E cient hydration of Cs ions scattered from ice lms

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1 Nuclear Instruments and Methods in Physics Research B 157 (1999) 191±197 E cient hydration of Cs ions scattered from ice lms T.-H. Shin, S.-J. Han, H. Kang * Department of Chemistry and Center for Ion±Surface Reaction, Pohang University of Science and Technology, Pohang, Gyeongbuk , South Korea Abstract Low energy (20±200 ev) beams of Cs ions are collided with a frozen water layer formed on a Si(1 1 1) surface at low temperature. The collision gives rise to e cient emission of Cs(H 2 O) n -type cluster ions (n ˆ 1±5) from the surface. The yield for the cluster formation is very high compared to the reactive scattering yield from chemisorbed surfaces, typically 100 times higher or even more. Such a large yield suggests that the clusters are created through condensed-phase reactions inside the ice layer. Ó 1999 Elsevier Science B.V. All rights reserved. PACS: D; S; J Keywords: Cs ; Cluster; Ice; Ion scattering; Reactive scattering 1. Introduction When a Cs beam is used to bombard a surface in secondary ion mass spectrometry (SIMS), it is known [1±4] that positive ions of the type CsX are produced in an appreciable amount from the surface, where X represents a surface element. An interesting feature in emission of these species is that the CsX yield is much less a ected by the ionization energy of X nor the matrix e ect of a surface. This observation has led to a proposal for the appropriate mechanism of CsX formation [2± 4]: the association of a neutral atom X and a Cs ion in the sputtered ux. The Cs ions are introduced into the sputtered ux from the surface * Corresponding author. Fax: ; sur on@postech.ac.kr deposits accumulated during continuous bombardment of kev Cs ions. In this way the CsX yield scales with the sputtering ux of the neutral atom X, and therefore, is little changed by the factors governing the ionization probability of X. A seemingly analogous, but fundamentally di erent process of CsX formation has been reported recently [5±9]. In these studies, a Cs beam of much lower energy (20±200 ev or hyperthermal energy) is collided with a surface that is not contaminated with Cs. The collision also gives rise to emission of CsX ions from the surface, but in this case X can be an adsorbed molecule as well as atom. Apparently, at these low energies the Cs collision can eject adsorbed molecules with a lesser degree of molecular fragmentation [7±9]. This phenomenon, called reactive scattering of low-energy Cs, can be explained in terms of a two-step mechanism [5,6]: X/99/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S X ( 9 9 ) X

2 192 T.-H. Shin et al. / Nucl. Instr. and Meth. in Phys. Res. B 157 (1999) 191±197 Cs g X-surface! Cs g X g surface 1 Cs g X g! CsX g 2 At rst, the collision of a Cs ion onto a surface causes desorption of X from the surface (reaction (1)). The collisionally desorbed X combines with the scattered Cs ion via electrostatic attraction forces, and forms a CsX ion complex (reaction (2)). Chemisorbed species like CO, OH, H 2 O, and C 6 H 6 have been investigated with this technique [5±8], from which the yield for reactive scattering (Y rs ) is found to be 10 3 ±10 4 for these species at Cs incident energies of 20±50 ev. Dividing Y rs into individual contributions of the two steps, the yield for collisional desorption (Y d ) is about 0.1±1, and the association probability of X with Cs (Y assoc ) is in the order of It is interesting that Y assoc represents the cationization e ciency of desorbed neutrals by Cs ions, and that the number 10 3 far exceeds an ionization e ciency of an electron impact ionizer. In this paper we report an application of this Cs reactive scattering technique to a di erent type of surface, an ice overlayer deposited on a low-temperature substrate. Ice layers have recently been a research topic of large interest [10±13] in relation to astrophysics and chemistry of upper atmosphere. Vapor deposition of water molecules onto a substrate surface at a very low temperature generates a frozen water layer in an amorphous phase, which, when warmed above the glass transition