J. Phys. Chem. B 2006, 110, 23497-23501
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Chlorine Interactions with Water Ice Studied by Molecular Beam Techniques Liza S. E. Romero Lejonthun, Patrik U. Andersson, Mats B. Någård,† and Jan B. C. Pettersson* Department of Chemistry, Atmospheric Science, Go¨teborg UniVersity, SE 412 96 Go¨teborg, Sweden ReceiVed: August 31, 2006
The kinetics of chlorine interactions with ice at temperatures between 103 and 165 K have been studied using molecular beam techniques. The Cl2 trapping probability is found to be unity at thermal incident energies, and trapping is followed by rapid desorption. The residence time on the surface is less than 25 µs at temperatures above 135 K and approaches 1 s around 100 K. Rate constants for desorption are determined for temperatures below 135 K. The desorption kinetics follow the Arrhenius equation, and activation energies of 0.24 ( 0.03 and 0.31 ( 0.01 eV, with corresponding preexponential factors of 1012.08(1.19 and 1016.52(0.38 s-1, are determined. At least two different Cl2 binding sites are concluded to exist on the ice surface. The observed activation energies are likely to be the Cl2-ice binding energies for these states, and the Cl2-surface interactions are concluded to be stronger than earlier theoretical estimates. The surface coverage of Cl2 on ice under stratospheric conditions is estimated to be negligible, in agreement with earlier work.
1. Introduction The interaction of gases with ice surfaces has in recent times received considerable attention.1,2 In addition to the fundamental interest, the studies are often motivated by the need to understand key heterogeneous processes on atmospheric ice particles. In particular, the discovery that depletion of ozone in the Antarctic stratosphere involves heterogeneous chlorine chemistry has created considerable interest. Polar stratospheric clouds are formed under cold wintertime conditions, and HCl and ClONO2 are believed to react on the surface of cloud particles according to the following major reaction pathways:3
HCl + ClONO2 f Cl2 + HNO3
(1)
H2O + ClONO2 f HOCl + HNO3
(2)
The reactions activate the chlorine reservoir molecules HCl and ClONO2 and transform them into compounds that may later contribute to the destruction of ozone. Cl2 and HOCl are released into the gas phase, while HNO3 is trapped in the cloud particles.3 In this study we focus on the interaction of Cl2 with water ice. The nonpolar molecule Cl2 binds less strongly to the ice surface compared to HCl, ClONO2, and HOCl, and it is therefore expected to have less influence on the heterogeneous chemistry. The Cl2 interaction with water ice is, however, not trivial, and detailed studies contribute to an improved understanding of the rich and complex chemistry on ice surfaces. While it at present appears unclear if Cl2 may react with the ice surface, it is interesting to note that Cl2 readily reacts with H2O in the liquid phase:
Cl2 + H2O f HCl + HOCl
(3)
a process that is commonly used for disinfection and wastewater treatment.4 * Corresponding author. Telephone: +46 31 772 28 28. Fax: +46 31 772 31 07. E-mail:
[email protected]. † Present address: Physical and Computational Chemistry, DMPK and Bioanalytical Chemistry, AstraZeneca R&D Mo¨lndal, SE-431 83 Mo¨lndal, Sweden.
