Decomposition of solid amorphous hydrogen peroxide by ion irradiation

Transcription

1 THE JOURNAL OF CHEMICAL PHYSICS 124, Decomposition of solid amorphous hydrogen peroxide by ion irradiation Mark J. Loeffler, Ben D. Teolis, and Raul A. Baragiola a Laboratory for Atomic and Surface Physics, Thornton Hall, University of Virginia, Charlottesville, Virginia Received 15 September 2005; accepted 11 January 2006; published online 10 March 2006 We present laboratory studies of the radiolysis of pure 97% solid H 2 O 2 films by 50 kev H + at 17 K. Using UV-visible and infrared reflectance spectroscopies, a quartz-crystal microbalance, and a mass spectrometer, we measured the absolute concentrations of the H 2 O, O 2,H 2 O 2, and O 3 products as a function of irradiation fluence. Ozone was identified by both UV and infrared spectroscopies and O 2 from its forbidden transition in the infrared at 1550 cm 1. From the measurements we derive radiation yields, which we find to be particularly high for the decomposition of hydrogen peroxide; this can be explained by the occurrence of a chemical chain reaction American Institute of Physics. DOI: / INTRODUCTION Solid hydrogen peroxide, like water ice, is a suitable substance for fundamental studies of radiation chemistry because of the small number and limited complexity of the radiation products. Extended radiolysis of both water and H 2 O 2 leads to solids containing similar radiation products, but we expect significant differences because of the different atomic composition; e.g., the extra O atom causes H 2 O 2 to be much more reactive. To date, the radiolytic behavior of highly reactive molecules in the solid phase, such as condensed H 2 O 2, has not been explored extensively. Our interest in the radiolytic behavior of solid H 2 O 2 stems from the discovery of this molecule on Europa, 1 one of the Galilean satellites around Jupiter, whose surface is composed mostly of water and is heavily irradiated with ions from the inner magnetosphere. Most icy bodies in the outer solar system and interstellar space lack atmospheres that can shield impacts of energetic particles and photons, and for this reason, we expect that hydrogen peroxide is ubiquitous. In particular, it has been predicted to occur with abundances as high as 5% in the icy mantles on grains in interstellar molecular clouds. 2 In this environment, H 2 O 2 can be produced by radiolysis initiated by cosmic rays, which can penetrate the clouds. Previous experimental studies have shown that irradiation of pure water ice with energetic photons, 3 low energy electrons, 4 and energetic ions 5,6 can produce small but detectable concentrations of H 2 O 2, which is consistent with the observations on Europa and with the predictions for the interstellar medium. Of additional relevance in astrophysics is that the destruction of H 2 O 2 leads to the formation of O 2,a process that has been postulated to be a source of condensed O 2 observed on icy extraterrestrial surfaces such as Europa. 7 Here we report experimental studies of the radiolysis of solid amorphous H 2 O 2 films in vacuum at 17 K by 50 kev protons. A distinctive feature of our work is the combination of several characterization techniques: infrared, visible, and ultraviolet spectroscopies, microbalance gravimetry, and mass spectrometry that allow us to measure quantitatively the amount of H 2 O, O 2, and O 3 in the films, during irradiation and during subsequent warming. This combination of techniques was possible by a specially designed port arrangement in our vacuum system and multitasking LABVIEWbased software. EXPERIMENTAL SETUP All experiments were performed in a stainless steel vacuum chamber Fig. 1 on a radiation-shielded cryostat. The base pressure of the chamber was Torr and one to two orders of magnitude lower inside the shield. H 2 O 2 films were grown by vapor deposition on an optically flat gold mirror electrode of a 6 MHz quartz-crystal microbalance QCM. The areal mass Q mass/unit area of the films was determined by the change in the resonance frequency of the crystal, which was measured with an Inficon IC/5 cona Electronic mail: raul@virginia.edu FIG. 1. Experimental setup /2006/ /104702/6/$ , American Institute of Physics

