Electron-Induced Decomposition of Condensed Acetone Studied by

Apr 2, 2015 - are significantly higher for neutral species,19 ESD is more easily applied to monitor volatile neutral products of electron-induced...
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Electron-Induced Decomposition of Condensed Acetone Studied by Quantifying Desorption and Retention of Volatile Products J. Warneke and P. Swiderek* Fachbereich 2 (Chemie/Biologie), Institute for Applied and Physical Chemistry, University of Bremen, Leobener Straße/NW 2, Postfach 330440, D-28334 Bremen, Germany S Supporting Information *

ABSTRACT: The electron-induced decomposition of thin condensed layers of acetone was studied by a combination of electron-stimulated desorption (ESD) experiments and thermal desorption spectrometry (TDS) monitoring both the decay of acetone and the formation of volatile products. A reaction mechanism for the decay is proposed and its validity verified by modeling the TDS and ESD data on the basis of a set of resulting elementary rate equations. The results show that ESD data as a function of electron exposure also give indirect evidence of the presence of nonvolatile species. In particular, formation and desorption of CO is delayed because part of the carbonyl groups become incorporated in larger nonvolatile compounds. These, in turn, decompose upon further electron exposure to release CO. ESD curves obtained below the desorption temperature of the small volatile products CO and CH4 have also been reproduced well assuming that acetone layers have a limited capacity for uptake of volatile species and become saturated with these products during decomposition. Finally, we show that the quantitative evaluation of ESD data obtained above and below the desorption temperature of small volatile products in principle offers an approach to a quantitative comparison of product amounts desorbing in ESD and postirradiation TDS.

1. INTRODUCTION Electron-induced reactions are relevant to a wide variety of processes1 ranging from the fundamental chemistry of radiation damage as studied in radiobiology2−6 and the formation of molecules in space dealt with by astrochemistry7−9 to technologies for nanofabrication such as focused electron beam induced deposition (FEBID),10,11 only to name a few. In such processes, the interaction of electrons with molecules often results in the formation of radical anions or cations and their subsequent fragmentation which, in the gas phase, i.e., under single collision conditions, is routinely used as an analytical tool in electron impact (EI) mass spectrometry. In the condensed phase or in adsorbates with sufficiently large surface coverage, the resulting reactive ions and their fragments can react with surrounding material leading to further products and possibly also the synthesis of larger molecules.12,13 Electron-stimulated desorption (ESD) experiments have frequently been applied to study the formation of negative or positive ion fragments in condensed molecular layers as a function of the electron incident energy (E0) and thus to identify dissociative electron attachment (DEA) or dissociative ionization (DI) processes5,14−17 but also reactive scattering in the condensed phase.18 However, as desorption probabilities are significantly higher for neutral species,19 ESD is more easily applied to monitor volatile neutral products of electron-induced surface reactions.10,20−22 Generally, ESD accounts for small and highly volatile products of an electron-induced reaction. In contrast, large molecules usually do not acquire sufficient © 2015 American Chemical Society

kinetic energy to leave the surface. Less volatile products formed by reaction of fragments with neighboring molecules thus do not desorb during electron exposure. Also, depending on their actual volatility and on the sample temperature, chemical composition,23,24 and morphology,17 even small molecules may remain within the condensed layer or adsorbate. To account for products that remain at the surface or in a condensed phase during exposure, postirradiation analysis can be performed by thermal desorption spectrometry (TDS).1 Here, products are separated according to their desorption temperatures. Combining ESD with TDS allows for the observation of all volatile reaction products that are generated during irradiation. This approach which requires only one analytical instrument thus offers valuable insight into the details of an electron-induced reaction. A combination of ESD and TDS has in fact been applied previously.21 In this earlier work, the electron-induced decomposition of monolayer adsorbates of azomethane (CH3NNCH3) was investigated but, in addition, XPS results were consulted to obtain quantitative information. More specifically, an exponential decay of the parent compound with increasing electron exposure observed in postirradiation TDS was matched by an equally exponential decrease of the N2 release rate, as evident from the most intense ESD signal at Received: February 4, 2015 Revised: March 16, 2015 Published: April 2, 2015 8725

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The Journal of Physical Chemistry C m/z 28. XPS further revealed that the percentage of nitrogen left on the surface after a certain exposure was the same as that of the parent compound as derived from TDS. These combined results lead to the conclusion that all nitrogen desorbed during electron irradiation and that N2 is an immediate product of azomethane decomposition. However, ESD of CH3 terminated before the parent compound was fully depleted pointing to concurrent reactions within the adsorbate, in line with a substantial amount of carbon remaining at the surface according to XPS. We have recently obtained a similar result when investigating the electron-induced decomposition of condensed layers of acetone (CH3COCH3).22 As for azomethane,21 ESD signals of CH3 and CH4 leveled off much more rapidly after the initial increase at the start of electron exposure than the desorption rate of the main product, in this case CO,22 suggesting at first sight and in the absence of a quantitative study for acetone that the decomposition processes may be similar for the two compounds. On the contrary, the same experiment, when performed with acetylacetone, yielded a very different ESD behavior with generally much lower intensities that increased further after the initial rise of the desorption rate at the start of irradiation. This behavior was traced back to reactions of fragments resulting from the initiating electron-molecule interaction with adjacent molecules to yield nonvolatile products which, in turn, decomposed to release CO and other volatile species at later stages of irradiation. This result prompted us to also revisit in more depth the electron-induced decomposition of acetone with the aim of obtaining detailed insight into the processes that occur in condensed layers during electron irradiation. Acetone is a good benchmark system because the production of CO as retained in condensed layers of this compound was previously quantified by high-resolution electron energy loss spectroscopy (HREELS).25 However, the fate of the methyl groups of acetone after dissociation and, in particular, ESD of these products has not been investigated. Furthermore, our previous work has shown that fragmentation products may alternatively desorb or remain within the condensed phase upon electron exposure.22 In consequence, the present study also aims at determining the branching ratio between retention and desorption of products by establishing a correlation between product quantities observed in ESD and in later TDS experiments.