The interaction of Cl2 with water ice has been investigated in a few earlier studies. Molina et al.5 carried out flow-tube experiments of Cl2 interactions with ice at 185 K and concluded that it was not retained by the ice. They also concluded that Cl2 produced by the reaction between HCl and ClONO2 on ice is released to the gas phase on a time scale of at most a few milliseconds. Banham et al.6 used reflection-absorption infrared spectroscopy and temperature programmed desorption mass spectrometry (TPD), and reported a desorption temperature of 110 K for Cl2 on ice. Graham and Roberts7 also used TPD and studied the interaction of Cl2 with amorphous water ice. A heating rate of 4 K/s was used and desorption of Cl2 was mainly observed at 125 K; a weak peak at about 155 K was also found. Cl2 and HCl desorption was also observed at 185 K, but was considered an artifact of the experiment. Donsig et al.8 studied the interactions of Cl2 with water ice using secondary ion mass spectrometry. At 90-100 K, Cl2 adsorbed molecularly on the ice surface, while Cl2 was concluded to react with the ice to form HCl and HOCl at temperatures above 130 K. In related work Yabushita et al.9,10 studied the photodissociation of Cl2 adsorbed on amorphous and crystalline ice. The interaction between Cl2 and a single H2O molecule has been studied experimentally by pulsed jet Fourier transform microwave spectroscopy11 and matrix isolation spectroscopy,12,13 and ab initio calculations of the system have also been performed.11,14 In the equilibrium structure Cl2 points toward the lone pair on the O atom of H2O, with a small charge transfer from O to the chlorine molecule. Geiger et al.15 performed ab initio calculations of Cl2 interacting with four water molecules representing an adsorption site on the surface of hexagonal ice. They determined a binding energy of 0.12 ( 0.01 eV for a structure where one of the Cl atoms interacts with a water O atom in the simulated surface. Liu et al.16 studied liquid-phase interactions by ab initio molecular dynamics simulations of Cl2 interacting with 19 H2O molecules, and reported the direct formation of an Cl- and H2OCl+ ion pair. In related work, Ramondo et al.17 studied Br2-ice interactions. In addition, the experimental study by Yabushita et al.9 indicated that the
10.1021/jp065656e CCC: $33.50 © 2006 American Chemical Society Published on Web 10/13/2006
23498 J. Phys. Chem. B, Vol. 110, No. 46, 2006
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Figure 1. Experimental setup. A pulsed Cl2 beam was directed toward an ice surface in the central ultrahigh-vacuum chamber. The beam pulse was mechanically chopped in the second chamber in order to select a central well-defined part of the pulse from the source. The ice surface was situated in a separate inner chamber that had a finite water vapor pressure e2 × 10-4 mbar. The beam passed into the inner vacuum chamber containing the ice surface through a thin slit, and molecules emitted from the ice left through the same slit and were subsequently detected by a differentially pumped quadrupole mass spectrometer.
interaction between chlorine and free OH groups on amorphous ice is stronger than the Cl2‚‚‚OH2 interaction on crystalline ice. Our research group has previously carried out molecular beam studies of Ar,18 HCl,19 CO,20 CO2,21 and He22 interacting with ice at surface temperatures of 100-190 K. The results show that molecular collisions with ice are highly inelastic and characterized by efficient transfer of energy to surface modes.18-21 HCl molecules form strong bonds with water ice either at impact or after diffusion on the surface.19 For a pure ice surface the sticking probability was 1.00 ( 0.02 for thermal incident kinetic energies, while a HCl coverage on the surface at least partially blocked the strongly bonded sites and allowed a fraction of the HCl molecules to desorb within a short time. Carbon dioxide interacts less strongly with the ice surface, and a finite residence time on the surface could be resolved and quantified.21 The desorption kinetics for CO2 on crystalline ice at 100-125 K were well described by the Arrhenius equation with an activation energy of 0.22 ( 0.02 eV and a preexponential factor of 1013.32(0.57 s-1. Below 120 K, CO2 populated strongly bonded sites on amorphous ice, resulting in surface residence times on the order of minutes at 100 K. In a related recent study, Suter et al.22 used low-energy elastic helium scattering to follow the onset of surface disorder in the ice surface at 180 K. We here report results from molecular beam studies of the interactions between molecular chlorine and water ice surfaces in the temperature range 103-165 K, with the aim to deepen the fundamental understanding of heterogeneous chemistry on water ice. Chlorine is adsorbed at thermal kinetic energies, and the following desorption step is studied by mass spectrometry. Rate constants for desorption are determined as a function of temperature below 135 K, and the data show that at least two different adsorption states on the surface influence the desorption behavior. 2. Experimental Section The experimental setup used for studies of Cl2 collisions with ice surfaces has been described elsewhere,18-21 and is only
