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Initial Reaction Probability and Dynamics of Ozone Collisions with a Vinyl-Terminated Self-Assembled Monolayer Jessica W. Lu, Larry R. Fiegland,† Erin Durke Davis,‡ William A. Alexander,§ Alec Wagner, Richard D. Gandour, and John R. Morris* Department of Chemistry, Virginia Tech, Blacksburg, Virginia 24061, United States ABSTRACT: The gassurface reaction dynamics of ozone with a model unsaturated organic surface have been explored through a series of molecular beam scattering experiments. Well-characterized organic surfaces were reproducibly created by adsorption of CdC-terminated long-chain alkanethiols onto gold, while the incident molecular beams were created by supersonic expansion of ozone seeded in an inert carrier gas to afford control over collision energy. Time-of-flight distributions for the scattered molecules showed near complete thermal accommodation of ozone for incident energies as high as 70 kJ/mol. Reflectionabsorption infrared spectroscopy, performed in situ with ozone exposure, revealed that oxidation of the double bond depends significantly on the translational energy of O3. For energies near room temperature, 5 kJ/mol, the initial reaction probability (γ0) for the formation of the primary ozonide was determined to be γ0 = 1.1 105. As translational energy increased to 20 kJ/mol, the reaction probability decreased. This behavior, along with a strong inverse relationship between γ0 and surface temperature, demonstrates that the room-temperature reaction follows the LangmuirHinshelwood mechanism, requiring accommodation prior to reaction under nearly all atmospherically relevant conditions. However, measurements show that the dynamics transition to a direct reaction (analogous to the EleyRideal mechanism) for elevated translational energies.
I. INTRODUCTION Interfacial reactions of O3 with organic particulates and surfactant-covered aerosols affect the balance of pollutants throughout the troposphere.15 The initially hydrophobic surfaces of many airborne particles can be rendered hydrophilic by oxidation reactions that convert vinyl functional groups into oxygen-containing groups, such as carboxylic acids.6,7 Such oxidative processing of organic surfaces influences the transport properties of particles along with the concentrations of ozone and other gases that impact atmospheric chemistry.8,9 In addition, reactions between oxidizing pollutants and unsaturated organic compounds facilitate the formation of secondary organic aerosols that affect the balance between incoming and outgoing radiation levels across the planet.1 Developing a comprehensive understanding of atmospheric chemistry requires elucidating the mechanisms and rates of ozone oxidation reactions with organic surfaces. The reaction of gas-phase O3 with organic surfaces found in the environment generally proceeds through the oxidation of unsaturated sites within the material.10,11 Ozone adds across double bonds to form a highly unstable cyclic intermediate, the primary ozonide, which readily decomposes to products that include a diradical (Criegee intermediate) and an aldehyde.12,13 Several recent experimental investigations have used selfassembled monolayers (SAMs) as well-characterized model substrates in studies of ozone reactions on organic surfaces.1012,14 In particular, vinyl-functionalized SAMs have been used to track the formation of surface acid and acid-anhydride species upon r 2011 American Chemical Society
exposure to O3.14 Similar model systems revealed that the reaction probability between O3 and surface-bound alkene groups is enhanced relative to gas-phase analogues due to surface trapping,10,12 a suggested prerequisite for interfacial oxidation by O3. Further experimental studies into the reactions of ozone with vinyl-terminated trichlorosilane SAMs have characterized reactive uptake rates as well as the gas-phase and surface-bound products.15,16 Complementary molecular dynamics (MD) simulations17 have reported a surface residence time for ozone on the vinyl-terminated SAM of picoseconds. According to the MD studies, the reaction probability of O3 on the surface is an order of magnitude higher than for gas-phase reactions. Beyond aliphatic systems, experiments with O3 and cycloalkenes tethered to a metal oxide surface18 reveal reaction probabilities remarkably close to numerous measurements of ozone reactions with various organic substrates. Analogous studies employing thin films of polycyclic aromatic hydrocarbons have also been used to determine reaction probabilities and mechanisms.19,20 These experiments highlight the interplay among surface accommodation, diffusion, reaction, and desorption. Despite the many advances in the overall understanding of ozone chemistry, scientists are only beginning to understand the full mechanistic details of ozone reactions in collisions on organic surfaces, and many aspects of the reactions remain unexplored. Received: August 18, 2011 Revised: November 1, 2011 Published: November 01, 2011 25343
