REACTION ROUTES IN THE CO H 2 CO-d n CH 3 OH-d m SYSTEM CLARIFIED FROM H(D) EXPOSURE OF SOLID FORMALDEHYDE AT LOW TEMPERATURES

The Astrophysical Journal, 702:291 300, 2009 September 1 C 2009. The American Astronomical Society. All rights reserved. Printed in the U.S.A. doi:10.1088/0004-637x/702/1/291 REACTION ROUTES IN THE CO H 2 CO-d n CH 3 OH-d m SYSTEM CLARIFIED FROM H(D) EXPOSURE OF SOLID FORMALDEHYDE AT LOW TEMPERATURES H. Hidaka, M. Watanabe, A. Kouchi, and N. Watanabe Institute of Low Temperature Science, Hokkaido University, Sapporo, Hokkaido 060-0819, Japan; hidaka@lowtem.hokudai.ac.jp Received 2009 June 4; accepted 2009 July 6; published 2009 August 10 ABSTRACT Grain surface reaction has been expected to be a key process for deuterium enrichment in interstellar molecules. We focus on formaldehyde, which is predicted to be formed on cold grain surface in astrophysical models and is known to be deuterium-enriched in a molecular cloud. Reaction routes and effective reaction rate constants are experimentally investigated when H 2 CO and D 2 CO are exposed to D and H atoms on amorphous solid water (ASW) at 10 20 K, respectively. For D + H 2 CO on ASW, H 2 CO was converted to HDCO and D 2 CO by the H D substitution reactions. Although CD 3 OD was slightly observed, doubly and triply deuterated methanol, CH 2 DOD and CHD 2 OD, were not observed. This implies that D addition to H 2 CO (formation of deuterated methanol) is a minor reaction route. On the other hand, for H + D 2 CO, H addition reactions to form CHD 2 OH proceed at a significant rate. Simultaneously, a competitive reaction, the substitution reaction by H atoms and subsequent H addition (D 2 CO HDCO H 2 CO CH 3 OH) also proceed at a significant rate. However, no H addition to HDCO was observed. The effective surface reaction routes when CO is exposed to H and D atoms are summarized using the present experimental results and the previous results of our group. Key words: astrochemistry dust, extinction ISM: molecules molecular processes 1. INTRODUCTION The surface chemistry on low temperature solids is one of the subjects of great interest in astrochemistry. At low temperatures, the contribution of the quantum mechanical tunneling reaction is relatively enhanced compared to the thermally activated reaction, especially reactions involving light particles such as the hydrogen atom. The tunneling reactions on cold solid surfaces are key processes for chemical evolution in very cold regions of molecular clouds. Successive hydrogenation of CO on a low temperature grain surface was theoretically proposed to explain the formation mechanism of formaldehyde (H 2 CO) and methanol (CH 3 OH) in molecular clouds (Tielens & Allamandora 1987; Tielens & Whittet 1997) as follows: CO HCO H 2 CO CH 3 O CH 3 OH. (S-1) Since the first and third reactions were estimated to have a large activation barrier (over a thousand Kelvins; Woon 2002), they are expected to occur accompanied by the quantum tunneling reaction. The formation of CH 2 OH in the third addition is also energetically possible, and it is more of an exothermic reaction than the formation of CH 3 O. However, the CH 2 OH formation reaction has an activation barrier higher than that for CH 3 O formation (Woon 2002; Osamura et al. 2005). Thus, CH 2 OH formation is less likely than CH 3 O. Experimentally, Hiraoka et al. (2002) sprayed H atoms on solid CO and found H 2 CO formation only. Using quantitatively controlled H source, Watanabe & Kouchi (2002) first demonstrated the efficient formation of H 2 CO and CH 3 OH by the successive hydrogenation of CO on CO H 2 O mixed amorphous ice at 10 K under the condition of interstellar environments. While the intermediates, namely, HCO and CH 3 O, in the reaction sequence (S-1) were not observed in their experiment, it can be understood that the reaction rate constants of the first and third reactions are much smaller than those of the second and forth radical radical reactions, respectively. Very recently, Fuchs et al. (2009) also confirmed H 2 CO and CH 3 OH formation by using thermal gas cracker H atomic source. For several years, these hydrogenation reactions were investigated for the dependence on surface temperatures (Watanabe & Kouchi 2002; Watanabe et al. 2003), compositions (Watanabe et al. 2004, 2006; Hidaka et al. 2004), and hydrogen isotope effects (Nagaoka et al. 2005; Hidaka et al. 2007). The formations of H 2 CO and CH 3 OH inside bulk ice mixtures by the energetic processes such as photolysis (d Hendecourt et al. 1986; Schutte et al. 1996) and proton bombardment (Hudson & Moore 1999) are well confirmed experimentally. However, photolysis is inefficient to produce the observed abundances of H 2 CO and CH 3 OH (Schutte et al. 1996; Watanabe et al. 2007). Although the effective formation of CH 3 OH was reported by Hudson & Moore (1999), this process is expected to be effective on the comet surface predominantly due to existing sufficient proton flux. A high gas-phase abundance of deuterated formaldehyde (formaldehyde-d 1,2 ) and methanol (methanol-d 1 3 ) have been observed in molecular clouds (Loinard et al. 2002; Parise et al. 2004, 2006) and comets (Crovisier et al. 2004). Although the cosmic abundance of deuterium is quite low (D/H 10 5 ; Linsky 1998), deuterium enrichment in various molecules (e.g., D 2 CO/H 2 CO, CHD 2 OH/CH 3 OH) has been reported to be several tens of percent. Theoretical attempts to explain the mechanisms of this deuterium enrichment were carried out using the pure-gas phase model. However, the estimated abundance of the deuterated molecules by the pure-gas phase chemical network calculations did not reach the observed deuteration levels, especially in multiply deuterated species. Charnley et al. (1997) and Stantcheva & Herbst (2003) studied these mechanisms by an approach using gas grain chemistry, so that it is expected that a high gas-phase atomic D/H ratio (over 0.1) is required for reproducing the observed abundances of deuterated molecules. In the gas-phase model, a high atomic D/H ratio is achieved by the following chemical sequence of exothermic 291

