Sulfur dioxide adsorption on I-covered Ag(111): metastable characteristics

Sulfur dioxide adsorption on I-covered Ag(111): metastable characteristics

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Surface Science 296 (1993) 36-48 North-Holland

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Sulfur dioxide adsorption on I-covered Ag( 111): metastable characteristics Z.-J. Sun, R.S. Mackay and J.M. White Department of Chemistry and Biochemistry, University of Texas at Austin, Austin, TX 78712, USA

Received 12 March 1993; accepted for publication 14 June 1993

SO, adsorption on iodine-covered (exceeding 1 monolayer) Ag(lll) has been studied by TPD (temperature programmed desorption), isothermal desorption, TOF (time-of-flight) and AES (Auger electron spectroscopy). A metastable state (with a desorption activation energy of 18 * 2 kJ/mol) co-exists with the SO, multilayer state. Molecules in the former interact both with each other and with the substrate. The formation of the metastable state depends heavily on the coverage, temperature and condition of the surface; higher coverages (exceeding 2 monolayers), higher surface temperatures (> 96 K) and surface impurities and disorder diminish the metastable state concentration. Uptake experiments indicate that interactions of SO, with I are weaker than SO, with SO,, but that SO,-1 interactions are important in the metastable state. Photodesorption and reaction experiments indicate that extensive inter-adsorbate interactions exist in both states. A wetting two-dimensional island model is used to describe the structure of metastable state.

1. Introduction Adsorption of gas molecules on crystal surfaces is the topic of numerous works [l-3], yet the adsorption-desorption characteristics of molecules are still not easily predictable. Many components are involved, including the incident encounter, surface diffusion (both at low and high coverage), sticking, surface reaction, phase transition, and outbound trajectories. In simple terms, two classes of interactions, substrate-adsorbate and adsorbate-adsorbate, determine the properties of these processes. Most investigations have involved stronger adsorbate-substrate than adsorbate-adsorbate interactions. Here, we report on a system SO,/I/Ag(lll) where the reverse holds; adsorbate-adsorbate interactions are stronger. Numerous studies show that different adsorbate phases can co-exist and inter-convert [l-31. In adlayers deposited on single crystal substrates, LEED and other experimental techniques have revealed a variety of two-dimensional (2D) or three-dimensional (3D) phases differing in their 0039-6028/93/$06.00

atomic structures and physicochemical properties [l]. Co-existence of several different adsorbed phases can occur. It is clear that the adsorptiondesorption parameters and their mechanisms must depend not only on the structure of the adlayer itself, but also on the nature of the interaction of adsorbed sp,ecies with surrounding species. For example, by changing the temperature and initial conditions of diffusion, one can arrive at preferential spreading of selected phases over the surface. If the initial state of an adlayer is metastable, e.g. the coverage is produced by means of low-temperature condensation to produce an amorphous state [4,5], then its transition into the equilibrium state, i.e. ordering, occurs at temperatures for which adspecies exhibit sufficient mobility, and contain enough thermal energy to overcome potential energy barriers. Sulfur dioxide, the adsorbate investigated here, is strongly chemisorbed on clean Agflll). TPD (temperature programmed desorption) shows desorption peaks at 204, 176 and 145 K, corresponding to adsorption at surface defects, on monolayer sites and within a compressed mono-

0 1993 - Elsevier Science Publishers B.V. All rights reserved

Z-J. Sun et al. / Sulfur dioxide adsorption on I-covered Ag(Ill):

layer structure [6]. The calculated desorption activation energies from monolayer and defect sites are 44.2 and 51.5 kJ/mol. At higher coverages, SO, multilayer desorption occurs at 130 K, corresponding to a desorption activation energy of 24 + 4 kJ/mol. In this system, SO, on Ag(lll), the adsorbate-substrate interaction is stronger than the adsorbate-adsorbate interaction and, thermodynamically, the monolayer is more stable than the multilayer. Atomic iodine is chemisorbed on Ag(ll1) and the TPD peak, atomic I, is at 850 K [7]. I multilayers are strongly bound; multilayer I TPD peaks at 560 K. Up to monolayer coverages, the I structure is dictated by the substrate structure, and leads to an ordered fi x 6 R30” superstructure on Ag(ll1) [8,9]. The properties of SO, on these two layers, which have different structures and electronic properties, are expected to differ. Based on the known properties of SO, and I on Ag(lll), we have studied the adsorption-desorption properties of SO, on I (2 1 monolayer, ML) covered Ag(lll). Using coverage (both SO, and I>, temperature and photon irradiation as variables, our study demonstrates the co-existence of a saturable metastable state (desorption peaks at around 120 K) and a non-saturable stable state (desorption peaks at around 130 K). After a brief description of experimental methods and conditions in section 2, experimental results are presented in section 3, with subsections on coverage, temperature, photon irradiation and surface conditions. In section 4, we present and discuss a phenomenological model.