70 eV. The sample temperature was measured using a type E thermocouple press-fitted to the Au substrate. The minimum temperature reached by the cryostat varied between roughly 35 and 38 K. Experiments performed under these conditions are simply denoted by 35 K. A second set of ESD experiments was performed at a sample temperature of 70 K. A temperature ramp of 1 K/s was applied by resistive heating during TDS experiments. Electron exposure was performed with a commercial flood gun (SPECS FG 15/40) delivering electrons with tunable kinetic energy (E0) at an estimated resolution of the order of 0.5−1 eV and currents of the order of a few μA/cm2, as measured at the substrate. All electron exposures in this study were performed at E0 = 23 eV. 2.2. Calibration of Film Thickness. The film thickness of acetone was estimated by thermal desorption spectrometry (TDS) of films with increasing coverage as described previously.22,24,26 The acetone desorption data at the dominant mass peak, namely m/z 43, show a weak but characteristic peak between 150 and 220 K that rapidly saturates and is therefore ascribed to the monolayer. A second peak with a maximum at 140 K starts to increase upon saturation of the monolayer peak and is hence attributed to the successive layers no longer in contact with the substrate. In line with the previous thickness calibrations, a pressure drop of 0.12 ± 0.03 mTorr in the calibrated volume was deduced to lead to deposition of a monolayer (ML) of acetone.

3. RESULTS AND DISCUSSION 3.1. Film Thickness and Effective Electron Penetration Depth. ESD experiments measure desorption rates of a given compound during electron exposure whereas TDS determines the amount of material remaining on the surface after a given exposure. In the simplest case, a given reactant is deposited at monolayer coverage (see, for example, ref 21) and decomposes fully to products that do not react within the layer and are volatile at the temperature at which electron irradiation is performed. In this situation, the rate of decay of the reactant (here acetone = Ac) at a given time t is determined by its number density at the surface at this time, nAc(t), the impinging number of electrons per unit area I0/S0, and the cross section for loss of the reactant σ−Ac as given by eq 1.27 dnAc(t ) I = −σ −Ac·nAc(t ) · 0 dt S0

2. EXPERIMENTAL SECTION 2.1. Experimental Setup. All experiments were performed in an ultrahigh vacuum (UHV) setup with a base pressure of about 10−10 Torr described previously.26 Thin films of acetone (Sigma-Aldrich, 99.5%) were deposited on a polycrystalline Au sheet cooled by a closed-cycle helium refrigerator (Leybold Vacuum). To produce these films, the vapor of acetone was introduced via a gas handling manifold consisting of precision leak valves and a small calibrated volume where the absolute pressure is measured with a capacitance manometer. For each film deposition a calibrated amount of vapor was leaked via a stainless steel capillary opening onto the metal substrate. Prior to each deposition the substrate was cleaned by heating to 400 K using two thin Ta resistive heating ribbons spot-welded to the thicker Au sheet. Electron-stimulated desorption (ESD) experiments and thermal desorption spectrometry (TDS) were performed by use of a quadrupole mass spectrometer (QMS) residual gas analyzer (Stanford, 200 amu) with electron impact ionization at

(1)

As the current density is constant during the experiment, we can substitute dx = I0·dt/S0; i.e., the change in acetone quantity is related to an infinitesimal change in injected charge per area (eq 2), x being denominated as electron exposure in the following. dnAc(x) = −σ −Ac·nAc(x) dx

(2)

The consequent ESD rate of a product P, dnP,des(x)/dx, stated as the change in number density nP(t) at the surface, is given by the total rate of formation of the product dnP(x)/dx and should be proportional to the rate of decay of the reactant: dn (x) d n (x ) d nP,des(x) = P = −gP · Ac dx dx dx

(3)

Here gP represents the fraction of reactant molecules that yield the given product P when fragmenting under electron impact. On the basis of the constant current density I0/S0, an 8726

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Figure 1. Evolution with electron exposure at E0 = 23 eV of ESD signals recorded at (a) m/z 28 and (b) m/z 15 on acetone layers of increasing thickness (8 ML, 17 ML, 25 ML, 34 ML, 50 ML as indicated by the arrows) deposited at 35 K and held at 70 K during the experiment. Electron irradiation was started at (A) and stopped at (B) by switching off and on a negative bias on the sample. Data in (b) have been corrected for contributions of CH4 by measuring simultaneously ESD at m/z 16 and subtracting from the m/z 15 curve the corresponding intensity as known from the mass spectrum of pure CH4.

