substrate (9) for the heating case. Similar distributions are seen for Norrish reaction cases. These distribution show step jumps when a part of the substrate ejects and it is reclassified from substrate to plume. Figure 3 shows the presence of a substantially higher number of radicals in the plume vs the substrate, but the substrate appears to have a higher rate of production of radicals. This difference is noted in the higher slope of the number of radicals in the substrate vs their number in the plume in between the jumps. We now examine the reasons for these trends, focusing first on the

dynamics in the polymer matrix, discussing the ejection mechanism next, and finally looking at the yield and product distribution in the plume. A discussion on how well the bond breaking and radical formation algorithm works is vital to the understanding of the results presented in this article. As stated in the previous section, not all of the beads with overstretched bonds are immediately tagged as radicals. Some of those beads require a large energy change, due to unfavorable local configuration, and can stay in the “un-bonded” state for a long time. The total number of beads with stretched bonds grows analogous to the growth in the total number of radicals in a system. The distribution of these beads is similar to that of radicals as shown in Figure 3, i.e., a majority of the beads with stretched bonds are found in the plume. These beads include regular nonradicals as well as radicals, because any of them can have one or many bond(s) stretched, not including the one already broken in the case of radicals. Thus, the total number of such beads is not exactly equal to twice the total number of stretched bonds, i.e., breaking all stretched bonds will yield slightly less than two radicals per stretched bond as expected. The ratio of the number of broken bonds to the number of beads with broken bonds ranges from 2.0-2.5, but only in the top part of the substrate and during ejection events. Rapid relaxation in both the newly ejected cluster and the newly emerged surface of the substrate is the primary cause of this disparity. This increase occurs almost exclusively due to C and CH2 beads (and their variations formed via bond breaks and chemical reactions), which form the backbone of the polymer chains and are strained during the ejection process. The total number of all types of beads with more than one stretched bond, expressed as the fraction of total number of beads with stretched bonds, reaches a maximum value of ∼12% for the pure heating case (red solid line) and about 3-7% for Norrish reaction cases (green dashed and blue dotted lines), as shown in Figure 4. Also, the total number of radicals with stretched bonds, expressed again as the fraction of total number of beads with stretched bonds, forms a fraction at roughly 15% for the heating case (9), about 25% for Norrish type I ((), and Norrish type II (1) cases from 200-1500 ps and thereafter shows an increasing trend as seen in Figure 4. Furthermore, only about 10% or less of the total number of such beads, i.e., radicals with broken bonds and nonradicals with multiple broken bonds, are found in the substrate. Therefore, the larger fraction of these beads and their increase in Norrish reaction cases is occurring due to

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Figure 4. Fraction of beads with multiple broken bonds, for heating (red solid line), Norrish type I (green dashed line), and Norrish type II (blue dotted line) runs in the current data set. Also shown using symbols is the fraction of radicals with broken bond(s), for heating (9), Norrish I ((), and Norrish II (1), in the same setup. The simulation parameters are same as those given in Figure 3 and the denominator in computing each fraction is the total number of beads with broken bonds including radicals.

dynamics in the plume. This fact may explain why many more bond breaks do not occur in the plume as it appears already saturated with radicals. Overall, both of these fractions, namely the fraction of beads with one or more stretched bonds and the fraction of radicals with stretched bonds, are only a small fraction (∼3-5%) of the total number of beads ejecting. This analysis reveals that for a small number of beads the overstretched bonds are not tagged as broken, irrespective of whether the bead is a radical or not. The average energy change in implementing bond breaks, i.e., tagging two beads linked by a stretched bond as radicals, is only about 0.15 eV per event, comparable to the thermal energy available to the beads. This artificial procedure introduced in our simulation strategy to eliminate unfavorable energy change is related to the potential energy functions used for switching from a bonded state to a radical state, and it adds a minuscule barrier to the reassignment of beads with stretched bonds as radicals. The dynamics of the ablation process is also not affected by this procedure as it involves only a small fraction of beads and most of those are located in the plume, leaving the substrate largely unaffected by the protocol. The thermo-mechanical stress induced bond breaks play a vital role in radical generation in the current set of runs. In Figure 5, we examine a 10 Å thick substrate slice at the depth of 100 Å from the initial surface showing the number of beads with stretched bonds (length >5 Å) and the number of radicals present in that layer. A direct comparison can be made between the earlier set of runs with no thermo-mechanical bond breaks and the current set. In the previous pure heating run, there are a significant number of stretched bonds (Figure 5(a) - red solid line), but no radicals are allowed to be generated in this case. In contrast, the Norrish type I and II simulations (blue dashed and orange dotted lines at the bottom) show a small number of overstretched bonds, whereas the number of radicals shows an initial surge due to their creation via photoscission followed by rapid decline due to fast reactions. With the current set of runs, we see a higher number of beads with overstretched bonds in all three energy absorption pathways even with these overstretched bonds converting into radicals. The beads forming the main chain, i.e., -C- and -CH2-, form the majority (∼90%)

