19456
J. Phys. Chem. C 2008, 112, 19456–19464
Electron-Induced Reactions in Condensed Acetaldehyde: Identification of Products and Energy-Dependent Cross Sections E. Burean and P. Swiderek* UniVersita¨t Bremen, Institute for Applied and Physical Chemistry, Fachbereich 2 (Chemie/Biologie), Leobener Strasse/NW 2, Postfach 330440, 28334 Bremen, Germany ReceiVed: August 19, 2008; ReVised Manuscript ReceiVed: October 14, 2008
Thermal desorption spectrometry (TDS) is used to measure cross sections for reactions induced in condensed thin films of acetaldehyde by low-energy electrons with incident energies ranging from 4 to 15 eV. The obtained values for the decay of acetaldehyde and the formation of CO and CH4 are in reasonable agreement with cross sections obtained previously for similar samples by high-resolution electron energy loss spectroscopy (HREELS). In addition to propionaldehyde that has been identified as additional product previously, evidence of the formation of 2-propanol, ethanol, CO2, and ethylene is obtained here. Whereas the former two may result directly from an attack of methyl and H radicals released upon electron-induced fragmentation of acetaldehyde on an adjacent molecule, CO2 is shown to be formed through electron-induced reactions of the initial product CO. The production of ethylene can be explained by rapid elimination of H2O from ethanol under electron exposure. 1. Introduction The control of chemical modifications in condensed matter by use of electron beams is important in material science with potential applications in surface chemical patterning1,2 and the creation of structures down to the nanometer length scale.3 In such applications, high-energy radiation is usually predominant, although energies as low as 50 eV are applied in patterning applications using a mask.1 The actual chemical reaction, however, is nowadays thought to be induced by secondary electrons produced under the effect of high-energy radiation.4,5 Such low-energy electrons offer the best perspective toward control of electron-induced chemical reactions. For example, many gas-phase studies have shown that molecules interacting with low-energy electrons undergo bond-specific dissociation depending on the incident energy (E0) of the electrons as, for example, demonstrated by a recent suite of experiments on the nucleobase thymine.6 In the condensed phase or on surfaces, the reactive fragments released under electron impact can attack adjacent molecules or the surface and further recombine with other fragments, thus yielding new, sometimes complex molecules.7,8 Fundamental investigations of the mechanisms and rates of reactions induced by low-energy electrons in condensed molecular samples form the basis for an assessment of the capability of low-energy electron beams to be used as a tool in controlling surface modification and patterning. Thus, we perform studies aimed at both the identification of products and the measurement of cross sections, with the latter being important for identifying the dominant pathways of the chemical transformations. Recently, new methods to identify products of low-energy electron-induced reactions in condensed molecular samples were established. High-resolution electron energy loss spectroscopy (HREELS) monitors the spectral changes upon electron exposure7,9 whereas thermal desorption spectrometry (TDS) gives evidence of the formation of new species after electron * Author to whom correspondence should be adressed. E-mail: swiderek@ uni-bremen.de.
exposure from their specific desorption behavior.9,10 Furthermore, it was shown that cross sections for the formation of products can be obtained from both methods.11,12 Specific emphasis was placed on electron-induced reactions in condensed acetaldehyde7,12 because this molecule is simple enough to produce only small reactive fragments upon electron-induced bond scission. However, the dense molecular environment leads to a variety of possible intermolecular reactions. Using TDS, the cross sections for the formation of CO, CH4, and propionaldehyde (CH3CH2CHO) at E0 ) 15 eV could be obtained.12 Furthermore, it was shown that additional products must be present. Here we report a study aimed at the energy dependence of the cross sections obtained previously for electron-induced reactions in condensed acetaldehyde and at a more comprehensive identification of the products. The energy dependence arises because, depending on E0, interactions of low-energy electrons with matter may lead to different processes, such as dissociative electron attachment (DEA), electronic excitation, or dissociative ionization.13 Negative ion formation in gaseous acetaldehyde via DEA has been investigated in the 0-10 eV range13 and must be accompanied by the complementary formation of radicals. It is thus interesting to investigate whether such reaction channels, which would occur at somewhat lower energies because of polarization effects, do survive in the condensed phase. The identification of additional reaction products, however, is important because it is shown here that the initial products are themselves consumed by electron exposure after the initial stages of the reaction. Electron exposure of condensed acetaldehyde thus produces a complex mixture of products. The identification of as many of these products as possible helps us to understand the mechanisms of these reactions. 2. Experiment The experiments were performed in a stainless steel ultrahigh vacuum (UHV) chamber with a base pressure near 10-10 Torr. Details of the apparatus were published previously.7,10 The