temperatures (120±140 K), becomes a viscous liquid [10]. This liquid phase is known [12] to coexist with the cubic crystalline ice phase over temperatures of 140±210 K. In this work, an ice overlayer is prepared by water vapor deposition below the glass transition temperature, and we call this overlayer a frozen water or amorphous ice layer. On this surface we have collided a low-energy Cs beam, and observed an increase of the reactive scattering yield by more than 100 times compared to chemisorbed molecules on covalent or metallic surfaces. Moreover, Cs ions can pick up more than one water molecule during the collision. Such unusual characteristics suggest that the nature of Cs collision is quite di erent with the ice overlayer, and so is the mechanistic feature of the reactive scattering. 2. Experimental The reactive ion±surface scattering apparatus consists of a low energy Cs ion beamline and an ultrahigh vacuum (UHV) scattering chamber with a base pressure of Torr. This apparatus has previously been described in detail [5,6]. Cs ions were produced from CsCl powder heated inside the Colutron ion source, and mass selected in the beamline by a Wien lter. The Cs beam had a current density of 1±10 na cm 2 as measured using a Faraday cup. The target sample, a Si(1 1 1) wafer with dimension of mm 3, was located inside a eld-free scattering region of the UHV chamber. The sample was attached to a UHV manipulator that can control the sample temperature between 100 and 1500 K via liquid nitrogen cooling and radiative heating from a Ta foil. A clean Si surface was obtained by a standard procedure of mild annealing at 950 K for over 5 h followed by ash heating to 1350 K for about 1 min. Surface cleanness was checked by Auger electron spectroscopy and the Cs reactive scattering technique. The cleaned sample was cooled to 100±120 K, then exposed to water vapor introduced into the chamber in order to generate ice layers on the surface. The thickness of the deposited layer was estimated from an ionization gauge reading, assuming a sticking coe cient of unity for water molecules on the surface below 120 K. However, there existed a large pressure di erence between the locations of the sample and the ionization gauge, because the gas inlet tube was directed toward the sample surface. For active gas like H 2 O, the actual pressure at the sample could be one order of magnitude higher than the ionization gauge reading. After deposition of the ice overlayer, the sample was collided with Cs beams at energies of 20±200 ev, and the positive ion products emitted from the surface were analyzed using a quadrupole mass spectrometer (QMS) operated in the ion sampling mode. The angle between the incident Cs beam and the QMS detector was instrumentally xed at 90, but the

3 T.-H. Shin et al. / Nucl. Instr. and Meth. in Phys. Res. B 157 (1999) 191± sample could rotate varying the beam incidence angle. Unless speci ed otherwise, the experiments were carried out for beam incidence and detector angles of 60 and 30, respectively, both measured with respect to the surface normal. This setup was an optimal geometry for the best signal intensity. No charging e ect at the ice overlayer was observed during the Cs scattering experiment. 3. Results The mass spectra of the ions produced as a result of Cs reactive scattering from a frozen water layer are shown in Fig. 1. A series of cluster ions of the type are readily observed, in Fig. 1. Mass spectra of the positive ions generated by Cs reactive scattering from a frozen water layer: (a) 20 ev Cs collision onto an H 2 O-ice layer; (b) 50 ev collision onto a D 2 O-ice layer. The intensities of larger clusters are magni ed by the factors indicated. addition to elastically scattered Cs ions at 133 amu. Fig. 1(a) was obtained by a 20 ev Cs beam impinging onto an H 2 O-covered surface. clusters with n ˆ 1±4 are shown in the spectrum, with their intensities gradually decreasing with increasing number of water molecules. No secondary ions are detected in the mass region below 133 amu, implying that 20 ev Cs beam energy is too low for generating secondary ions. The frozen H 2 O layer was deposited at substrate temperature