Prasad et al. of the beads, based on bead types, which have stretched bonds and also lead in radical formation. The carbonyl group (CO bead) is the next most targeted site for attrition. The number of radicals in each case (Figure 5b) is also much higher when compared to the previous set of runs, especially during the later stages. Clearly, the net thermo-mechanical damage to the substrate is significantly higher in the current set of runs, with the bond break from the initial set of stretched bonds leading to more subsequent damage in the substrate similar to that discussed in ref 67 (and later in the text). Therefore at longer times, a majority of the radicals present in the system are a product of the bond break process. In addition, a rapid rise in the number of beads with broken bonds and number of radicals is seen immediately preceding the ejection events indicating a role of bond breaks and radical generation in the ejection process. The overall trends in the generation of radicals along all three pathways are similar, like those shown in Figure 3 for heating and Norrish type I cases, with the difference in their profiles seen in Figure 5, panels a and b, appearing only because substrate ejection occurs at different times and involves different amounts of material for each pathway. 3.2. Reactions and Substrate Transformation. A significant difference is also seen in the substrate temperature evolution between the previous and current set of runs. The profiles from the earlier data set show (Figure 6, orange, red, and blue lines) an initial rise in surface temperature during the laser heating phase followed by an exponential decrease due to cooling and evaporation from the matrix.36 However, the current data (green, magenta, and cyan symbols) shows that following the end of the pulse the temperature starts to rise again around 300 ps reaching from 2500 K to about 3000 K over the simulation period. A larger increase in temperature, from 1500 K to about 3000 K, is seen at the depth of 200 Å below the initial substrate surface (not shown), where previously the temperature held steady followed by a slow decline. The same trend is seen in the deeper parts of the sample as well. The likely source for this intermediate term rise in temperature is the thermomechanical bond break and radical creation process. Throughout the top layers of the sample, the initial high temperature results in thermo-mechanical stress and bond breaks, leading to generation of additional radicals as seen in Figure 5b, which triggers more reaction and energy release creating a synergistic feedback loop. Conduction of this extra energy to greater depths also results in higher temperature increases followed by increased probability of bond breaks. The formation of reaction products, e.g., small gas molecules and MMA monomers, which are volatile and/or have higher per unit atomic volume also adds stress and assists in fragmenting the substrate. The higher temperatures and radical concentrations seen in the current set of runs together result in a large number of reactions transforming a significant fraction of the top layers of substrate. This transformation can be seen in Figure 7, where the total sum of all non-hydrogen abstraction reactions is plotted for all three pathways and compared with the previous data set. The hydrogen abstraction reactions are excluded as they are numerous and a significant majority of them involve hydrogen atoms transferring back and forth between radicals (except CH4 and CH3OH formation), which does not result in net energy input or lead to permanent change in the substrate. In the Norrish reaction cases in previous simulations (Figure 7, orange and blue lines), the number of reactions reaches saturation soon after the end of the pulse, which coincides with the end of ablation events, the reduction in the number of radicals via termination reactions and the fall in substrate temperature. In contrast, with



Figure 5. (a) Number of beads with overstretched bonds of length >5 Å, and (b) the number of radicals present in a 10 Å thick substrate layer at the depth of 100 Å from the original surface for heating (9), Norrish I ((), and Norrish II (1). These results are compared with those from the previous set of runs with no thermo-mechanical bond breaks for heating (red solid line), Norrish type I (blue dashed line) and Norrish type II (orange dotted line). Simulation conditions are same as those given in Figure 3. No radicals are generated for the heating case in the previous data set. Missing data at certain times is due to ejection of that part of the substrate.