10.1021/jp807419k CCC: $40.75 2008 American Chemical Society Published on Web 11/14/2008
Electron-Induced Reactions in Condensed Acetaldehyde samples consisted of thin molecular films condensed on a polycrystalline Au substrate maintained at a temperature of 35 K by means of a closed-cycle He cryostat. The polycrystalline Au foil was cleaned by resistive heating to 273 K prior to each experiment. The temperature is measured using a thermocouple type E press-fitted to the Au substrate. For a TDS experiment, the substrate is heated at a rate of 1 K/s up to a temperature of 273 K. The desorbing molecules are ionized by electron-impact ionization (electron energy 70 eV) and detected by a quadrupole mass spectrometer (QMS) residual gas analyzer (Stanford, 200 amu). To produce the thin molecular films, calibrated amounts of gases or vapors are introduced via a gas-handling manifold with a volume of approximately 45 cm3. For each film deposition, a calibrated amount of gas or vapor measured as a pressure drop by use of a capacitance manometer is leaked via a stainless steel capillary whose end is located just in front of the substrate. The film thickness was estimated by performing TDS measurements on pure and nonexposed films for increasing amounts of deposited vapor and identifying the monolayer from its characteristic desorption temperature. Acetaldehyde films in most experiments were produced from an amount of vapor corresponding to a pressure drop of 2 mTorr. This produces a film thickness of 20 to 30 layers as reported previously.7,12 However, and as noted in the respective Figure captions, some experiments on acetaldehyde films were performed at higher thickness (pressure drop of 4 mTorr) to enhance the sensitivity for the detection of products. For additional experiments on electroninduced reactions in condensed CO, an amount of vapor corresponding to a 1 mTorr pressure drop was found to be sufficient. As shown previously, this also produces a film with thickness in the multilayer regime.14 In the exposure experiments, after the deposition of pure acetaldehyde (CH3CHO) multilayers, the film was irradiated with electrons using a commercial flood gun with an energy resolution of 0.5-1 eV. The electron beam energy was varied in the range of 4-15 eV. The electron current on the substrate (surface area 2.8 cm2) was monitored throughout the irradiation so that the electron exposure in µC can be deduced. The electron gun delivers currents on the order of a few µA/cm2 as measured at the substrate. The product molecules formed after irradiation were detected by recording TDS curves for different molecular masses comprising the parent ion of acetaldehyde (44 amu) and the most important positive fragment ions corresponding to the suspected product. Results obtained from nonexposed samples are denoted as 0 µC. For the purpose of product identification, additional TDS measurements without irradiation were performed on reference mixtures of the starting material containing a certain percentage of a suspected product. The stated purity of acetaldehyde (CH3CHO, Fluka) was 99.95%, and 98% isotopic purity in the case of acetaldehyded4 (CD3CDO, Aldrich). CO (Linde) was of 2.0 quality. CO2 and ethylene (C2H4) were obtained from Air Liquide at purities of 99.995 and 99.95%, and CH4 (99.995%) was obtained from Messer-Griesheim. 2-Propanol (Riedel-de Ha¨en) and propionaldehyde (CH3CH2CHO, Acros Organics) had purities of 99.8 and 99+%, respectively. All compounds were used without further purification. Liquid samples were degassed by repeated freeze-pump-thaw cycles under vacuum. 3. Results and Discussion Thin molecular films of acetaldehyde are decomposed efficiently upon exposure to low-energy electrons, and the reaction products can be monitored by TDS7,12 as shown in Figure 1 for
J. Phys. Chem. C, Vol. 112, No. 49, 2008 19457
Figure 1. TDS curves recorded at 28 and 16 amu on multilayer films of acetaldehyde before (0 µC) and after various electron exposures (2000, 8000, and 16 000 µC) at E0 ) 15 eV. The signal at 114 K is due to fragments of remaining acetaldehyde formed upon electron impact ionization in the mass spectrometer.