2 Loeffler, Teolis, and Baragiola J. Chem. Phys. 124, troller to a resolution of 0.1 Hz. 8 The measured Q can be converted to film column density molecules/cm 2 if the film composition is known and converted to thickness if the mass density is known. 8 Since the frequency of the QCM depends on temperature, we measured this dependence in a blank experiment with no film deposited and used it to correct the data shown here. Hydrogen peroxide, 97% pure, was prepared by vacuum distillation of a commercial grade 50 wt % H 2 O 2 aqueous solution Fischer Scientific in a glass ampoule and vapor deposited onto the microbalance using an effusive glass doser aimed at near-normal 2.5 incidence. The choice of glass was dictated by the need to prevent the catalytic decomposition of H 2 O 2 that occurs on metal surfaces. Films were deposited at 110 K to a column density = H 2 O 2 /cm 2 about 910 nm thick, chosen to be larger than the maximum ion penetration depth H 2 O 2 /cm 2 or 700 nm for 50 kev H + used in these experiments 9 to prevent alteration of the substrate. The relatively high deposition temperature was chosen so that impurities, formed when scattered H 2 O 2 reacts with the chamber walls, would not stick on the substrate, as verified by infrared spectroscopy and mass spectrometry. After growth, the films were cooled to 17 K and irradiated at an incident angle of 9. The proton beams were produced by an ion accelerator, mass analyzed, and scanned uniformly over the film. A thin wire collector placed in the ion beam path monitored the proton current and fluence. A Dycor M200 quadrupole mass spectrometer MS monitored the species ejected sputtered during irradiation or desorbed during heating of the film. The specular reflectance of the films on the gold mirror was measured at an incident angle of 35 at infrared wavelengths and 22.5 at visible and UV wavelengths. The infrared spectra were recorded with a Thermo-Nicolet Nexus 670 Fourier transform infrared spectrometer at 2 cm 1 resolution, and the UV-visible reflectance by an Ocean Optics S2000 charge-coupled device CCD grating spectrometer in the range = m. The spectra were divided by the reflectance of the gold mirror substrate taken before film deposition. The ratios R were then converted to optical depth units, ln R. Absorption band areas were derived after subtraction of base lines that matched the continuum. In Fig. 2 we show infrared reflectance spectra taken before and after irradiation with H + /cm 2, and in Table I we list the frequencies of absorption band maxima. We note that irradiation leads to the appearance of water bands, the 1550 cm 1 O 2 stretch band, several ozone bands, and a few unidentified features. RESULTS Quantification methods QCM and MS: Oxygen and water at high irradiation fluences Following irradiation with H + /cm 2, we heated the film at a constant rate of 0.2 K/min, measuring the mass loss due to desorption with the QCM Fig. 3 and the desorbed flux with the MS Fig. 4 b. The reading of the MS at FIG. 2. Infrared spectra of a solid H 2 O 2 sample before 1 and after 2 irradiation to a fluence of H + /cm 2 at 50 kev. We abbreviate H 2 O 2 as HP and H 2 OasW. 32 amu was due not only to O 2 but also to O 3 Ref. 10 and H 2 O 2, since these molecules break up in collisions with the walls and in the ionizer of the mass spectrometer. Based on previous measurements with water films, we attribute the small mass loss peak between 35 and 90 K to the desorption of O 2 and O 3 from the surface. Most of the oxygen produced by radiolysis remained in the film until the temperature reached 155 K, when it left abruptly, as shown in the data of Figs. 3 and 4. We note that the desorption of O 2 from oxygen-water mixtures also shows a sharp peak near 155 K Ref. 13. Integration of the peak in Fig. 4 a after subtracting a base line yields 35 g/cm 2. Adding this value to that for the mass loss below 120 K, we obtain 39.3 g/cm 2 for the total areal mass of oxygen both O 2 and O 3 produced by radiolysis of the 147 g/cm 2 hydrogen peroxide film. At temperatures above that of the oxygen outburst, three distinct changes in the sublimation rate were detected by the QCM Fig. 3. We attribute these changes to desorption of H 2 O bound to other H 2 O molecules, of H 2 O bound in the H 2 O 2 2H 2 O dihydrate compound, and of pure H 2 O 2 most of which was beyond the penetration depth of the ions, based on separate infrared measurements of water-h 2 O 2 mixtures made in our laboratory 14 and in previous reports. 15 Then, from the QCM data of Fig. 4, we derived the amount of water produced during the irradiation by integrating the rate of mass loss curve up to 176 K, after subtracting the peak of oxygen at 155 K. We find that the amount of water produced is 59.4 g/cm molecules/cm 2, after a small correction for H 2 O 2 desorbed between 150 and 165 K.