Figure 6. Substrate temperature in K, for the 10 Å thick substrate layer at a depth of 100 Å from the original surface, for all three photoexcitation pathways in the previous and the current data sets. The simulation conditions and legends are same as those given in Figure 5.

Figure 7. Total number of non hydrogen abstraction reactions occurring in the old and the current data set for heating, Norrish type I, and Norrish type II cases. The simulation parameters and legends are same as those given in Figure 5. The pure heating case from the old data set had zero reactions.

the current data set (green, magenta, and cyan symbols), we see a continuous increase in the number of reactions, with all three pathways showing higher total number of reactions during the laser pulse. We also get approximately the same number of reactions per step after the end of the laser pulse among the three pathways, as indicated by the slope of the curves in Figure 7, which points toward similar mechanism of action governed by the thermo-mechanical bond breaking process. Differences in number for various types of reactions occurring in the matrix and resulting product concentration are also observed. In the earlier set, the types of reaction products clearly identified the photochemical pathway, with Norrish type I identified by formation of gaseous species like CO, CO2, CH3OH, and HCOOCH3, whereas Norrish type II was identified by formation of large amount MMA monomers. In the current set, the thermomechanical bond breaks blur this distinction allowing all reactions to occur in each case and also add additional reaction pathways as shown in Figure 2. The bond breaks also curtail certain reaction channels such as the decarboxylation reaction, because a CO-O bond break prevents the likelihood of CO2 formation. Consequently, the number of reactions generating carbon monoxide is up among all three pathways and CO2

production is down for the Norrish type I case. Higher production of another small gaseous molecule CH3OH is also observed in each case. This relative distribution is contrary to the results from the earlier set,36 where CO2 production dominated among all other small molecules, due to favorable energy of reaction. The creation of double bonded carbon centers via unzipping, radical-radical recombination, and •CH3 radical elimination is also up compared to the previous set. These reactions form the majority of reactions occurring in the substrate. The progression of these individual reactions over time is qualitatively identical to that shown in Figure 7. The increased number of reactions indicates not only greater damage in general but also damage created deeper in the substrate. For example, formation of double bonded carbon centers in the earlier set of Norrish type II runs resulted in an almost complete breakdown of the original polymer in the top layers of the substrate, implying more reactions can only occur deeper in the substrate. The chemical damage or transformation of the original polymer substrate has been macroscopically characterized earlier using two variables:36 fraction of transformed material and fraction of small molecules formed. The first variable counts

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Figure 8. Number of (a) transformed beads including beads with broken bonds and radicals, and (b) beads forming small molecules, both expressed in terms of fraction of the total number of beads present in the 10 Å thick substrate layer at a depth of 100 Å from the original surface, for all three photoexcitation pathways in the previous and the current data sets. The simulation conditions and legends are same as those given in Figure 5. Missing data at certain times is due to ejection of that part of the substrate.

the number of beads, as a fraction of total number of beads in the 10 Å layer, that are altered via bond breaks and chemical reactions taking place in the substrate, and the second one counts only newly created gas molecules. In Figure 8, we show the evolution of these variables in a 10 Å slice of substrate at the depth of 100 Å from the original surface for both sets of runs. With the current set of runs, we see similar total fraction of transformed substrate for all three pathways (green, magenta, and cyan symbols), with the heating run approaching that value over a period of 1 ns. The value is slightly higher than the maximum reached in the previous set for the Norrish type II case (orange dotted line), where almost all of the beads in top layers (i.e., above the 100 Å depth) were transformed. In the previous Norrish type I case (blue dashed line), the maximum conversion was not achieved as it lacked reaction pathways to transform the polymer main chain.36 At the depth of 200 Å, the profiles are similar, but at greater depth, the fraction for the heating case diminishes quickly due to the lack of direct thermal damage by the laser. In the case of small molecules as well, we see all three different pathways in the current set reach the maximum fraction achieved in the previous set of runs. This value, in the old data set, was only attained in the Norrish type I case, where the reactions resulting from side chain scission yielded most of the gaseous species. The net damage, therefore, in each of the current set of runs is unmistakably greater, with every laser excitation channel reaching the maximum observed fraction of total chemical transformation and small molecule generation at certain times, than in all the previous set of runs put together. The decomposition pathway taken by the original polymer substrate can also be characterized in terms of reduction in the length of polymer main and side chain, as a function of depth and time, as shown in the contour plots in Figure 9. The steps in these plots correspond to an ablation event when a part of the substrate is ejected and thereafter labeled as plume. For the older set of runs, the pure heating case shows no reduction in the main and side chain length as the bonds on the polymer backbone can only stretch and separate (which are not counted as broken in our scheme). For the Norrish type I pathway, where the side chain breaks to yield gas molecules via reactions, the side chain length is reduced from 4 beads to 2-3 beads in the top layers of the substrate as shown in contour plot in Figure 9a. The main chain does not show significant destruction in this case (not shown). The Norrish type II pathway on the other