an electron incident energy (E0) of 15 eV. Signals occurring at 48 and 55 K in the TDS curves recorded after exposure at 28 and 16 amu give evidence of the formation of CO and CH4 as important reaction products. Acetaldehyde desorbing at 114 K gives rise to additional signals in these curves. Propionaldehyde (not shown) was also observed previously as an additional product.7,12 To follow and quantify the course of the reaction, the integral intensity of the desorption peaks can be plotted as a function of electron exposure (Figure 2). It was recently shown that TDS can be used to determine cross sections not only for the loss of the starting material but, from the initial linear increase of the integral signals, also for the products that are formed either immediately upon irradiation with electrons via dissociation (CO) or following subsequent reactions of the complementary fragments within the film (f.e. CH4 and propionaldehyde).12 Whereas the cross sections for the loss of acetaldehyde and for the production of CO at E0 ) 15 eV were obtained as 1.7 × 10-16 and 7.4 × 10-17 cm2, respectively, the formation of CH4 and proprionaldehyde proceeded with very similar cross sections of 6.6 × 10-18 and 6.2 × 10-18 cm2. Here, after a brief summary of the method for determining the cross sections (section 3.1), we extend these measurements to different E0 ranging from 4 to 15 eV (section 3.2) before identifying additional reaction products to complete the study of electron-induced chemistry in condensed acetaldehyde (section 3.3). 3.1. Determination of Cross Sections for Electron-Induced Reactions. The procedure for obtaining cross sections for reactions initiated by exposure to low-energy electrons is summarized briefly here. We start by assuming pseudo-firstorder kinetics A + e- f P + e-, taking into account that the incident electron density is essentially constant throughout the exposure experiment. Then the approach for the measurement
19458 J. Phys. Chem. C, Vol. 112, No. 49, 2008
Burean and Swiderek
Figure 2. Integrated intensities of the desorption peaks of acetaldehyde (95-183 K, 44 amu), CO (38-63 K, 28 amu), CH4 (42-68 K, 16 amu), and propionaldehyde (125-210 K, 58 amu) from multilayer films of acetaldehyde as a function of electron exposure at E0 ) 15 eV. Data from the same experiment as shown in Figure 1 except the bottom curve for which the films were produced from amounts of vapor corresponding to a pressure drop of 4 mTorr in the gas inlet. The intensity scales are different for the four data sets.
of the cross section for the loss of A initially present in the film with number density nA0 (i.e., at t ) 0) and the formation of a product P under exposure to electrons is based on the following rate equations:
dnA I0 ) -σlossnA dt S0
(F1)
dnP I0 ) σPnA dt S0
(F2)
Here, the current density is represented by the ratio of the incident electron current I0 and the irradiated area S0. nA and nP are the number densities of the starting material and the product, respectively. After integration, eq F1 gives
ln
nA nA0
) -σloss
I0t S0
(F3)
The product of incident current and exposure time, I0t, represents the electron exposure. Because both number densities nA0 and nA are directly proportional to the area under the desorption peak appearing in the TDS run, the slope of a linear fit to the logarithmic plot of the integral intensity nA normalized to nA0, which was obtained for the nonexposed film, versus the electron exposure (converted to number of electrons) yields the cross section σloss for the degradation of the starting material. During the initial stages of exposure (i.e., as long as nA ≈ nA0 (linear regime)), the cross section σP for the formation of a specific product can be obtained from
σP )
nP S0 nA I0t
(F4)
In each TDS run, nA and nP are directly proportional to the area under the desorption peak of the initial compound and a
Figure 3. Plot of the integral peak intensity of the desorption signal at 48 K after exposure, normalized to the integral peak intensity at 114 K of CH3CHO for the same TDS run, as a function of electron exposure at E0 ) 15 eV and plot of the integral peak intensity of the desorption signal at 48 K in reference mixtures, normalized to the integral peak intensity at 114 K of CH3CHO for the same TDS run, as a function of CO content. The inset shows TDS measurements at 28 amu for multilayer films of CH3CHO before and after various electron exposures at E0 ) 15 eV.
product monitored at a specific mass. Because of different ionization efficiencies, however, these two integral intensities do not directly reflect the ratio of the amount of the two species within the film. Thus, nP/nA must generally be determined by comparing the signal of a specific product (i.e., a characteristic integral TDS feature) obtained after a given exposure I0t within the linear regime to those obtained from TDS curves of a series of reference mixtures containing the same compound in different proportions (i.e., having different ratios nP/nA). In this case, the slope of a plot of the product signal intensity as a function of exposure yields an intensity increase per injected charge (in µC) whereas the intensity of the same signal as a function of the quantity of product in the reference mixture yields an intensity increase per percent (Figures 3 and 4). The ratio of the two values yields the increase in percentage of product with exposure (%/µC) that can be converted to the fraction of product formed per injected charge (i.e., the number of electrons). Multiplication of this value by S0 gives the cross section for the formation of the product under consideration. Intensity fluctuations of the overall instrumental signal as well as those due to slight changes in the film thicknesses were eliminated by using an internal normalization procedure.12 Briefly, the signal characteristic of a specific product is given by the ratio of the peak area of the product desorption signal to that of acetaldehyde in the same film. The ratio of the increase of this signal with exposure and of the increase with the percentage of product in the reference mixtures yields the increase in the percentage of product upon exposure that can finally be converted to the cross section. Using this procedure, the cross section for the loss of starting material can be obtained to within (5%, and those for product formation can be obtained to within (25%.12
Electron-Induced Reactions in Condensed Acetaldehyde
J. Phys. Chem. C, Vol. 112, No. 49, 2008 19459
Figure 6. Cross section for electron-induced formation of CO in acetaldehyde multilayer condensed films determined from TDS measurements as a function of incident electron energy.