note that ESD of D2 obtained as a function of coverage on amorphous solid water on Pt(111) has shown enhanced yields at intermediate coverages before dropping to a constant value.28 This effect was ascribed to a reaction proceeding at the water− Pt interface that is driven by excitons produced by the incident electron beam and migrating to the inner interface of the water layer. Above a certain film thickness, however, these migrating energetic states did not reach the inner interface anymore so that this reaction channel was no longer accessible. The present ESD data (Figure 1) do not show evidence of such an enhancement within a certain thickness range, suggesting that the inner interface does not make a noticeable contribution in the experiments described here. In line with the observed saturation of the ESD yields (Figure 1), the amount of acetone remaining after the exposure experiments increases more strongly above this thickness (Figure 2). In particular, a significantly larger amount of

exponential decrease is then expected for both the rate of product formation and the resulting rate of desorption.21 However, in the case of samples with thickness in the multilayer regime, the limited mean free path of electrons within the condensed phase leads to an increasing probability that an electron experiences an inelastic scattering event and is consequently slowed down. This restricts the layer thickness into which the impinging electron beam can deposit energy that is required to decompose the molecules.28 The desorption rate of the products as measured by ESD and the remaining amount of reactant as seen in postirradiation TDS will then not show the same direct correlation as for low coverage. Therefore, we have first investigated the dependence of ESD and TDS signals on the thickness of the deposited acetone layers to identify conditions under which a complete processing of the initial reactant layer is possible. Figure 1 shows the evolution of ESD of CO and CH3 with electron exposure at E0 = 23 eV for acetone films of increasing thickness held at 70 K. The electron-induced formation of CO and CH3 has been investigated previously.22,25 CH3 is the immediate fragmentation product of α-cleavage following ionization whereas neutralization of the residual CH3CO+ fragment and its subsequent dissociation releasing another CH3 is the most likely mechanism for production of CO.22 Secondary electrons (SEs) produced in the ionization process might in principle induce further chemistry via dissociative electron attachment (DEA) but are not expected here to make a significant contribution to the formation of CH3 and CO. DEA to acetone is only efficient at energies around 9 eV.22 However, at most one SE having an energy around 9 eV can be released by an impinging 23 eV electron. Also, ESD yields are very low near this energy.22 Therefore, we can safely assume that DEA does not make a significant contribution. According to previous TDS results,22 small products desorb from acetone below 70 K with maxima of the desorption peaks located near 50 K for CO and near 55 K for CH4. The data shown in Figure 1 should thus account for the total production at least of CO whereas CH3 may still react to other products such as CH4 within the molecular layer. Figure 1 reveals that the ESD intensities increase linearly with increasing amounts of deposited vapor up to a film thickness corresponding to around 25 ML. They saturate rapidly at larger coverage indicating that the effective depth into which the impinging electrons can deposit their energy is exceeded. Here, it is also interesting to

Figure 2. TDS curves recorded at m/z 43 on acetone films of increasing thickness (8 ML, 17 ML, 25 ML, 34 ML, 50 ML as indicated by the arrow) after the electron exposure of 2400 μC/cm2 at E0 = 23 eV shown in Figure 1.

acetone remained at a thickness corresponding to 50 ML than at 34 ML, despite the fact that the total desorption of CO and CH3 was the same for both experiments as deduced from the coincidence of the ESD curves for these thicknesses (Figure 1). In line with these results the thickness for all experiments described in the following sections was chosen as 8 ML, i.e., within the regime where the desorption rate of the products 8727

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part of the loss of acetone as shown in Figure 3. We also note that the incident current in the experiments shown in Figure 3 did not change significantly with increasing electron exposure, thus ruling out effects of film charging. As observed previously in condensed layers of acetaldehyde,27,31 larger carbonyl containing molecules can be formed under electron exposure. In acetone, the analogous reaction involves CH3 radicals released by α-cleavage.22 These can abstract a hydrogen atom from an intact adjacent molecule to form CH4. The resulting acetonyl radical can recombine with further CH3 to yield methyl ethyl ketone or, in later stages of the reaction, even larger carbonyl containing products that, upon further α-cleavage, do not release CH3 but larger alkyl radicals. Although other reactions leading to larger species cannot be ruled out, 22 this scenario rationalizes the observations made from Figure 1, namely, the faster decay of the CH3 signal in ESD as compared to the decay of CO. Also, the larger products are less volatile than acetone and thus remain within the condensed layer. We show now that such a nonvolatile residue can in fact explain the deviations from the simple exponential decay of acetone seen in Figure 3. The nonvolatile residue accumulates in the condensed layer as the initial acetone is decomposed. However, as the molecules of this residue are structurally related to acetone, they are equally decomposed under continued electron exposure. We take nR(x) as the number density including the combined amounts of such nonvolatile residues R as present after exposure x and consider that according to the mechanism outlined above, a certain fraction gR of the acetone molecules reacts with radical species within the condensed film to produce the residue. The change in R content within the film can then be expressed as

depends linearly on the amount of deposited acetone as seen clearly at the onset of irradiation (Figure 1). A closer inspection of the data shown in Figures 1 and 2 reveals deviations from the simple behavior described in eqs 2 and 3. In particular, although the ESD signal of CH3 decays roughly to the baseline within the applied electron exposure of 2400 μC/cm2 for the 8 ML acetone film, desorption of CO remains at a significant level of about 40% of its initial rate (Figure 1). Also, a noticeable amount of acetone remains at the surface after this exposure (Figure 2) despite the conclusion that the layer is still fully accessible to the electron beam at this thickness. The more rapid decay of CH3 ESD is reminiscent of the previous results for azomethane.21 We thus perform in the following sections a quantitative analysis of the kinetics of acetone decay and the resulting formation of products to obtain insight into the origin of this behavior. 3.2. Kinetics of Acetone Decomposition. The decomposition kinetic of 8 ML films of acetone was investigated in more detail by a series of postirradiation TDS experiments. The residual amount of acetone was determined after varying electron exposures by integrating the characteristic desorption signal at m/z 43 (Figure 2). The result for electron exposures performed at both 35 and 70 K is shown in Figure 3. The two

d n (x ) dnR (x) = −gR · Ac − σ −R ·nR (x) dx dx

(4)

