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The effect of the structure of ice on the aggregation state of co-adsorbed formic acid
Thin Solid Films 327–329 (1998) 499–502
The effect of the structure of ice on the aggregation state of co-adsorbed formic acid S. Trakhtenberg*, R. Naaman Department of Chemical Physics, Weizmann Institute of Science, Rehovot 76100, Israel
Abstract In the present work we have investigated the effect of the structure of ice on the adsorption of formic acid on its surface. Single crystal silicon wafers (100) coated by organized organic thin films (OOTFs) were used as the substrate on top of which ice was deposited. The amount of water required to fully cover the surface was determined by measuring the reactivity of the OOTF with O(3P) atomic beam as a function of ice coverage. It was found that the structure of vapor deposited formic acid is affected by the phase of the underlying ice and that the phase transition in the ice affects the infrared absorption spectrum of the formic acid. 1998 Elsevier Science S.A. All rights reserved Keywords: Formic acid; Organized organic thin films; Ice coverage
1. Introduction Recently atmospheric heterogeneous processes has been the focus of much attention [1–3]. Since the polar stratospheric clouds consist of small ice particles [4], understanding the processes occurring on their surfaces is of major importance for modelling the chemistry that takes place. It has been established that heterogeneous reactions of chlorine and nitrogen containing species on the surfaces of these particles may affect the ozone depletion in polar regions [5]. However, it was also suggested that the chemistry of organic molecules on ice surfaces is relevant to cloud chemistry and to cirrus clouds, where organic material is expected to be adsorbed on droplets [6]. The chemistry of adsorbed organic molecules may differ substantially from that of gas phase molecules. For example, it was found that the reaction between adsorbed alkyl chain and ground state atomic oxygen is much faster than the gas phase reaction between the oxygen atom and alkanes [7]. In the case of molecules adsorbed on ice, the phase of the ice may affect their adsorption rate and reactivity. For example, it was found that uptake of HCl into amorphous ice is more rapid than into crystalline ice [8]. In earlier studies [9] we have established the effect of * Corresponding author. Tel.: +972 8 9343409; fax: +972 8 9344123; e-mail: [email protected]
0040-6090/98/$ - see front matter PII S0040-6090 (98 )0 0696-8
substrate morphology on the structure of adsorbed ice. In those studies we modified the surface morphology by organized organic thin films (OOTFs). The OOTFs made it possible to modify the surface morphology while keeping the same chemical interaction between the substrate and the adsorbed ice. In the present work we extend the past studies and probe how the structure of the ice affects the formic acid adsorbed on its surface. In order to ensure that indeed the formic acid is adsorbed only on ice and not on the underlying organized organic layers we have firstly investigated how much water is required to fully cover OOTF surface. This was done by reacting atomic beam of ground state oxygen, O(3P), with the OOTF and probing how much water has to be deposited on top of the OOTF in order to block the access of the oxygen atoms to the organic layer.
2. Experimental The experimental setup allows in situ Fourier transform infrared spectroscopy (FTIR) measurements of adsorbed layers under ultrahigh vacuum conditions and it was described in detail previously [7,9]. In the present work two types of measurements were performed. In the first type of measurements, the FTIR spectrum of the OOTF was determined after it was covered with a controlled amount of water. Here, the OOTF was exposed to a beam
1998 Elsevier Science S.A. All rights reserved
500
S. Trakhtenberg, R. Naaman / Thin Solid Films 327–329 (1998) 499–502
3. Results and discussion
Fig. 1. The schematic presentation of the investigated system.