hand, rapidly depolymerizes from the original chain length of 19 MMA units into monomers via the unzipping process, causing reduction in the main chain length (Figure 9b), while the side chain remains largely intact (not shown). In the current set of runs, the polymer rapidly depolymerizes into monomers and polymer fragments for all pathways, with their contour plots for main chain length evolution virtually identical to the Norrish type II case in the old data set; only the damage is limited in depth, and occurs in stage-wise manner for the heating case, as shown in Figure 9d. The side chain destruction occurs rapidly for Norrish type I as seen in the previous runs, and relatively slowly to about 3-3.5 beads for other cases, as shown for the heating case in Figure 9c. The destruction of the chain lengths in a stage-wise manner can also be seen as paving the way for the ejection events leading to destruction deeper in the sample, for all three cases, as shown in the Figure 9c,d. The reduction of polymer main and side chain lengths via generation of the reaction products causes near complete decomposition of the polymer matrix greatly reducing the highly entangled and cohesive nature of the polymer matrix. This process has been shown to cause rapid release of gaseous species and to assist in the ejection of larger clusters via a thermally activated process.36 The increased fragmentation and decomposition of the substrate also sets the stage for much larger ablation yield in the current set of runs. 3.3. Ejection Process and Mechanism. Snapshots of the earliest ejection for each photoexcitation pathway are shown in Figure 10, showing substantial presence of gaseous particles (green beads) and transformed double bonded carbon centers (blue beads) in the top layers and the ejecta in each case. The initiation of laser pulse instantly leads to destruction of polymer chains and creation of radicals for the Norrish reaction cases. For the pure heating case the bond break and radical generation process starts around 50 ps. Temperature rise and swelling of the substrate are also concurrent. These generated radicals react quickly as their survival times are not longer than several picoseconds. The reactions result in nearly complete transformation of the top layers of the substrate as seen in Figure 10. More than 30% of this transformed substrate is made up of volatile small molecules, which eject rapidly upon formation, within 100-170 ps of the runtime, with ejection velocities of small gas molecules in excess of 1000 m/s in all cases. The double bond carbon centers are formed upon rapid decay of the polymer chains into monomers and polymer fragments as seen in Figures



Figure 9. Contour plot as a function of time and substrate depth from original surface, indicating (a) side chain length evolution in old Norrish type I run and (b) main chain length evolution in the old Norrish type II run. Also shown are (c) side chain length and (d) main chain length evolution for the current heating run. The simulation conditions for all runs are same as those given in Figure 5. The evolution of main chain length for Norrish type reactions under current set of runs is indistinguishable to (b) above. Likewise the side chain length evolution, in the current set Norrish type II run is similar to current heating run given in (c) above. The side chain lengths are indicated in number of beads and the main chain lengths in number of MMA units.

Figure 10. Snapshot at 170 ps, showing the first ejection in each case, for (a) heating, (b) Norrish type I, and (c) Norrish type II runs. The simulation conditions correspond to that given in Figure 3. The zero value on the scale (in Å) corresponds to the surface, in each case, identified at that instant in the simulation based on density. The original polymer matrix is shown in light colored and smaller sized beads as seen in (a). The beads in yellow represent radicals and beads with broken bonds, the green beads represent gas molecules, and blue beads represent double bonded carbon centers.