Here, the cross section σ−R represents the overall probability that the nonvolatile residue molecules are decomposed under the electron beam. As the nonvolatile residue accumulates within the acetone layer, the probability increases that an impinging electron is scattered at a molecule R and in consequence does not effectively reach an acetone molecule. Taking into account that the impinging current density remains constant, the probability for decomposition of the remaining acetone under the beam then decreases more strongly than expected from eq 2. The nonvolatile residue thus has a decelerating effect on the decay of acetone. This effect is taken into account by adding to eq 2 a term that is proportional to nR(x):

Figure 3. TDS peak areas measured at m/z 43 (compare Figure 2) from 8 ML acetone films after varying electron exposure at E0 = 23 eV at 35 K (open circles) and 70 K (filled squares). Single exponential functions cannot fit the data as shown for several examples (dashed lines). The fit function nAc(x) (section 3.1) results from numerical solution of rate eqs 4 and 5 as described in section 3.2. TDS peak areas are accurate within ±5%, the error resulting mainly from uncertainties in the amount of leaked vapor.

data sets coincide, revealing that the rate of decay of acetone is roughly independent of temperature. However, attempts to fit these data by a single exponential decay as predicted by eq 2 were unsuccessful, as obvious from several examples included in Figure 3. This suggests that the decomposition kinetics of acetone is not adequately described by eq 2. To unravel the fate of acetone, it is important to note that ESD is not a significant decay channel. In fact, the ESD intensity of m/z 43 (acetone) is only 5% of the intensity of m/z 28 (CO). Taking into account that the partial electron impact ionization cross sections are about 5 Å2 for the m/z 43 fragment of acetone29 and 1.93 Å2 for m/z 28 of CO,30 the relative ESD yields for CO and acetone result as roughly 50:1. Electron-induced decomposition thus accounts for the major

dnAc(x) = −σ −Ac·nAc(x) + c·nR (x) dx

(5)

Here, c is a parameter that accounts for the shielding effect of R on acetone. Although a microscopic model describing the actual trajectories of electrons scattered within the condensed layer would certainly be more accurate, it would require detailed knowledge of the single scattering events. As this information is not easily available, we have chosen here to model the decomposition of the investigated acetone layers on the basis of rate eqs 4 and 5. However, we note that care must be taken not to ascribe a microscopic interpretation to the fit parameter c. 8728

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Here, it is important to note that the same fit to the TDS data; i.e., nAc(x) was obtained as long as the product gR·c remained constant. The actual choice of both parameters, on the contrary, determines the absolute intensity of nR(x). This finding is explained by an analytical solution of the coupled differential eqs 4 and 5 (Supporting Information). Here, it is important to note that, in consequence, only the shape but not the absolute intensity of nR(x) is physically meaningful. In contrast, the slope of nAc(x) at low electron exposure yields a value for σ−Ac of (1.0 ± 0.1) × 10−16 cm2. This is a reasonable result compared to the cross section of roughly 5 × 10−17 cm2 reported previously for production of CO in condensed films of acetone held at 18 K.25 3.3. Kinetics of Methyl, Methane, and CO Production at 70 K. According to eq 3, ESD rates of decomposition products should, in the simplest case, be directly proportional to the rate at which the reactant acetone decays under electron exposure. Therefore, we have tried to model the ESD rates as a function of exposure of the three volatile products, namely, CH3 radicals, CH4, and CO using the function nAc(x) determined in the previous section (Figure 5). Here, the contribution of CH4 as deduced from the ratio of m/z 15 and m/z 16 in its mass spectrum was subtracted from the data recorded at m/z 15 to obtain the ESD curve for CH3. nAc(x) was then numerically differentiated and the resulting function dnAc(x)/dx was scaled for better comparison to the same height as the experimental ESD data at x = 0, i.e., at the start of irradiation. Note that this is an arbitrary normalization

The coupled system of differential eqs 4 and 5 was solved numerically and the proportionality factors gR, σ−R, σ−Ac, and c were adjusted to obtain a function nAc(x) that fits the integrated desorption peaks at m/z 43 obtained by TDS (Figure 3). This procedure also yields a function nR(x), included in Figure 4.

Figure 4. Fit function nAc(x) to the TDS peak areas obtained at m/z 43 from 8 ML acetone films after varying electron exposure at E0 = 23 eV (compare Figure 3) and arbitrarily scaled resulting function nR(x) representing the evolution with electron exposure of the nonvolatile residue R. The fit results from numerical solution of rate eqs 4 and 5 as described in the text.

Figure 5. ESD of (a) CH3, (b) CH4, and (c) CO from 8 ML acetone films held at 70 K during electron exposure at E0 = 23 eV and comparison with the decay rate of acetone dnAc(x)/dx (black line) as described by eq 3 and obtained from nAc(x) shown in Figure 4. The dropping lines relate to the end of irradiation in each of a set of overlaid experimental data that together reveal the good reproducibility of the experiments. ESD data in (a) have been corrected for contributions of CH4 by measuring simultaneously ESD at m/z 16 and subtracting from the m/z 15 curve the corresponding intensity as known from the mass spectrum of pure CH4. (d) Difference between measured ESD data for CO and dnAc(x)/dx as plotted in (c). 8729

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acetone by formation of other products whereas σCO,R is a cross section for release of CO from the residue R. Using again dnAc(x)/dx as well as nR(x), a much better fit to the experimental ESD data for CO is obtained with only small deviations above 6000 μC/cm2 (Figure 6).