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The Journal of Physical Chemistry C As an ozone molecule impinges on an organic surface, it can impulsively scatter from the interface or be deflected enough times along the surface to dissipate excess energy and become trapped.21,22 The accommodated species might simply desorb back into the gas phase or diffuse to react at unsaturated sites on the surface or within the bulk of the material.23,24 The requirement for accommodation prior to reaction, as described by the LangmuirHinshelwood (LH) mechanism, appears to apply to many types of gassurface interactions on organic and metallic surfaces.19,25 In the LH mechanism, an impinging species quickly loses memory of its incident trajectory and dissipates its initial energy such that reaction rates are controlled by the surface temperature for any specific system.26 Alternatively, the Eley Rideal (ER) mechanism describes a process that involves direct product formation and scattering in a single impulsive collision.26 Scattered products arising from ER processes may retain angular or energetic information from the incident gas-phase molecule.27 In addition, the incident translational28 and vibrational energies29,30 are often coupled to the reaction coordinate and are critical to overcoming barriers to product formation. The ER mechanism occurs for many species incident on metallic surfaces;31,32 however, there are no demonstrations, to our knowledge, of an ER, or even a direct insertion-type, reaction during collisions of a closed-shell molecule on an organic surface. Researchers speculate that the geometric constraints and tight transition states for organic reactions require full accommodation, reorientation, and diffusion to occur en route to product formation, implying that interfacial reactions on organic surfaces are generally governed by LH dynamics.12 Additionally, recent studies into how surface temperature affects the rate of ozone oxidation of organic surfaces suggests that this reaction may, in fact, follow the LH mechanism.19 Numerous laboratory studies of heterogeneous reactions between gas-phase ozone and organic surfaces suggest that many systems follow the LangmuirHinshelwood mechanism. The kinetics and major products of the ozonolysis of polycyclic aromatic hydrocarbons (e.g., naphthalene, anthracene, benzo[a]pyrene) and soot have been examined extensively in previous work.19,3337 In particular, these studies probed the rate dependence on different reaction parameters, such as ozone concentration, ambient relative humidity, and surface temperature, and propose that the reaction between thermal O3 and an organic surface most likely proceeds through the LH mechanism. Likewise, ozone reactions with thin films of complex organic molecules, cypermethrin and chlorophyll, and laboratory-generated aerosols with significant organic fractions appear to occur via the surface-mediated reaction mechanism.3843 These previous experiments indirectly show that under atmospherically relevant conditions, when both gas and surface molecules possess thermal energies, ozone molecules react with organic surfaces through the LH mechanism. The studies described below are among the first molecular beam probes of ozone oxidation reactions, where the translational energy of the incident O3 is controlled, the position of the double bond is placed precisely at the gassurface interface, and the reactions are not complicated by impinging background gases or other ambient contaminants. The data provide a direct measure of the reaction probability for O3 on a vinyl-terminated SAM as a function of incident translational energy. These results reveal that the oxidation reaction is governed by an LH mechanism at thermal collision energies; however, the dynamics transition to a direct mechanism, one that is initiated upon the
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Figure 1. Schematic of the UHV molecular beam chamber for surface analysis of ozone reactions with vinyl-terminated SAMs.
initial collision (analogous to the ER mechanism), for ozone molecules with greater than 20 kJ/mol of translational energy.