of ground state atomic oxygen (O3P) and the destruction of the organic layer (due to reaction with the oxygen) was monitored as a function of the average water coverage. In the second type of measurements, the FTIR spectrum of formic acid was determined after it was deposited on organic films covered with ice. In this case, the dependence of the formic acid spectra on the ice phase was monitored. The investigated system is schematically presented on Fig. 1. 2.1. The reaction between O(3P) and OTS in the presence of water Silicon wafers covered with octadecyltrichlorosilane (OTS) were used as substrates. Their quality was ensured by wettability measurements. Samples with water contact angles of 113 ± 2 and bicyclohexyl contact angles of 49 ± 1 were used in the experiments. Substrate covered with either amorphous or partially crystalline ice, or the substrate itself was exposed to O(3P) flow of about 2 × 1015 atoms/s for 60 min at a substrate temperature of 100°K. In all cases presented the ice was deposited as amorphous ice. To make a crystalline ice, the ice was annealed at 173°K and was cooled again to 100°K. The amount of water adsorbed was determined by its IR absorption. As a result the value stated for the thickness of adsorbed ice refers to an ‘average’ value. Namely, the surface is not uniform and the water can be organized, for example, as big clusters, leaving part of the surface uncovered with water. The FTIR spectra of the sample were monitored during the reaction with O(3P) in order to determine the reactivity of OTS.
In the first part of the project we have determined how much water is required to fully cover the organic surface under our experimental conditions. This finding is essential for the second part of the work, in order to ensure that formic acid deposition occurs on the ice and not on the organic surface. Generally, when molecules are adsorbed on a substrate they can form either a uniform layer or they may cluster to form relatively thick islands of adsorbed molecules, leaving parts of the surface uncovered. In order to investigate how ice is adsorbed on OTS surfaces, we used the reaction between O(3P) and the organic layer as a probe. By monitoring the depletion of the OTS absorption spectrum we could probe the ability of the ice to block the penetration of the oxygen to the organic layer. Hence, if the ice layer covers completely the OTS surface, we do not expect to see any reaction occurring. However, as long as the coverage is not full and the ice layer is not thick enough, reaction will take place. Therefore, we have a quite sensitive tool to probe the way the ice layer is adsorbed on the surface and to check how this coverage depends on the ice structure. Fig. 2 shows the IR absorption spectrum of OTS in the region corresponding to the C–H stretching. The peak at 2962 cm − 1 corresponds to the absorption of the methyl groups and the peaks at 2917 and 2850 cm − 1 corresponds to the absorption of the methylene groups.
2.2. Formic acid deposition on ice Silicon wafers covered with OTS were used as substrates. Formic acid was deposited at 113°K on top of either amorphous or partially crystalline ice or on the OTS itself. After deposition of formic acid the temperature of the substrate was raised with a rate of 0.75°K/min and the changes in the infrared absorption of the both adsorbed species were monitored.
Fig. 2. The FTIR spectrum of OTS in the region corresponding to the absorption of the methylene and methyl C–H stretching. The peak at 2962 cm − 1 corresponds to the absorption of the methyl groups and the peaks at 2917 and 2850 cm − 1 corresponds to the absorption of the methylene groups.
S. Trakhtenberg, R. Naaman / Thin Solid Films 327–329 (1998) 499–502
Fig. 3 presents the changes in the absorption of the OTS due to the reaction with the O(3P) beam. Namely, we present the result of subtracting the spectra taken after the exposure to oxygen for 60 min, from the spectra taken before the exposure. In Fig. 3A no water were adsorbed on the OTS, in Fig. 3B,C the OTS was covered ‘on average’ by a 10 nm of amorphous and crystalline ice, respectively. In Fig. 3D,E the OTS was covered ‘on average’ with 30 nm of amorphous and crystalline ice, respectively, while in (3F) 50 nm of amorphous ice were adsorbed. The results indicate that in order to inhibit the reaction between OTS and O(3P) the OTS has to be covered with 30 nm of crystalline ice or 50 nm of amorphous ice. Hence, we established the conditions under which the OTS surface is fully covered with ice. In the second stage of the work we investigated the effect of the ice phase on the structure of formic acid adsorbed on its surface. The focus of our study centred on the IR absorption band of formic acid corresponding to the C=O stretching vibration. Millikan et al. [10] proposed that this band splits upon crystallisation. Hence, the splitting in this band serves as an indication for the formic acid phase. IR spectra were measured for acid deposited on the substrate (silicon wafer covered with OTS) with no ice coverage, on substrate covered ‘on average’ with 75 nm partially crystalline ice and on substrate covered with 80 nm thick amorphous ice (Fig. 4A–C, respectively). In all cases the substrates were kept at 113°K. Since the formic acid molecules are much larger than oxygen atoms, it is fair to assume
Fig. 3. The FTIR spectra of OTS monolayer after reaction with O(3P) at 100°K for 60 min, subtracted from the spectra of the same monolayer before reaction. In (A) The OTS is not covered with ice; (B) OTS covered with 10 nm of amorphous ice; (C) covered with 10 nm of partially crystalline ice; (D) covered with 30 nm of amorphous ice; (E) covered with 30 nm of partially crystalline ice; (F) covered with 50 nm of amorphous ice.