that helps to visualize the quality of the fit to the experimental data but does not imply that each molecule acetone that is decomposed yields one molecule of the respective products. This procedure yields a good fit to the ESD curves of both CH3 and CH4 (Figure 5a,b). This result is not obvious although CH3 radicals are the immediate product of α-cleavage in acetone following electron impact ionization (eq 6a). In fact, concurrent decay channels exist for decay of CH3 in the molecular layer and desorption of CH4 results from a sequence of elementary reactions steps: e−

Ac → [Ac−CH3]+ + CH3

(6a)

CH3 → CH3(des)

(6b)

CH3 + Ac → [Ac−H] + CH4

CH4 → CH4(des) (6c)

2CH3 → C2H6

(6d)

CH3 + [Ac−H] → R

(6e)

Figure 6. ESD of CO from 8 ML acetone films held at 70 K during electron exposure at E0 = 23 eV and comparison with a fit obtained from eq 7 using the functions dnAc(x)/dx and nR(x) from Figure 4 (black line).

The fact that ESD of CH3 is directly proportional to the decay of the reactant acetone indicates that the branching ratios between desorption (eq 6b) and the sum of the three other decay channels of CH3, namely, reaction with intact acetone to yield CH4 (eq 6c), recombination with another CH3 to produce C2H6 (eq 6d),22 and recombination with acetonyl radicals to yield the residue R (eq 6e) remains constant. In consequence, also the probability for formation and subsequent desorption of CH4 remains constant. This is rationalized by considering that the residue R is structurally very similar to acetone and can thus replace acetone in the second and third CH3 decay channel. In contrast to the cases of CH3 and CH4, the ESD curve for CO cannot be fitted by the acetone degradation rate. As seen in Figure 5c, the ESD rate for CO decays much more slowly than the rate at which acetone is decomposed. After an exposure of 8000 μC/cm2, the rate of acetone decay has nearly dropped to zero, but ESD of CO still takes place with about one-fourth of its initial intensity. Stopping the electron irradiation at the end of an ESD experiments resulted always in an immediate drop of the intensity, clearly showing that the delayed desorption of CO is not a result of product molecules being temporarily retained within the layer as such and slowly outgassing over a certain time. The formation of larger ketones, summarized as residue R in section 3.2, provides a reasonable explanation. These nonvolatile residues should equally undergo α-cleavage and subsequently release CO thus being a source of CO even after depletion of acetone. This interpretation is supported by comparing more closely the ESD data for CO and the function dnAc(x)/dx (Figure 5c). In fact, the shape of the difference between these two curves (Figure 5d) resembles the curve nR(x) (Figure 4) suggesting that the additional CO release stems from decomposition of the residue R. Taking into account release of CO from the nonvolatile residue R, eq 3 must be modified to yield eq 7 describing ESD of CO at 70 K: d n (x ) d nCO,des,70K (x) = CO dx dx d n (x ) = −gCO· Ac + σCO,R ·nR (x) dx

The small deviations between the experimental data and fit functions as seen in Figures 5a,b and 6 are not unexpected given the simplicity of the applied model. In fact, however, around 80% of the acetone is decomposed at electron exposures around 6000 μC/cm 2 (Figure 3) and thus unaccounted side reactions may start to contribute to the experimental ESD of CO (Figure 6). Also, the small differences between measured and simulated curves for ESD of CH3 and CH4 seen between 2000 and 4000 μC/cm2 (Figure 5a,b) can be explained by contributions from electron-induced decomposition of the nonvolatile residue R that, according to our model, reaches its maximum yield in this range of exposures (Figure 4). In fact, at early stages of the reactions, R must to a significant amount be represented by molecules that still contain one methyl end group and thus can yield CH3 and consequently also CH4 as a result of further α-cleavage. However, again, a complete model accounting for all possible individual reactions is clearly beyond the scope of the present study. 3.4. Kinetics of Methyl, Methane, and CO Production at 35 K. When a condensed molecular layer is held at sufficiently low temperature, even small and highly volatile products can be retained in the layer. This has been used to quantify the amount of product formed by techniques such as TDS but also high-resolution electron energy loss spectroscopy (HREELS).1,25,27,31 However, as outlined above, ESD is a concurrent process that can remove product from the condensed phase so that techniques monitoring the surface composition may only be able to account for a part of the total product yield. To evaluate the extent of product removal from the condensed layer by ESD as compared to the total product yield, we have investigated the decomposition of acetone layers held at 35 K, i.e., below the desorption temperatures of about 50 K for CO and 55 K for CH4.22 Under these conditions, the ESD rates do not simply drop with increasing electron exposure but show a more complex behavior (Figure 7). Most strikingly and in contrast to the result obtained at 70 K (section 3.2), the ESD rate of CO is initially lower than at 70 K but then increases slightly but reproducibly during the initial stages of

(7)

Here gCO is the branching ratio between decay of acetone by direct CO release as described above and possible decay of 8730