3 Decomposition of amorphous HOOH J. Chem. Phys. 124, TABLE I. Peak positions for the absorption features present in the film before irradiation and after reaching H + /cm 2. The numbers in parentheses are the uncertainties in the least significant digits. Virgin sample at 17 K Sample irradiated by ions/cm 2 Position cm 1 Position cm 1 Position cm H 2 O H 2 O CO H 2 O H 2 O H 2 O H 2 O O H 2 O, H 2 O H 2 O O Dangling bond H 2 O ?? Dangling bond H 2 O ?? Dangling bond H 2 O sh H 2 O H 2 O H 2 O O H 2 O H 2 O H 2 O H 2 O H 2 O O H 2 O H 2 O H 2 O Some of this desorbed water originates from the decomposition of the dihydrate compound. This is the equilibrium phase of hydrogen peroxide in the depth penetrated by the protons, since the H 2 O 2 concentration was much lower than the eutectic value of 33%. 16 Between 168 and 176 K, after the sublimation of pure water ends, we observe a broad structure in the sublimation of water that we attribute to the decomposition of the dihydrate Fig. 4 a. We then find that H 2 O/cm g/cm 2 are bound to H 2 O 2 in the dihydrate compound. Therefore, from the stoichiometry of the compound, H 2 O 2 /cm 2 or 10.1 g/cm 2 exists in the irradiated ice. This value is an upper limit since some of the unirradiated H 2 O 2 below the proton penetration depth will likely diffuse into the irradiated region during warming above 125 K, as seen by Loeffler and Baragiola. 14 UV-visible spectroscopy: Quantification of O 3 The areal mass Q for oxygen species, 39.3 g/cm 2,is the sum of the values for O 2 and O 3 since these molecules are indistinguishable with the QCM and also with the MS, due to the efficient cracking of ozone before detection mentioned above. In Q, we neglect atomic oxygen because of its high reactivity. 12 The contribution of O 3 to Q is obtained by using the strong Hartley absorption band of ozone that appears in the UV-visible reflectance at 255 nm O 2 has neg ligible absorption in the nm region we observe. Figure 5 a depicts the UV-visible spectral reflectance of the film before and after irradiation; the large oscillations result from interference between reflections from the substrate and the film surfaces. By matching this interference pattern with a theoretical calculation based on the Fresnel equations, 17 we derive the density, 1.6 g cm 3, for unirradiated H 2 O 2 and the optical constants of the film Fig. 5 b. Using the peak cross section of cm 2 for the Hartley band, 18 we calculate a column density of O 3 /cm 2, which corresponds to 5.9 g/cm 2. Subtracting this value from the total Q, we obtain the areal mass of O 2 to be 33.4 g/cm 2, cor- FIG. 3. Mass loss on the microbalance due to sublimation of a H 2 O 2 film irradiated to H + /cm 2. 1 g/cm 2 = O 2 /cm 2 or H 2 O 2 /cm 2. FIG. 4. Sublimation of a hydrogen peroxide film irradiated to H + ions/cm 2 a Mass loss rate during heating at 0.2 K/min. b Mass spectrometer reading at mass 32. The mass 32 signal below 140 K was below the noise level in the MS. The large rise in b beginning at 180 K is O 2 from H 2 O 2 decomposition off the chamber walls.