II. EXPERIMENTAL SECTION Below, we describe a series of ultrahigh vacuum (UHV) molecular beam scattering experiments designed to provide direct insight into the initial reaction probabilities, energy transfer dynamics, and mechanisms in collisions of ozone molecules on unsaturated organic surfaces. The instrumental setup (Figure 1) provides the ability to characterize the mass and final translational energy distributions of gas-phase reaction products while, simultaneously, tracking the breaking and formation of bonds on a surface. In this work, well-characterized model organic surfaces are provided by vinyl-terminated SAMs of alkanethiols on gold and the UHV environment eliminates background gases from altering the surface during a particular study. Molecular beams of O3 provide a source of reactant molecules with well-characterized collision energy and flux, both critical parameters for studying interfacial reaction mechanisms and probabilities.44 The translational energy and mass of the scattered products are monitored with pulsed beam time-offlight (TOF) techniques,45 while the surface-bound species are tracked in situ with reflectionabsorption infrared spectroscopy (RAIRS). In addition to RAIRS, an X-ray photoelectron spectrometer (XPS) facilitates elemental analysis to identify surface-bound products. A. Preparation of Vinyl-Terminated Self-Assembled Monolayers. The 17-octadecene-1-thiols and 14-pentadecene-1-thiols
used in this work were synthesized according to established procedures.62 The molecules were verified to be >99% pure by using 1 H NMR. The SAMs were created on vapor-deposited gold on silica. The gold slides were purchased from EMF Corp. A thin layer (∼50 Å) of chromium is coated onto the glass to strongly bind a 1000 Å thick layer of gold to the substrate. Prior to experimental use, the gold slides were cleaned in piranha solution (70/30 (v/v) mixture of H2SO4/H2O2) for 1 h to remove trace organics. Caution: piranha solution is extremely explosive in the presence of organics. Following generous rinsing with deionized water (Nanopure Millipore, 18 MΩ), the slides were dried under a stream of ultrahigh purity (UHP) N2 and immersed in freshly prepared 1 mM solutions of alkanethiols in HPLC grade hexanes. The SAMs were placed in solution for 2448 h to achieve a well-ordered, tightly packed monolayer. Upon removal, the SAMs were 25344
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The Journal of Physical Chemistry C rinsed with hexanes and dried under a stream of UHP N2 before being placed in the UHV chamber through the load lock system. B. Ozone Molecular Beam Generation and Scattering. Following alignment and IR characterization of the SAM, the gassurface reaction dynamics were studied by performing a series of molecular beam scattering experiments. Ozone, unstable at room temperature, required careful preparation and storage prior to each scattering experiment. The ozone storage, purification, and gas seeding system46 involved initially flowing O2 (UHP, 99.993%) through a commercial ozone generator. The effluent from the generator was passed through a bed of silica particles held at 78 C where O3 was trapped. Prior to each experiment, the condensed O3 was purified by heating to 55 C under vacuum to remove trapped O2. Subsequent to purification, the molecular beams were created by flowing an inert carrier gas over the condensed O3. The temperature of the silica particles and the backing pressure of the carrier gas were carefully controlled to regulate the seeding ratio within the gas source. The purity of O3 in the source was periodically verified via an inline UVvis spectrometer by using the absorption transition at 254 nm and the partial pressure of the carrier gas. By employing pure O3 seeded in Ar, N2, Ne, and He carrier gases, we created stable, >99% pure, O3 beams with peak translational energies of 5 (pure O3), 10, 15, 20, 70, and 85 kJ/mol (fwhm = 2.5 kJ/mol). Creating the ozone beams in a triply differentially pumped source chamber enabled the flux of molecules to be varied over a wide range (10131015 cm2 s1), while maintaining a low mainchamber pressure ( 10 carbon atoms,48 (ii) the scattering dynamics over the range of impact energies used in this work are insensitive to the underlying gold substrate for long-chain SAMs,49 and (iii) oxidation of the AuS bond during molecular beam exposure to O3 does not occur (on the time scale of our measurements) for long-chain vinyl-terminated SAMs. RAIR spectra of unexposed n = 16 and n = 13 H2CdCHSAMs are shown in Figure 2. The peak position of the methylene asymmetric stretch at 2919 cm1 is similar to that of a crystalline hydrocarbon and indicative of a well-ordered monolayer. The narrow peak widths indicate that the SAMs were uniform across the 1 cm2 surface.50 In addition, the spectral characteristics are consistent with many previous studies51 that indicate long-chain alkanethiol molecules adsorbed to gold surfaces occupy a surface area of 21.4 Å2, which accounts for a surface density of 4.7 1014 molecules/cm2. The absorbance at 3085 cm1 is attributed to the asymmetric CH2 stretching mode of the terminal group.52 Three additional peaks at 1644, 994, and 911 cm1 are due to the CdC stretch, CdC out-of-plane deformation, and the CH out-of-plane deformation of the =CH2 group, respectively.52 Because the surface selection rule for RAIRS dictates that only modes with a transition dipole moment component oriented perpendicular to the plane of the surface gives rise to an 25345
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Figure 3. Time-of-flight distribution for 85 kJ/mol ozone scattering from vinyl-terminated SAMs with n = 13 number of methylene carbons. The corresponding P(Ef) distribution for data shown in panel A is provided in panel B. The solid curve underneath the distribution represents a Maxwellian distribution at the temperature of the surface; scaled to fit the experimental data.