292 HIDAKA ET AL. Vol. 702 reactions, were produced by thermal cracking of paraformaldehyde (purity H + 3 +HD H 2 +H 2 D +, 99.8%, Merck) and paraformaldehyde-d 2 (purity 99%, ACROS) powders. The powders were heated to 59 C in glass vacuum and tubes. H 2 CO and D 2 CO vapors were directed to the ASW surface through different lines for preventing contamination by H 2 D + + e H+H+D, H 2 +D, formaldehyde isotopologues. The compositions of the samples were monitored by a Fourier HD + H. transform infrared spectrometer (FTIR) with a resolution of 1cm 1, and the absorption spectra were measured by reflection absorption spectroscopy. The amount of solid molecules in units of the column density is given by following equation: The model of Roberts et al. (2003) yielded a high atomic D/H ratio, 0.3, by incorporating HD + 2 and D+ 3 in H 2D + formation. However, current gas grain models do not quantitatively account for the observed multiply deuterated species with any single D/H ratio. The development of gas grain models is still desirable, especially to account for further formation mechanisms (routes) of deuterated molecules on the grain surfaces. Recently, H D substitution reactions on a cold surface were experimentally investigated and proposed for the formation routes of deuterated methanol (Nagaoka et al. 2005, 2007): CH 3 OH CH 2 DOH CHD 2 OH CD 3 OH. (S-2) The backward processes (e.g., CH 2 DOH CH 3 OH) were found not to proceed. In their experiment, deuterium enrichment of interstellar methanol, including multiply deuterated methanol, is reproduced by the H D substitution reaction on a solid methanol surface with an atomic D/H ratio of 0.1. However, the formation routes of formaldehyde-d 1,2 and the contribution of the hydrogenation (deuteration) of formaldehyde-d 1,2 to the formation of methanol-d 1 3 on the cold grain surface are still unclear. In this paper, the reactions of the hydrogen isotope exchange and the hydrogenation (deuteration) of solid formaldehyde on amorphous solid water (ASW) at 10 20 K are investigated experimentally and the formation routes of formaldehyde-d 1,2 and methanol-d 1 3 are discussed. 2. EXPERIMENTAL Experiments on the exposure of solid H 2 CO and D 2 CO on ASW to D and H atoms, respectively, were carried out by using an apparatus named Laboratory Setup for Surface reaction in Interstellar Environment (LASSIE). Details of LASSIE have been described previously (Watanabe et al. 2003; Hidaka et al. 2004). The cold H and D atoms were produced in an atomic source chamber by the dissociation of H 2 or D 2 molecules in a microwave-induced plasma in a Pyrex grass tube and transferred to a cold aluminum pipe connected to the cold head of a He refrigerator. Before reaching the sample surface, the kinetic energies of the atoms were cooled to about 100 K by collisions with the wall of the cold aluminum pipe. The fluxes of these atomic beams were about 2 10 14 cm 2 s 1. At the end of the aluminum pipe, a deflector was mounted to eliminate any charged particles and long-lived metastable H and D atoms from the plasma. Solid H 2 CO and D 2 CO on the ASW samples were prepared using a cold aluminum substrate mounted at the center of the chamber. The substrate was connected to another He refrigerator and can be cooled down to 8 K. The underlying ASW layer was formed by H 2 O vapor deposition through a capillary plate on the substrate at 10 20 K. H 2 CO and D 2 CO layers were also formed by vapor deposition on ASW at the same temperatures as those used for the ASW formation. The H 2 CO and D 2 CO vapor N = cos θ ζ (ν)dν, (1) 2A log e where N is the column density (molecules cm 2 ), θ is the IR incident angle of 83, A is the integrated absorption coefficient (cm molecule 1 ), and ζ (ν) is the absorbance at a wave number ν. Using the adsorption coefficients of 2.0 10 16 cm molecule 1 for the OH-stretching mode of H 2 O (Hagen et al. 1981) and 9.6 10 18 cm molecule 1 for the CO-stretching mode of H 2 CO (Schutte et al. 1993), the column densities of the H 2 O and H 2 CO layers were approximately 30 10 15 and 9 10 15 molecules cm 2, respectively. The absorption coefficient for D 2 CO has not been previously reported in literature. Therefore, the absorption coefficient for D 2 CO (A D2 CO) was estimated by the following method. The area of the temperature programmed desorption (TPD) spectrum versus absorbance is plotted for several amounts of deposited H 2 CO and D 2 CO. The plots show good linearity and provide the proportionality factor α for the relation between the amount of deposited molecules and absorbance. A D2 CO can be calculated with A H2 CO by using the following equation: A D2 CO = α