briefly presented here. The ultrahigh-vacuum apparatus is schematically shown in Figure 1. Short gas pulses were produced by a solenoid-valve molecular beam source (General Valve) in the first chamber. A fraction of the gas flow was selected with a skimmer (orifice diameter 0.5 mm) placed 3 cm from the pulsed source, and a molecular beam was formed that entered the second differentially pumped vacuum chamber. A mechanical chopper was used to select the central part of each gas pulse, giving a square wavelike pulse with a width of 90-1130 µs depending on operating conditions. The translational energy of the beam was 0.11 eV. The beam then entered the main chamber, where the scattering experiments were performed. This chamber was pumped by four turbomolecular pumps and a helium cryostat, and the base pressure was about 5 × 10-9 mbar during the experimental period. The background pressure mainly consisted of water vapor produced by the experiments since the pressure could rise to the 10-8 mbar range at the highest ice temperatures employed in the experiments. The ice surface was prepared on a 12 × 12 mm graphite substrate (Advanced Ceramics Corp., grade ZYB) positioned inside a small cylindrical chamber, which was enclosed within the main ultrahigh-vacuum chamber. The substrate could be maintained at a temperature in the range 100-200 K with a stability of (0.1 K. Water vapor was introduced into the inner chamber through a leak valve and the vapor pressure could be increased up to the 10-4 mbar range, which allowed the ice surface to be maintained in a dynamic state with evaporation and condensation taking place at high rates. A slit opening in the chamber allowed the molecular beam to reach the surface, and the flux from the surface to reenter the main chamber. The flux from the ice was monitored with a differentially pumped quadrupole mass spectrometer (QMS) that was used to perform time-of-flight measurements in the scattering plane defined by the beam and the surface normal. The angular resolution was less than (1.5°. The signal from the QMS was stored on a multichannel scaler with a dwell time of 10-80 µs.
Chlorine Interactions with Water Ice
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Figure 3. Time-of-flight distributions for Cl2 desorption from ice on a logarithmic intensity scale. Notice the change in time scale compared to Figure 2.
Figure 2. Time-of-flight distributions for molecular chlorine desorbing from a water ice surface at surface temperatures from 105 to 150 K.
Initially, 400-1000 monolayers (ML) of water were deposited at 150 or 165 K, with a buildup rate of 0.2-1.5 ML/s, before surface scattering measurements started. The ice was first maintained at 150 K (or 165 K), before being cooled to temperatures as low as 103 K during the experiments. The surface buildup temperature did not affect the obtained data. The ice thickness was monitored by detecting the reflectance of a diode laser beam directed at the ice surface.18 The ice buildup procedure has previously been concluded to give stable crystalline ice.18-21 A certain degree of polycrystallinity or the partial existence of dense forms of amorphous ice cannot, however, be completely ruled out with the procedure. 3. Results and Discussion The experimental results consist of time-of-flight spectra for molecular chlorine emitted from the ice surface at temperatures in the range from 103 to 165 K. The incident angle of the molecular beam was 45°, and the incident kinetic energy was 0.11 eV. The results are completely dominated by trapping of Cl2 followed by desorption. No indication of an inelastic scattering component has been observed in the time-of-flight distributions taken for different scattering angles. The lack of an inelastic scattering channel is expected, considering the low incident kinetic energy and the relatively high mass of Cl2 that favor trapping over scattering,18 and it also agrees with the behavior observed for other molecules on ice.18-21 All displayed time-of-flight data are therefore measured in the surface normal direction since the signal from the trapping-desorption channel is largest in that direction. Figure 2 shows time-of-flight spectra for Cl2 desorbing from ice in the temperature range 105-150 K. The distribution observed at 150 K is typical for a trapping-desorption process, and the shape of the distribution is mainly given by the relatively broad Maxwell-Boltzmann velocity distribution of the desorbing molecules. The detailed peak shape is also to some degree influenced by the shape of the incoming gas pulse, while the residence time on the surface was too short at this temperature to influence the results. In the temperature interval 135-165 K, the time-of-flight distributions were only slightly changed due to small changes of the velocity distribution with temper-
