ARTICLE pubs.acs.org/JPCC
Adsorption of Organic Isomers on Water Ice Surfaces: A Study of Acetic Acid and Methyl Formate M. Bertin,*,†,‡ C. Romanzin,†,‡,§ X. Michaut,†,‡ P. Jeseck,†,‡ and J.-H. Fillion†,‡ †
UPMC Univ Paris 6, Laboratoire de Physique Moleculaire pour l’Atmosphere et l’Astrophysique (LPMAA) UMR 7092, F-75252 Paris, France ‡ CNRS, UMR 7092, F-75252 Paris, France ABSTRACT: The adsorption of two isomers, acetic acid (CH3COOH) and methyl formate (HCOOCH3), on water ice surfaces is investigated using temperature-programmed desorption and infrared spectroscopy in the 80200 K temperature range. For each compound, multilayer and submonolayer adsorption regimes are distinguished. In the multilayer regime, the adsorption conformations of the adlayer molecules as a function of the sample temperature are analyzed, providing the identification of phase transitions and associated thermal desorptions. In the submonolayer regime, different behaviors are found in the adsorption of each isomer. The acetic acid first layer desorbs simultaneously with the water substrate itself, whereas the methyl formate monolayer desorption is found 15 K below the onset of water sublimation. This difference in adsorption energies is discussed and associated with the ability of each isomer to perform hydrogen bonding with the dangling OH surface water bonds. Such a differential adsorption process between organic isomers might be of interest in systems where these molecules can be found in interactions with water ices, such as stratospheric or interstellar media. Indeed, it might play a role in the observation of gas-phase relative abundances of these isomers in such media.
1. INTRODUCTION The interaction between small organic molecules and water ices attracted much attention because of its relevance in several fields of interests, from fundamental molecular physics to the chemistry of the polar stratosphere or interstellar medium (ISM). Because of their presence as volatile organic compounds (VOCs) in regions of the upper tropospherelower stratosphere where water ice particles are also present, the adsorption of small oxygen-bearing molecules, such as acetic acid, on water surfaces is an important subject for atmospheric chemistry.1 Small oxygen-bearing organics in interaction with water-rich ices are also of importance in the colder parts of the interstellar medium (molecular clouds), where molecules are accreted (or formed) at the surface of dust grains.2 Accretion and evaporation processes are of particular interest in the production of saturated organic species in star-forming regions, where a complex and rich chemistry, involving a strong coupling between gas and grains, is taking place during the warming-up of these icy mantles in the vicinity of a protostar. Various large molecules, including methyl formate (HCOOCH3) and acetic acid (CH3COOH), have been detected in numerous hot cores.38 It is much likely that these species have been formed both in the gas phase and, to a larger extent, in the condensed phase (i.e., at the surface or in the bulk of the ices), as it has been pointed out in several experimental and theoretical studies.913 The interaction of such species with water at low temperatures is, therefore, playing a role in this context, both because molecules present (or formed) in the gas r 2011 American Chemical Society
phase can condense onto the surface of cold water-rich icy mantles (T ∼ 2040 K) and because molecules present (or formed) in these mantles are released in the gas phase at higher temperatures (T ∼ 100200 K).1416 Thermal desorption studies of ice mixtures of organic molecules and water, mimicking ISM ices compositions and phases, have accordingly been intensively studied (e.g., refs 17 and 18). Here, our experimental study focuses on the surface adsorption and thermal desorption of two organic isomers, acetic acid (AA) (CH3COOH) and methyl formate (MF) (HCOOCH3), deposited on water ice films. The experiments have been carried out in the 80200 K temperature interval, which includes a typical range of desorption temperatures for organics on ice, and water ice itself, under laboratory conditions. In this study, a distinction has been made between (i) a multilayer coverage regime, where we mainly probe the interactions between the organic molecules in condensed phase, and (ii) a (sub)monolayer regime, where the adsorption/desorption is expected to be dominated by surface waterorganics interactions. In the case of acetic acid, the multilayer adsorption and subsequent desorption from water ice has already been studied,1922 but data for submonolayer adsorption on water are, to our knowledge, lacking. In the case of methyl formate, whereas its reactive adsorption on metallic substrates Received: February 15, 2011 Revised: May 28, 2011 Published: June 02, 2011 12920
dx.doi.org/10.1021/jp201487u | J. Phys. Chem. C 2011, 115, 12920–12928
The Journal of Physical Chemistry C attracted much attention (e.g., refs 2325), no study on its molecular adsorption on water is available despite its relevance for ISM or stratospheric chemistry. As both isomers AA and MF have the same molecular weight, and a comparable dipolar moment,26 any difference observed in the desorption behavior cannot be explained by kinetic consideration or by van der Waals-type interactions but has to be associated with specific intermolecular interactions related to the different chemical functions of the molecules (COOH for AA and >CdO for MF). Comparing desorption energies between the two isomers is, therefore, expected to bring insights on the ability of each molecule to be involved in hydrogen bonding within the pure compound or with the surface water molecules. The experiments have been carried out using temperatureprogrammed desorption (TPD), which permits the detection of desorbing molecules as a function of the ice temperature during a constant warming-up, while the condensed phase molecules are simultaneously probed by means of reflection absorption infrared spectroscopy (RAIRS). In the following, the results obtained on the acetic acid and methyl formate deposited on crystalline and/or amorphous water ice will be presented separately. For each compound, care has been taken to distinguish between the multilayer and the submonolayer regimes, extending the findings for the submonolayer in order to estimate the single molecule limit adsorption energy and configuration. Finally, a comparison between the two systems is made, and the implications of the results are considered.