Figure 4. Plot of the integral peak intensity of the desorption signal at 55 K after exposure, normalized to the integral peak intensity at 114 K of CH3CHO for the same TDS run, as a function of electron exposure at E0 ) 15 eV and plot of the integral peak intensity of the desorption signal at 55 K in reference mixtures, normalized to the integral peak intensity at 114 K of CH3CHO for the same TDS run, as a function of CH4 content. The inset shows TDS measurements at 16 amu for multilayer films of CH3CHO before and after various electron exposures at E0 ) 15 eV.
Figure 7. Cross section for electron-induced formation of CH4 in acetaldehyde multilayer condensed films determined from TDS measurements as a function of incident electron energy.
Figure 5. Cross section for electron-induced degradation of acetaldehyde in multilayer condensed films determined from TDS measurements as a function of incident electron energy.
3.2. Energy Dependence of Cross Sections for ElectronInduced Reactions in Acetaldehyde. Using the procedure described in section 3.1 and discussed in detail in ref 12, cross sections for the decomposition of acetaldehyde and the formation of both CO and CH4 under electron exposure were systematically measured in the incident energy (E0) range from 4 to 15 eV. The E0 dependence of the cross sections for the loss of acetaldehyde is shown in Figure 5. The decomposition of acetaldehyde starts at E0 ) 6 eV, but its rate increases strongly above a threshold of 9 eV. Similarly, the cross sections for the formation of products CO and CH4 increase steeply above a threshold of about 9 eV (Figures 6 and 7), but a smaller
production rate is already observed above 6 eV. The reactivity around 6 eV may be related to a dissociative electron attachment process observed earlier between 6 and 7 eV in gaseous acetaldehyde.13 The general shape of the energy dependence of the cross section for the formation of CO resembles that for the production of CO from acetone and methanol obtained earlier by means ofhigh-resolutionelectronenergylossspectroscopy(HREELS).11,15 The maximum cross section for the production of CO from acetone was determined to be 6.8 × 10-17 cm2 around 16 eV,15 which is similar to the maximum value of 10.9 × 10-17 cm2 at 15 eV observed here. Concerning the decomposition of the starting material (Figure 5), the maximum cross section also agrees favorably with the value of 2.3 × 10-16 cm2 (15 eV) obtained for propionaldehyde using HREELS.16 The overall good agreement shows again that TDS is suitable for measuring meaningful cross sections for electron-induced reactions. The energy dependence of the cross sections for the formation of CO from acetone has been discussed in detail previously. It was suggested that core-excited negative ion resonances decaying into either repulsive neutral excited states or repulsive positive ion states are responsible for the molecular fragmentation.15 Because of the similarity of the molecular structures of acetone and acetaldehyde, the low-energy electron-induced
19460 J. Phys. Chem. C, Vol. 112, No. 49, 2008
Burean and Swiderek
reactions of the latter are very likely dominated by similar excitation pathways. 3.3. Identification of Other Reaction Products. As observed previously at E0 ) 15 eV,12 the cross section for CO formation accounts for roughly 50% of the observed loss of acetaldehyde. The formation of CH4, however, amounts to only 10% of the value for CO. These results indicate that other products must be present after electron exposure. Propionaldehyde with a quantity comparable to that of CH4 was observed previously.12 Also, as seen in Figure 2, the amount of initially dominant products CO and CH4 also decreases at longer electron exposures. Therefore, more comprehensive experiments have been performed that aim to identify additional products and thus offer more insight into the reaction mechanism. Neglecting the charge state of the involved species, the formation of propionaldehyde has been explained7 by a scenario implying the electron-induced fragmentation of acetaldehyde (R1)
CH3CHO f · CH3 + CO + · H
(R1)
followed by the recombination of methyl and H radicals to form methane (R2) or the abstraction of an H from the methyl group of an adjacent acetaldehyde molecule (R3a), followed by recombination of the remaining [acetaldehyde-H] radical with methyl to produce propionaldehyde (R3b):
· CH3 + · H f CH4
(R2)
· H + CH3CHO f · CH2CHO + H2
(R3a)
· CH2CHO + · CH3 f CH3CH2CHO
(R4a)