absorbance,47 analysis of relative peak intensities for well-organized SAMs provides insight into the orientation of the surface functional groups. Interestingly, the intensity of the CdC stretch at 1644 cm1 is insensitive to whether there is an even or an odd number of carbon atoms along the backbone of the alkane chains, but chain length affects the intensity of the CH2 stretching mode at 3085 cm1 for the terminal carbon. These observations suggest that the orientation of the double bond, relative to the surface normal, does not depend on methylene chain length, which may be due to a slight azimuthal rotation along the backbone of the chain.48 Therefore, one does not anticipate (and we do not, in fact, observe) an evenodd chain length dependence to the overall reactive scattering dynamics. B. Dynamics of O3 Scattering from H2CdCHSAMs. The time-of-flight (TOF) data for a pulsed molecular beam of 85 kJ/mol ozone scattering from the n = 13 H2CdCHSAM are shown in Figure 3. The data are a plot of the detector signal at m/e = 48 versus the flight time for molecules to traverse the distance between the surface and the ionizer of the mass spectrometer. The raw signal is proportional to number density N(t) and is used to compute the probability P(Ef) that an O3 molecule leaves the surface with final energy Ef. The translational energy distributions are computed from the relations Ef = (1/2)m(L/t)2 and P(Ef) ∼ t2N(t), where m is the mass of O3, t is the O3 flight time, and L is the flight length. The translational energy distribution, P(Ef), for ozone scattering from the SAM is presented in Figure 3B. In molecular beam scattering, the P(Ef) distributions are typically separated into a direct inelastic scattering (IS) component and a thermal component by assigning the latter to the data that falls within a Boltzmann distribution: PTD(Ef) = Ef(RTs)2 exp(Ef/RTs). The solid curves in Figure 3A,B show that the majority of the data are fit well by a Boltzmann distribution. The strong overlap of the Boltzmann curve to the lowest energy portion of the scattering data demonstrates that energy transfer is extensive, leading to broad, near-thermal energy distributions for the scattered parent molecules, similar to that for argon and carbon dioxide scattering from long-chain CH3SAMs.53 This is not unexpected, as it has been shown previously that gases with similar masses exhibit similar scattering behavior.21,54,55 Because complete thermal (or mass) accommodation and extensive energy transfer are indistinguishable in these TOF measurements, the entire portion of the final energy distribution that can be modeled by a Boltzmann distribution is typically referred to as the thermal desorption (TD) component.56 The direct
inelastic component to the energy distributions are assigned to the difference between P(Ef) and PTD(Ef). The TD fraction, defined as the weighting coefficient, β, in the relation P(Ef) = βPTD(Ef) + (1 β)PIS(Ef),21 is very large for this system. That is, our results show that the gassurface collision is dominated by nearly complete energy transfer and the majority of the scattered molecules leave the surface with translational energies very close to that of the surface, ∼5 kJ/mol at 298 K. We measure a β fraction of 0.90 ( 0.01 and a fractional energy transfer to the surfaces in the IS channel, (Ei ÆEISæ)/Ei, of 0.88 ( 0.01 for the 85 kJ/mol ozone scattering from the H2Cd CHSAM (the error bars denote one standard deviation). Results for the lower energy beams are very similar, highlighting the efficient energy transfer and thermal accommodation processes in ozone collisions on the organic surface. The extensive energy exchange and thermal accommodation observed for ozone collisions with the H2CdCHSAM result from very efficient coupling of the translational energy of the impinging molecule to isolated and concerted motions of the methylene chains within the monolayer. The SAM provides several degrees of freedom into which the translational energy of the impinging molecules can be partitioned. The bending motions of the chains, vibrational modes along the chains, and torsion of the terminal groups can all be excited by the O3SAM collision.57 Extensive gassurface energy transfer and thermal accommodation have also been observed for high-energy Ar and Kr scattering from long-chain CH3 and H2CdCHSAMs58 as well as CO2, NO2, and O3 scattering from CH3 and OH SAMs.59 However, translational energy dissipation is further facilitated for molecular collisions because the ozone molecule can partition energy into rotational and