2. EXPERIMENTAL SECTION The present work has been realized in the newly designed setup SPICES (surface processes and ICES) dedicated to the study of astrophysically relevant molecular ices. A helium-compression closed-cycle cryostat is housed in an ultra-high-vacuum (UHV) chamber with a base pressure of ∼1 1010 Torr. A polycrystalline gold substrate is fixed at the cold end of the cryostat where its temperature can be varied from ∼10 to 350 K by resistive heating. The heating regulation allows the control of the temperature ramps between 0.1 and 10 K 3 min1. Water ices are grown as follows. The H2O (high-purity, liquid chromatography standard from Fluka) was purified by several freezepumpthaw cycles prior to use. The cold Au substrate is exposed in situ to a background partial pressure of water during a given time, resulting in an exposure expressed in Langmuir (1 L = 106 Torr 3 s), which can be converted to a coverage expressed in bilayers (BL). Depending on the dosing temperature of the sample, the resulting ice can present different phases. In our case, we prepared amorphous water ice by dosing 3 108 Torr of H2O vapor onto the sample kept at 80 K. Such experimental conditions are expected to lead to the growth of an amorphous ice film.2729 The resulting amorphous film is expected to be relatively compact, because no clear signature of OH dangling bonds, expected in the case of highly porous water ice film,27 can be observed in IR spectroscopy. Crystalline water ice is obtained via dosing the water at T = 140 K and then by quickly decreasing the sample temperature below 100 K. The crystalline character of the resulting H2O film is confirmed by the absence of the desorption signal related to the amorphous phase in the TPD curves.28,29 In the case of amorphous water growth, the dosing temperature (80 K) being much lower than the desorption temperature for water (T g 140 K), one can assume the sticking
ARTICLE
coefficient to be close to one. Therefore, the thickness of the resulting ice is directly proportional to the exposure and can be derived as in the following: 1 BL corresponds to 2.4 L taking one bilayer to be 1.15 1015 molecules 3 cm2.30 In the case of crystalline ice films grown at higher temperature, one cannot assume perfect sticking and the thickness dependency with the exposure has been determined by calculating the area below the TPD curves, which has been calibrated as a coverage using the desorption curves obtained for the amorphous water. Commercially available acetic acid (AA, 99.99% purity, Sigma-Aldricht) and methyl formate (MF, >99.8% purity, Fluka) are also purified with several freezepumpthaw cycles. The MF or AA overlayers are prepared via dosing a background pressure of each product on top of the water icecovered gold substrate. The dosing temperature of the organics has been chosen at 80 K. A better modeling of the ISM ices would require depositing the organics on ices kept below 40 K. However, in our laboratory conditions, the desorption of these species takes place between 100 and 180 K. To perform our thermal desorption experiments, the sample has, therefore, to be heated-up from the deposition temperature to the desorption temperature at a given rate. We expect that a warming-up of the sample between 40 and 80 K results in little difference compared to an ice where the organics have been directly prepared at 80 K. Temperature-programmed desorption (TPD) curves have been obtained by recording the mass signal of the desorbing species while keeping a constant sample heating rate of 1 K 3 min1. The mass spectrometer, located in front of the sample, is a quadripolar mass spectrometer QMS200 by Balzers in channeltron detection mode and allows routinely the simultaneous detection of several masses. Thus, desorption of both water and AA or MF overlayers was simultaneously probed. The TPD curves shown below only exhibit the signal related to the most intensive ion signal detected in mass spectrometry originating from the electron impact ionization/fragmentation of each species, that is, H3COþ (m = 31 amu) for the MF, H3C2Oþ (m = 43 amu) for the AA, and H2Oþ (m = 18 amu) for the water.31 The curves of the intact ionized molecules have also been monitored for MF and AA (m = 60 amu) and are identical to the ones obtained for the most intensive fragments. Although electron impact ionization also leads to the production of H2Oþ from both the acid and the ester, the resulting signal is found to be more than 100 times weaker than the signal due to the water desorption itself and, therefore, does not significantly contribute to the observed signal in our TPD curves of H2O. Caution has been taken to ensure that the desorption features that we observe originate from the surface and not from other cold parts of the experimental setup during the warming-up phase. The fact that (i) our TPD curves are identical, for a given thickness and ramp, to other available literature data and (ii) each desorption feature observed corresponds concomitantly to a change in the IR spectra of the remaining molecules on the surface gives further evidence that the molecules we probe originate from the sample sublimation. Reflection absorption infrared spectroscopy (RAIRS) is used simultaneously with the TPD to probe the condensed molecules during the warming-up. This well-known technique is already described in the literature.32 In our case, a Fourier transform spectrometer (Bruker Vector22) provides the ingoing IR beam that is focused on the sample surface with an incident angle of 75° with respect to the surface normal. The reflected outgoing beam 12921