D 2 CO A H2 CO. (2) α H2 CO The absorption coefficient for D 2 CO is estimated to be 1.3 10 17 cm molecule 1, and D 2 CO was deposited in the same amount as H 2 CO on ASW in the present experiment. Assuming the surface number density of 10 15 molecules cm 2,the column densities of the formaldehyde samples correspond to 9 monolayers (MLs). In the reflection absorption spectroscopy, the infrared reflectance of films with few-μm thickness were studied by Teolis et al. (2007). They show that the optical depths and integrated band areas are not proportional to the film thickness and the column density calculated from the band area using Equation (1) includes a large error. In the case of very thin films (severalnm thickness), it was reported that the ratio of the reflectance is linearly proportional to the mean film thickness (Kolb & McIntyre 1971). Our experimental condition obviously corresponds to be the latter case. In fact, the good linearity was found between the integrated band area and the deposition time of the gas when the sample was prepared by vapor deposition. Reflection absorption spectroscopy has good detection sensitivity compared to the transmission method. The sensitivity depends on the incident angles and molecular species, and is approximately calculated with the refractive index r using (4sin 2 θ/cosθ)(r 1 /r 2 ) 3 (Kolb & McIntyre 1971), where r 1 and r 2 indicate the refractive indices of vacuum and the adsorbed species, respectively. Equation (1) considers only the difference in infrared transmission volume in the samples and not the refractive index. Using a simple corrected Equation (1), we

No. 1, 2009 REACTIONS OF H(D) ATOMS WITH SOLID FORMALDEHYDE ON ASW 293 CO DCO HCO (R-16) (R-17) (R-18) (R-19) (R-4) (R-3) (R-2) (R-1) (S-1) (S-3) D CO 2 (R-10) HDCO (R-9) (R-22) (R-23) (R-24) (R-25) H CO 2 (S-4) (R-8) (R-7) (R-6) (R-5) CD 3 O (S-5) CHD 2 O CH 2DO CH 3 O (R-12) (R-11) CD OD 3 CD 3OH CHD OH CH DOH 2 2 CH OH 3 Figure 1. Surface reaction network when CO is exposed to H and D atoms. The numbers indicate the reactions and reaction sequences discussed in the text. The short black and gray arrows show the H and D atom addition reactions, and the short dashed-black and -gray arrows show the D and H atom abstraction reactions, respectively. The chain arrows indicate the H D direct exchange reactions. The reaction sequences are represented by the long arrows. Molecules in rectangular frames are known to exist based on astronomical observations. (S-2) checked the difference between the column densities estimated by the reflection method and that by the transmission method in pure amorphous solid H 2 O and CO. The results indicated that the difference in column densities of both species was within an approximately factor of 2, and the differences were not significant. On the other hand, in the estimation of column densities using the refractive index correction, the difference between the two methods was approximately a factor of 5, with the assumption of r 2 = 1.3 for solid H 2 O. Although the reasons for better correction of Equation (1) and for species-independent behavior are unknown, this could be attributed to the extremely rough surface of the sample (roughness comparable to the thickness of the sample is expected). The determination of the exact column density is difficult even by the transmission method in the layered and the mixed samples, because the absorption coefficient depends on the coexisting molecular species and quantitative ratio in the sample. Thus, we believe that estimation of column densities using Equation (1) is reasonably acceptable in the present experimental conditions. It should be noted that the discussions in the present study are based on relative column densities, therefore any uncertainties due to the measurement methods do not affect the reliability of the quantitative values in the discussion below. 3. RESULTS AND DISCUSSION Numerous chemical reactions appear in this section, therefore the reactions and reaction sequences are summarized in Figure 1 as a guide for. 3.1. Exposure of H 2 CO on ASW to D Atoms Figure 2(a) shows the infrared absorption spectrum of H 2 CO on ASW sample at 15 K (top), and the change in the sample spectrum after exposure to cold D atoms (bottom). The observed peak positions and assignments are listed in Table 1. The newly formed molecules are HDCO, D 2 CO, and CD 3 OD. CH 2 DOD and CHD 2 OD were not observed, indicating that D atom addition to H 2 CO and HDCO is negligible in the D atom exposure of H 2 CO. During D atom exposure of H 2 CO, peaks of deuterated formaldehyde appeared at an early stage, and that of CD 3 OD appeared at the last stage. Figure 3(a) shows the variations in the column densities of H 2 CO, HDCO, D 2 CO, and CD 3 OD calculated using the integrated absorbance of CO-stretching bands with each absorption coefficient. In these plots, the coefficient for HDCO was assumed to be a value intermediate between that for H 2 CO and D 2 CO. The absorption coefficient of CD 3 OD was calculated to be 6.4 10 18 cm molecule 1 from the data of Nagaoka et al. (2007) in which the coefficient of CD 3 OD relative to CH 3 OH was determined. The plots show that HDCO is the first product and reaches a maximum value as the exposure time of D atoms is increased. D 2 CO arises subsequently and increases with a decrease in HDCO. Finally, the formation of