vibrational modes as it collides with the surface. Previous molecular beam studies of carbon dioxide scattering show that the internal energy modes can play a significant role in determining the extent of energy transfer during collisions on organic surfaces.60 The nearly complete thermal accommodation efficiency, even for the highest energy ozone beam, suggests that the gassurface reaction rates for O3 on organic surfaces are not limited by inefficient energy dissipation during the collision. However, the TOF studies do not provide direct insight into the mass accommodation efficiency or the reaction probability for O3 on the surface. That is, we find that surface-bound or gas-phase product formation occurs at a rate that is far below the limits of detection during a typical pulsed molecular beam scattering experiment. As described below, fewer than 1% of the surface 25346
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Figure 4. (A) Difference spectra of n = 16 H2CdCHSAMs exposed to O3. The spectrum shown on top in blue is that of the SAM prior to ozone exposure using a clean Au sample as the background. Upon ozone exposure, the spectra in black show that modes associated with the CdC moiety decrease, particularly the peaks at 1644 and 911 cm1. (B) Reaction mechanism showing the formation of the primary ozonide, which decomposes to an aldehyde and Criegee diradical intermediate. (C) Plot of the intensity of the CdC mode at 1644 cm1 as a function of ozone exposure. The solid curve represents the best fit to the Langmurian model, described in the text for an initial reaction probability of 1.1 105. The dashed curves provide an indication of model sensitivity by showing the expected data response for initial reaction probabilities of 1.4 105 and 9.8 106.
functional groups react with O3 during the time required to record the TOF data (Figure 3). The majority of molecules simply desorb before a reaction can occur. Therefore, the reaction rates were studied by removing the chopper wheel from the molecular beam path to increase the ozone flux. With a continuous beam of O3 impinging on the surface, RAIRS was used as a realtime probe of product formation. C. Reaction Probability of O3 with the Surface Vinyl Groups. Product Analysis. Initial insight into the reaction of O3 with the surface-bound vinyl groups is provided in Figure 4A, which shows the RAIR difference spectra for the n = 16 H2CdCHSAM during exposure to a continuous beam of low-energy, 5 kJ/mol, O3. In this experiment, the original SAM was used as the background, such that negative features in the spectra indicate the removal of modes from the surface and positive bands reveal peak broadening, an increase in the absorptivity, or the development of new modes. The gassurface reaction has three primary effects on the H2CdCHSAM: (1) all modes associated with the double bond decrease in intensity, while (2) the peaks associated with the methylene chain broaden and shift to higher wavenumber (as reflected by the peaks in the difference spectra around 2925 cm1), and (3) new weak modes appear at 1740 and 950 cm1. Control experiments with an 18C CH3SAM (the same overall chain length as the n = 16 H2CdCHSAM) show no changes in the spectra during this exposure regime, indicating that the changes in the IR spectra (Figure 4A) are due to reactions at the double bond. These observations are consistent with oxidation of the double bond to form a surface-bound carbonyl, as previously reported in a series
of high-vacuum and ambient-exposure studies of unsaturated hydrocarbons to ozone.12,14 The well-known first step in ozone reactions with a vinyl group is addition across the double bond to form a primary ozonide (Figure 4B). The highly unstable primary ozonide quickly decomposes to an aldehyde and the Criegee diradical intermediate. The formaldehyde product may either react with the diradical to form the secondary ozonide or desorb from the surface. If formed, the secondary ozonide is expected to give rise to IR absorption modes at approximately 1100 and 1385 cm1.12 The broad peak in the difference spectra of Figure 4A at 1000 cm1 may be due to the emergence of a new CO mode; however, the absence of any strong signal in the 1385 cm1 region suggests that this new mode is not due to the secondary ozonide. Rather, we speculate that the majority of the formaldehyde product desorbs into the vacuum before formation of the secondary ozonide. Following formaldehyde desorption, the Criegee intermediates likely rearrange to form carboxylic acid groups and react with adjacent chains, as observed previously.14 The structural changes to the monolayer during oxidation lead to disordering of the methylene groups along the backbone of the SAM. The disordering produces a 10 cm1 blue shift in the CH2 stretching frequencies and broadens these peaks relative to the original SAM. The changes in the peak profile are reflected in the difference spectra by an increase in absorbance at 2930 cm1. However, the changes to the methylene modes are small compared to their original intensity, and the overall organization of the chains within the SAM changes little over the time scale of the reactions. These observations, coupled with XPS data that 25347