501
Fig. 4. The IR absorption spectra of 25 layers of formic acid deposited at 113°K on (A) OTS on Si wafer with no ice coverage; (B) 75 nm thick partially crystalline ice; (C) 80 nm thick amorphous ice. The spectra were measured at 113°K.
that for an ‘average’ ice coverage that is thicker than 50 nm, the formic acid molecules interact only with ice and not with the underlying organic substrate. From Fig. 4 it is evident that the absorption band shape is strongly dependent on the substrate nature. The acid adsorbed on the low energy, well-ordered OTS surface is the most crystalline and the acid adsorbed on amorphous ice is the most amorphous one. The possible explanation for this observation is that in the first case, the interaction between the acid and the substrate is the weakest and the acid is free to arrange itself in the most stable, i.e. crystalline way. The interaction between the acid and the ice is stronger than between the acid and the OTS due to both dipole-dipole interactions and hydrogen bonds between the acid and water molecules. This suggest that the interaction between the formic acid molecules and amorphous ice is stronger than the interaction with crystalline ice. This may result from the larger surface area [11] and the larger number of ice molecules with dangling OH groups [8] in amorphous versus the crystalline ice. The stronger the interaction between the water molecules and the substrate, the more difficult it is for the water molecules to rearrange themselves to the more stable crystalline state. After the acid deposition, the samples were slowly heated. Fig. 5 presents the three formic acid samples after heating the surface to 175°K. The splitting of the bands is now evident. However one can observe the differences in both width and position of the peaks. Millikan et al. [10] reported the band peaks at 1703 and 1609 cm − 1 for crystalline formic acid. The spectrum of formic acid deposited on
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S. Trakhtenberg, R. Naaman / Thin Solid Films 327–329 (1998) 499–502
OTS with no ice shows identical bands. However, the peaks in the spectra of formic acid deposited on both crystalline and amorphous ice are shifted to higher frequencies (1718 and 1609 cm − 1, and 1721 and 1612 cm − 1, respectively). In addition, the bands are wider than in the case of formic acid deposited on OTS. These differences are probably due to the interaction between the acid and the ice. The position of the high frequency peak, corresponding to the out-of-phase vibrations, was chosen as a quantitative measure for the ice-acid interaction. Fig. 6 shows the dependence of the absorption peak position on surface temperature. When formic acid was adsorbed on the OTS itself and the surface was heated, almost no shift in the peak position could be observed. When formic acid was adsorbed on partially crystalline ice, the peak starts to shift at about 160°K and keep shifting until the complete desorption of the formic acid. In the case of formic acid adsorbed on amorphous ice, the abrupt change of the peak position occurs between 167 and 173°K simultaneously with the crystallisation of the amorphous ice. The ice crystallisation was observed by monitoring its FTIR spectrum [9]. The differences between the shifts observed for acid adsorbed on the two type of ice results, most probably, from the fact that while in the case of amorphous ice the phase transition in ice is abrupt, in the case of partially crystalline ice, the amorphous portion undergo the phase transition in each domain at slightly different temperature. Hence, by probing the absorption of the formic acid it is
Fig. 6. The position of the C=O stretching band peak of formic acid as function of the surface temperature. The acid was deposited at 113°K on: (K) OTS on Si wafer with no ice coverage; (B) (W) 75 nm thick partially crystalline ice; (C) (B) 80 nm thick amorphous ice.
possible to determine the ice phase on which it was adsorbed, even after the ice is completely crystalline.