CD 3 OD starts after significant formation of D 2 CO. The behavior of these column densities implies the following reaction scheme as a result of D atom exposure of H 2 CO: H 2 CO HDCO D 2 CO CD 3 OD. (S-3) Figure 4 shows the surface reaction network when H 2 CO on ASW was exposed to D atoms at a low temperature. This is a part of the reaction network for CO formaldehyde(-d) methanol(-d) (Watanabe & Kouchi 2008). There are three energetically allowed routes for H D substitution reaction in formaldehyde. Among these, two routes are composed of a combination of abstraction and addition reactions: H 2 CO + D HCO + HD, (R-1) HCO + D HDCO, (R-2) HDCO + D DCO + D 2, (R-3) DCO+D D 2 CO, (R-4) and H 2 CO + D CH 2 DO, (R-5) CH 2 DO + D HDCO + HD, (R-6) HDCO + D CHD 2 O, (R-7) CHD 2 O+D D 2 CO + HD. (R-8) The third route is a direct exchange via metastable states of methoxy radicals: H 2 CO + D CH 2 DO HDCO + H, (R-9)

294 HIDAKA ET AL. Vol. 702 (a) (b) Figure 2. Infrared absorption spectra of the initial samples (top) and variations in the absorption spectra (bottom) for (a) D atom exposure of H 2 CO on ASW and (b) H atom exposure of D 2 CO on ASW at 15 K, respectively. Peaks below and above the base lines represent a decrease and increase in the absorbance, respectively. HDCO + D CHD 2 O D 2 CO + H. (R-10) The reactions (R-6) and (R-8) compete with the following D atom addition reactions to produce deuterated methanol, and CH 2 DO + D CH 2 DOD CHD 2 O+D CHD 2 OD, (R-11) (R-12) respectively. The ab initio calculations show that CH 3 O+H CH 3 OH is a non-barrier reaction in gas phase whereas the abstraction reaction CH 3 O+H H 2 CO + H 2 has a small activation barrier of 55 85 K (Xu et al. 2007;Lietal.2004). If the contribution of the route via reactions (R-5) (R-8) is significant in the H D substitution of formaldehyde, the formation of doubly and triply deuterated methanol CH 2 DOD, CHD 2 OD should also occur due to non-barrier reactions. However, CH 2 DOD and CHD 2 OD were not observed in the present experiments. There is a possibility that the deuterated methanol was lost by the H D substitution reactions of the methyl side in methanol (reaction sequence (S-2); Nagaoka et al. 2005, 2007 to CD 3 OD). However, since the substitution reactions in methanol are much slower than the radical radical reaction, CHD 2 OD should be observed even in such a case. Therefore, the routes (R-5) (R-8) are unlikely to be the main route for H D substitution of formaldehyde and the substitution mainly proceeds through reaction routes (R-1) (R-4) and/or (R-9) (R-10). The activation barriers for reactions (R-1) and (R-9) were determined to be 1899 and 1766 K, respectively, by gas-phase experiments (Oehlers et al. 2000). The contributions of these two routes cannot be evaluated in the present experiments. For the reasons mentioned above, the contribution of H D substitution in singly Table 1 List of Peak Positions in the Infrared Absorption Spectra of the D + H 2 CO System Wavenumber (cm 1 ) Timing a Molecule Assignment b 977.70 Late CD 3 OD CO stretching 988.44 Early D 2 CO CD 2 wagging 1029.64 Early HDCO CH 2 rocking 1064.91 Late CD 3 OD CD 3 asymmetry bending 1101.44 Early D 2 CO CD 2 scissoring 1123.94 Late CD 3 OD CD 3 symmetry bending 1176.26 H 2 CO CH 2 rocking 1247.36 H 2 CO CH 2 wagging 1396.71 Early HDCO CH 2 scissoring 1498.24 H 2 CO CH 2 scissoring 1676.26 Early D 2 CO CO stretching 1694.11 Early HDCO CO stretching 1733.20 H 2 CO CO stretching 2076.57 Late CD 3 OD CD 3 symmetry stretching 2087.43 Early D 2 CO CD stretching 2139.73 Early HDCO CH asymmetry stretching 2201.41 Early D 2 CO CD 2 scissoring overtone 2213.05 Late CD 3 OD CD 3 asymmetry stretching 2244.97 Late CD 3 OD CD 3 asymmetry stretching Notes. a Timing of the peak appearing during D atom exposure. b Assignments of vibration modes were made by comparison to references (Tso &Lee1984; Falk & Whalley 1961). and doubly deuterated methanol is negligible small in the formation of CD 3 OD. Therefore, the formation of CD 3 OD proceeds by successive D atom addition reaction to the substituted D 2 CO: D 2 CO CD 3 O CD 3 OD. The chemical reactions with D 2 should be mentioned here, since undissociated D 2 molecules are present in our atomic

No. 1, 2009 REACTIONS OF H(D) ATOMS WITH SOLID FORMALDEHYDE ON ASW 295 (a) (b) Figure 3. Variation in column densities normalized to the initial amount of formaldehyde due to (a) D atom exposure of H 2 CO and (b) H atom exposure of D 2 CO, respectively, as a function of exposure time at 10 20 K. The error bars represent standard errors. CD OD D CO 2 CHD OD DCO 3 2 2 CO HDCO CH DOD HCO CD 3 O CHD 2 O CH 2DO H CO 2 Figure 4. Surface reaction network when H 2 CO on ASW was exposed to D atoms. Molecules enclosed in rectangular square frames are those observed in the present experiments. Species written in smaller font represent radicals. The dashed and solid arrows show the H atom abstraction reactions with formation of HD molecules and D atom addition reactions, respectively. Shaded arrows indicate the H D direct exchange processes in the reaction (R-9) and(r-10). The reactions shown by all arrows are exothermic. The bold arrows represent the dominant routes of the chemical reactions. beam. We performed experiments on D 2 molecule exposure of H 2 CO on ASW samples; no changes were observed in the IR spectrum. There are intermediate radical reactions