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show minimal AuS oxidation during ozone exposure, suggest that the reactions are isolated to oxidation of the terminal vinyl group. Reaction Probability. Figure 4C shows a plot of the intensity of the CdC mode at 1644 cm1 as a function of ozone exposure. The data fit well to a Langmuirian model for the rate of vinyl group elimination during exposure:61 AðtÞ ¼ A0 exp½vnγ0 =4L
ð1Þ
where A(t) is the absorbance of the CdC mode at time t, A0 is the initial intensity of the CdC mode, v is the velocity of the impinging gas, n is the ozone number density, L is the area of the sample, and γ0 is the probability for reaction at the double bond. The one-parameter fit to the data provides an initial reaction probability of γ0 = (1.1 ( 0.3) 105 for the low-energy ozone beam. The initial reaction probability we have measured for the lowest-energy ozone beam impinging on the CH2dCHSAM is bracketed by previously reported values.12,13,18,19 Most notably, investigations of ozone reactions with vinyl-terminated SAMs under ambient temperature and pressure12 reported an initial reaction probability of 7 106. Similarly, ozone reactions with octenyltrichlorosilane-functionalized SiO2 particles19 show an uptake coefficient of 7 105. The general agreement of the UHV scattering experiments and the previous work performed under ambient pressure conditions12,13,18,19 suggest the intriguing conclusion that background gases play only a minor role in the overall chemistry. Higher reaction probabilities are reported15 in experiments that employ a cylindrical flow reactor coupled to a chemical ionization mass spectrometer. Specifically, vinylterminated trichlorosilane SAMs13 exhibit a reactive uptake coefficient of 1.7 104 when exposed to O3. Furthermore, ozone reactions with frozen 1-hexadecene18,19 and solid 1-pentene18,19 yielded uptake coefficients of approximately 1 104 and 1 103. The higher reaction probabilities in cylindrical flow tube reactors are likely due to increased surface roughness for these samples as compared to the planar SAMs. In addition to providing a precise measurement of the initial reaction probability on a well-defined model surface, the observation of surface oxidation by O3 in UHV is an intriguing result, given the extremely short residence time for O3 on the surface. That is, the average residence time for O3 can be estimated from transition state theory, assuming an activation energy and a typical pre-exponential factor for desorption. Energy minimization calculations63 for the O3alkene van der Waals complex in the gas phase predict an association energy of 7.8 kJ/mol, which leads to a characteristic residence time of only 2 ps for a typical pre-exponential factor of 1013 s. MD simulations17 predict similar residence times for O3 on organic SAMs. The ozone molecules that react must either diffuse into the correct orientation during this short time or react upon the initial collision. Furthermore, because the reaction barrier for the formation of the primary ozonide may be as high as 20 kJ/mol,64 the low-energy ozone molecules can only react if they gain energy from the surface over this short duration. Therefore, these studies reveal a strong competition among energy transfer, thermal accommodation, desorption, and reaction. We have investigated this competition further by using molecular beams to control the incident gas energy and the surface temperature to regulate the thermal energy available from the substrate.
Figure 5. Initial reaction probability for O3 impinging on an n = 13 H2CdCHSAM as a function of collision energy and surface temperature. Data points are shown with error bars representing one standard deviation. The two left-most open circles are the upper limit for γ0 for low-energy O3 scattering (65 kJ/mol) from the colder SAMs are very similar to γ0 for room-temperature surfaces.