Alternatively, H might be abstracted from the aldehyde group (R3b), followed by recombination with the methyl radical at this site to yield acetone (R4b):
˙
· H + CH3CHO f CH3CO + H2
˙
CH3CO + · CH3 f CH3COCH3
(R3b) (R4b)
Finally, the H radical may also add to the oxygen of an adjacent acetaldehyde (R3c) to produce 2-propanol upon recombination of the resulting radical with methyl (R4c):
˙
· H + CH3CHO f CH3CHOH
(R3c)
˙
(R4c)
CH3CHOH + · CH3 f CH3CHOHCH3
Neither acetone nor 2-propanol could be identified with certainty in the previous study. Furthermore, a desorption signal at 156 K in the 31 amu TDS curve could not be assigned.7 Here, more convincing evidence could be obtained by performing the TDS experiments at twice the film thickness used previously. In contrast to the earlier experiments,7 a new desorption signal in the 140-200 K range becomes visible in the TDS curve recorded at 45 amu (Figure 8). The maximum of this broad peak shifts from 173 K after an exposure of 500 µC to 165 K after 4000 µC whereas its intensity increases in this exposure regime. Both observations are consistent with an increasing amount of product at increasing exposures. As discussed previously,14 a product formed in small quantities remains at submonolayer coverage after the desorption of the parent compound if the latter has a lower desorption temperature than the product. The desorption peak of the product can thus shift to lower temperature upon transition from the submonolayer to multilayer coverage (i.e., upon an increase in the amount of product).
Figure 8. TDS curves recorded at 45 amu on multilayer films of acetaldehyde before (0 µC) and after various electron exposures (500, 1000, 2000, 4000, 8000, and 16 000 µC) at E0 ) 15 eV. The signal at 114 K is due to an isotopic impurity of acetaldehyde. The films were produced from amounts of vapor corresponding to a pressure drop of 4 mTorr in the gas inlet.
To identify the product observed in the 45 amu curves (Figure 8) and to analyze in more detail the origin of the previously observed 31 amu signal at 156 K, we summarize in Table 1 the most important fragments of the products expected from reactions R4a, R4b, and R4c as well as of some additional alcohols (methanol, ethanol, and 1-propanol) together with the desorption temperature of some of these compounds either as pure films or from mixtures with acetaldehyde. Among these products, 45 amu is characteristic of 2-propanol. More importantly, the desorption temperatures observed after electron exposure agree closely with those of 2-propanol from mixtures with acetaldehyde with up to 8% of 2-propanol (Figure 9). This indicates that the observed product is in fact 2-propanol and that it is the last product remaining on the substrate in an important quantity upon an increase in the temperature. Also, this finding explains the previous observation that the desorption peak of propionaldehyde does not shift when its quantity increases upon exposure.7 As concluded earlier,14 propionaldehyde is embedded in a thin film of another product, identified here as 2-propanol, and thus desorbs at a constant temperature. A closer inspection of the desorption signal of 2-propanol and comparison with the desorption behavior from the reference mixtures with acetaldehyde (Figure 9) show that the desorption from the reference mixture sets in at slightly higher temperature than does the onset of the 45 amu desorption signal from the exposed acetaldehyde film. This points to an overlap with the desorption of another product. The temperature range of this additional contribution (150-160 K) coincides with the desorption maximum in the 31 amu TDS curve at 156 K.7 Table 1 shows that 31 amu is a characteristic signal of 1-alcohols. Whereas methanol could clearly be excluded previously7 because of the lack of signal at 32 amu and 1-propanol has
Electron-Induced Reactions in Condensed Acetaldehyde
J. Phys. Chem. C, Vol. 112, No. 49, 2008 19461
TABLE 1: Mass Spectral Data and Desorption Temperatures from an Acetaldehyde Matrix for Different Anticipated Reaction Products
acetaldehyde (CH3CHO) propionaldehyde (CH3CH2CHO) acetone (CH3COCH3) 2-propanol (CH3CHOHCH3) 1-Propanol (CH3CH2CH2OH) ethanol (CH3CH2OH) methanol (CH3OH)
desorption temperature from mixture with CH3CHOb
mass spectruma
product 29 29 43 45 31 31 31
(100%), (100%), (100%), (100%), (100%), (100%), (100%),
44 58 58 43 29 45 32
(82%), (85%), (64%), (10%), (18%), (52%), (74%),
43 31 31 31 42 29 29
(48%), 31 (