4. Conclusions The experimental results provide an insight on the affect of the ice structure on the structure of molecules adsorbed on its surface. The phase of the ice influences the aggregation state of coadsorbed species. The temperature induced phase transition in the adsorbed species is affected by phase transition in ice.
References
Fig. 5. The IR absorption spectra of 25 layers of formic acid deposited at 113°K on (A) OTS on Si wafer with no ice coverage; (B) 75 nm thick partially crystalline ice; (C) 80 nm thick amorphous ice. The spectra were measured at 173°K.
[1] M.J. Molina, T.-L. Tso, L.T. Molina, F.C.-Y. Wang, Science 238 (1987) 1253. [2] J.G. Anderson, D.W. Toohey, W.H. Brune, Science 251 (1991) 39. [3] F.G. Dentener, P.J. Crutzen, J. Geophys. Res. 98 (1993) 7149. [4] M.A. Tolbert, M.J. Rossi, D.M. Golden, Science 240 (1988) 1018. [5] S. Solomon, R.R. Garcia, F.S. Rowland, D.J. Wuebbles, Nature 321 (1986) 755. [6] J.-P. Chen, P.J. Crutzen, J. Geophys. Res. 99 (1994) 18847. [7] Y. Paz, S. Trakhtenberg, R. Naaman, J. Am. Chem. Soc. 116 (1994) 10344. [8] J.D. Graham, J.T. Roberts, Geophys. Res. Lett. 22 (1995) 251. [9] S. Trakhtenberg, R. Naaman, S.R. Cohen, I. Benjamin, J. Phys. Chem. B 101 (1997) 5172. [10] R.C. Millikan, K.S. Pitzer, J. Am. Chem. Soc. 80 (1958) 3515. [11] L.F. Keyser, M. Leu, J. Coll. Interface Sci. 155 (1993) 137.
The effect of the structure of ice on the aggregation state of co-adsorbed formic acid S. Trakhtenberg*, R. Naaman Department of Chemical Physics, Weizmann Institute of Science, Rehovot 76100, Israel
Abstract In the present work we have investigated the effect of the structure of ice on the adsorption of formic acid on its surface. Single crystal silicon wafers (100) coated by organized organic thin films (OOTFs) were used as the substrate on top of which ice was deposited. The amount of water required to fully cover the surface was determined by measuring the reactivity of the OOTF with O(3P) atomic beam as a function of ice coverage. It was found that the structure of vapor deposited formic acid is affected by the phase of the underlying ice and that the phase transition in the ice affects the infrared absorption spectrum of the formic acid. 1998 Elsevier Science S.A. All rights reserved Keywords: Formic acid; Organized organic thin films; Ice coverage
1. Introduction Recently atmospheric heterogeneous processes has been the focus of much attention [1–3]. Since the polar stratospheric clouds consist of small ice particles [4], understanding the processes occurring on their surfaces is of major importance for modelling the chemistry that takes place. It has been established that heterogeneous reactions of chlorine and nitrogen containing species on the surfaces of these particles may affect the ozone depletion in polar regions [5]. However, it was also suggested that the chemistry of organic molecules on ice surfaces is relevant to cloud chemistry and to cirrus clouds, where organic material is expected to be adsorbed on droplets [6]. The chemistry of adsorbed organic molecules may differ substantially from that of gas phase molecules. For example, it was found that the reaction between adsorbed alkyl chain and ground state atomic oxygen is much faster than the gas phase reaction between the oxygen atom and alkanes [7]. In the case of molecules adsorbed on ice, the phase of the ice may affect their adsorption rate and reactivity. For example, it was found that uptake of HCl into amorphous ice is more rapid than into crystalline ice [8]. In earlier studies [9] we have established the effect of * Corresponding author. Tel.: +972 8 9343409; fax: +972 8 9344123; e-mail: [email protected]
0040-6090/98/$ - see front matter PII S0040-6090 (98 )0 0696-8
substrate morphology on the structure of adsorbed ice. In those studies we modified the surface morphology by organized organic thin films (OOTFs). The OOTFs made it possible to modify the surface morphology while keeping the same chemical interaction between the substrate and the adsorbed ice. In the present work we extend the past studies and probe how the structure of the ice affects the formic acid adsorbed on its surface. In order to ensure that indeed the formic acid is adsorbed only on ice and not on the underlying organized organic layers we have firstly investigated how much water is required to fully cover OOTF surface. This was done by reacting atomic beam of ground state oxygen, O(3P), with the OOTF and probing how much water has to be deposited on top of the OOTF in order to block the access of the oxygen atoms to the organic layer.