2. Experimental The experiments were carried out using a standard ultrahigh vacuum chamber described elsewhere [61. Briefly, it is equipped with a single pass cylindrical mirror analyzer and a coaxial electron gun for Auger electron spectroscopy (AES); a quadrupole mass spectrometer for temperature programmed desorption (TPD), time-of-flight (TOF), and residual gas analysis (RGAI; an ion gun for sputtering; and a pinhole (10 pm> doser,

metastable characteristics

37

which terminated within about 1 mm of the surface. The working pressure during the experiments was 3 X lo- ” Torr. The Ag(ll1) surface was cleaned by Ar+ ion sputtering and subsequently annealed at 690 K. Cleanliness was verified by AES. The temperature was monitored by a chromel-alumel thermocouple spot welded to a tantalum loop that was pressed into a hole in the edge of the sample. The crystal could be cooled to 89 K by thermal contact with a liquid nitrogen reservoir and could be resistively heated to 950 K. Sulfur dioxide, Mattheson (99%0), was dosed after purification, the latter involving several freeze-pump-thaw cycles in liquid nitrogen. Methyl iodide, Aldrich (9.5%), a liquid at room temperature, was purified by pumping into a container immersed in a saturated salt water-ice bath ( - 20°C). An excimer laser provided pulses (15 ns) of 193 and 248 nm photons incident at 60 off the surface normal. The dosing rate was controlled by the pressure behind the pinhole; unless specifically noted, this pressure was 1.4 Torr. Much of our work compares TPD peak areas. Monolayer SO, is defined as the peak area measured when both the 176 and 204 K TPD peaks are saturated on Ag(lll), i.e., no iodine [6]. Other SO, surface coverages are calculated by comparing the measured TPD area with this reference area. Using known procedures [lo], the I coverage is prepared by condensing multilayer CH,I on Ag(ll1) at 90 K, photolyzing with 193 or 248 nm irradiation, and flashing the surface to 537 K to remove CH,, as C2Hs, as well as intact CH,I, leaving only I on the surface. To accumulate sufficient I, this procedure was repeated several times. In the final surface preparation step, upon flashing at least twice to 600 K, a small amount of multilayer I desorbs and the overlayer becomes more highly ordered. I (chemisorbed) bound to Ag is characterized by a broad atomic, not molecular, desorption peak from 700 to 900 K [lo]. A monolayer coverage of I was prepared by flashing to 626 K, removing the multilayer. The I coverage was verified by comparing I TPD after finishing SO,based experiments with the area measured for a coverage prepared thermally by condensing CH,I

38

Z.-J. Sun et

al. / Sulfur dioxide adsorption on I-covered Ag(ll1):

multilayer on Ag(ll1) surface and flashing to 400 K, a procedure known to deposit 0.33 ML of I [7]. This coverage was confirmed by comparing the AES I(521 eV)/Ag(351 eV> peak intensity ratios with TPD areas.

K dominates (fig. la). With increasing coverage, the population of this state first increases (0.1 to 1.7 ML), but after a critical coverage (N 2 ML), it decreases (fig. lb). Simultaneously, a new state appears around 130 K (denoted as state B), continuously intensifies and, after the critical coverage (2 ML), becomes dominant. Fig. Id compares TPD spectra at 1.7 and 3.2 ML coverages. Clearly, the leading edge is suppressed at 3.2 ML, indicating that the population of A is decreasing. Above 3.2 ML, there is no evidence of state A, while the state B population (fig. lc) grows without bound. These spectra cannot be characterized as a merging of peak A into peak B. State B around 130 K is indistinguishable from multilayer SO, on clean Ag(lll) [6]. The desorption activation energy, obtained by fitting the leading edge of the TPD curves at coverages of more than 5 ML, is 24 + 3 kJ/mol, which is the same as the activation energy measured for multilayer SO, on clean Ag(ll1) [61. Thus, we identify state B as multilayer SO,. State A desorption is fascinating. Unlike typical chemisorbed monolayers, its desorption peak temperature is lower than for the multilayer. The

3. Results In the following, a metastable SO, state is identified and its conversion to a stable state is investigated. Coverage, substrate temperature, photon irradiation and the iodine surface condition are varied. 3.1. Coverage effects We first identify the metastable state and demonstrate that it converts into the non-saturable multilayer state at high total coverages (> 2 ML). Fig. 1 shows a series of TPD for various SO, coverages on I multilayer. The substrate temperature during dosing was 89 K. At low coverages from 0.1 to 1.7 ML, a low temperature desorption state (denoted as state A> around 120

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8=0.81

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.%

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metastable characteristics

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120

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Temperature (K) d

Temperature (K) Fig. 1. TPD of indicated

n

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$

2

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130

I-covered

Ag(ll1)

surface

dosed at 89

K.