with D 2 molecules are follows: HCO + D 2 HDCO + D, DCO + D 2 D 2 CO + D, CD 3 O+D 2 CD 3 OD + D. (R-13) (R-14) (R-15) In the hydrogen system, HCO + H 2 H 2 CO + H is endothermic (+4800 K; Woon 2002), while CH 3 O+H 2 CH 3 OH + H is slightly exothermic ( 120 K; Kerkeni & Clary 2006). Although methanol formation through the CH 3 O+H 2 reaction is energetically allowed, the activation barrier of that reaction was calculated to be over 6000 K (Kerkeni & Clary 2006). Thus, the methanol formation process of CH 3 O+H 2 CH 3 OH + H is less likely than the barrier-free formation process of CH 3 O+H CH 3 OH. Since the difference in the zero-point energy in isotopic reaction systems would be much smaller than the heat of reaction and the activation barrier for the three intermediate radical reactions (R-13) (R-15), these reactions can be negligible. 3.2. Exposure of D 2 CO on ASW to H Atoms Figure 2(b) shows the infrared absorption spectrum of D 2 CO on ASW sample at 15 K (top) and the variation in the spectrum after exposure to cold H atoms (bottom). The observed peak positions and assignments are listed in Table 2. The molecules formed were HDCO, H 2 CO, CH 3 OH, and CHD 2 OH. The presence of CHD 2 OH implies that the H atom addition reactions to D 2 CO proceed at a significant rate. Figure 3(b) shows the variations in the column densities of D 2 CO, HDCO, H 2 CO, CH 3 OH, and CHD 2 OH. The CO-stretching bands for CH 3 OH and CHD 2 OH overlap in the IR spectra. For the peak area separation of CH 3 OH and CHD 2 OH in the CO-stretching mode, the area for CHD 2 OH was calculated by using the area of the CD 2 -wagging mode for CHD 2 OH and the ratio of peak areas between the CD 2 wagging and CO stretching for CHD 2 OH (Nagaoka et al. 2007), so that the area for CH 3 OH was separated from the area of complex CO-stretching bands. The absorption coefficient of 6.9 10 18 cm molecule 1 for CHD 2 OH, derived

296 HIDAKA ET AL. Vol. 702 Table 2 List of Peak Positions in the Infrared Absorption Spectra of the H + D 2 CO System Wavenumber (cm 1 ) Timing a Molecule Assignment b 948.20 Early CHD 2 OH CD 2 wagging 975.04 Late?? 988.35 D 2 CO CD 2 wagging 1032.72 Early CHD 2 OH, CH 3 OH CO stretching 1087.45 Early CHD 2 OH CD 2 bending 1100.76 D 2 CO CD 2 scissoring 1130.34 Late CH 3 OH CH 3 rocking 1303.38 Early CHD 2 OH CD 2 rocking 1328.53 Early CHD 2 OH CD 2 rocking 1395.53 Early HDCO CH 2 scissoring 1679.05 D 2 CO CO stretching 1698.28 Early HDCO CO stretching 1722.75 Early H 2 CO CO stretching 1880.20 D 2 CO CD 2 rocking overtone 1974.85 D 2 CO CD 2 wagging overtone 2094.72 D 2 CO CD stretching 2127.28 Late CHD 2 OH CD 2 symmetry stretching 2177.60 Late CHD 2 OH CD 2 bending overtone 2210.16 D 2 CO CD 2 scissoring overtone Notes. a Timing of the peak appearing during H atom exposure. b Assignments of vibration modes were made by comparison to references (Tso &Lee1984; Serrallach et al. 1974; Barros et al. 2005). by the data of relative integrated band area to CH 3 OH (Nagaoka et al. 2007), was used in the estimation of the column density. The negative column density for CH 3 OH at the early exposure time results from the error associated with the above-mentioned peak separation. In Figure 3(b), HDCO was found to be the first product as well as the result of D atom exposure of H 2 CO. H 2 CO and CH 3 OH were formed successively with a decrease in HDCO. In contrast to the results of D atom exposure of H 2 CO, CHD 2 OH was formed immediately. The time variations in these observed column densities imply the following two reaction routes: D 2 CO HDCO H 2 CO CH 3 OH (S-4) and D 2 CO CHD 2 O CHD 2 OH. (S-5) Figure 5 shows the surface reaction network when D 2 CO on ASW was exposed to H atoms at low temperatures. The plausible route for the D H substitution reaction in formaldehyde is a combination of abstraction and addition reactions, D 2 CO + H DCO + HD, (R-16) DCO + H HDCO, (R-17) HDCO + H HCO + HD, (R-18) HCO + H H 2 CO. (R-19) Direct exchange processes via metastable states of methoxy radicals, D 2 CO + H CHD 2 O HDCO + D (R-20) HDCO + H CH 2 DO H 2 CO + D, (R-21) cannot occur at low temperatures because these processes are endothermic reactions due to the difference in zero-point energy between formaldehyde isotopologues. D CO 2 DCO CO HDCO HCO CHD 2 O CH 2DO CHD OH 2 H CO 2 CH DOH 2 CH O 3 CH OH 3 Figure 5. Surface reaction network when D 2 CO on ASW was exposed to H atoms. Molecules enclosed in rectangular frames are those observed in the present experiments. Species written in smaller font represent radicals. The dashed and solid arrows show the D atom abstraction reactions with formation of HD molecules and the H atom addition reactions, respectively. The reactions shown by all arrows are exothermic. The bold arrows represent the dominant routes of the chemical reactions. Also, in this reaction system, there is the other energetically possible route of sequential reactions, D 2 CO + H CHD 2 O, CHD 2 O+H HDCO + HD, HDCO + H CH 2 DO, (R-22) (R-23) (R-24) CH 2 DO + H H 2 CO + HD. (R-25) The large yield of CHD 2 OH indicates that D 2 CO converts to CHD 2 OH through CHD 2 O by successive H atom addition reactions. That is, reaction (R-22) proceeds significantly. After the formation of CHD 2 O, the reaction (R-23) competes with the H atom addition reaction CHD 2 O+H CHD 2 OH. As discussed in the previous section, H atom addition to the methoxy