2. Experimental The experimental setup allows in situ Fourier transform infrared spectroscopy (FTIR) measurements of adsorbed layers under ultrahigh vacuum conditions and it was described in detail previously [7,9]. In the present work two types of measurements were performed. In the first type of measurements, the FTIR spectrum of the OOTF was determined after it was covered with a controlled amount of water. Here, the OOTF was exposed to a beam
1998 Elsevier Science S.A. All rights reserved
500
S. Trakhtenberg, R. Naaman / Thin Solid Films 327–329 (1998) 499–502
3. Results and discussion
Fig. 1. The schematic presentation of the investigated system.
of ground state atomic oxygen (O3P) and the destruction of the organic layer (due to reaction with the oxygen) was monitored as a function of the average water coverage. In the second type of measurements, the FTIR spectrum of formic acid was determined after it was deposited on organic films covered with ice. In this case, the dependence of the formic acid spectra on the ice phase was monitored. The investigated system is schematically presented on Fig. 1. 2.1. The reaction between O(3P) and OTS in the presence of water Silicon wafers covered with octadecyltrichlorosilane (OTS) were used as substrates. Their quality was ensured by wettability measurements. Samples with water contact angles of 113 ± 2 and bicyclohexyl contact angles of 49 ± 1 were used in the experiments. Substrate covered with either amorphous or partially crystalline ice, or the substrate itself was exposed to O(3P) flow of about 2 × 1015 atoms/s for 60 min at a substrate temperature of 100°K. In all cases presented the ice was deposited as amorphous ice. To make a crystalline ice, the ice was annealed at 173°K and was cooled again to 100°K. The amount of water adsorbed was determined by its IR absorption. As a result the value stated for the thickness of adsorbed ice refers to an ‘average’ value. Namely, the surface is not uniform and the water can be organized, for example, as big clusters, leaving part of the surface uncovered with water. The FTIR spectra of the sample were monitored during the reaction with O(3P) in order to determine the reactivity of OTS.
In the first part of the project we have determined how much water is required to fully cover the organic surface under our experimental conditions. This finding is essential for the second part of the work, in order to ensure that formic acid deposition occurs on the ice and not on the organic surface. Generally, when molecules are adsorbed on a substrate they can form either a uniform layer or they may cluster to form relatively thick islands of adsorbed molecules, leaving parts of the surface uncovered. In order to investigate how ice is adsorbed on OTS surfaces, we used the reaction between O(3P) and the organic layer as a probe. By monitoring the depletion of the OTS absorption spectrum we could probe the ability of the ice to block the penetration of the oxygen to the organic layer. Hence, if the ice layer covers completely the OTS surface, we do not expect to see any reaction occurring. However, as long as the coverage is not full and the ice layer is not thick enough, reaction will take place. Therefore, we have a quite sensitive tool to probe the way the ice layer is adsorbed on the surface and to check how this coverage depends on the ice structure. Fig. 2 shows the IR absorption spectrum of OTS in the region corresponding to the C–H stretching. The peak at 2962 cm − 1 corresponds to the absorption of the methyl groups and the peaks at 2917 and 2850 cm − 1 corresponds to the absorption of the methylene groups.