dx.doi.org/10.1021/jp201487u |J. Phys. Chem. C 2011, 115, 12920–12928
The Journal of Physical Chemistry C
ARTICLE
is detected using an external liquid-N2-cooled MCT detector. Both the detector and the focusing optics are housed in an external vacuum system whose pressure is kept below 102 mbar, the incoupling/outcoupling with the UHV chamber being performed with two differentially pumped KBr windows, while the interferometer is purged with a constant gaseous N2 flow. The spectra presented in the present work have all been acquired in the 8004500 cm1 range, with a resolution of 2 cm1. RAIRS has been carried out on each sample during its warmingup from 80 to 200 K at 1 K 3 min1, accumulating 50 spectra every 5 K. At this rate, the accumulation time of one spectrum represents a temperature variation of the sample of about 1.5 K.
3. RESULTS AND DISCUSSION 3.1. Acetic Acid Adsorption on Water Ice. The interaction between acetic acid molecules and the water ice surface has already been studied both theoretically and experimentally.1922 It is well known that evaporating liquid CH3COOH results in the production of cyclic AA dimers in the gas phase, which would, in our case, be directly deposited on the ice surface during the overlayer growth.20,33 According to Bahr et al.,22 it is predicted that the deposition of the first layer of AA molecules on the water surface leads to a strong bonding of the surface dimers, via two hydrogen bonds, with an adsorption energy lying in the same range as the water ice cohesion energy. The AA multilayers have also been shown to remain in a cyclic dimer organization. With increasing temperature, the thick AA film undergoes a phase transition from the cyclic dimers to chainlike hydrogen-bonded polymers, allowing the underlying water molecules to desorb almost freely while the AA remains in the condensed phase. However, these results concerned experiments carried out on thick multilayers of acetic acid deposited on water. We conducted very similar experiments looking at the adsorption/desorption of the AA molecules as a function of the overlayer coverage. Our aim was to study the system in the submonolayer regime, and to extend our findings to the limit of the single molecule adsorbed on the water ice surface, more relevant in the stratospherical/ astrophysical context. Figure 1 shows thermal desorption curves of several exposures of acetic acid on top of 15 BL of crystalline ice. The desorption of the crystalline water exhibits only one feature, between 140 and 160 K, associated with the sublimation of pure crystalline ice. No change in the TPD curves of H2O has been observed, at any acetic acid exposure: the TPD curves are found to be identical to the ones obtained from pure crystalline water films of 15 BL. The TPD curves of the acid overlayers are more complex. One can distinguish three different contributions to the desorption. The first one (R), between 135 and 145 K, shifts toward higher temperature when the coverage decreases and is not observed for exposures below 2.5 L. The second one (β), from 150 to 160 K, is very similar to the desorption feature of the underlying water ice and is observed even at very low exposure. Finally, the last contribution (γ), extending up to 180 K, is not seen below 2.5 L. Because, for low exposures, only the contribution β is seen, we attribute it to the AA first monolayer desorption. This attribution is also supported by the fact that the area under this peak is constant above 2.5 L and decreases monotonically for lower exposures (submonolayer regime). The contributions R and γ are then associated with multilayer desorption. According to Bahr et al.,22 the γ feature is related to desorption of the AA organized in chainlike hydrogen-bonded polymers. Nevertheless,
Figure 1. TPD curves of CH3COOH deposited on 15 BL of crystalline water ice at 80 K, for several acid exposures. Desorption of both the AA overlayer (upper panel) and the water substrate (lower panel) were simultaneously probed, while the sample was heated-up with a 1 K 3 min1 rate. Only one water desorption curve is shown because they were all identical at any AA overlayer exposure. The different features observed in the CH3COOH overlayer desorption are labeled R, β, and γ for discussion.