Z.-J. Sun et al. / Sulfur dioxide adsorption on I-covered Ag(lll):

desorption activation energy, estimated by fitting the leading edge of TPD of a 0.1 ML coverage (the coverage where state A is dominant) is 18 _+2 kJ/mol. Just as striking, near 2 ML, the state A peak starts to decrease, and finally completely disappears above 3.2 ML. We interpret this in the following way. With increasing coverage, inter-adsorbate interactions increase and these drive the 2D metastable islands formed at low coverages to the 3D multilayer state. As a result, state A disappears. Thus, we identify state A as a metastable configuration of SO,. Since the coverage is referenced to monolayer coverage on I-free Ag(lll), there is no guarantee that saturation of the first layer on I-covered Ag(ll1) corresponds exactly to our definition of 1 ML. We might expect that somewhat more SO, could be packed into the first layer on I-covered Ag. The monolayer on Ag(ll1) is ordered [ll] and desorbs with first order kinetics [6,12], indicating that inter-adsorbate interactions do not dominate. But on multilayers of I, the desorption is near zeroth order (fig. la), indicating that attractive inter-adsorbate interactions prevail in state A and close packing into a 2D state A is plausible. In addi-

metastable characteristics

39

tion, state A might have double layer features, like the so-called “compressed monolayer” observed on Ag(ll1) [6,12]. 3.2. Temperature effects To gain further insight and to confirm the metastable character of state A, we investigated the following thermal effects: (1) different heating rates to assess the amount of inter-conversion during TPD; (2) dosing at 114 K on clean and I-covered Ag(ll1) to show that molecules in the metastable state have extensive interaction with underlying I; (3) dosing at different substrate temperatures to show that state A population is variable; (4) annealing after adsorption; and (5) isothermal desorption to extract an activation energy for inter-conversion from A to B. 3.2.1. Heating rate variation The amount of conversion which occurs during dosing and during the TPD experiment can be assessed by varying the TPD heating rate. The left-hand panel of fig. 2 shows the TPD of 0.5 ML SO, (dosed at 89 K> obtained for heating rates,

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Temperature (K) a Fig. 2. (a) TPD of 0.5 ML SO,,

dosed

at 89 K and taken peak heights

Heating rate (K/s) b at different as a function

heating

rates (from 0.7 to 7.5 K/s);

of heating

rate.

(b) the ratio of A to B

40

Z-J. Sun et al. / Sulfur dioxide adsorption on l-covered Ag(lll):

p, between 0.7 and 7.5 K/s. As expected [3], peak intensities and peak desorption temperatures increase with the heating rate. The state A peak position as a function of the heating rate has been used to estimate the desorption activation energy by a method provided in ref. [3]. The estimated value (20 rt_5 kJ/mol) is uncertain but comparable to that calculated from leading edge fitting (18 + 2 kJ/mol>. The relative intensities of A and B, obtained by comparing their peak intensities, are plotted (fig. 2, right-hand). Varying the ramp rate more than an order of magnitude only changes the relative intensity of A and B about 20% (A/B ratio is 2.3 at 0.7 K/s and is 2.87 at 7.5 K/s). Further, between 3 and 7.5 K/s the A/B ratio changes by less than 8%. This indicates that conversion is much slower than desorption at heating rates above 3 K/s and that, as long as the ramp rate is higher, conversion of A to B during TPD can be neglected. 3.2.2. Dosing at 114 K and uptake experiments In this section, we show that state A originates from desorption of those molecules which are in contact with I. By trial and error, we determined that 114 K was the highest substrate temperature for which multilayer SO, would accumulate on I-free Ag (111) when 5.6 Torr of SO, was placed behind the pinhole doser. This pressure gives 8 ML in 60 s at low temperatures (< 100 K), on both clean and I-covered surfaces. Fig. 3 shows the SO, TPD after dosing for 60 s at 114 K, terminating the dose, and promptly retooling to 89 K. On AgUll) there is multilayer condensation on top of a chemisorbed state ( - 176 K> and the defect sites (- 200 K) [61. For I-covered Ag(lll), there is no metastable state A or multilayer state B in fig. 3. Only a very small amount of SO, (2% of a monolayer) desorbs. It may be due to adsorption on imperfections (defects or impurities) in the 1 surface. The absence of multilayer accumulation on I-covered Ag(ll1) can be explained as follows. Before multilayer adsorption commences, there must be adsorption of molecules directly adjacent to the substrate. When SO, is dosed onto Ag(lll), the SO,-Ag interaction is strong and a mono-

metastable characteristics

7

I

Temperature

I

/

/Substrate temperature

114 K]

-

(K)

