The Astrophysical Journal, 577:265–270, 2002 September 20 # 2002. The American Astronomical Society. All rights reserved. Printed in U.S.A.
FORMATION OF FORMALDEHYDE BY THE TUNNELING REACTION OF H WITH SOLID CO AT 10 K REVISITED Kenzo Hiraoka, Tetsuya Sato, Shoji Sato, Norihito Sogoshi, Tatsuya Yokoyama, Hideaki Takashima, and Shinichi Kitagawa Clean Energy Research Center, Yamanashi University, Takeda-4, Kofu 400-8511, Japan;
[email protected] Received 2001 October 22; accepted 2002 May 24
ABSTRACT The reaction of H atoms with the solid CO thin film was reinvestigated in the temperature region of 10–25 K. H2CO was found to be only the reaction product, and no other products such as CH3OH were detected. This indicates that the tunneling reactions of H with H2CO to form CH3OH are even slower than the slow reaction of H with CO to form H2CO. The CH3OH found in the envelopes of the dark clouds may have other sources for their production in addition to reactions 2H þ H2 CO ! CH3 OH, e.g., reaction of O(1D) with CH4 trapped on the dust grains to form CH3OH. The yield of H2CO from the reaction H with solid CO showed a steep increase with a decrease of temperature from 25 to 10 K. This indicates that the dark clouds whose temperature is kept at as low as 10 K are the favorable place for the chemical evolution via tunneling reactions. The erosion of the solid CO film was not observed with the spray of the H atoms over the CO solid film in the temperature range of 10–25 K. This finding suggests that the contribution of the highly exothermic reaction 2H þ H2 to desorption of the grain mantle may not be as large as thought before. Subject headings: astrochemistry — ISM: molecules — methods: laboratory — molecular processes
detected. The yield of H2CO was found to increase steeply with a decrease of temperature from 25 to 10 K.
1. INTRODUCTION
Formaldehyde (H2CO) is one of the most complex molecules for which specific gas-phase reaction sequences have been proposed (Watson 1977): CH3 þ O ! H2 CO þ H :
2. EXPERIMENTAL
ð1Þ
The general experimental procedures were similar to those described previously (Hiraoka et al. 2001). The cryocooler (Iwatani Plantech, type D310) and a quadrupole mass spectrometer (Leda Mass, Microvision 300D) were housed in a vacuum manifold. The vacuum chamber was evacuated by two turbomolecular pumps (ULVAC, UTM500, 500L/s and Seiko Seiki, STP-H200, 200L/s) connected in tandem. The base pressure of the vacuum system under the current experimental conditions was 5 1010 torr after baking the vacuum system. The sample gas CO was deposited on the silicon substrate ([100] surface with the size of 30 50 0:5 mm3), which was firmly pressed onto the cold head of the cryocooler using indium foil between the mating surfaces. After the deposition of the sample, the H2 gas was introduced to the bottleneck discharge (Fig. 1) tube through the stainless steel capillary (1 m long and 0.1 mm inner diameter). The reagent H2 gas (Iwatani Gas, UHP grade, 99.99999%) was purified ˚ molecular sieve trap kept at 77 by passing it through a 3 A K. The H atoms produced by the DC discharge of a few torr H2 in the bottleneck discharge tube were sprayed over the sample film. The sample film was completely prevented from being bombarded by the charged particles and UV photons produced by the plasma. The temperature of H atoms sprayed over the sample film was about 27 K. The flux of the H2 molecules sprayed over the film was about 1016 molecules s1 cm2. The measurement of the flux of H atoms sprayed over the solid film was not made in the present experiment. It is generally believed that a few percent of reagent gas is decomposed into atoms in the ordi-
The production of H2CO is then crucially dependent on adequate production of CH3. On the other hand, the significant abundance of H2CO in the diffuse envelopes of dark clouds cannot be explained by purely gas-phase processes. The production of CH3 is insufficient in these regions to drive equation (1) to form enough H2CO (Sen, Anicich, & Federman 1992). After H2, CO is the most abundant molecule in dense clouds and thus has special importance. Tielens et al. (1991) suggested that most of the CO accreted in H2O-rich mantles has reacted with other species on the grain surface. When the H-atom accretion rate is high, this leads to HCO, H2CO, and possibly CH3OH (eq. [2]), accounting for the observed large abundance of CH3OH in grain mantles (Tielens 1989) and comets (Crovisier 1998): H
H
H
H
CO > HCO > H2 CO > H2 COH > CH3 OH :
ð2Þ
Van Ijzendoorn et al. (1983) measured the absorption spectrum of HCO in Ar, Kr, Xe, CH4, CO, and N2 matrices. They found that the absorbance of HCO in Ar, Kr, and CH4 matrices grew several-fold while that for trapped H atoms decreased on warm-up from 10 to 15 K. Hiraoka et al. (1994) studied the reactions of H atoms with solid thin film of CO at 10 K. They found that H2CO and also CH3OH were formed from the reaction with small yields. In the present work, the reaction of H atoms with solid CO was revisited. Although the formation of H2CO was confirmed in the current work, no trace amount of CH3OH could be 265
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Fig. 1.—Schematic diagram of the experimental system (not to scale)