the contribution R has not, to our knowledge, been pointed out in previous TPD experiments, presumably because of the high temperature rate that has been used (1 K 3 s1) compared with the one of the present study (1 K 3 min1) and because of the important thickness of the AA film used in previous works giving an intense desorption feature β, which blurs the low intensity signals. To make a more accurate assignment of the observed multilayer and monolayer desorption features, RAIRS has simultaneously been carried out during the desorption. Figure 2 shows RAIR spectra for different coverages of CH3COOH deposited on crystalline water as a function of the sample temperature during the continuous warming-up at 1 K 3 min1. In the following, the case of the multilayer regime (exposure g 2.5 L, Figure 2a,b) and submonolayer regime (exposure < 2.5 L, Figure 2c) will be separately described. The IR spectrum of solid acetic acid on water ice is, at present, wellknown, allowing us to attribute each peak to a given vibration of the condensed AA molecule (Table 1). In particular, it is possible to distinguish between monomers, dimers, and crystalline AA film looking at the stretching vibrations of CdO and CO. Indeed, these vibrations are very sensitive to the environment of the molecule, the COOH function being mainly responsible for the intermolecular interactions because it is involved in hydrogen bonding.19 Figure 2a,b shows IR spectra of multilayers of AA on crystalline water ice in the 18501280 cm1 region. Between T = 80 and 130 K, before any desorption of acetic acid, the stretching vibration of CdO is mainly observed via a single peak at 1720 cm1, which is associated with the cyclic dimer organization of the AA molecules, and a weak signal at 1790 cm1 related to some surface-located monomers. Upon 12922
dx.doi.org/10.1021/jp201487u |J. Phys. Chem. C 2011, 115, 12920–12928
The Journal of Physical Chemistry C
ARTICLE
Figure 2. RAIR spectra for different sample temperatures of (a) 10, (b) 2.5, and (c) 1.3 L of CH3COOH deposited at 80 K onto 15 BL of crystalline water ice. The spectra have been acquired at a regular time spacing during the warming-up of the sample from 100 to 200 K at 1 K 3 min1. The dotted lines represent the IR spectra of the underlying pure crystalline water ice at 80 K.
Table 1. Assignment of the Spectral Features of Multilayers of CH3COOH Deposited on Crystalline Ice at 80 K in the Region of 12801800 cm1b CH3COOH on
CH3COOH on CH3COOH solid on
crystalline water
crystalline water
polycrystalline
(this work)
(ref 22)
Cu (ref 19)
assignmenta
1285, 1320
1280, 1301
1284, 1301, 1323
νCO (C)
1302
1322
1314
νCO (D)
1364
1363
1363
δsCH3
1415
1412
1418
δasCH3
1445
1440
1436, 1447
in-plane δOH
1647, 1660, 1700
1646, 1660
1646, 1660, 1700
νCdO (C)
1720 1750, 1790
1699, 1720 1747, 1791
1732 1749, 1790
νCdO (D) νCdO (M)
a Abbreviations/symbols: ν, stretching; δ, bending; s, symmetric; as, asymmetric; (C), related to crystalline AA; (D), related to AA cyclic dimers; (M), related to surface or bulk-located trapped AA monomers. b All values are given in cm1.