Fig. 3. TPD of SO,, dosed at 114 K and then retooled to 89 K. The two curves are for dosing onto clean Ag(ll1) and multilayer I-covered Ag(ll1). The pressure behind the pinhole doser was 5.6 Torr. The dosetime was 60 s. This dose deposits 8 ML SO, when the substrate temperature is < 100 K.

layer forms, upon which the multilayer subsequently grows. On an I-covered surface, the first layer never forms because the adsorption temperature lies in the desorption regime of state A. Thus, multilayer growth cannot be realized. In other experiments not shown, we find that at 90 K, SO, accumulates at the same adsorption rate on both surfaces, while at 105.2 K, it accumulates slower on the I-covered surface (70% of the clean Ag). This is consistent with weaker binding of SO, on I than on SO,. 3.2.3. Dosing at different substrate temperature The relative coverage of states A and B also depends strongly on the substrate temperature during dosing (fig. 4). In these experiments, the substrate was held between 92.7 and 101.2 K during dosing, but was cooled to 90 K before TPD. The pressure behind the pinhole doser was 2.8 Torr, dosetime 30 s, and the coverage reached at 89 K was 2 ML. The ratio of state A to state B steadily decreases with increasing substrate temperature. The total coverage, measured by total TPD area, is constant below 97.7 K, but drops slowly for higher temperatures. Clearly, the relative population of state drops sharply above 94.5 K and becomes very difficult to distinguish above 101.2 K.

Z.-J. Sun et al. / Sulfur dioxide adsorption on I-covered Ag(lll):

3.2.4. Annealing experiments

The relative amounts of states A and B were also studied by annealing after adsorption. This experiment requires great care because, as shown in fig. 4, the peak intensities are very sensitive to temperature. To reach the designated temperatures, a heating current is applied that is proportional to the difference between the measured instantaneous temperature and the desired temperature. Within 20 s, it is possible to heat the surface from 89 K to within -0.5 K of the designated temperature. The stable designated temperature is reached within 45 s (_+0.2 K>. Fig. 5 shows four sets of experiments conducted at different temperatures. Fig. 5a shows TPD after dosing 0.5 ML of SO, at 89 K, with and without waiting for 10 min before doing TPD. These two curves are indistinguishable, indicating that at 89 K, the conversion of A to B is

110

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Temperature ( K ) Fig. 4. TPD of SO*, from a multilayer I-covered surface, dosed at different substrate temperatures and retooled to 89 K after dosing is finished. The pressure behind the pinhole was 2.8 Torr and dosetime was 30 s. The coverage desorbed is indicated on each curve.

metastable characteristics

41

negligible and there is no loss of adsorbate. At 96.7 K and higher, the conversion of A to B steadily increases with annealing time, with an accelerated rate at higher temperatures (figs. 5b5d). Total conversion of A to B can be accomplished by heating the substrate long enough (top curve in fig. 5~). Due to the heating, a small amount of adsorbed SO, is lost to gas phase (the maximum was 10% for annealing 270 s at 100.7 K). The loss is faster at higher temperatures, though the conversion amount is the same (e.g., the top two curves in figs. 5c and 5d). This observation, agreeing with the results shown in fig. 2, indicates that the rate of desorption is much smaller than the rate of conversion at low temperatures, but becomes competitive with the conversion at higher temperatures. An activation energy for the conversion of A to B was estimated from this, and other, annealing data. The largest amount of loss ( < 20%) was at 105.7 K when heated for 147 s (not shown). Thus, the influence of desorption on the conversion process was neglected and the total areas were normalized to the area with no heating loss. The state A area remaining after selected annealing times was obtained by comparing the TPD curve with those obtained after total conversion of A to B. Fig. 6 (upper panel) shows the fractional remaining state A area. Tentatively, treating the conversion as a first order process, the corresponding rate coefficient, k(T), was determined by fitting the curve to A(t)/A, = exp( -kt). Plotted as In(k) versus lOOOR/T (fig. 6, lower panel), the data are reasonably linear and the calculated activation energy is 17 f 4 kJ/mol, a little smaller, but lying within the uncertainty of, the desorption activation energy of the state A. The pre-exponential factor is about lo9 s-l. This analysis is informative, but it does not account for-coverage dependences which, as shown in fig. 1, are important. Moreover, as discussed below, the applicability of first order kinetics can be questioned. Fig. 7 shows the TPD of 0.5 ML SO, after annealing to 98.7 K for 600 s (lower curve), and following the same procedure, but re-dosing the annealed surface with 0.15 ML SO, before TPD (upper curve). The re-population of the state A is