nary glow discharge plasma (Cherigier et al. 1999). In the present experiment, the H2 gas plasma flows through the restricted narrow bottleneck (0.2 mm internal diameter and about 2 mm long) and then passes through a 90 bent Pyrex glass tubing with a 10 mm diameter and about 5 cm long before being sprayed over the solid film. From the flow rate of the H2 flux (1016 cm2 s1) and the estimated diffusion rate constant of H atoms (100 s1; Payne & Stief 1976), the H-atom flux may be crudely estimated to be 1013 atoms cm2 s1 under the present experimental conditions. During the spray of H2 gas containing H atoms over the CO sample film, the H2 molecules must be adsorbed in the CO film and the morphology of the film may change with reaction time. At present, we could not estimate the effect of H2 contamination for the CO sample film. In this work, product analysis was made by the thermal desorption mass spectrometry. The quantitative analysis of gaseous products using a mass spectrometer is very sensitive and highly suitable for such experiment that the absolute quantity of the reactants and products is only limited to be a few monolayer (ML) thick samples condensed on the cold substrate. Strictly speaking, however, there is no guarantee that the products detected by the thermal desorption mass spectrometric method are formed at the reaction temperature but not during the warming of the sample for thermal desorption analysis. To perform in situ and real-time product analysis in the low-temperature solid-phase reactions, infrared absorption spectra of CO film being reacted with H atoms were measured. However, no absorption bands due to the formation of reaction products were detected. This was solely due to the low yields of the reaction products
including the major product H2CO. We think that the main product, H2CO, was formed at 10 K but not during the warm-up of the sample because almost all of the radical products are likely to be converted to stable products during the H-atom spray (H atom itself is a radical scavenger). In the tunneling reactions of H atoms with solid C2H2, C2H4, C3H6, and SiH4 at 10 K, stable reaction products could be detected by means of Fourier transform IR spectroscopy owing to the high yields of products. However, no radical products were detected by the Fourier transform IR spectroscopy in those measurements. This indicates that the radical products were efficiently annihilated to closed-shell products by recombination or H-atom abstraction reactions.
3. RESULTS AND DISCUSSION
3.1. Formation of H2CO from the Reaction of H with Solid CO In our previous work, the reaction of H atoms with solid CO molecules was studied (Hiraoka et al. 1994). It was found that H2CO as well as CH3OH were formed as reaction products. In that work, the sample CO film was being irradiated by the UV light from the H2 discharge tube during the H-atom spray. We also found that the inner wall of the bottleneck discharge tube was gradually contaminated with the strayed CO molecules during the repetitive CO gas introduction into the vacuum chamber. The CO molecule is known to be highly adsorptive. The inner wall of the H-atom spray bottleneck discharge tube was found to be
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rather quickly contaminated by the CO molecules. With the contamination of the discharge tube, several hydrogenated products such as CH4 and CH3OH were formed when the H2 plasma was generated in the bottleneck discharge tube. The gaseous products formed in the H2 plasma were sprayed over the silicon substrate and condensed there as contaminants. In the present experiments, the drawbacks of the previous reaction systems were completely eliminated. As shown in Figure 1, the bottleneck discharge tube with two optical traps was made. By coating the discharge tube with thick colloidal graphite, no light emission could be visible by the naked eye. The tungsten coil electrode was installed inside of each optical trap. By applying 100 and 0 V (ground potential) on the first and second coil electrodes, respectively, the charged particles produced by the H2 plasma could be completely annihilated inside the discharge tube. Every time the sample was deposited on the silicon substrate, the exit of the H-atom spray bottleneck discharge tube was plugged by the flat stainless steel lid that prevents the sample CO molecules from entering into the discharge tube. By using this improved experimental system, no discharge products were detected for a long period of the repetitive experiments. Figure 2 shows the thermal desorption mass spectrum of the 10 ML thick CO solid sample reacted with H atoms for 1 hr at 10 K. The peak with m/z 30 appearing at 130 K is due to the formation of H2CO. If methanol is formed as the reaction product, the peak with m/z 31 must appear in the desorption mass spectrum (the mass spectral pattern of methanol is m/z 31 [100%], m/z 32 [67%], m/z 29 [65%], m/ z 28 [6%], and m/z 18 [2%]). However, no ion signals with m/z 31 could be detected in the thermal desorption mass spectra as shown in Figure 2. This suggests that the consecutive addition reactions of H atoms with H2CO to form CH3OH are slower than those with CO to form H2CO under the present experimental conditions. The carbonyl group of H2CO seems to be less reactive toward H atoms to form the saturated compound CH3OH.