2.2. Formic acid deposition on ice Silicon wafers covered with OTS were used as substrates. Formic acid was deposited at 113°K on top of either amorphous or partially crystalline ice or on the OTS itself. After deposition of formic acid the temperature of the substrate was raised with a rate of 0.75°K/min and the changes in the infrared absorption of the both adsorbed species were monitored.
Fig. 2. The FTIR spectrum of OTS in the region corresponding to the absorption of the methylene and methyl C–H stretching. The peak at 2962 cm − 1 corresponds to the absorption of the methyl groups and the peaks at 2917 and 2850 cm − 1 corresponds to the absorption of the methylene groups.
S. Trakhtenberg, R. Naaman / Thin Solid Films 327–329 (1998) 499–502
Fig. 3 presents the changes in the absorption of the OTS due to the reaction with the O(3P) beam. Namely, we present the result of subtracting the spectra taken after the exposure to oxygen for 60 min, from the spectra taken before the exposure. In Fig. 3A no water were adsorbed on the OTS, in Fig. 3B,C the OTS was covered ‘on average’ by a 10 nm of amorphous and crystalline ice, respectively. In Fig. 3D,E the OTS was covered ‘on average’ with 30 nm of amorphous and crystalline ice, respectively, while in (3F) 50 nm of amorphous ice were adsorbed. The results indicate that in order to inhibit the reaction between OTS and O(3P) the OTS has to be covered with 30 nm of crystalline ice or 50 nm of amorphous ice. Hence, we established the conditions under which the OTS surface is fully covered with ice. In the second stage of the work we investigated the effect of the ice phase on the structure of formic acid adsorbed on its surface. The focus of our study centred on the IR absorption band of formic acid corresponding to the C=O stretching vibration. Millikan et al. [10] proposed that this band splits upon crystallisation. Hence, the splitting in this band serves as an indication for the formic acid phase. IR spectra were measured for acid deposited on the substrate (silicon wafer covered with OTS) with no ice coverage, on substrate covered ‘on average’ with 75 nm partially crystalline ice and on substrate covered with 80 nm thick amorphous ice (Fig. 4A–C, respectively). In all cases the substrates were kept at 113°K. Since the formic acid molecules are much larger than oxygen atoms, it is fair to assume
Fig. 3. The FTIR spectra of OTS monolayer after reaction with O(3P) at 100°K for 60 min, subtracted from the spectra of the same monolayer before reaction. In (A) The OTS is not covered with ice; (B) OTS covered with 10 nm of amorphous ice; (C) covered with 10 nm of partially crystalline ice; (D) covered with 30 nm of amorphous ice; (E) covered with 30 nm of partially crystalline ice; (F) covered with 50 nm of amorphous ice.
501
Fig. 4. The IR absorption spectra of 25 layers of formic acid deposited at 113°K on (A) OTS on Si wafer with no ice coverage; (B) 75 nm thick partially crystalline ice; (C) 80 nm thick amorphous ice. The spectra were measured at 113°K.