9 and 10. These main chain fragments do not appear as volatile and are ejected in the form of clusters, in a layer-wise manner as reported earlier.36 The ejection of these larger chunks of material is aided by the rapid ejection of gaseous species which erodes the cohesive polymer matrix. These larger chunks move slowly, with average velocity of about 100 m/s, independently of the fast moving gaseous molecules. This first stage of ejection in each case, including the pure thermal pathway, is very similar to the ejection seen in pure photochemical cases in the earlier set of runs and discussed in detail in ref 36. The photochemical nature is also apparent in Figure 11d-f, described below, which

shows the complete transformation of the top layers of the substrate during the initial stages of laser ablation. The first stage of ejection is over by 250 ps for the heating case and about 300-400 ps for the Norrish reaction cases, after which all the initially and predominantly photochemically generated radicals are converted to gases, monomers and ejected. This stage is followed by a long pause of around 500 ps or so during which subsurface events prepare the substrate for next phase of ejection events. During this time frame, the substrate is hot with temperatures ranging from 2500-3000 K. The newly exposed surface, with high temperature and stress, facilitates

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Figure 11. Contour plot as a function of time and substrate depth from original surface, indicating evolution of fraction of (a-c) beads with broken bonds and (d-f) transformed beads in the substrate. The plots are for the current set of heating, Norrish type I and Norrish type II runs from top to bottom in both columns. The simulation conditions correspond to that given in Figure 3.

breaking of stretched bonds and formation of radicals. This increase is already shown in Figure 5. Additionally, contour plots of two variables, namely the fraction of beads with broken bonds and the fraction of substrate transformed, as a function of depth and time, are now shown in Figure 11 for all three photoexcitation pathways. These plots highlight the process of rapid initial and direct laser induced transformation of the polymer matrix, which has been discussed in our earlier publication,36 as well as the thermal and stress generated damage occurring later during the run. In each of these plots, as shown in Figure 11a-c, there is increase in the fraction of radicals and beads with broken bonds going from zero to a value of 0.3-0.35 (blue-cyan to yellow-red in the figure), prior to the ejection events, which can be seen in the subsurface layers. This change is followed by more reactions (Figure 7) among thermally generated radicals and the generation of more transformed species as seen in Figure 8. As the contour plot in Figure 11d-f shows, chemical reactions change the substrate from original blue indicating zero damage to orange and red indicating complete transformation of the matrix. The transformation also extends deep into the substrate, up to 400 Å for Norrish reaction cases within first 300-400 ps, while for the pure heating case it continues to slowly extend deeper with each stage of ejection. The breaking of bonds and radical generation is a continuous process, consequently, a steady stream of gas molecules are produced within the subsurface layers and these particles incessantly attempt to eject outward. The unzipping and other carbon double-bond forming reactions also occur, depending upon availability of the reaction pathway, further fragmenting the substrate. Together these two processes lead to segregation

Figure 12. Contour plot as a function of time and substrate depth from original surface, indicating evolution of total density (g/cm3) of the substrate for the heating case. The simulation parameters correspond to that given in Figure 3. The red region at initial time and deep in the sample correspond to original untransformed polymer sample.

in the top layers of the polymer matrix, with the lower density, highly volatile gas rich layer formed just underneath the surface, trying to eject outward and the higher density, less volatile region made up of monomers and polymer fragments in its way. This partitioning is shown in Figure 12, for the heating pathway, in terms of the density, with the higher density (yellow-red) regions rich in monomers and polymer fragments and the lower density (green-blue) regions rich in gaseous species. Similar partitioning is evident in all three pathways and can be perceived