D. Translational Energy and Surface Temperature Dependence of γ0. Figure 5 shows the energy dependence to the initial
reaction probability for molecular beam scattering of O3 from the CH2dCHSAM with the surface held at 298 K. As described above, we find identical results for scattering from the n = 16 and n = 13 SAMs, as predicted by the insensitivity of the orientation of the double bond to the chain length in this system. The low reaction probability (105), observed for the lowest energy beam, decreases when the incident energy increases to 20 kJ/mol. This result strongly suggests that the reaction pathway, at incident energies near thermal temperatures, follows the LH mechanism, where gassurface accommodation is required prior to reaction. The overall reaction probability decreases with incident energy because the extent of mass accommodation, along with the residence time, decreases as the collision energy increases. Although there is much more overall energy available for the reaction, the more energetic gas-phase molecules recoil from the surface before a reaction can occur. The results observed for low-energy molecular beam scattering suggest that the reaction of O3 with the terminal vinyl groups is mediated by the formation of a precursor state.29 For a reaction to occur, the O3---CH2dCHprecursor complex must form prior to desorption. In addition, the complex must surmount the energy barrier for reaction, which may be as high as 20 kJ/mol.64 For the low-energy molecular beams, there is simply insufficient energy in the incident gas source to overcome this barrier; therefore, the reaction energy originates from the thermal energy of the surface. Molecules that do not obtain sufficient energy from the surface, on the time scale of the collision, simply desorb before a reaction can occur. Because residence time and accommodation efficiency decrease with incidence energy, the overall reaction probability decreases as incident energy increases. Further evidence for the precursor-mediated reaction mechanism also emerges from the surface temperature dependence to the reaction probability. We have repeated the reaction probability measurements, shown in Figure 5, for surface temperatures of 250 K to find that, for incident energies below 20 kJ/mol, surface temperature significantly affects the reaction probability. Specifically, for the lowest-energy beams, the reaction probability for Ts = 250 K is below the detection limits of our experiment, 25348
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The Journal of Physical Chemistry C ∼3 106. Because, on the cold surface, there is insufficient energy in the incident beam or in the surface to activate the reaction, the reaction does not appear to occur on the time scale of our measurements. Although γ0 is immeasurably low for the ozonevinyl reaction on the 250 K surface, γ0 is dramatically greater for the higher energy beams at all surface temperatures. The reaction probability for 70 and 85 kJ/mol ozone is ∼2 orders of magnitude larger than that for the low-energy beams on the cold surface. The observation of increasing reaction probability with incident energy is directly opposed to the results expected for the LH mechanism operates near thermal beam energies. Despite the significantly lower accommodation rates that accompany high beam energies, the reaction probability is dramatically greater for the highest energy incident gases. This trend demonstrates that, at elevated translational energies, the reaction energy originates from the incident gas rather than from the surface. Further evidence for these dynamics comes from the observation that, in contrast to the low-energy beams, γ0 is independent of surface temperature for the high-energy beams. For the 70 and 85 kJ/mol beams, the data points for γ0 fall at approximately the same values as for the room temperature sample, 9.0 105. Therefore, we find that oxidation of the surface double bond can be activated by translational energy and that this collision energy is not dissipated into the surface prior to reaction. For these beams, the reaction must occur prior to thermal accommodation, implying that the dynamics follow a direct reaction mechanism at high energies. The trend reported in Figure 5 for the dependence of initial reaction probability on incident energy is consistent with observations of precursor-mediated reactions that were previously studied on metallic surfaces. However, there are few demonstrations of a translational energy-activated reaction on an organic surface.65 Although experimental challenges preclude accurate measurements of γ0 for intermediate incident collision energies, in the range of 2565 kJ/mol, future experimental and theoretical work over this range will help reveal the energy at which the dynamics transition from the LH mechanism to the direct reaction. This transition energy will provide direct insight into the reaction barrier.
IV. CONCLUSION Our exploration of the ozone translational energy and surfacetemperature dependence of the oxidation of surface vinyl groups by O3 reveal insight into three key aspects of this reaction. First, gassurface energy transfer is extensive for collisions of O3 on the model organic surface. The efficient energy exchange results from many low-energy degrees of freedom within the surface and the molecule that provide effective sinks for the energy of the incident particle. Second, the probability for reaction is low, ∼105, for near-thermal incident energies on room-temperature surfaces. The reaction is inefficient because the energy for reaching the transition state is above the available energy in the incident beam. Therefore, the molecule must gain energy from the surface and reach the correct transition state geometry for a reaction to occur before desorbing, which is a rare event for the short residence times of these gassurface interactions. Lastly, the reaction of O3 with the surface-bound vinyl groups can be activated by high translational energies. The translationally activated process proceeds via a direct reaction where O3 reaches the transition state upon the initial gassurface collision. These
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observations show that the reaction follows a LH mechanism at thermal energies but transitions to a direct reaction at elevated incident energies. Together, these insights may inform the development of more accurate models for understanding the mechanistic and energetic details of ozone reactions. Ultimately, this understanding will lead to more accurate simulations for predicting the fate of O3 in the environment.