that for an ‘average’ ice coverage that is thicker than 50 nm, the formic acid molecules interact only with ice and not with the underlying organic substrate. From Fig. 4 it is evident that the absorption band shape is strongly dependent on the substrate nature. The acid adsorbed on the low energy, well-ordered OTS surface is the most crystalline and the acid adsorbed on amorphous ice is the most amorphous one. The possible explanation for this observation is that in the first case, the interaction between the acid and the substrate is the weakest and the acid is free to arrange itself in the most stable, i.e. crystalline way. The interaction between the acid and the ice is stronger than between the acid and the OTS due to both dipole-dipole interactions and hydrogen bonds between the acid and water molecules. This suggest that the interaction between the formic acid molecules and amorphous ice is stronger than the interaction with crystalline ice. This may result from the larger surface area [11] and the larger number of ice molecules with dangling OH groups [8] in amorphous versus the crystalline ice. The stronger the interaction between the water molecules and the substrate, the more difficult it is for the water molecules to rearrange themselves to the more stable crystalline state. After the acid deposition, the samples were slowly heated. Fig. 5 presents the three formic acid samples after heating the surface to 175°K. The splitting of the bands is now evident. However one can observe the differences in both width and position of the peaks. Millikan et al. [10] reported the band peaks at 1703 and 1609 cm − 1 for crystalline formic acid. The spectrum of formic acid deposited on
502
S. Trakhtenberg, R. Naaman / Thin Solid Films 327–329 (1998) 499–502
OTS with no ice shows identical bands. However, the peaks in the spectra of formic acid deposited on both crystalline and amorphous ice are shifted to higher frequencies (1718 and 1609 cm − 1, and 1721 and 1612 cm − 1, respectively). In addition, the bands are wider than in the case of formic acid deposited on OTS. These differences are probably due to the interaction between the acid and the ice. The position of the high frequency peak, corresponding to the out-of-phase vibrations, was chosen as a quantitative measure for the ice-acid interaction. Fig. 6 shows the dependence of the absorption peak position on surface temperature. When formic acid was adsorbed on the OTS itself and the surface was heated, almost no shift in the peak position could be observed. When formic acid was adsorbed on partially crystalline ice, the peak starts to shift at about 160°K and keep shifting until the complete desorption of the formic acid. In the case of formic acid adsorbed on amorphous ice, the abrupt change of the peak position occurs between 167 and 173°K simultaneously with the crystallisation of the amorphous ice. The ice crystallisation was observed by monitoring its FTIR spectrum [9]. The differences between the shifts observed for acid adsorbed on the two type of ice results, most probably, from the fact that while in the case of amorphous ice the phase transition in ice is abrupt, in the case of partially crystalline ice, the amorphous portion undergo the phase transition in each domain at slightly different temperature. Hence, by probing the absorption of the formic acid it is
Fig. 6. The position of the C=O stretching band peak of formic acid as function of the surface temperature. The acid was deposited at 113°K on: (K) OTS on Si wafer with no ice coverage; (B) (W) 75 nm thick partially crystalline ice; (C) (B) 80 nm thick amorphous ice.
possible to determine the ice phase on which it was adsorbed, even after the ice is completely crystalline.
4. Conclusions The experimental results provide an insight on the affect of the ice structure on the structure of molecules adsorbed on its surface. The phase of the ice influences the aggregation state of coadsorbed species. The temperature induced phase transition in the adsorbed species is affected by phase transition in ice.
References
Fig. 5. The IR absorption spectra of 25 layers of formic acid deposited at 113°K on (A) OTS on Si wafer with no ice coverage; (B) 75 nm thick partially crystalline ice; (C) 80 nm thick amorphous ice. The spectra were measured at 173°K.
[1] M.J. Molina, T.-L. Tso, L.T. Molina, F.C.-Y. Wang, Science 238 (1987) 1253. [2] J.G. Anderson, D.W. Toohey, W.H. Brune, Science 251 (1991) 39. [3] F.G. Dentener, P.J. Crutzen, J. Geophys. Res. 98 (1993) 7149. [4] M.A. Tolbert, M.J. Rossi, D.M. Golden, Science 240 (1988) 1018. [5] S. Solomon, R.R. Garcia, F.S. Rowland, D.J. Wuebbles, Nature 321 (1986) 755. [6] J.-P. Chen, P.J. Crutzen, J. Geophys. Res. 99 (1994) 18847. [7] Y. Paz, S. Trakhtenberg, R. Naaman, J. Am. Chem. Soc. 116 (1994) 10344. [8] J.D. Graham, J.T. Roberts, Geophys. Res. Lett. 22 (1995) 251. [9] S. Trakhtenberg, R. Naaman, S.R. Cohen, I. Benjamin, J. Phys. Chem. B 101 (1997) 5172. [10] R.C. Millikan, K.S. Pitzer, J. Am. Chem. Soc. 80 (1958) 3515. [11] L.F. Keyser, M. Leu, J. Coll. Interface Sci. 155 (1993) 137.