ature. This indicates that Cl2 molecules on average stayed less than 25 µs on the ice surface at temperatures above 135 K, since this was the smallest time shift that could be resolved in the present experiments. As the temperature was decreased below 135 K, the maximum signal intensity decreased and the distributions became more extended in time with decreasing temperature. This change cannot be explained by changes in the Maxwell-Boltzmann velocity distribution with temperature. Instead, the data illustrate the effect of a finite residence time on the surface that becomes longer when temperature decreases. Figure 3 shows time-of-flight spectra on a logarithmic intensity scale and on a longer time scale than in Figure 2. The data clearly illustrate that more than one surface state is involved in the desorption process. A fraction of the Cl2 molecules adsorbed on the surface leave the surface within 10 ms, while a second fraction slowly decays over a period of tens of milliseconds. The fast component dominates at high temperature, while the slow desorption process becomes more important at low temperatures. Time-of-flight data of the type shown in Figures 2 and 3 have been simulated assuming that Cl2 thermally desorbs from the ice surface at a rate described by first-order desorption. The velocity distribution of the desorbing flux is described by
FTD(V) ) c1V2 exp(-mV2/2kBTs)
(4)
where V is the velocity, c1 is a scaling factor, m is the Cl2 mass, and Ts is the surface temperature. The distribution takes into account that the mass spectrometer is density sensitive. Using the Jacobian
dV ) -(L/t2) dt
(5)
for conversion of velocity to time space, eq 4 is converted into a time-of-arrival distribution G(t):
GTD(t) ) c1(L3/t4) exp(-m(L/t)2/2kBTs)
(6)
where L is the distance from the surface to the detector and t is the flight time.23 Since the Cl2 molecules spend a measurable time on the ice surface, the density profile of Cl2 molecules on the surface, θ(t), influences the measured time-of-flight profile. The density profile of Cl2 molecules on the surface, θ(t), is obtained by convoluting the beam density profile at the surface S(t) over the residence time distribution of Cl2 on the surface, FRES(t):
θ(t) )
∫0tS(τ) FRES(t-τ) dτ
(7)
23500 J. Phys. Chem. B, Vol. 110, No. 46, 2006
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Figure 4. Time-of-flight spectrum for Cl2 desorbing from water ice at 110 K: experimental data (open circles); calculated distributions using eq 10 for a fast (---) and a slow (- -) desorbing component and for the sum of the two components (s).
For temperatures above 125 K, the residence time distribution for Cl2 on the surface is well described by
FRES(t) ) exp(-kt)
(8)
where t is the residence time and k is a first-order desorption rate constant. The simulated intensity profile at the surface is finally given by
I(t) )
∫0tθ(τ) G(t-τ) dτ
(9)
and I(t) is fitted to the experimental results by varying c1 and the first-order rate constant, k. The surface temperature is treated as a constant in the convolution process. As is obvious from the data in Figure 3, time-of-flight data measured at temperatures e122.5 K cannot be fitted with a single-exponential decay, and instead a sum of two terms has been used to simulate the data:
FRES(t) ) a exp(-k1t) + b exp(-k2t)
(10)
where a and b are scaling factors; k1 and k2 are first-order desorption rate constants. Equation 10 is not directly related to a physical model of the Cl2-ice system, but the approach is an effective way to quantify the observed temperature dependence of the desorption process. Fits using eq 10 give excellent agreement with experimental data below 125 K, as illustrated in Figure 4 for Ts ) 110 K. In this case, 39% of the desorbing molecules are described by the faster exponential decay and 61% by the slower one. The observed average lifetime, τ, of Cl2 on the ice surface ranges from 13 µs at 135 K to 38 ms at 104 K. We first note that the desorption energy must be considerably larger than the binding energy of 0.12 eV found in ab initio calculations of Cl2 interacting with four water molecules,15 representing an adsorption site on hexagonal ice. We assume that the desorption kinetics follow the Arrhenius equation:
k ) A exp(-Edes/kBT)
(11)
where A is the preexponential factor, Edes is the desorption energy, and kB is Boltzmann’s constant. If a desorption energy of Edes ) 0.12 eV and a typical preexponential factor A ) 1013 s-1 are used, the residence time on the surface would be 4.5 ns at 130 K. The experimentally determined value is 28 µs at 130 K, about 4 orders of magnitude larger than the calculated value. The desorption rate constants observed for the temperature range 104-135 K are considerably lower than expected if the binding energy was 0.12 eV. The low desorption rate constants observed cannot be explained by a large barrier for adsorption since measurements at a scattering angle of 45° indicate a near cos θ distribution and the rise in the time-of-flight distributions
Figure 5. 5: Arrhenius plot of rate constants for desorption of Cl2 from water ice at temperatures 104-135 K. Lines illustrate the results of least-squares fits of the Arrhenius equation to the data indicated by (b) and (4). A third group of data (O) was not included in the analysis. See text for further details about the analysis.