’ AUTHOR INFORMATION Present Addresses †
JILA, National Institute of Standards and Technology, University of Colorado, Boulder, CO 80309.
‡
US Army, Edgewood Chemical Biological Center, Edgewood, MD, 21010.
§
Department of Chemistry and Biochemistry, Montana State University, Bozeman, MT 59717.
’ ACKNOWLEDGMENT This work has been supported by the NSF (CHE-0948293). The authors are grateful for the 14-pentadecene-1-thiols provided by the group of T. Randall Lee. In addition, the authors thank Professor Diego Troya for numerous insightful discussions. ’ REFERENCES (1) Finlayson-Pitts, B. J.; Pitts Jr., J. N. Chemistry of the Upper and Lower Atmpsohere; Academic Press: New York, 2000. (2) Ravishankara, A. R. Science 1997, 276, 1058–1065. (3) Novakov, T.; Penner, J. E. Nature 1993, 365, 823–826. (4) Cruz, C. N.; Pandis, S. N. J. Geophys. Res., [Atmos.] 1998, 103, 13111–13123. (5) Gray, H. A.; Cass, G. R.; Huntzicker, J. J.; Heyerdahl, E. K.; Rau, J. A. Environ. Sci. Technol. 1986, 20, 580–589. (6) Claeys, M.; Wang, W.; Ion, A. C.; Kourtchev, I.; Gelencser, A.; Maenhaut, W. Atmos. Environ. 2004, 38, 4093–4098. (7) Hoffmann, T.; Odum, J. R.; Bowman, F.; Collins, D.; Klockow, D.; Flagan, R. C.; Seinfeld, J. H. J. Atmos. Chem. 1997, 26, 189–222. (8) Rohrer, F.; Berresheim, H. Nature 2006, 442, 184–187. (9) Ellison, G. B.; Tuck, A. F.; Vaida, V. J. Geophys. Res., [Atmos.] 1999, 104, 11633–11641. (10) Wadia, Y.; Tobias, D. J.; Stafford, R.; Finlayson-Pitts, B. J. Langmuir 2000, 16, 9321–9330. (11) Lai, C. C.; Yang, S. H.; Finlayson-Pitts, B. J. Langmuir 1994, 10, 4637–4644. (12) Dubowski, Y.; Vieceli, J.; Tobias, D. J.; Gomez, A.; Lin, A.; Nizkorodov, S. A.; McIntire, T. M.; Finlayson-Pitts, B. J. J. Phys. Chem. A 2004, 108, 10473–10485. (13) Moise, T.; Rudich, Y. J. Phys. Chem. A 2002, 106, 6469–6476. (14) Fiegland, L. R.; Saint Fleur, M. M.; Morris, J. R. Langmuir 2005, 21, 2660–2661. (15) Moise, T.; Rudich, Y. J. Geophys. Res. 2000, 105, 14667–14676. (16) Thomas, E. R.; Frost, G. J.; Rudich, Y. J. Geophys. Res. 2001, 106, 3045–3056. (17) Vieceli, J.; Ma, O. L.; Tobias, D. J. J. Phys. Chem. A 2004, 108, 5806–5814. (18) Stokes, G. Y.; Buchbinder, A. M.; Gibbs-Davis, J. M.; Scheidt, K. A.; Geiger, F. M. J. Phys. Chem. A 2008, 112, 11688–11698. (19) McCabe, J.; Abbatt, J. P. D. J. Phys. Chem. C 2009, 113, 2120–2127. (20) Gross, S.; Bertram, A. K. J. Phys. Chem. A 2008, 112, 3104–3113. (21) Saecker, M. E.; Nathanson, G. M. J. Chem. Phys. 1993, 99, 7056–7075. (22) Rettner, C. T.; Auerbach, D. J. Science 1994, 263, 365–367. (23) Ringeisen, B. R.; Muenter, A. H.; Nathanson, G. M. J. Phys. Chem. B 2002, 106, 4988–4998. 25349
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