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saturation of the integrated TDS peak areas is reached more gradually than for CH4. After saturation within the layer has been reached according to TDS, ESD rates should thus be the same at 35 and 70 K. Therefore, we directly compare in Figure 9 the ESD curves obtained at these two temperatures. This comparison suggests that at the beginning of electron irradiation at 35 K about 60% of produced CH4 and 70% of CO desorbs while the rest remains in the acetone layer. However, after a certain electron exposure, the ESD curves recorded at the two temperatures coincide, indicating that the sticking probability at 35 K for CO and CH4 decreases strongly during irradiation. Only for CH3 do the ESD curves coincide throughout the investigated range of exposures. In this case, the desorption rate is not limited by simple physical retention in the layer but rather by concurrent reactions of CH3, as summarized in eq 6 and obvious from production of CH4. This chemical retention must be equally efficient at both temperatures, leading to the negligible difference between the ESD intensities at 35 and 70 K. The obvious decrease of the sticking coefficient of CO and CH4 with ongoing degradation of the acetone layer (Figure 9) must be taken into account when the ESD results at 35 K are modeled. As already suggested by TDS, this requires a description of the saturation of the condensed layer by the volatile products. The ratio of the desorbing and retained amounts of product would then depend on the amount of product already contained in the layer. Furthermore, the layer has only a certain capacity for uptake of the product. To verify if saturation of the condensed layer by small products is in fact responsible for the decreasing retention and to exclude an effect of chemical changes of the layer during electron exposure, we performed ESD experiments with intermittent heating sequences. This represents approximately the hypothetical situation that during irradiation CO, CH3, and CH4 are always formed in a film free of products despite the low temperature of 35 K. To achieve this, the film was irradiated for only a few seconds, which is the time required for acquisition of one or two ESD values. Then, irradiation was stopped and the target was heated above the desorption temperature of CO and CH4. After the target was cooled back to 35 K, the procedure was repeated until an electron exposure of 2000 μC/cm2 was reached. The resulting ESD curves (Figure 9) reveal that the desorption probability is in fact considerably lower under these conditions. Furthermore, the ESD signal decreases continuously following the initial onset in close accord with the situation at 70 K, supporting that saturation of the layer is the decisive factor. Taking into account the saturation effect in the ESD curves of CO and CH4 at 35 K as described above, the amount of product molecules P retained in the layer is modeled by eq 8.

Figure 7. ESD of CO, CH3, and CH4 from 8 ML acetone films held at 35 K during electron exposure at E0 = 23 eV. ESD data for CH3 have been recorded at m/z 15 and corrected for contributions of CH4 by measuring simultaneously ESD at m/z 16 and subtracting from the m/ z 15 curve the corresponding intensity as known from the mass spectrum of pure CH4. Refer to Figure 1 for details of the experiment.

irradiation before decreasing slowly (see also Figure 10). Also, the ESD rate of CH4 decreases less steeply than at 70 K. Retention of part of the products in the condensed layer of acetone and of its nonvolatile reaction products is an obvious explanation for the differences between ESD at 70 and 35 K. The accumulation of CO and CH4 is obvious from the integrated desorption signals as observed in postirradiation TDS after increasing electron exposures (Figure 8). As shown

Figure 8. TDS peak areas measured at m/z 28 (CO, filled squares) and m/z 16 (CH4, open circles) as obtained from 8 ML acetone films after varying electron exposure at E0 = 23 eV at 35 K.

above, the decay of acetone proceeds with roughly the same rate at 35 K and at 70 K. We thus assume that the probability of the initial electron-induced fragmentation event is also to a good approximation independent of temperature at least between 35 and 70 K and only the branching ratio between the subsequent processes changes, namely, desorption or accumulation in the layer. However, a comparison of Figures 7 and 8 suggests that this branching ratio changes during electron exposure. In fact, the amount of CH4 in the condensed layer as seen in TDS starts to saturate after about 1000 μC/cm2 (Figure 8) whereas desorption as seen in ESD still proceeds (Figure 7). A similar situation is also observed for CO although

npmax d ,film − nP,film,35K (x) dnP(x) nP,film,35K (x) = g film,35K · · dx npmax dx ,film (8)

Here gfilm,35K represents the maximum fraction of the total production rate dnP(x)/dx of molecules P that leads to retention in the layer at 35 K. Upon temperature increase, this fraction must drop to yield at 70 K gfilm,70K = 0. dnP(x)/dx in the case of CH4 results immediately from eq 3 whereas production of CO is described by eq 7. nmax P,film is the saturation number density of P and nP,film,35K(x) the actual number density 8731

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Figure 9. ESD of (a) CO, (b) CH3, and (c) CH4 from 8 ML acetone films held alternatively at 70 K (red) and 35 K (blue) during electron exposure at E0 = 23 eV. In a third experiment (green), ESD was performed at 35 K but interrupted after short intervals to perform an annealing cycle at 70 K. ESD data in (b) recorded at m/z 15 have been corrected for contributions of CH4 by measuring simultaneously ESD at m/z 16 and subtracting from the m/z 15 curve the corresponding intensity as known from the mass spectrum of pure CH4. Refer to Figure 1 for details of the experiment.

Figure 10. ESD of (a) CO and (b) CH4 from 8 ML acetone films held at 35 K during electron exposure at E0 = 23 eV and comparison with the decay rate of acetone (red line) as described by eqs 8 and 9 based on the total rates of product formation of CH4 given by eq 3 and of CO given by eq 7. The dropping lines relate to the end of irradiation in each of a set of overlaid experimental data that together reveal the good reproducibility of the experiments.