heating-up above 140 K, the vibrational spectra change dramatically. New stretching vibrations of CdO appear at 1647, 1660, and 1700 cm1, while two new contributions to the CO stretching are observed at 1285 and 1320 cm1. These features are the signature of a phase transition of the AA molecules, from a cyclic dimer organization to the crystalline organization, where each different contribution to the stretchings is associated with the environment of the molecule in the chainlike polymers. These features become dominant in the IR spectra with ongoing warming-up, together with the CdO peak at 1750 cm1 related to trapped monomers within the chainlike organization. After the desorption of the water (T > 160 K), the crystalline phase and the trapped surface or bulk monomers are still observed and vanish
after the desorption of the multilayer (i.e., T > 180 K). One can remark that the crystallization process is observed later in temperature for decreasing AA overlayer thickness. Indeed, the crystalline features of CdO stretching (1647, 1660, and 1700 cm1) is seen from 140 K for 10 L exposure (Figure 2a), whereas for 2.5 L (Figure 2b), such peaks begin to appear only at 150 K. This temperature shift with decreasing coverage is very similar to the one observed for the R desorption signal in TPD experiments (Figure 1), which is also observed in the same temperature range (130150 K). Therefore, we attribute the R desorption feature to the evaporation of AA monomers during the dimer-to-polymer phase change of CH3COOH. This behavior can be explained as follows. For high coverage (e.g., 10 L), the multilayer dimers being close to one another, the phase transition can occur quickly once the available thermal energy is high enough to trigger the process. On the other hand, when the coverage gets close to the monolayer, as it is in the case for 2.5 L exposure (Figure 1), very few multilayer dimers are present. Hence, the system needs time for the dimers to diffuse and meet before the crystallization can take place, even if the activation energy is reached. The warming-up being dynamical, the temperature still increases during the dimer diffusion time, and the crystallization is thus artificially observed at higher temperature. This low-temperature multilayer desorption process could then explain the decay in intensity already observed via RAIRS by Bahr et al.22 between 115 and 145 K, which they attributed to a decrease of the dipole moment of the active groups of the molecules due to their hydration. Contrary to the CH3COOH multilayers' vibrational spectra, which exhibit many changes during the warming-up, the submonolayer IR spectrum evolution with temperature (Figure 2c) is more straightforward to analyze. After deposition, only one of each CdO and CO stretching vibration is seen at 1717 and 12923
dx.doi.org/10.1021/jp201487u |J. Phys. Chem. C 2011, 115, 12920–12928
The Journal of Physical Chemistry C
ARTICLE
Figure 3. TPD curves of HCOOCH3 deposited at 80 K on (a) 15 BL of crystalline water ice and (b) 15 BL of amorphous water ice, for several methyl formate exposures. Desorption of both the MF overlayer (upper panel) and the water substrate (lower panel) were simultaneously probed, while the sample was heated-up with a 1 K 3 min1 rate. Only one water desorption curve is shown because they were all identical at any MF overlayer exposure. The different features observed in the HCOOCH3 overlayer desorption are labeled R0 , β0 , γ0 , and δ0 for discussion.
1290 cm1, respectively. These values are somehow different from those observed for the AA multilayer regime. The CdO stretching vibration is found close to the one expected for cyclic dimers (1720 cm1), whereas the CO vibration is more comparable to that of the acid molecules in the crystalline conformation (1285 cm1). This finding indicates that, even if the AA molecules remain in a cyclic dimer organization in the first layer, the interaction with the surface water molecules is strong enough to distort the dimer, resulting in a more constrained CO stretching vibration than in the case of the multilayers. This result is, therefore, in good agreement with refs 19 and 22, where the dangling OH bonds of the water surface are involved in hydrogen bonding with the COOH function, which is expected to result in such effects in the CO vibrational features. Upon warming up, no change in the CdO stretching is observed up to 150 