4 Loeffler, Teolis, and Baragiola J. Chem. Phys. 124, FIG. 6. The production of water, dioxygen, and ozone in a film of H 2 O 2 /cm 2 irradiated with 50 kev H + to a fluence of H + /cm 2. The lines in the figure are the fits described in the text. For H 2 O 2, we only include the H 2 O 2 that is in the path of the ion beam, and thus we subtract the column density corresponding to the underlying unirradiated hydrogen peroxide 5± H 2 O 2 /cm 2. FIG. 5. Top: The UV-vis reflectance before and after - irradiation with 50 kev H + of a film initially of H 2 O 2 /cm 2 to a fluence of H + /cm 2. The oscillations are a result of the interference in the film. Bottom: absorption coefficient before and after - irradiation. responding to O 2 /cm 2. The results of all the measurements discussed above are summarized in Table II. Infrared spectroscopy and the fluence dependence of H 2 O 2, H 2 O, O 2, and O 3 We now normalize the area of the infrared bands of H 2 O 1655 cm 1,O cm 1, and O cm 1 measured during irradiation to the column densities calculated above for high fluences to obtain the fluence dependence of the column densities of those species. To quantify the destruction of H 2 O 2, we do not use the strong 2800 cm m band of H 2 O 2, because the absorbance of this overtone depends strongly on the molecular environment. 14 Rather, we used two other infrared absorption bands: the sharp O O stretch feature at 886 cm 1 that shifts less than 2 cm 1 and does not change shape during irradiation and the 1347 cm 1 bending mode absorption. The fluence dependence of H 2 O 2 TABLE II. The second column gives radiation yields determined from the initial slope of the fluence dependence in Fig. 6; the numbers in the parentheses are uncertainties in the least significant digits. are the column densities and C the concentrations for the irradiated layer at H + /cm 2. G molecules/100 ev molecules cm 2 H 2 O O O H 2 O C given in Fig. 6 is an average of the similar results derived using these two bands. The band areas are scaled to the column density of H 2 O 2 measured with the QCM before irradiation. In Table III, we give values for the effective band strength A * as a function of the column density : A * = B/ k, where B is the band area of the infrared absorption feature at zero fluence for H 2 O 2 and at H + /cm 2 for the other species. Here the factor k is the ratio of the length of infrared beam path to the film thickness: 2.14 for our geometry. We use the term effective band strength, to distinguish it from the usual band strength A, measured in transmission experiments, since B is not directly proportional to column density, i.e., Beer s Law is, in general, not followed. This is a consequence of strong interference effects that are present in infrared reflection absorption spectroscopy IRAS. To use our values of A * in a different geometry or with another film thickness, they must first be corrected for changes in the interference effect. 5,19 Finally, we note that there is an error in the column densities from using the same value of A * for all fluences since absorption band strengths are affected by the changes in the optical properties and thickness of the films, which in turn change with irradiation. In the case of H 2 O 2, this error is larger at high fluences and it is estimated to be less than 50% from comparisons of the infrared data from the two H 2 O 2 bands mentioned above and the QCM data. 5,19 With this analysis we obtain the fluence dependence for H 2 O, O 2, and O 3 production and H 2 O 2 destruction shown in Fig. 6. Table II gives saturation concentrations and radiolytic G values, defined as the number of molecules produced or destroyed for H 2 O 2 per 100 ev of deposited energy, in the limit of low fluence. The definition of G implies that the initial production is linear with fluence. This is observed for