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In our previous work (Hiraoka et al. 1998a), the reaction of H atoms with C atoms seeded in the CO solid was investigated. It was found that the H atoms diffuse deep inside the CO solid film and hydrogenate the C atoms embedded in the CO matrix efficiently. This clearly indicates that the H atoms can migrate in the CO matrix and have ample chance to react with seeded C atoms. In other words, the reactivity of H atom toward the CO molecule is low enough to allow the long diffusion of H atoms in the CO matrix. Figure 3 shows the elementary processes for equation (2). The solid arrow means that the reaction of H with the left-hand reactant forms the right-hand product. The number represents the heat of reaction in kcal mol1. The absence of CH3OH clearly indicates that the rate of the process (e) is a slow process. We conjecture that this process is even slower than (a). The predicted slow process (e) is in accord with the fact that 2-propanol was not detected from the reaction of H atoms with solid acetone at 10 K (Hiraoka et al. 1998b). It has been generally assumed that CH3OH in the interstellar medium is formed by the hydrogenation reaction of H2CO formed on the grain mantles (Tielens & Allamandola 1987). However, the present result suggests that the formation of CH3OH via equation (2) on the grain surface in the dark clouds may not be the major process. CH3OH was found to be the major component as well as H2CO in recent comets Hyakutake and Hale-Bopp (Crovisier 1998). In the present experiment, the reaction of H with pure CO solid was investigated. The interstellar molecules adsorbed on the dust grains are embedded mainly in the H2O matrix. In such circumstances, the hydrogen bond between the polar molecules and the surrounding H2O matrix molecules must be formed. In the process of the H-atom addition to H2CO, the hydroxyl group OH is newly formed in the intermediate product CH2COH. The energy barrier for the process of the tunneling reaction, H2 CO þ H ! H2 COH, would be lessened at least to some extent by reorganization of the surrounding H2O molecules to form the hydrogen bonds with the newly formed H2COH. Such an effect cannot be
Fig. 2.—Thermal desorption mass spectrum for the 10 ML thick CO solid sample reacted with H atoms for 1 hr at 10 K
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Fig. 3.—Elementary processes for eq. (2). The arrow means that the reaction of H with the left-hand reactant forms the right-hand product. The corresponding number represents the heat of reaction in kcal mol1.