K, showing that the AA molecules remain in the same conformation; that is, no phase transition takes place, presumably because of the important interaction between the surface AA and H2O molecules and the low density of acid molecules. The intensity of the acid-related peaks is then vanishing for higher temperatures, which corresponds to the β desorption signal in Figure 1 and to the underlying water desorption. Thus, one cannot observe the desorption of the first layer of AA deposited on water, because its release in the gas phase proceeds via the evaporation of the water substrate. This implies that the interaction energy between the CH3COOH submonolayer molecules and the surface H2O molecules is greater than the cohesion energy of the crystalline water ice, which has been evaluated on the gold substrate to be ∼48 kJ 3 mol1.34 This common H2OCH3COOH desorption strongly supports the fact that the first AA layer on water is composed of tightly bonded AAwater surface complexes. This monolayer desorption feature is also observed in the case of the multilayer regime, where, due to the thermally induced crystallization of the upper AA layers into chainlike polymers, the water
can desorb through the polymer network and can transport the first layer of AA molecules with it. However, in these cases, the signal related to the first layer is difficult to see in RAIRS because of its low intensity compared with the multilayer signal. 3.2. Methyl Formate Adsorption on Water Ice. Whereas acetic acid adsorption on the water ice surface already gave rise to several studies, the methyl formate adsorption on H2O and its temperature-induced sublimation have not, to our knowledge, been specifically studied, despite its relevance to the ISM astrochemistry. A recent experimental study on the system of HCOOCH3 on water ice has been performed by means of IR spectroscopy.11 This latter work is more dedicated to its formation from ion irradiation of condensed molecular mixtures, and the desorption process of the methyl formate from water has not been studied. Because MF is also interesting in the field of heterogeneous chemistry, and especially in the catalytic formation of methanol, its thermally induced desorption from monocrystalline metal surfaces (Ag(111), Ni(111)...) attracted much attention (e.g., refs 2325). These studies focused mainly on its chemisorption and thermal decomposition at relatively high temperature (>200 K), and very little information on its physisorption process and molecular desorption at low temperature is available. We conducted in the present work the same experimental protocol we used for the study of acetic acid in order to get an insight in the MF adsorption/desorption processes from a water ice surface in the multilayer and submonolayer regimes. Figure 3 shows TPD curves of several exposures of HCOOCH3 deposited at 80 K on 15 BL of crystalline (Figure 3a) or amorphous H2O (Figure 3b). The desorption of the crystalline water substrate is the same as the one previously shown in Figure 1; that is, only one desorption feature is observed between 140 and 160 K, whereas the desorption of the amorphous ice exhibits, not surprisingly, two contributions. This double structure is wellknown in the case of metastable H2O ice and is explained by the higher desorption rate of the amorphous water compared with that 12924
dx.doi.org/10.1021/jp201487u |J. Phys. Chem. C 2011, 115, 12920–12928
The Journal of Physical Chemistry C
ARTICLE
Figure 4. RAIR spectra of 5 L of HCOOCH3 deposited at 80 K on 15 BL of (a) crystalline and (b) amorphous water ice, as a function of the sample temperature. The spectra have been acquired regularly during the warming-up of the sample from 80 to 200 K at 1 K 3 min1. For better clarity, the spectra at 125, 140, and 165 K have been multiplied by a factor of 2. For these spectra, the CdO stretching peak related to the surface monolayer regime is marked using “/”. The dotted lines represent the IR spectra of the underlying pure water ice at 80 K.