indicates a barrier less than 0.02 eV. We therefore conclude that all molecules interact relatively strongly with the ice surface. Figure 5 shows an Arrhenius plot of the desorption rate constants obtained from fits to the experimental data. For temperatures higher than 135 K, the time resolution in the experiment was not sufficient to determine the desorption rates. The data for the temperature range 125-135 K are obtained using eq 8 and are well described by a straight line (this line is not shown in the figure), giving an activation energy of 0.27 ( 0.01 eV and a preexponential factor of 1014.86(0.38 s-1. A simple desorption process usually results in a preexponential factor of about 1013 s-1. The high preexponential factor observed here indicates that the desorption process is influenced by two or more Cl2 states on the ice surface with different characteristics with respect to binding energy and Cl2 population.24,25 Below 125 K, eq 10 was used to simulate the time-of-flight data, and two rate constants where obtained from each fit. In the temperature range 115-125 K two groups of constants with characteristic slopes are observed in the Arrhenius plot. In this temperature range the slow process has an activation energy of 0.24 ( 0.03 eV and a preexponential factor of 1012.08(1.19 s-1. As the temperature is decreased below 115 K, the surface residence time for the slow process becomes comparable to the time between consecutive Cl2 pulses in the experiment. The more strongly bound surface sites related to the slow process are therefore not emptied before the next Cl2 pulse reaches the surface and a surface population builds up. The strongly bound sites become partially saturated in the temperature range 110115 K, and are expected to be completely saturated below 110 K. Below 110 K, Cl2 thus interacts with an ice surface for which strongly bound sites are believed to be saturated. The slowest decay process seen below 110 K appears to have Arrhenius parameters similar to those of the fast process observed above 115 K, and an overall fit to the data (black dots in Figure 5) gives an activation energy of 0.31 ( 0.01 eV and a preexponential factor of 1016.52(0.38. In addition, a new and faster desorption process is observed below 110 K. To summarize, Cl2 exists in two (or more) states on the ice surface. The states are strongly coupled, as revealed by the complex kinetics and the observation of preexponential parameters considerably larger than 1013 s-1. The two activation energies of 0.24 ( 0.03 eV and about 0.3 eV are likely related to the binding energy of the two states on the surface. The strongly bound state becomes
Chlorine Interactions with Water Ice saturated at the lower surface temperatures employed here, and a new and less strongly bound state appears on the Cl2-covered ice surface. The experimental data indicate that Cl2 interacts more strongly with the ice surface than predicted by earlier theoretical calculations, and surface models of higher complexity than used before are possibly required to simulate the Cl2-ice interactions. Ramondo et al.17 showed that strong polarization of Br2 occurs when it is complexed by simultaneous Br‚‚‚OH2 and Br‚‚‚HOH bonds resulting in an increased binding energy to an ice surface, and this may apply also to Cl2. The present results are consistent with earlier TPD experiments where significant Cl2 desorption was observed in the temperature range 110-125 K.6,7 By extrapolation to higher temperatures the present results can be used to describe Cl2-ice interactions in the wintertime stratosphere over Antarctica. Based on the Arrhenius parameters determined at 125-135 K, the residence time on pure ice at 185 K is 31 ns. Assuming a Cl2 concentration of 2 ppbv, the calculated surface coverage is 2 × 10-9 ML at the