5 Decomposition of amorphous HOOH J. Chem. Phys. 124, TABLE III. Effective band strengths for our film in the infrared obtained from irradiation at H + /cm 2. H 2 O a 1655 cm 1 H 2 O cm 1 H 2 O cm 1 O cm 1 O cm 1 A * cm molecule a We note that this band overlapped with the 1389 cm 1 H 2 O 2 band see Fig. 2, so this strength could be an underestimate. H 2 O but not for ozone where the dependence is quadratic or for O 2 for which the very weak absorption does not allow us to obtain data at low fluences. DISCUSSION Radiolytic processes Irradiation effects in solid H 2 O 2 have not been studied previously but there are a number of reports of radiolysis of hydrogen peroxide in various concentrations of liquid water irradiated by weakly exciting rays, x-ray photons, and electrons and a study of UV photolysis on frozen H 2 O 2 :H 2 O mixtures. 24 Different effects are expected for ionic projectiles that deposit a large amount of energy per unit path length in the material large linear energy transfer LET, such as the overlap of reaction chains in the ion track due to a high density of electronic excitations. The initial decomposition reaction is believed to be X +H 2 O 2 OH+OH, 1 where X symbolizes the projectile. In addition, one photochemical study 22 suggested, based on the results from H 18 2 O:H 2 O 2 experiments, that the decomposition under 2537 Å light might result from X +H 2 O 2 H 2 O+O, 2 O+H 2 O 2 OH+HO 2, and that, in addition, the products in 2 could result from reaction 1 followed by a subsequent reaction in the water cage: 25 2OH H 2 O+O. In the condensed phase, the OH formed can either react with surrounding molecules or recombine as a result of the cage effect. Experiments in aqueous solutions show that the rate constant for H 2 O 2 decomposition is very high and depends on concentration. 23 This behavior has been explained by the chain reaction 26 OH+H 2 O 2 H 2 O+HO 2, HO 2 +H 2 O 2 H 2 O+O 2 +OH. Therefore, if a hydroxyl radical encounters a H 2 O 2 molecule it will likely destroy it, allowing the chain to propagate. We can combine 5 and 6 to determine that the energy released in each cycle of the chain reaction is 2.5 ev: OH+2H 2 O 2 OH+2H 2 O+O ev This exothermic chain reaction 7 explains the large value measured for the destruction of H 2 O 2, G H 2 O 2 =21, in comparison to H 2 O, G H 2 O =0.48 at 73 K, Ref. 27. Itis important to note that since at most 1.5% of the mass of the irradiated region was eroded by sputtering in our experiments, the majority of the products in the final reaction 6 remain in the ice. The radicals required to start or propagate the chain reaction can also be provided by X +H 2 O OH+H, 8 H+H 2 O 2 H 2 O+OH, H+O 2 HO The termination step of the chain reaction has been debated in literature; 21 23,28 it appears that it could either be HO 2 +HO 2 H 2 O 2 +O 2 11 or 2HO 2 +H 2 O 2 2H 2 O 2 +O Note that reactions 6, 11, and 12 are sources of O 2, which can then contribute to the production of O 3. We note the lack of detection of the intermediate species HO 2. This may be due to efficient destruction in the tracks of high LET particles as in our experiments by radical-radical reactions such as reaction 6, 11, and 12, or OH+HO 2 H 2 O+O Unidentified species So far we have shown the detection of the radiation products: H 2 O, O 2, and O 3. In addition, there are a few unidentified absorption features Table I, which disappear when the irradiated film is warmed to temperatures in the range of K. These features may be due to radicals or other volatile molecules in the film. Possible candidates might be HO 2,H 2 O 3, and H 2 O O. Of the three, HO 2 would be the most likely if one assumes that the small feature at 1138 cm 1 corresponds to the 1101 cm 1 absorption of HO 2 in solid Ar. However, there are no features within 100 cm 1 of the strongest HO 2 absorption at 1389 cm 1 or its O H stretch at 3419 cm 1 Ref. 29. The largest unidentified absorption shown here is at 1257 cm 1 ; this absorption was also produced by laser irradiation 266 nm ofah 2 O:O 3 mixture at 17 K 30 and identified previously as the 6 mode of H 2 O 2. However, we do not use this assignment since we do not observe the 1257 cm 1 absorption in dilute mixtures of