Laser Excitation of Poly(methyl methacrylate) as the source of all late stage ejections. The segregation and ejection processes can be described to occur as follows: the matrix layers closer to the surface readily allow gases trapped in it to eject and in the course allow the leftover fragmented substrate to rise/swell as well as come together as a denser, less permeable region. This process relieves the compressive stresses on the layers just beneath the surface allowing many stretched bonds to break to form radicals and the radicals to undergo reactions forming gases and other polymer decomposition products. The inability of the gases to escape either above or below this region allows buildup of a gas rich region. The composition of gas molecules, in these regions, at or before the ejection event in each of the late stage runs, reaches up to 20% in terms of the number of the beads, which coupled with lower density of these regions creates complete disentanglement of the parts of the substrate above and below it. It is the combination of the subsurface gas molecules and fragmented polymer chains that guides the ejection of larger chunks. The ejection event itself is driven by atomistic scale fluctuations in these high temperature regions, such as in the local pressure gradients and/or in velocity profiles producing concerted motion of particles with net upward momentum, both of which are seen in the ablation profiles.36 The data for the current set of simulations, as shown in Figure 5-11, illustrates presence of both kinds of damages identified previously to lead to ablation36 - thermo-mechanically generated damage, in terms of fraction of broken bonds and radicals formed, as well as chemical damage, in terms of fraction of gas molecules and transformed species formed. For the current set of runs, either of these coupled with high surface temperatures can lead to ejection of large amounts of material. As quantitatively shown in Figure 11a-c, more than 20% of the beads have broken bonds in the top surface layers during the initial stages, i.e., the critical number of bond breaks required for ejection is reached. But, the surface layer of the polymer matrix, in each case, is also nearly 100% disintegrated. The ejection in these cases therefore is determined to be based on chemical destruction, due to the role played by gaseous species in hollowing out the polymer matrix and causing the ejection of chunks of polymer substrate. This mechanism is similar to that for pure photochemical cases in the previous set of runs,36 where ejection was described using a two-step model by Kalontarov and Marupov.56 Similarly, the second ejection in the heating case, all of the ejections for the Norrish type I case and the Norrish type II case, are judged to originate largely via a chemical disintegration pathway as seen in Figure 11e,f. Here, the segregation process plays a major role in disengaging the fully disintegrated top layers of the substrate from the bulk underneath. The ejection mechanism in this case is also thermal as it is the high temperature that causes parts of the substrate to move upward, like a piston, driven by the gaseous region present under it. For the later stage ejections in the heating case, the fraction of transformed substrate reaches only 0.60, whereas the fraction of beads with broken bonds reaches 0.30, indicating the dominant role for the thermo-mechanical bond break process to cause ejection. As previously noted in ref 36 and in section 3.1, a large fraction of these bond breaks are the main chain bond breaks, as also seen in Figure 9d, which serves as the source of disentanglement of the polymer matrix, reduction of long-range cohesive interaction energy, and aids ablation process. The diminished presence of gaseous species also emphasizes a direct thermally activated ejection process, which has been seen in the earlier set of pure thermal simulations.36 Based on this evidence it can be stated that a clear identification

J. Phys. Chem. C, Vol. 113, No. 27, 2009 11501 of the process leading to ablation, as either photochemical or photothermal means of substrate damage, cannot be made. For the current set of runs, both of these processes exist synergistically and contribute to the destruction of polymer substrate and aid in ablation process over a much longer time scale (t . τ). The extended ablation time scale can explain similar results obtained in experiments especially with the reactive substrates.87 A key factor responsible for ejection is the continued presence of high temperature in the substrate. A minimum temperature of 2600-2800 K for the pure heating case, 2100-2300 K for Norrish type I case, and 2400-2600 K for Norrish type II case, is observed in the top layers of the substrate throughout the simulation and leads to ejection in each case. The differences in these temperature values are most likely due to the way in which initial laser energy was utilized, how it resulted in heating the substrate, and then propagating that heat, thermo-mechanical damage and the resulting chemical reactions deep into the substrate. It is worth noting that the temperatures quoted here are not the absolute minimum value necessary for ablation, but with reduction in substrate temperatures, it will take exponentially longer, tens of nanoseconds, microseconds, to milliseconds or more, to cause ejection, timescales which are beyond the scope of MD simulations. It is highly likely that below a certain temperature, lower than the 2100 K value given above, the thermal process might cease to engineer ejection. This temperature is expected to be the same for all three pathways as it is the thermal process sustaining ejections long after the end of the pulse. It is also important to note here that the temperature values quoted should not be directly compared with the values observed in real ablation experiments, as our simulations make use of coarse-grained molecular dynamics. This model lacks appropriate electronic and atomic degrees of freedom and is consequently devoid of accurate physical description of polymer’s heat capacity and conductivity. As discussed earlier, the high temperatures lead to thermomechanical stresses which couples with additional thermochemical stresses generated by the presence of large number of small molecules.37 The resulting background pressure from these stresses is in the range of 10-500 MPa similar to that seen in the earlier set of simulations.36 Together these conditions, coupled with the bond break physics added in the current set of simulations, create an environment for continued destruction of the polymer chains and their ejection. Another key variable, pressure gradients and the resulting pressure waves, does not play a role in these simulations as τ ) 150 ps pulse width is used here, outside of the stress confinement regime, leading to the absence of pressure gradients. Even for the shorter τ ) 5 ps pulse width, where pressure gradients were found important,36 no additional physics is observed in the current set of simulations. The stress confinement time scale is short (50 ps); thus, the added bond break and radical generation physics do not contribute to buildup of pressure gradients and pressure waves. 3.4. Effect of Longer Wavelength (Lower Energy) Photons. The use of lower energy photons, equivalent to λ ) 248 nm, using the same laser pulse width of τ ) 150 ps, results in effectively more photons incident upon the substrate at any given fluence. In the previous set of runs, this resulted in a relatively higher ablation yield for Norrish reaction cases (with all other simulation parameters held constant) due to a higher number of reactions and chemical transformation deeper in the substrate,48 but no ablation was achieved for the heating case as