radical would be much faster reaction than the D atom abstraction reaction (R-23) due to non-barrier radical radical reaction. Furthermore, since singly deuterated methanol CH 2 DOH was not detected, it can also be predicted that the reaction (R-24) is negligible. As a result, we conclude that the sequential route (R-22) (R-25) is inefficient for the conversion of D 2 CO to H 2 CO. TheformationofCH 3 OH proceeds by successive H atom addition reactions to the substituted H 2 CO: H 2 CO CH 3 O CH 3 OH. The formation of CH 3 OH by the D H substitution reactions in deuterated methanol, which proceeds by the combination of addition and abstraction, is the other energetically allowed reaction route: CHD 2 OH CH 2 DOH CH 3 OH. However, it was demonstrated that these reactions do not induce any change in the IR spectra when solid CH 2 DOH and CHD 2 OH were exposed to H atoms at 10 K (Nagaoka et al. 2005). 3.3. Quantitative Evaluation of the Effective Rate Constants Using Effective Mass A large surface temperature dependence of the reactivity was observed in both experiments, and the reactivities for all of the observed substitution and addition reactions drop above 20 K where the sticking coefficient of the H (D) atom becomes

No. 1, 2009 REACTIONS OF H(D) ATOMS WITH SOLID FORMALDEHYDE ON ASW 297 very small. This implies that solid formaldehyde reacts with the adsorbed H (D) atoms on the sample surfaces via the socalled Langmuir Hinshelwood process. If the reactions occur via the Eley Rideal process, then the reactivity should have little dependence on the surface temperature. For quantitative analysis, the experimental data were fitted by the following single exponential function: [ΔX] t = β exp( k i n m t), (3) [formaldehyde-d 0,2 ] t=0 where β is the saturation value, t is the exposure time, n m (m = H or D) is the number density of atoms on the surface, and k i is a reaction rate constant. The reaction rate constant k i cannot be separated from the fitting parameter of k i n m due to the difficulty in estimating the absolute value of n m. Therefore, we discuss the quantitative determination using the effective rate constant defined as k i n m. Here, n m is assumed to be time-independent and is governed mainly by the balance between the supply of adsorbed atoms from the beam and loss by H H (D D) recombination and desorption of H (D) atoms from the surface. The effective rate constants of the H D substitution reaction for H 2 CO HDCO (k 1 ) are determined by the fitting of the H 2 CO decay curves. The obtained values are regarded as effective rate constants of reactions (R-1) and/or (R-9)with the activation barriers. Thus, k 1 may include the contributions of two chemical reactions (R-1) and (R-9). The effective rate constants for the D H substitution reaction of D 2 CO HDCO (k 2 ), namely, the first step of the reaction sequence (S-4), are derived by subtracting the rates of CHD 2 OH formation, k 3,bythe reaction sequence (S-5) from the decay rates of D 2 CO, because D 2 CO is consumed by both sequences (S-4) and (S-5). Since, in the reaction sequence (S-5), the formation of CHD 2 O radical is a rate-limiting reaction, the rates of CHD 2 OH formation are almost equivalent to the rate of D 2 CO consumption by H atom addition reaction to D 2 CO (R-22). Figure 6 shows the fitting results for the two substitution reactions and the addition reaction, and the effective rate constant of these reactions are summarized in Table 3. All effective rate constants k i n m drop at over 15 K. Since the reaction rate constant generally increases with increasing surface temperature, the observed inverse relation of the effective rate constants at over 15 K is attributed to a decrease in n m with increasing surface temperature. In D atom exposure of H 2 CO, the activation energies for the first three competitive reactions (R-1), (R-5), and (R-9) were estimated to be 1897, 1958, and 1765 K, respectively, by gas-phase experiments above room temperature (Oehlers et al. 2000), with values close to each other. In the H atom exposure of D 2 CO, the activation energy for reaction (R-16)was experimentally reported to be 1.0 kcal mol 1 higher than that for the isotopically different reaction H + H 2 CO H 2 +HCO (McNesby et al. 1960), which can be estimated to be 2233 K by using the activation energy for the isotopically different reaction of 1730 K (Oehlers et al. 2000). On the other hand, the activation energies for the reaction (R-22) have not been previously reported. However, the differences in the activation energies by the isotope effect are expected to be small; for instance, the activation energies in the isotopically different reactions of H 2 CO + H CH 3 O, H 2 CO + D CH 2 DO (R-5), and HDCO + H CH 2 DO (R-24) were narrowly distributed between 1958 and 2232 K (Oehlers et al. 2000). Thus, all the activation energies values discussed above in both experiments are expected to be close to each other. Since these Figure 6. Data fitting for the effective rate constants of (a) H 2 CO HDCO, including the contributions of reactions (R-1) and(r-9) (k 1 n D ), (b) D 2 CO + H HD + DCO (k 2 n H ), and (c) D 2 CO + H CHD 2 O(k 3 n H ), respectively. The fitting function is shown in Equation (3). activation energies are significantly higher, compared to the surface temperature of 10 20 K, the observed reactions are considered to proceed by quantum tunneling. The probability of potential tunneling strongly depends on the tunneling mass in the reaction coordinate, because the tunneling reaction results from the wave nature of matter. Therefore, the probability is enhanced when the tunneling mass is small, but it drastically drops with increasing tunneling mass. In fact, the addition of D atom to CO was found to be significantly slower than H addition (Hidaka et al. 2007). In the present experiments, not only the D H substitution but also the H atom addition reactions were observed in the D 2 CO + H system, whereas mainly only the H D substitution reaction was observed in the H 2 CO + D system. If the differences in the activation energies for the addition and the substitution reactions in both reaction systems are small, as described (a) (b) (c)