6 Loeffler, Teolis, and Baragiola J. Chem. Phys. 124, water and H 2 O 2. Furthermore, we also dismiss H 2 O 3 or H 2 O O since the reported infrared spectra of these species in rare gas matrices do not show absorptions near these two unidentified features, after accounting for reasonable 30 cm 1 matrix-related frequency shifts. CONCLUSIONS We have shown that the combination of several experimental techniques allows a fairly complete quantification of the radiation products of hydrogen peroxide. We find that understanding the high initial radiation yields requires considering a chemical chain reaction. At high fluences, the radiation products that we can detect are water, molecular oxygen, and ozone; but we cannot exclude OH, which we cannot detect because of interference of its infrared bands with those of water and hydrogen peroxide. ACKNOWLEDGMENTS This research was supported by the NASA Cosmochemistry Program. We thank W. H. Shoup for constructing the glass manifold used in this article. One of the authors M.J.L. thanks the Virginia Space Grant Consortium for a fellowship. 1 R. W. Carlson, M. S. Anderson, R. E. Johnson et al., Science 283, T. I. Hasegawa and E. Herbst, Mon. Not. R. Astron. Soc.nnu. Rep. NMR Spectrosc. 263, ; A. G. G. M. Tielens and W. Hagen, Astron. Astrophys. 114, M. S. Westley, R. A. Baragiola, R. E. Johnson, and G. A. Baratta, Nature London 373, ; P. A. Gerakines, W. A. Schutte, and P. Ehrenfreund, Astron. Astrophys. 312, X. Pan, A. D. Bass, J.-P. Jay-Gerin, and L. Sanche, Icarus 172, M. J. Loeffler, U. Raut, R. A. Vidal, R. A. Baragiola, and R. W. Carlson, Icarus 180, O. Gomis, M. A. Satorre, G. Strazzulla, and G. Leto, Planet. Space Sci. 52, ; O. Gomis, G. Leto, and G. Strazzulla, Astron. Astrophys. 420, ; M. H. Moore and R. L. Hudson, Icarus 145, M. T. Sieger, W. C. Simpson, and T. M. Orlando, Nature London 394, ; P. D. Cooper, R. E. Johnson, T. I. Quickenden, Icarus 166, ; T. M. Orlando and M. T. Sieger, Surf. Sci. 528, N. J. Sack and R. A. Baragiola, Phys. Rev. B 48, J. F. Zeigler, Stopping and Range of Ions in Matter SRIM2003, 2003 available at 10 M. Famá, D. A. Bahr, B. D. Teolis, and R. A. Baragiola, Nucl. Instrum. Methods Phys. Res. B 193, R. A. Baragiola and D. A. Bahr, J. Geophys. Res. 103, ; D. A. Bahr, M. Famá, R. A. Vidal, and R. A. Baragiola, ibid. 106, R. A. Baragiola, C. L. Atteberry, D. A. Bahr, and M. M. Jakas, Nucl. Instrum. Methods Phys. Res. B 157, R. A. Vidal, D. Bahr, R. A. Baragiola, and M. Peters, Science 276, M. J. Loeffler and R. A. Baragiola, Geophys. Res. Lett. 32, L P. A. Giguere and K. B. Harvey, J. Mol. Spectrosc. 3, ; R.L. Miller and D. F. Horning, J. Chem. Phys. 34, ; J.A.Lannon, F. D. Verderame, and R. W. Anderson, Jr., ibid. 54, W. Foley and P. A. Giguere, Can. J. Chem. 29, M. S. Westley, G. A. Baratta, and R. A. Baragiola, J. Chem. Phys. 108, A. J. Sedlacek and C. A. Wright, J. Phys. Chem. 93, ; A.V. Benderskii and C. A. Wight, J. Chem. Phys. 101, B. D. Teolis, M. J. Loeffler, U. Raut, and R. A. Baragiola unpublished. 20 E. R. Johnson, J. Chem. Phys. 19, ; M. Ebert and J. W. Boag, Discuss. Faraday Soc. 12, ; F. S. Dainton and J. Rowbottom, Nature London 168, J. Weiss, Discuss. Faraday Soc. 12, J. P. Hunt and H. Taube, J. Am. Chem. Soc. 74, E. J. Hart and M. S. Matheson, Discuss. Faraday Soc. 12, V. S. Gurman, V. A. Batyuk, and G. B. Sergeev, Kinet. Katal. 8, T. J. Hardwick, Can. J. Chem. 30, F. Haber and R. Willstatter, Chem. Ber. 64, J. A. Ghormley and A. C. Stewart, J. Am. Chem. Soc. 78, ; C. J. Hochandel, Comparative Effects of Radiation Wiley, New York, 1960, Chap W. C. Schumb, Hydrogen Peroxide Reinhold, New York, M. E. Jacox and D. E. Milligan, J. Mol. Spectrosc. 42, H. Chaabouni, L. Schriver-Mazzuoli, and A. Schriver, J. Low Temp. Phys. 26,