Z.-J. Sun et al. / Sulfur dioxide adsorption on l-covered Ag(ll1):

42

desorption proceeds. However, if desorption is from islands, e.g. surface of a 3D island or edges of a 2D island, and the islands are shrinking due to the loss of mass, fractional order is expected. It should be noted that, due to this fractional order character, the measured desorption rate of state A molecules will be influenced by the conversion of A to B. A more detailed kinetic study would consider this influence.

evident. This result shows that, at low coverages, conversion of A to B does not preclude the formation of additional A. 3.2.5. Isothermal desorption Fig. 8 shows isothermal desorption scans for 0.5 ML (upper) and 5 ML (lower) of SO,. At the same temperature (107.7 IQ, thermal desorption from 5 ML SO, is much slower than from 0.5 ML (lower panel). For 0.5 ML, the desorption rate decreases to almost the same rate for 5 ML coverage after 250 s, indicating that state A has been totally consumed, either by thermal desorption or conversion into state B. For 0.5 ML, fitting the initial process to de/dt

3.3. Photon-driven effects

Photodesorption and photochemical reaction studies of SO, using 193 and 248 nm excimer laser pulses [13] indicate extensive lateral adsorbate interactions in both state A and B. Fig. 9 shows TPD before and after irradiation (248 nm) of 0.5 ML SO, on an I-covered surface. States A and B are both depleted to the same extent. A time-of-flight study of the photodesorption of SO,, as well as reaction products 0 (for 193 nm

= -k(T)B”

results in a fractional

desorption

metastable characteristics

order between

n = 0.07 to 0.23. The fact that the order is near

zero, but not zero, should not be surprising, since a zero order process assumes that the number of active desorption sites does not change as the

C

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0.5 ML SOz, from multilayer I-covered surface dosed at substrate temperature 89 K, annealed to a different temperature (indicated in each panel) for a different amount of time (indicated on each curve), and retooled to 89 K before TPD. The TPD coverage is indicated on each curve.

Z.-J. Sun et al. / Sulfur dioxide adsorption on I-covered Ag(Ill):

_&-=7

K

=103.7K 400

!

t

600

600

]

43

metastable characteristics

travel along the surface to react with another ground state SO, molecule. This would influence the production yields and possibly the translational energies of the products, contrary to our observation. A model describing state A as twodimensional islands of high local SO, coverage bound to I is consistent with all our data. In this model for A, as in multilayer state B, there is extensive inter-adsorbate interaction. 3.4. Dependence on I surface condition

Time (s)

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The presence of state A and its conversion to state B also depends on the I surface condition. As mentioned in the experimental section, the I coverage was prepared by photolyzing multilayer CH,I and then flashing to 537 K to remove methyl radicals as well as CHJ, leaving atomic I on the surface. The topmost curve in fig. 10 (upper panel) shows the SO, TPD (0.5 ML) on a surface prepared in this way, but without flashing. Only state B appears. After flashing the 1.26

lOOOR/T

Fig. 6. Upper panel: The relative coverage in state A after annealing for various times. At each temperature, the state A area was fit to a first order inter-conversion (A + B) process (solid lines). Lower panel: Arrhenius plot of fits obtained from the upper panel.

only), SO and SO,, was done at coverages where either A dominates (0.5 ML) or B dominates (5 ML) at both wavelengths. No differences in product translational energies and production yields were observed. Previous work in the absence of I [131, shows that the photoreaction product, SO,, is produced only from a bimolecular reaction *SO,(ad)

+ SO,(ad)

-+ SO(g) + SO,(g),

(1)

where *SO,(ad) is an excited molecule. For this reaction to occur, extensive adsorbate-adsorbate interaction is necessary. However, there are no observable differences in the product translational energies and yields for the two coverages (0.5 and 5 ML). If state A were isolated SO, adsorbed on I, then reaction (1) would either not occur or would have a much smaller cross section, since the excited *SO, molecule would need to

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Temperature

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(K)

Fig. 7. Lower curve, TPD of 0.5 ML S02, dosed at substrate temperature of 89 K, annealed to 98.7 K for 600 s, retooled to 89 K. For upper curve, repeat above procedure, then add 0.15 ML SO,.