298 HIDAKA ET AL. Vol. 702 Table 3 Effective Rate Constants and Relative Reaction Rate Constants Surface Temperature (K) 10 15 20 Effective rate constant (minute 1 ) k 1 n a D 0.064 ± 0.01 0.22 ± 0.03 0.12 ± 0.02 k 2 n b H 0.026 ± 0.0085 0.085 ± 0.011 0.047 ± 0.003 k 3 n c H 0.1 ± 0.029 0.15 ± 0.021 0.072 ± 0.009 Relative reaction rate constant d k 1 0.97 k 2 0.38 k 3 0.66 Notes. Top: Effective rate constants for the H D substitution reaction of H 2 CO (k 1 n D ), D H substitution reaction of D 2 CO (k 2 n H ) and H atom addition reaction to D 2 CO (k 3 n H ). Bottom: Relative reaction rate constants k 1,2,3 to that for H + CO HCO in CO (1 ML) on ASW (Hidaka et al. 2004) at15kareshownin parentheses. a H atom abstraction and H D exchange processes: D + H 2 CO HCO + HD and HDCO. b D atom abstraction process: H + D 2 CO DCO + HD. c H atom addition process: H + D 2 CO CHD 2 O. d The values of the relative rate constants were determined by low coverage experiments with reactant molecules of 1 ML present on ASW (10 MLs). The value for normalization used was 0.34 minute 1 for hydrogenation of CO (1 ML) on ASW (10 MLs). See the text for details. above, the discrepancy in reaction routes could be understood by considering the magnitude of the tunneling mass in the respective reactions. For the addition reactions (R-5) and (R-22), the tunneling mass in the reaction coordinate is simply described by the reduced mass. Unlike the two-body addition reaction, for the abstraction processes of reactions (R-1) and (R-16), the reduced mass cannot be defined due to the three free particles involved in the reaction system. Hence, the effective mass is used to evaluate the tunneling mass in the potential tunneling between the initial and final states. In a linear triatomic molecule, AXB, the effective mass is represented by the following equation with the parameter c: m c = m am b (1 + c) 2 + m b m x c 2 + m a m x, (4) M(1 + c 2 ) where m a, m x, and m b are the atomic masses of A, X, and B atoms, respectively, M is the mass of the molecule M = m a + m x + m b, and c is a ratio of the infinitesimal change of one bond distance R 1 to that of the other bond distance R 2 : c = dr 2 /dr 1 (Johnston 1966). The variation in effective mass with the angle in the R 1 R 2 plane is given in Figure 7 for the triatomic complex: m a = 1or2,m x = 2or1,m b = 30 or 29 representing the structures H D DCO and D H HCO. Unfortunately, the potential energy surfaces of H D DCO and D H HCO for atom transfer in the R 1 R 2 plane have not been previously reported. However, the transition between the two states by tunneling on the potential surface for atom transfer normally proceeds by the tunneling of the barrier at a potential ridge around 45 in the R 1 R 2 coordinate. In Figure 7, the effective masses for the H (dashed line) and D (solid line) atom transfer are approximately 0.5 and 1 atomic masses at around 45, respectively. For D 2 CO + H, the tunneling mass value of 1 (effective mass) for the D atom abstraction reaction (R-16; D atom transfer) is Figure 7. Variation of the effective mass for A X B molecules calculated using Equation (4) when the bond lengths of R 1 (A X) and R 2 (X B) change. The horizontal axis indicates the angle with tangent c = dr 2 /dr 1. Curves for A = H, X = D, B = DCO and A = D, X = H, B = HCO are shown by the solid and dashed lines, respectively. close to the mass of 0.97 for reduced mass of the H addition reaction to D 2 CO (R-22). Therefore, it is reasonable that the contributions of reactions (R-16) and (R-22) are competitive. In the same manner, for H 2 CO + D, the tunneling mass of 0.5 (effective mass) for the D atom abstraction reaction (R-1; H atom transfer) is much smaller than the value of 1.9 for the reduced mass of the D atom addition reaction to H 2 CO (R-5). This is the reason why the contribution of reaction (R-5) is negligible. 