same temperature. Sticking of chlorine on clean ice is thus negligible under polar stratospheric conditions, in agreement with earlier work.5 4. Conclusions The kinetics of Cl2 interactions with ice at temperatures between 103 and 165 K have been studied using molecular beam techniques, and the main conclusions can be summarized as follows: (a) Thermal collisions between chlorine molecules and crystalline ice at 103-165 K result in trapping on the ice surface. Inelastic scattering is negligible. (b) Trapping of Cl2 is followed by rapid desorption. The residence time on the surface is less than 25 µs at temperatures above 135 K and approaches 1 s when the temperature is decreased to 100 K. (c) At least two different Cl2 binding sites exist on the surface. The observed desorption activation energies of 0.24 and 0.31 eV are likely to be the Cl2-ice binding energies for these two states. These binding energies are considerably larger than previous estimates based on quantum mechanical calculations. (d) In agreement with earlier work, the surface coverage of Cl2 on ice under stratospheric conditions is estimated to be negligible. Although Cl2 interactions with ice may be regarded as simple when considering the chlorine chemistry in the wintertime
J. Phys. Chem. B, Vol. 110, No. 46, 2006 23501 stratosphere over Antarctica, the present study shows that the Cl2-ice interactions display considerably complexity. To better understand the chlorine states on the ice surface, detailed quantum mechanical calculations that include a relatively large set of surface water molecules and allow for crystal structure flexibility should be performed. Acknowledgment. We thank Mr. Benny Lo¨nn for construction work and technical support. This work was supported by the Swedish Research Council. References and Notes (1) Abbatt, J. P. D. Chem. ReV. 2003, 103, 4783. (2) Girardet, C.; Toubin, C. Surf. Sci. Rep. 2001, 44, 159. (3) Solomon, S. ReV. Geophys. 1999, 37, 275, and references therein. (4) Wang, T. X.; Margerum, D. W. Inorg. Chem. 1994, 33, 1050. (5) Molina, M. J.; Tso, T.; Molina, L. T.; Wang, F. C.-Y. Science 1987, 238, 1253. (6) Banham, S. F.; Horn, A. B.; Koch, T. G.; Sodeau, J. R. Faraday Discuss. 1995, 100, 321. (7) Graham, J. D.; Roberts, J. T. J. Phys. Chem. B 2000, 104, 978. (8) Donsig, H. A.; Herridge, D.; Vickerman, J. C. J. Phys. Chem. 1998, 102, 2302. (9) Yabushita, A.; Inoue, Y.; Senga, T.; Kawasaki, M. J. Phys. Chem. B 2002, 106, 3151. (10) Yabushita, A.; Kawasaki, M.; Sato, S. J. Phys. Chem. A 2003, 107, 1472. (11) Davey, J. B.; Legon, A. C.; Thumwood, J. M. A. J. Chem. Phys. 2001, 114, 6190. (12) Engdahl, A.; Nelander, B. J. Chem. Phys. 1986, 84, 1981. (13) Johnsson, K.; Engdahl, A.; Ouis, P.; Nelander, B. J. Phys. Chem. 1992, 96, 5778. (14) Dahl, T.; Røeggen, I. J. Am. Chem. Soc. 1996, 118, 4152. (15) Geiger, F. M.; Hicks, J. M.; de Dios, A. C. J. Phys. Chem. A 1998, 102, 1514. (16) Lui, Z. F.; Siu, C. K.; Tse, J. S. Chem. Phys. Lett. 1999, 311, 93. (17) Ramondo, F.; Sodeau, J. R.; Roddis, T. B.; Williams, N. A. Phys. Chem. Chem. Phys. 2000, 2, 2309. (18) Andersson, P. U.; Någård, M. B.; Bolton, K.; Svanberg, M.; Pettersson, J. B. C. J. Phys. Chem. A 2000, 104, 2681. (19) Andersson, P. U.; Någård, M. B.; Pettersson, J. B. C. J. Phys. Chem. B 2000, 104, 1596. (20) Suter, M. T.; Andersson, P. U.; Pettersson, J. B. C. Phys. Scr., T 2004, T110, 350. (21) Andersson, P. U.; Någård, M. B.; Witt, G.; Pettersson, J. B. C. J. Phys. Chem. A 2004, 108, 4627. (22) Suter, M. T.; Andersson, P. U.; Pettersson, J. B. C. J. Chem. Phys. Accepted. (23) Atomic and Molecular Beam Methods; Scoles, G., Ed.; Oxford University Press: Oxford, 1988; Vol. 1. (24) Nordholm, S. Chem. Phys. 1985, 98, 367. (25) Bethune, D. S.; Barker, J. A.; Rettner, C. T. J. Chem. Phys. 1990, 92, 6847.