of the crystalline one, the shoulder observed at 152 K arising from the thermally induced transition between the two phases.35 As it was in the case for AA/water, the TPD curves of H2O are all identical to the ones obtained for pure water films, independent from the MF exposure. The TPD of methyl formate is also structured and presents three contributions at ∼112, 120, and 130 K, labeled R0 , β0 , and γ0 , respectively, which are seen independently from the underlying water ice phase. The desorption of MF from the amorphous water substrate also exhibits a higher temperature feature, δ0 , not seen in the case of crystalline water, which is coincident to the desorption of the amorphous water before the phase transition. With decreasing MF exposure (5.0 to 0.6 L), the lower-temperature features R0 and β0 vanish in intensity, while the γ0 structure is still observed down to 0.3 L. In addition, whereas the curve area below the R0 and β0 peaks scales with the MF coverage, the integrated signal of the γ0 peak does not seem to evolve for exposures higher than 0.6 L. We, therefore, attribute the R0 and β0 structures to the contribution of the multilayer regime and the γ0 signal to the first monolayer of methyl formate interacting with the water ice surface. Finally, the δ0 feature, only observed when using amorphous water ice as the substrate, is seen at all overlayer exposures. Its intensity abruptly falls at the phase transition temperature (152 K), meaning that this contribution is due to MF molecules trapped within the highly corrugated amorphous water substrate and released in the gas phase during the crystallization of H2O. Because the surface monolayer contribution γ0 is also observed in the case of the amorphous water surface, these trapped molecules are believed to be located in the subsurface or the bulk of the ice substrate. This process is a well-known effect, referred to as the “volcano effect”, already observed for other molecular overlayers on amorphous H2O films.36,37 It is interesting to notice that this volcano desorption is still observed in the submonolayer regime (CdO, and its interaction with the surface water molecules should, therefore, be weaker. This effect, which has been theoretically predicted,41 is experimentally evidenced here. This differential desorption behavior implies that, at thermodynamic equilibrium, one should not observe the coexistence of gaseous CH3COOH and H2O ices, whereas the coexistence is possible between gaseous HCOOCH3 and solid H2O for temperatures between 125 and 140 K. One should keep in mind that these temperatures refer to our laboratory conditions, where the heating rate is 1 K 3 min1 and should not be generalized to the ISM, where the warming-up of the ices is much slower, leading to desorption at much lower temperatures. The observed differences in the adsorption energies that we derived from our study should result in a similar behavior. This effect might lead to a better understanding of observations in different regions of the ISM, especially regarding the relative AA/MF gas-phase abundances in the presence of icy grains. Moreover, this differential adsorption effect has also been highlighted in the case of another couple of isomers, that is, dimethyl ether and ethanol, on crystalline water ice surfaces,41 demonstrating that the findings obtained for AA/MF may be generalized to other simple organic molecules.26 In particular, a comparison between the hydrogenbonding strength of O-bearing simple organics on water surfaces would certainly help to quantify the role played by several chemical functions (aldehydes, ketones, ethers...) on the adsorption energy. For instance, the method presently used for AA and MF could be extended to other small organics with similar molecular weights, but with other chemical groups, such as propionaldehyde (CH3CH2CHO). This has been the case for acetone (CH3COCH3),42 for which an adsorption energy of ∼39 kJ 3 mol1 has been measured on the amorphous water surface, which is very close to the one we obtain for MF. This result tends to show a similar hydrogen-bonding strength between ketones and esters with the water surface. Nevertheless, further investigations with ices presenting higher porosity and comparison between pure, mixed, and layered ices17,18 are needed to estimate in detail the role played by this effect in an astrophysical context. The different chemical routes of formation of each isomer, presenting different efficiencies, should also be considered because they may significantly influence the gas-phase abundance ratio of organics in the ISM.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Present Addresses §
Univ. Paris Sud 11, Laboratoire de Chimie Physique (LCP), F-91400 Orsay, France.
ARTICLE