Z-J. Sun et al. / Sulfur dioxide adsorption on I-cowered Agclll):

44

Isothermal

metastable characteristics

scan at 64 amu.

w/o flash flash to 537 K flash to 600 K 1st time

b-

flash to 600 K 2nd time

I

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Temperature ( K )

107.7 K (0.5 ML) 107.7 K (5 ML)

500 0

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Time (s) Fig. 8. Iso-thermal scans of SO, signal from equal doses, corresponding to 0.5 and 5 ML SO, at 89 K, at different substrate temperatures. For the 0.5 ML SOz (upper panel), the desorption rate was fit to a fractional order desorption process, the order n was 0.15 + 0.08.

surface to 537 K (most of the methyl radicals and all the CH,I are removed, but there is no desorption of I; see lower panel fig. 10) and retooling to

Temperature (K) Fig. 9. TPD of 0.5 ML S02, dosed at substrate temperature 89 K, with and without photon irradiation (4000 laser shots at 248 nm).

600

700

800

900

Temperature (K) Fig. 10. Upper panel: TPD of 0.5 ML SO,, dosed at 89 K, from I-covered surfaces prepared by different methods. From top to bottom, the surfaces were prepared by dosing SO, onto: (i) a CH,I multilayer subjected to prolonged 193 nm photon irradiation; (ii) flash (i) to 537 K and re-cool to 89 K; (iii) flash (ii) to 600 K and re-cool; (iv) repeat (iii); and (v) flash (iv) to 620 K. Lower panel: The I+ and CH; TPD signals from a photon-irradiated CH,I multilayer surface that was flashed to 537 K.

89 K, the same amount of SO, was dosed. The TPD shows state B and a small amount of state A (shoulder in second curve). Repeating this process after further flashing to 600 K, where considerable multilayer I desorbs and presumably the remaining I becomes more ordered, a larger amount of state A SO, is present in TPD. Flashing to 600 K again, and redosing SO,, the subsequent TPD shows state A is dominant. Further flashing to 600 K does not change the state A population, but if the surface is flashed to 626 K (the temperature where all multilayer is removed and only I monolayer remains), the population of A decreases while B increases (bottom curve>. Imperfections, such as impurities and defects, can provide nuclei for the facile formation of

2-J.

Sun et al. / Sulfur dioxide adsorption on I-covered Ag(Il1):

state B. When the surface concentration of these imperfections is high enough, it can limit the formation of the metastable state A (the top three curves in fig. 10). Significantly, the strong relationship of state A population with the surface condition demonstrates that it is a state with extensive SO,-1 interaction. It should be stressed here that the I multilayer preparation procedure (outlined in the experimental section) is important. When the I surface sits overnight or is not well annealed, state A often disappears and the multilayer SO, uptake takes on characteristics of clean Ag(ll1) (see fig. 3). By flashing to 600 K one or two times to remove contaminants, state A reappears. The decrease of state A on monolayer I surface (bottom curve of upper panel of fig. 10) is not caused by the presence of imperfections, but by structural and electronic changes compared to multilayer I. Iodine monolayer on Ag(lll) is known to have a well-ordered fi X 6 R30 structure with a 1: 3 iodine-to-silver atom ratio [71. The monolayer differs from the multilayer not only in the surface structure, but in electronic properties as well. Consequently, the SO,-substrate potential energy surface may differ from the multilayer, and lead to relative population changes. Finally, rest~cturing the SO, might involve concerted motion of the SO, and I, but this is considered unlikely since the SO,-1 interaction is much weaker than the I-Ag or I-I interaction.

4. Discussion After a brief discussion of how surface diffusion plays a role in deciding the adsorption-desorption properties (4.0, we will propose a phenomenological adsorption model (4.2). The inter-conversion kinetics will be discussed further in section 4.3. 4.1. Su$zce difficsion A clear picture of the desorption-adsorption process requires a combined understanding of phase transition, surface diffusion (both at high and low coverages), thermal desorption and stick-