Figure 11. Fit functions to the ESD intensities as a function of electron exposure for (a) P = CO and (b) P = CH4 obtained at 70 K (A = (d/dx)nP,des,70K) and 35 K (B = (d/dx)nP,des,35K) as well as the difference between the two curves (A − B). The integral of this latter curve over electron exposure (∫ x0(A − B) dx) represents the amount of product that does not desorb during electron exposure but during the subsequent TDS experiment.

from acetone layers after electron exposure at 35 K (Figure 8) saturate at about the time when the ESD signal obtained at 35 K starts to coincide with the ESD signal recorded at 70 K (Figure 9), the latter temperature being higher than the desorption temperature of these products.22 This suggests that the difference between the integrated ESD curves obtained at the two temperatures reflects the amount of product that remains in the layer but is monitored in the postirradiation TDS experiment. We thus evaluate in the following if this

in the condensed layer. The amount of desorbing product P at 35 K can then be described by eq 9. d n (x ) d d nP,des,35K (x) = P − nP,film,35K (x) dx dx dx

(9)

By variation of gfilm,35K and nmax P,film, the experimental results were fitted as shown in Figure 10, gfilm,35K being 0.30 for CO and 0.42 for CH4. 3.5. Quantitative Comparison of Products in ESD and TDS. The integrated TDS peak areas of CO and CH4 obtained 8732

DOI: 10.1021/acs.jpcc.5b01167 J. Phys. Chem. C 2015, 119, 8725−8735

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and 35 K, indicating that the intensities obtained from TDS and ESD are quantitatively comparable. However, for CO the integrated TDS signals are larger by about a factor of 1.6 than the corresponding values obtained from the two ESD experiments. Three effects can possibly contribute to this deviation, namely, (i) TDS in fact accounting for more CO than reflected by the difference between the ESD data at 35 and 70 K, (ii) overlap of the TDS signal of CO with other products also yielding a fragment at m/z 28 but desorbing to a lesser extent during ESD, and (iii) a nonlinear relation between the desorption rate of CO and the simultaneously obtained MS signal. We discuss in the following these different phenomena. To evaluate the first effect, TDS data for CO and CH4 after increasing electron exposures are shown in Figure 13. The

correlation between the ESD and TDS data also holds quantitatively. Both ESD and TDS measure an MS signal at a given m/z ratio as a function of time. If this signal can be ascribed to a specific product, ESD monitors the yield of this product as desorbing during electron exposure whereas TDS accounts for the product amount desorbing af ter electron exposure and during continuous increase of the surface temperature. The integrals over time of the ESD and TDS signals should then directly relate to the fractions of the total product amount that desorbs and remains at the surface, respectively. Assuming that CH4 and CO do not further react with components of the condensed layer, ESD data recorded at m/z 16 and 28 during electron exposure at a temperature above the desorption temperature relate to the total amounts of these products. In the case of electron exposure at 35 K, however, only a certain fraction of these products desorbs to be detected in ESD whereas the rest should be recovered after electron exposure in a TDS experiment. For each product, the difference between the integral ESD intensities obtained at 70 and 35 K during a specific electron exposure should thus match the integral TDS intensities obtained after the same exposure. This relation is verified here for CH4 and CO. As summarized in Figure 11, the ESD intensities are, for clarity, represented by the fit functions shown in Figures 5, 6, and 10. The integrated difference between the ESD signals obtained at 70 and 35 K (Figure 11) can now be compared to the integrated TDS intensities for CO and CH4 obtained after varying electron exposure at 35 K. Here, it is important to note that integration in the case of the ESD data was performed over electron exposure x whereas integration was performed over time t when the TDS data are evaluated. The ESD integrals were thus transformed back to integrals over time by taking into account the definition of electron exposure dx = I0·dt/S0. The resulting curves for CO and CH4 are plotted together with the integrated TDS intensities in Figure 12. Note also that, although we omit the actual numbers on the intensity axis for clarity, all data are in fact plotted on the same scale. In fact, Figure 12 shows that the integrated TDS intensities for CH4 after varying exposure at 35 K agree well with the integrated difference between the ESD signals obtained at 70

Figure 13. TDS curves recorded at m/z 28 on 8 ML acetone films after increasing electron exposure at E0 = 23 eV.

integration of the desorption peak areas is typically performed between temperatures ranging from a value below the onset of the desorption signal to a value above the point where the signal returns to the baseline. This procedure includes only data points below 70 K for the lowest electron exposures whereas, after longer irradiation, the CO desorption signal broadens and develops a tail toward higher temperatures, most likely due to changes in the chemical nature of the deposit or to depletion of the acetone layer and consequent contact of product molecules with the underlying Au substrate. In this situation, a small fraction of the desorption signal extends to temperatures up to about 75 K, implying that not all product molecules have in fact desorbed during ESD at this temperature but are still accounted for by the integrated TDS data. Also, the baseline is somewhat difficult to locate above the desorption peak, causing a certain margin of error in the integration procedure. However, it is clear from Figure 13 that this effect is unlikely to cause the 60% deviation seen for CO. The second effect, namely, overlap with other products, concerns the formation of C2H6 but also of C2H4. The mass spectrum of both compounds32 is dominated by a signal at m/z 28. At low electron exposures, the TDS curve obtained at m/z 28 shows well separated signals with maxima at 48 and 80 K ascribed to CO and C2H6 (Figure 13 and ref 21). The desorption temperature of C2H4 is similar to that of C2H6.33,34 A mass scan performed previously during electron exposure has shown small desorption peaks of comparable intensity at m/z 26 and 27 but no signal at m/z 30, pointing in fact to formation of C2H4.22 As the CO desorption peak shifts to higher temperature upon increasing electron exposure (Figure 13), it starts to overlap with the desorption signals of C2H6 and the likely byproduct C2H4. This may also make a certain