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Figure 13. Yield, in number of MMA units, as a function of time when thermo-mechanical bond breaks are introduced for each of the dominant energy absorption pathways, using (a) λ ) 157 nm laser: heating (9), Norrish type I (2), and Norrish type II (1), and using (b) λ ) 248 nm laser: heating (solid right-facing triangle), Norrish type I ((), and Norrish type II (b). The other simulation parameters are τ ) 150 ps and a fluence of 15 mJ/cm2.

the critical energy threshold was not reached.47,49 In the current set of simulations, higher relative yields are seen as before (discussed in the next section), with heating cases resulting in ablation as well. With the initiation of the τ ) 150 ps laser pulse, the key difference with the λ ) 157 nm simulations is that the top layers of the substrate in these longer wavelength runs stay relatively cooler. The maximum temperature is in the range of 3200-3500 K for λ ) 248 nm vs 4000-4200 K for λ ) 157 nm (both at 15mJ/cm2 fluence). This temperature difference is due to the lower energy per photon and higher number of the photons, which, due to the constraints of single photon absorption only, causes photons to reach deeper in to the substrate. The lower temperature does not affect the initial (photochemical) radical formation and reactions in both Norrish types. After the first stage of ejection, which carries away the hottest chunk of the substrate, the temperature differences between the two cases are around (100-200 K, with the 248 nm case sometimes higher. Similarly at larger depths of 100-300 Å, the 248 nm cases might show higher temperature. Both of these results again are due to greater number of photons ending up deeper for lower wavelength case causing more reactions. Lower temperatures imply reduced thermo-mechanical stress to overstretch bonds, i.e., slower rate of creation of radicals. Consequently the concentration of stretched bonds and radicals are relatively lower. This step is the rate limiting process for further reactions, transformation and ejection. The evolution of the total number of reactions (in Figure 7) is higher at first for Norrish reaction cases and then settles down to a similar number of reactions per step, after the end of the first stage of ejection, owing to similar substrate temperature profiles later in the runs. For the thermal case, the profile of the number of reactions is similarly slow to catch up to that of the higher energy photons. The unzipping process appears to be unaffected, with a similar or higher number of polymer chains breaking to form double bonded carbons. The rate of gas formation is likewise unaffected, constrained only by the requirement of the presence of the source radicals (e.g., •CH3 for CH4 formation). The first stage of ejection, for the Norrish reaction cases, is similar to the pure photochemical case which has been discussed in detail in ref 48. For the pure heating case, which did not ablate in the previous set of simulations, the first stage of ejection occurs at around 500 ps, substantially later than in the Norrish reactions cases, and is thermo-chemical in nature. During most of this stage, the fraction of beads with broken bonds remains small (

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