expected in the reaction taking place in the neat CO matrix. This could be related to the absence of CH3OH in our experiment. The reaction of H with CO molecules seeded in the H2O matrix is now in progress in our laboratory. It was already pointed out that CH3OH could be synthesized when the H2CO molecules formed in the CO matrix were exposed to the UV light. This is likely to be due to the fact that the excited state of carbonyl compounds (electronic transition of n ! ) formed by the UV light absorption have a biradical character and must become highly reactive toward H atoms. Actually, H-atom–sprayed CO and (CH3)2CO solid films at 10 K gave some CH3OH and (CH3)2COH, respectively, when they were irradiated by UV light (Hiraoka et al. 1998b). This might explain the formation of CH3OH from H2CO on the dust grains where the electronic excitation by UV irradiation can take place near the envelope of the dark cloud. Lis et al. (2001) obtained high-resolution spectra toward the molecular cloud Sgr B2 at 63 lm, the wavelength of the ground-state fine-structure line of atomic oxygen, using the LWS instrument on the Infrared Space Observatory. They derived an atomic oxygen abundance of 2:7 104 in the dense gas phase. Although the ground-state O atom (3P) is unreactive toward saturated hydrocarbons, the first excited state O(1D) is known to be highly reactive toward hydrocarbons. Matsumi et al. (1993) investigated reactions of O(1D) with alkanes (RH) and alkyl chlorides. The O(1D) atom was found to insert indiscriminately into the primary and secondary CH bonds of alkanes, and there is no potential barrier in the entrance channel for Oð1 DÞ þ RH. If the gas-phase O(1D) impinges on the dust grains and reacts with the CH4 molecule trapped on the dust grains, the CH3OH molecule might be formed:
the solid-phase reactions of O(1D) with CH4 and other alkanes have not been studied so far. The investigation of the reactions of O(1D) with solid alkanes at cryogenic temperature is now in progress in our laboratory.
Oð1 DÞ þ CH4 ! CH3 OH :
Fig. 4.—Relationship between the yields of H2CO (%) and the reaction temperature for reaction H þ CO together with those of C2H6, C2H6, and solid product (mainly polysilane) for reactions H þ C2 H2 , H þ C2 H4 , and H þ SiH4 , respectively, obtained under similar experimental conditions. Film thickness: 10 ML; H-atom spray time: 1 hr.
ð3Þ
Equation (3) may be one of the candidates for the formation of CH3OH on the dust grains. To the best of our knowledge,
3.2. Temperature Dependence of the Yield of H2CO from the Reaction H þ CO Figure 4 displays the relationship between the yields of H2CO (%) and the reaction temperature for reaction
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H þ CO together with those of C2H6, C2H6, and solid product (mainly polysilane) for reactions H þ C2 H2 , H þ C2 H4 (Hiraoka et al. 2000), and H þ SiH4 (Hiraoka et al. 2001), respectively, obtained under similar experimental conditions. Here the yield means the ratio of the reaction product to the reactant deposited on the Si substrate. In these experiments, the sample gases were deposited first at 10 K, and then H atoms were sprayed over the solid films for 1 hr after the substrate temperature was raised from 10 K to the reaction temperature. The experiment above 25 K for H þ CO was not made because reactant CO starts to sublime above 28 K. The yield of H2CO in Figure 4 shows a steep increase with a decrease of temperature from 0.01% at 25 K to 0.08% at 10 K. In Figure 4, there is a general trend that the rates of all tunneling reactions dealt with increase with a decrease of temperature. In our previous paper (Hiraoka et al. 2000), the observed negative temperature dependence was suggested to be due to either the increase of the sticking probability of H atoms and/or the increase of the rate constants for the tunneling reactions with decrease of temperature. In Figure 4, the yield of C2H6 from reaction H þ C2 H2 is of the same order as that from H þ C2 H4 at 10 K. Interestingly, however, Bennitt & Mile (1973) predicted that the rate constant (kr) for reaction H þ C2 H4 is about 6000 times greater than that for H þ C2 H2 at 77 K. In case the rate constant (kr) for reaction H þ C2 H4 is also much larger than that for H þ C2 H2 at 10 K as predicted by Bennitt & Mile (1973), the H-atom concentration [H] on the C2H2 film must be orders of magnitude greater than that on the C2H4 film because the rate for the formation of the final product C2 H6 is mainly determined by kr ½H ½C2 Hn (n ¼ 2 or 4) for the first-step H-atom addition reactions, H þ C2 Hn ! C2 Hnþ1 (n ¼ 2 or 4; Hiraoka & Sato 2001). It was confirmed that the [H]’s for C2H2 and C2H4 films were of the same order at 10 K (Hiraoka & Sato 2001). This indicates that the steep increase of the yield of C2H6 from reaction H þ C2 H2 can only be explained by the keen negative temperaturedependent rate constant kr, for H þ C2 H2 . In general, a steeper increase in the rates of the tunneling reactions with a decrease of temperature was observed for molecules whose rates of reactions with H are small at higher temperature regions, e.g., C2H2 and CO in Figure 4. The observed increases of the rates of reactions H þ C2 H2 and H þ CO in Figure 4 are much steeper than that of the sticking probability of H atoms on amorphous carbon with a decrease of temperature (Pirronello et al. 2000); i.e., the observed increases of the yields of C2H6 and H2CO in Figure 4 cannot be explained only by the increase of the sticking probability of the H atom on the solid surface. The negative temperature dependence for rate constants of the tunneling reaction H þ H2 and its isotopic variants was also predicted theoretically by Takayanagi & Sato (1990).