metastable characteristics

45

ing on the surface. Below we assess how surface diffusion affects the adsorption-desorption properties. What is the magnitude of the diffusion barrier for isolated adsorbed SO, on I? First, the general rule that diffusion barriers for adsorbates are approximately 10% to 40% of the desorption barrier [14] leads to an upper limit for isolated molecules, denoted SO,,, between 1.6 and 6.4 W/mol (based on the desorption activation energy from the proposed 2D state A, the actual desorption activation energy of isolated SO,, should be even smaller). Assuming a typical average attempt rate of lOI3 s-i, isolated molecules will hop lo9 to lo** steps per second at 89 K [2]. Thus, diffusion of isolated SO,, will be rapid on the time scale of a typical experiment (3 to 15 min). At 0.1 ML coverage, these isolated SO,, molecules can rapidly travel over the surface until they attach to an SO, aggregate, stabilized by adsorbate-adsorbate interactions. This means SO, can rapidly form adsorbate-adsorbate bonded clusters (high local concentration) even at very low total coverages. In the present system, we propose that these two-diSO,/I,‘A~lll), mensional island clusters describe the state A. Overall, surface diffusion may not only depend on substrate-adsorbate interactions, but also on adsorbate-adsorbate interactions when there is extensive adsorbate correlation, i.e., diffusion could occur as groups of molecules migrating together [HI. The adsorbate geometry may significantly influence the rate, and even the mechanism, of surface diffusion. In general, a succession of two-dimensional adsorbate phases can form within the diffusion zone, their structure and other physical properties governing the diffusion rate f16]. In line with this, surface diffusion exhibits a pronounced multi-phase character akin to that observed in the volume of solids. In these terms, changing’the temperature and initial conditions of di~sion can alter the observed populations in states A and B. 4.2. A phenomenological adsorption model Based upon the above considerations and our data, the following model for adsorption and

46

Z.-J. Sun et al. / Sulfur dioxide adsorption on I-covered Ag(ll1):

metastable characteristics

fig. lld, complete conversion of A to B occurs only at high coverages (as evidenced in fig. lb-ld), and/or at elevated temperatures (as evidenced in figs. 4 and 5). According to this model, after finishing the A -+ B transition, fewer I sites should be occupied. Further dosing at low temperatures should repopulate the state A. Experimentally, re-population of state A is evident in fig. 7. A similar model was proposed to describe H 2O adsorption [17]. A drop of water spreading on a solid surface exhibits the shape shown in fig. lle. The “precursor film”, a thin lip of liquid, spreads outwards from the base of the drop. This lip is attributed (at least in part) to the short-range forces between water molecules and the solid surface. 4.3. Inter-conversion kinetics

Fig. 11. A simple schematic description (a)-(d) for adsorption and inter-conversion of SO, on I-covered AgUll); (e) a schematic description for water adsorption proposed by deGennes [17].

A + B transition is proposed (figs. lla-lld). For very low doses, low temperatures and short waiting times, isolated SO,, molecules are dominant (fig. lla). These travel across the surface and form two-dimensional islands - state A (fig. llb), particularly during the waiting time between dosing and TPD. For somewhat larger doses, extensive two-dimensional islands form during dosing (typical dose time is 0.5 to 10 min), due to a higher density of isolated SO, and a prolonged dosing period. Three-dimensional islands (multilayer state B), form during dosing when SO, adsorbs on the two-dimensional islands (fig. 11~). At 89 K and relatively low coverages ( < 2 ML), there is no conversion of A + B (as evident in fig. 5a), because an energy barrier for the transition of about 17 f 4 kJ/mol is present. Although the metastable state A is thermodynamically unstable, it is kinetically stable at 89 K. As depicted in

The difference in rates of thermal desorption and conversion at different temperatures, as shown in figs. 2, 4, 5 and 8, reflects different kinetics. The desorption of state A is a near zero fractional order process (as shown in fig. 8). The rate for this Arrhenius-type process (vO exp (-E/kT)) can be described in terms of an attempt frequency v0 and activation energy E. The attempt frequency is typically very high, e.g. 1013 s-l. With such a high attempt frequency, the rate of desorption will be significantly enhanced at high temperatures. Turning from desorption to adsorbate conversion, the attempt frequency describing the transition of A to B, as calculated from fig. 5, is about lo9 s-l. The large difference in pre-exponential factors is not surprising, since the entropies of the final states are different. The desorption process, with gas phase as the final state, has an infinite choice of final orientations. The A + B transition, with a multilayer as the final state, has a restricted range, and hence much lower entropy compared to the gas phase. The activation energy of the A + B transition (17 * 4 kJ/m o 1) is probably slightly smaller than the desorption activation energy (18 f 2 kJ/mol). As a result, the conversion rate will be faster than the desorption rate at low temperatures, while at high temperatures, where entropy factors domi-