4. IMPLICATIONS FOR THE SURFACE REACTION NETWORK The successive reactions of H and D atoms with CO on cold grain surfaces are key processes to understand not only formaldehyde and methanol formation but also deuterium enrichment in these molecules. The surface reaction network for the CO formaldehyde methanol system, including the hydrogenation, deuteration, and hydrogen isotope exchange of formaldehyde and methanol, and the relative reaction rate constants for H 2 CO + H CH 3 O, CO + D DCO, and H D substitution of CH 3 OH normalized to that for CO + H HCO on ASW at 15 K were summarized by Watanabe & Kouchi (2008). Obtaining the relative reaction rate constants for all chemical reactions in the reaction network has important implications for chemical modeling of molecular clouds including the grain surface reactions. From the present experiments, the relative reaction rate constants k 1, k 2, and k 3 for H 2CO HDCO ((R-1) and (R-9)), D 2 CO + H HD + DCO (R-16), and D 2 CO + H CHD 2 O(R-22) were determined, respectively. These values are normalized to the rate constant for H + CO HCO in CO on ASW at 15 K (Hidaka et al. 2004). The values of the relative rate constants were determined by low coverage experiments in which 1 ML of reactant molecules (cf. a ML is defined as 10 15 molecules cm 2 ) is deposited on ASW (10 MLs). The amounts of reactant molecules and ASW are the same as those of Hidaka et al. (2004). The amount of 1 ML reactant molecules does not correspond to a coverage of 100% due to the large surface area of the ASW (Kimmel et al. 2001). Thus, the coverage is expected to be quite low. In the low coverage condition, H (D) atoms preferentially

No. 1, 2009 REACTIONS OF H(D) ATOMS WITH SOLID FORMALDEHYDE ON ASW 299 0.38 D CO 2 DCO 0.66 CO HDCO HCO CD 3OH CHD OH CH DOH 2 1 = 0.34 min -1 0.97* 2 H CO 2 CD 2OH, CHD 2 O CHDOH CH 2OH, CH 3 O (0.78) (1) 0.5 (1.5) CH OH 3 Figure 8. Surface reaction network when CO is exposed to H and D atoms based on Watanabe et al. (2006), Watanabe & Kouchi (2008), and the present study. Molecules enclosed in rectangular frames are known to exist, based on astronomical observations. The solid black and gray arrows show the H and D atoms addition reactions, respectively. The dashed black and gray arrows show the D and H atoms abstraction reactions with formation of HD molecules, respectively. Shaded arrows indicate the H D direct exchange processes in reaction (R-9) and(r-10). The numbers shown are the reaction rate constants relative to those of H + CO HCO at 15 K (Hidaka et al. 2004). The number with an asterisk shows the results of the H atom abstraction and/or the H D direct exchange reactions. The relative rates in parentheses show the results of pure solid methanol experiments at 10 K (Nagaoka et al. 2007). adsorb at the H 2 O sites of ASW and the number densities have little dependence on the reactant molecular species. In contrast, the atoms will first land on the reactant molecules in the high coverage condition in which the sticking coefficients, and thus the surface number densities of atoms, n m (m = H or D), are strongly affected by the reactant species because of the different atom molecule interactions. In order to minimize the influence of reactant molecules on n m, we often adopt the relative reaction rate constants obtained in the low coverage experiments. Therefore, the experiment for D atom exposure of H 2 CO (1 ML) on ASW was performed under the same experimental condition (same coverage, atom flux, and surface temperature) as that for H atom exposure of CO (1 ML) on ASW (Hidaka et al. 2004), and consequently, the relative reaction rate constant k 1 is obtained by the ratio of the effective reaction rate constants. The other relative reaction rate constants (k 2, k 3 ) were calculated by using the ratios of effective rate constants k 2 n H /k 1 n D and k 3 n H /k 1 n D with k 1. The obtained k 1, k 2, and k 3 are summarized in Table 3. In our previous work (Nagaoka et al. 2005; Watanabe & Kouchi 2008), we demonstrated that the abundance of deuterated formaldehyde and methanol observed in molecular clouds are well reproduced by experiments in which the solid CO was exposed to H and D atoms simultaneously with a D/H ratio of 0.1. Also, the high abundances in formaldehyde and methanol observed can be partly explained by the grain surface reactions. However, these previous studies did not provide information about the dominant formation routes for deuterated formaldehyde. The present experiments suggest that the formation of HDCO and D 2 CO do not require the route of D atom addition to CO, namely, CO + D DCO, as the first step because the reaction rate constants of H atom addition to CO (Hidaka et al. 2007) and H D substitution of H 2 CO to form HDCO are much larger than those of D atom addition to CO. Therefore, D 2 CO would be produced by the H D substitution reaction of preexisting HDCO. Although D 2 CO converts to HDCO and H 2 CO by the D H substitution reactions, the formation of doubly deuterated methanol, CHD 2 OH, by H atom addition to D 2 CO proceeds 1.7 times faster than the H D substitution of D 2 CO to HDCO. In the present experiments, significant deuterated methanol formation by H or D atom addition reactions to HDCO were not observed. 5. CONCLUDING REMARKS The reaction routes, when H 2 CO and D 2 CO are exposed to D and H atoms on ASW at 10 20 K, respectively, were studied experimentally by IR spectroscopy. For D + H 2 CO, H 2 CO was converted to HDCO and D 2 CO by the H D substitution reactions, and CD 3 OD was slightly formed by D atom addition to D 2 CO. Doubly and triply deuterated methanol, CH 2 DOD and CHD 2 OD, were not observed in the IR spectra, indicating that D atom addition to H 2 CO and HDCO is much slower than the H D substitution reactions. Thus, the H D substitution reactions in formaldehyde (H 2 CO HDCO D 2 CO) were the dominant reaction route. For H + D 2 CO, D H substitution reactions (D 2 CO HDCO H 2 CO) and subsequent CH 3 OH formation by H atoms addition to H 2 CO proceed efficiently. 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