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that the lifetime of the decay of v ¼ 1 0 vibrational levels of N2 is as long as 1:5 0:5 s in liquid nitrogen. In the present experiment, the H-atom recombination reaction 2H ! H2 with the heat of reaction of 104 kcal mol1 as well as other exothermic reactions such as equation (2) take place on the solid surface. The occurrence of exothermic reactions on the solid surface may result in the local heating of the van der Waals solid. This could lead to the erosion of the sample film. Actually, this process is considered to be one possible mechanism for the desorption of adsorbed molecules into the gas phase in the dark clouds. In order to check whether the desorption of CO molecules takes place during the H-atom spray over the solid CO film, the recovery of CO deposited was measured after the solid CO film was sprayed by the H atoms for 1 hr by means of thermal desorption mass spectrometry. Figure 5 shows the temperature dependence of the ratio of the CO recovered after the H-atom spray for 1 hr at a certain temperature to that for the CO deposited at 10 K without spraying the H atoms. The experiment above 25 K was not performed because the solid CO starts to sublime above 28 K. To our surprise, the ratio is almost independent of the temperature up to 25 K. This suggests that the relaxation of the vibrational energy of H2 formed by the recombination reaction 2H ! H2 to the lattice phonons is inefficient as mentioned above. This finding is in accord with the recent theoretical calculation by Takahashi, Masuda, & Nagaoka (1999a, 1999b). They investigated the time and space dependence of the local temperature increase of icy mantles caused by the release of H2 formation energy in the vicinity of H2-forming sites using classical molecular dynamics computational simulations. They predicted that the H2 formation energy was partitioned to the vibrational energy (78.5%), the rotational energy (9.9%), and the translational energy (7.4%) for the
3.3. Does the van der Waals Solid Film Erode during the H-Atom Spray? In the low-temperature tunneling reactions, only the exothermic reactions could take place. A greater part of the heat of reaction evolved will eventually degrade to the phonon energy. There is ample evidence that multiphonon processes for the dissipation of the vibrational energies are slow because of the large difference in the vibrational and cohesive energies (Dressler, Oehler, & Smith 1975; Oehler, Smith, & Dressler 1977). Calaway & Ewing (1975) found
Fig. 5.—Temperature dependence of the ratio of the CO recovered after the reaction of CO film with H atoms for 1 hr at a certain reaction temperature to that for the CO deposited at 10 K without spraying the H atoms.
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desorbing H2 molecule and only 4.4% was absorbed to the amorphous ice. The contribution of the highly exothermic reaction 2H ! H2 to desorption of the grain mantle may not be as large as thought before. 4. CONCLUSIONS
The tunneling reaction of H atoms with solid CO was investigated in detail. Formaldehyde was found to be only the product, and no trace amount of methanol was detected. This suggests that the formation of CH3OH starting from the reactions of H with CO in the icy grain is a slow process in the cold dark clouds. However, if the H2CO molecules embedded in the mantle were irradiated by the UV light near the envelopes of dark clouds, the photolytic hydroge-
nation reaction would proceed to form CH3OH. In addition, the reaction Oð1 DÞ þ CH4 ! CH3 OH may also be conceivable for the formation of methanol (K. Hiraoka et al. 2002, in preparation). The desorption of CO solid film was found to be negligible up to just below the sublimation temperature (25 K) when the film was sprayed by the H atoms with a flux of about 1013 atoms cm2 s1. This is due to the slow energy transfer from the vibrationally excited H2 to the phonons of the matrix elements. It is likely that the exothermic reactions taking place on the dust grains may not necessarily lead to the direct desorption or erosion of the grain mantles because the coupling between the vibrational energies of the hot molecules and the cohesive energies of the matrix is inefficient.
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