Figure 12. Product amount for CO and CH4 remaining in the 8 ML condensed layers of acetone held at 35 K after varying electron exposures at E0 = 23 eV as represented by the difference integrated over time of the ESD intensities obtained at 70 and 35 K (solid lines) and by the TDS intensities integrated over time for CO (filled squares) and CH4 (open circles). Data for CO were measured at m/z 28 and for CH4 at m/z 16. 8733

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information on the amounts of product retained in the layer. In particular, it is shown here that, in principle, integral intensities for specific mass signals obtained by ESD and TDS can be directly compared. However, care must be applied with regard to the pumping speed when TDS experiments are performed. As shown for the case of CO, TDS intensities may appear too large when the desorption rate becomes sufficiently large. This can be traced back to insufficient pumping speed and consequent temporary accumulation of the desorbed product in the vacuum chamber. In this situation, the integrated ESD intensities obtained at two different temperatures as proposed in the present study provide an alternative and more precise approach to monitoring the product quantities in the film. The direct correlation with TDS data obtained at sufficiently low desorption rates may then be used to establish a standard for an absolute determination of volatile product quantities retained in a condensed layer. This approach can be applied in quantitative studies of electron-induced reaction such as those underlying the decomposition of precursor molecules in focused electron beam induced deposition (FEBID).

contribution to the integrated TDS signals for CO shown in Figure 12. However, as at least C2H4 also contributes to a certain extent to ESD, this counteracts an overestimate of the TDS signal. To quantify contributions of C2H6 and C2H4, ESD and TDS would have to be measured at their characteristic signal m/z 26, 27, and 30. However, this was beyond the scope of the present work. A more severe distortion of the results, however, is most likely related to the third effect. In desorption studies, signals may be overestimated if the pressure increases above a value at which desorbing material can be pumped away immediately. This effect is most relevant to CO as observed by TDS because, in this case, the amount of product desorbs within a few seconds and CO is the dominant desorbing product.22 This effect may also contribute to the broadening toward high temperatures as observed for the CO desorption signal. In contrast, during ESD, desorption is rate-limited by the impinging electron current density and all products desorb much more slowly than in TDS. In this situation, the pumping speed is sufficient to remove all desorbed gas before the next data point is acquired. In conclusion, the fast desorption of large amounts of CO is the most likely reason for the deviation between the relative product amounts remaining in the layer as deduced from TDS and from ESD data. In future studies, this can be investigated in more detail by varying the rate of temperature increase during the TDS experiments. However, our results show that the described analysis of ESD experiments performed at two different sample temperatures provides an alternative and also faster access to the product amounts remaining in the condensed layer during electron exposure.

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ASSOCIATED CONTENT

S Supporting Information *

Analytical solution to eqs 4 and 5. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors thank to Dr. Klaus G. Warneke for very helpful discussions. Financial support by DFG is gratefully acknowledged. This work was conducted within the framework of the COST Action CM1301 (CELINA).

4. CONCLUSIONS In this study we have revisited the electron-induced reactions of thin condensed layers of acetone with the aim of obtaining a detailed understanding of its decomposition kinetics. In particular, a combination of electron-stimulated desorption (ESD) experiments and thermal desorption spectrometry (TDS) was applied to quantitatively monitor both decay of acetone and formation of products. The results show that ESD data as a function of electron exposure reveal not only the formation of volatile products but indirectly also the presence of larger nonvolatile species. This insight was obtained by proposing a set of elementary rate equations for the underlying chemical reactions and using them to model the rate of acetone decay and the ESD rates of the volatile products as a function of electron exposure. The good overall agreement with the experimental data supports the validity of the proposed reaction mechanisms. In contrast to previous results for azomethane where electron-induced decomposition immediately removed N2 from an adsorbed layer leaving only hydrocarbon material behind,21 formation and desorption of CO is delayed because the carbonyl group partly becomes incorporated in larger nonvolatile carbonyl compounds. These, in turn, decompose upon further electron exposure to finally release CO. ESD curves obtained below the desorption temperature of the small volatile products CO and CH4 have also been reproduced well by assuming that the acetone layers have a certain capacity for uptake of volatile species and become saturated with these products during decomposition. Furthermore, the comparison of ESD experiments performed at such low temperatures with data obtained above the desorption temperature of the volatile products offer quantitative

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REFERENCES

(1) Arumainayagam, C. R.; Lee, H.-L.; Nelson, R. B.; Haines, D. R.; Gunawardane, R. P. Low-Energy Electron-Induced Reactions in Condensed Matter. Surf. Sci. Rep. 2010, 65, 1−44. (2) Boudaiffa, B.; Cloutier, P.; Hunting, D.; Huels, M. A.; Sanche, L. Resonant Formation of DNA Strand Breaks by Low-Energy (3 to 20 eV) Electrons. Science 2002, 287, 1658−1660. (3) Abdoul-Carime, H.; Gohlke, S.; Illenberger, E. Site-Specific Dissociation of DNA Bases by Slow Electrons at Early Stages of Irradiation. Phys. Rev. Lett. 2004, 92, 168103/1−4. (4) Swiderek, P. Fundamental Processes in Radiation Damage of DNA. Angew. Chem., Int. Ed. 2006, 45, 4056−4059 and references cited therein. (5) Dugal, P. C.; Abdoul-Carime, H.; Sanche, L. Mechanisms for Low-Energy (0.5−30 eV) Electron-Induced Pyrimidine Ring Fragmentation within Thymine- and Halogen-Substituted Single Strands of DNA. J. Phys. Chem. B 2000, 104, 5610−5617. (6) Abdoul-Carime, H.; Sanche, L. Alteration of Protein Constituents Induced by Low-Energy (