Z.-J. Sun et al. / Sulfur dioxide adsorption on I-couered Ag(ll1): metastable characteristics

nate, the desorption rate will be much faster than the conversion rate. Certainly, the desorption of A and the A 4 B conversion, described above with simple Arrhenius equations, may be more complex. For example, assuming state A is a two-dimensional island phase and state B is a three-dimensional island phase, description in terms of a phase transition may be more appropriate [l,lS]. In general, a phase transition initiates upon the formation of nucleation sites, and develops at “domain walls” [1,18], where the different phases meet. At elevated temperatures, a new phase might nucleate within the zone of another. Phase transitions can also be correlated with surface diffusion, especially surface diffusion of groups of adsorbates. This type of transition, since it develops at specific “domain walls” and involves a collection of molecules, loses its competitiveness with thermal desorption as the temperature is raised. However, at low temperatures, thermal energy can still drive such a phase transition and there can be negligible thermal desorption (fig. 51, provided the former requires a smaller activation energy than the latter. In light of the above discussion, the reasonable fit to first order kinetics may imply that nuclei formation controls the A to B transition, since their formation could be proportional to the coverage of state A. This must be regarded as a rough description; further discussion is not warranted without more detailed experimental and/or modelling work.

5. Summary, conclusions

and suggestions

To summarize, on I-covered Ag(lll), a thermodynamically metastable state A of SO, (desorption activation energy of 18 f 2 kJ/mol) can co-exist with the multilayer state B (desorption activation energy of 24 f 4 kJ/mol). The photodesorption and photodissociation behavior of molecules in states A and B indicate extensive inter-adsorbate interactions, i.e. no evidence for isolated SO,. Uptake experiments demonstrate that between 100 and 120 K, the effective sticking coefficient of SO, on an I-covered surface is lower than on an SO,-covered surface and, at 114

41

K, state A is not formed. This is consistent with a model in which SO, interacts more weakly with adsorbed I than with another SO, molecule. We propose that the metastable state A is a two-dimensional SO, island structure. The co-existence of states A and B depends heavily on total surface coverage. At high coverages (> 2 ML), conversion of A to B is evident and is attributed to extensive adsorbate-adsorbate interactions. Temperature, as well, has a significant effect on the co-existence of the states A and B. Through a variety of temperature-dependent experiments, we show that the conversion of A to B occurs for temperatures above 96 K, and for 0.5 ML SO,, we estimate an activation barrier for conversion of A to B of 17 & 4 kJ/mol. Surface conditions are also important; the existence of impurities and surface disorder inhibit the formation of state A, and adsorption on monolayer I results in less population of state A than adsorption on multilayer I. These studies of the adsorption and desorption of the metastable state A of SO, on I-covered Ag(ll1) and its relaxation to the more stable state B have manifested the intricate nature of these phenomena. Other experimental tools, such as HREELS, reflection-FTIR, LEED and helium-diffraction will be helpful in understanding the short and long range geometry of the adsorbate structures, as well as the I surface structure. Construction of a phase diagram as a function of surface concentration, temperature and surface conditions, will be helpful. Computer simulation, based on molecular dynamics and/or MonteCarlo methods, should give a better understanding of these aspects. A thorough study of this system could shed light on a number of problems such as wetting, corrugation, coating and material growth.

Acknowledgments

This work is supported by the National Science Foundation, Grant CHE 9015600. Z.-J. Sun would like to thank Dr. X.-L. Zhou for helpful discussions.

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Z.-J. Sun et al. / Sulfur dioxide adsorption on I-covered Ag(lll):

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[lo] X.L. Zhou and J.M. White, Surf. Sci. 241 (1991) 270. [ll] D.A. Outka, R.J. Madix, G.B. Fischer and G.L. DiMaggio, Langmuir 2 (1986) 406. [12] M.E. Castro and J.M. White, J. Chem. Phys. 95 (1991) 6057. [13] Z.-J. Sun and J.M. White, J. Chem. Phys., to be submitted. [14] R. Gomer, Field Emission and Field Ionization (Harvard Univ. Press, Cambridge, MA, 1961) pp. 134-135. [15] A.G. Naumovets and YuS. Vedula, Surf. Sci. Rep. 4 (1985) 365. [16] A.G. Fedorus, A.G. Naumovets and Yu.S. Vedula, Phys. Status Solidi (a) 13 (1972) 445. [17] P.G. deGennes, Rev. Mod. Phys. 57 (1985) 827. [18] A. Thorny, X. Duval and J. Regnier, Surf. Sci. Rep. 1 (1981) 1.