’ ACKNOWLEDGMENT The authors gratefully acknowledge support from the French National CNRS Program PCMI (Physique et Chimie du Milieu Interstellaire) and from the French National Agency for Research (ANR) under Contract No. ANR-09-BLAN-0066-01. ’ REFERENCES (1) Abbatt, J. P. D. Chem. Rev. 2003, 103, 4783. (2) Williams, D. A.; Herbst, E. Surf. Sci. 2002, 500, 823. (3) Remijan, A.; Snyder, L. E.; Friedel, D. N.; Liu, S. Y.; Shah, R. Y. Astrophys. J. 2003, 590, 314. (4) Cazaux, S.; Tielens, A.; Ceccarelli, C.; Castets, A.; Wakelam, V.; Caux, E.; Parise, B.; Teyssier, D. Astrophys. J. 2003, 593, L51. (5) Bottinelli, S.; Ceccarelli, C.; Williams, J. P.; Lefloch, B. Astron. Astrophys. 2007, 463, 601. (6) Arce, H. G.; Santiago-Garcia, J.; Jorgensen, J. K.; Tafalla, M.; Bachiller, R. Astrophys. J. 2008, 681, L21. (7) Shiao, Y. S. J.; Looney, L. W.; Remijan, A. J.; Snyder, L. E.; Friedel, D. N. Astrophys. J. 2010, 716, 286. (8) Remijan, A.; Snyder, L. E.; Liu, S. Y.; Mehringer, D.; Kuan, Y. J. Astrophys. J. 2002, 576, 264. (9) Horn, A.; Mollendal, H.; Sekiguchi, O.; Uggerud, E.; Roberts, H.; Herbst, E.; Viggiano, A. A.; Fridgen, T. D. Astrophys. J. 2004, 611, 605. (10) Bennett, C. J.; Kaiser, R. I. Astrophys. J. 2007, 661, 899. (11) Modica, P.; Palumbo, M. E. Astron. Astrophys. 2010, 519, A22. (12) Garrod, R. T.; Weaver, S. L. W.; Herbst, E. Astrophys. J. 2008, 682, 283. (13) Bennett, C. J.; Kaiser, R. I. Astrophys. J. 2007, 660, 1289. (14) Garrod, R. T.; Herbst, E. Astron. Astrophys. 2006, 457, 927. (15) Aikawa, Y.; Wakelam, V.; Garrod, R. T.; Herbst, E. Astrophys. J. 2008, 674, 984. (16) Herbst, E.; van Dishoeck, E. F. Complex Organic Interstellar Molecules. In Annual Review of Astronomy and Astrophysics; Annual Reviews: Palo Alto, CA, 2009; Vol. 47, p 427. (17) Burke, D. J.; Brown, W. A. Phys. Chem. Chem. Phys. 2010, 12, 5947. (18) Collings, M. P.; Anderson, M. A.; Chen, R.; Dever, J. W.; Viti, S.; Williams, D. A.; McCoustra, M. R. S. Mon. Not. R. Astron. Soc. 2004, 354, 1133. (19) Gao, Q.; Leung, K. T. J. Phys. Chem. B 2005, 109, 13263. (20) Picaud, S.; Hoang, P. N. M.; Peybernes, N.; Le Calve, S.; Mirabel, P. J. Chem. Phys. 2005, 122, 10. (21) Allouche, A.; Bahr, S. J. Phys. Chem. B 2006, 110, 8640. (22) Bahr, S.; Borodin, A.; Hofft, O.; Kempter, V.; Allouche, A.; Borget, F.; Chiavassa, T. J. Phys. Chem. B 2006, 110, 8649. (23) Sexton, B. A.; Hughes, A. E.; Avery, N. R. Surf. Sci. 1985, 155, 366. (24) Zahidi, E.; Castonguay, M.; McBreen, P. J. Am. Chem. Soc. 1994, 116, 5847. (25) Schwaner, A. L.; Fieberg, J. E.; White, J. M. J. Phys. Chem. B 1997, 101, 11112. (26) Lattelais, M.; Pauzat, F.; Ellinger, Y.; Ceccarelli, C. Astrophys. J. 2009, 696, L133. (27) Rowland, B.; Fisher, M.; Devlin, J. P. J. Chem. Phys. 1991, 95, 1378. (28) Dohnalek, Z.; Ciolli, R. L.; Kimmel, G. A.; Stevenson, K. P.; Smith, R. S.; Kay, B. D. J. Chem. Phys. 1999, 110, 5489. (29) Kimmel, G. A.; Stevenson, K. P.; Dohnalek, Z.; Smith, R. S.; Kay, B. D. J. Chem. Phys. 2001, 114, 5284. (30) Chakarov, D. V.; Osterlund, L.; Kasemo, B. Langmuir 1995, 11, 1201. (31) NIST. National Institute for Science and Technology Chemical Webbook. http://webbook.nist.gov/. (32) Greenler, R. G. J. Chem. Phys. 1966, 44, 310. (33) Chocholousova, J.; Vacek, J.; Hobza, P. J. Phys. Chem. A 2003, 107, 3086. 12927
dx.doi.org/10.1021/jp201487u |J. Phys. Chem. C 2011, 115, 12920–12928
The Journal of Physical Chemistry C
ARTICLE
(34) Fraser, H. J.; Collings, M. P.; McCoustra, M. R. S.; Williams, D. A. Mon. Not. R. Astron. Soc. 2001, 327, 1165. (35) Speedy, R. J.; Debenedetti, P. G.; Smith, R. S.; Huang, C.; Kay, B. D. J. Chem. Phys. 1996, 105, 240. (36) Smith, R. S.; Huang, C.; Wong, E. K. L.; Kay, B. D. Phys. Rev. Lett. 1997, 79, 909. (37) Ayotte, P.; Smith, R. S.; Stevenson, K. P.; Dohnalek, Z.; Kimmel, G. A.; Kay, B. D. J. Geophys. Res., [Planets] 2001, 106, 33387. (38) Katon, J. E.; Ranieri, N. L. Spectrosc. Lett. 1978, 11, 367. (39) Dejong, A. M.; Niemantsverdriet, J. W. Surf. Sci. 1990, 233, 355. (40) Dohnalek, Z.; Kimmel, G. A.; Joyce, S. A.; Ayotte, P.; Smith, R. S.; Kay, B. D. J. Phys. Chem. B 2001, 105, 3747. (41) Lattelais, M.; Bertin, M.; Mokrane, H.; Pauzat, F.; Pilme, J.; Pimot, C.; Ellinger, Y.; Romanzin, C.; Jeseck, P.; Michaut, X.; Fillion, J.H.; Chaabouni, H.; Congiu, E.; Dulieu, F.; Baouche, S.; Lemaire, J.-L. Astron. Astrophys. Manuscript in press. (42) Schaff, J. E.; Roberts, J. T. Langmuir 1998, 14, 1478.
12928
dx.doi.org/10.1021/jp201487u |J. Phys. Chem. C 2011, 115, 12920–12928