of 110 K and with a water exposure of 1.5 L (1 L ˆ Torr s), according to an ionization gauge reading. The actual water exposure at the sample probably reached 10±20 L due to the pressure di erence between sample and ionization gauge locations, mentioned in Section 2. Fig. 1(b) presents data obtained with 50 ev Cs impinging onto a D 2 O-ice layer deposited to the same thickness as in Fig. 1(a). The intensity distribution for Cs D 2 clusters is qualitatively similar to the one in Fig. 1(a), with the peak positions shifted by the isotopic mass di erences. The yields for larger Cs D 2 clusters are overall increased upon increase of Cs energy to 50 ev, the clusters up to n ˆ 5 being detected. An interesting di erence from Fig. 1(a) is that hydrated protons, i.e., D D 2 clusters, are detected in the low mass region (not shown in Fig. 1). These species indicate production of D secondary ions from D 2 O by 50 ev Cs collision and its subsequent hydration. It is improbable that D D 2 clusters exist in the overlayer as inherent species and as such become emitted during the Cs collision, because the concentration of autoionized water (10 15 molecules cm 3 ) is too low to account for the observed peak intensities. D D 2 clusters are not observed when Cs collision energy is below 35 ev, and thus this energy corresponds to an ``instrumental'' threshold for D production from D 2 O by Cs impact. Several novel features are evident for Cs reactive scattering from the ice overlayer, compared to the results obtained from chemisorbed species on surfaces [5±9]. First, the yield for reactive scattering is extremely high from the ice overlayer. The yield for Cs(H 2 O) production [Y rs (H 2 O)], which is de ned as the intensity ratio for Cs(H 2 O) /Cs, is 0.3±1.0 for Cs incidence

4 194 T.-H. Shin et al. / Nucl. Instr. and Meth. in Phys. Res. B 157 (1999) 191±197 energies of 20±200 ev. For comparison, Y rs (H 2 O) of 10 3 ±10 4 has been measured [5,6] for H 2 O molecules chemisorbed on a Si(1 1 1) surface at a room temperature, when similar Cs energies are employed. Second, multi-hydrated cluster ions, with n > 1, are produced with substantial intensities. Such large cluster ions require multiple association reactions and have not been observed in Cs reactive scattering from other surfaces. Third, upon collision onto the ice overlayer, intensity of the scattered Cs ions decreases greatly. The second and third features will be addressed in more detail in the following. In Fig. 2, the yields of clusters are shown as a function of the number of water molecules for Cs incidence energies of 50, 100, and 200 ev. In these data the cluster intensities are not calibrated with respect to the variation of the QMS detection sensitivity with mass. In Fig. 2(a), showing the cluster intensity variation in a linear scale, the cluster intensity decreases rapidly as the cluster size increases. In the logarithmic plot of Fig. 2(b), the decrease in cluster intensity is closer to a linear behavior, although a certain extent of upward deviation from linearity is evident. Upon scattering from the ice surface, a Cs beam undergoes a signi cant decrease in its intensity. The degree of the beam attenuation is far greater than from a clean Si surface. Fig. 3 presents in situ monitoring of the scattered Cs intensity as the ice overlayer grows in thickness, by exposing a Si surface to a constant H 2 O vapor pressure of Torr at 115 K. The Cs intensity scattered from the ice surface is normalized to the value from a clean Si(1 1 1) surface. The Cs intensity drops to a few percent of the initial value when the amount of H 2 O exposure exceeds 3 L in ionization gauge reading, or roughly 30 L in the real exposure at the sample surface. The logarithm of the Cs intensity linearly decreases with the amount of water exposure, indicating an exponential attenuation function, I s Cs ˆ I 0 Cs e c d 3 where I s (Cs ) and I 0 (Cs ) are the Cs intensities scattered from the ice and the clean Si surface, Fig. 2. Variation in the yield of clusters as a function of the number of water molecules: (a) cluster intensity shown in the linear scale; (b) in the logarithmic scale. Cs beam energies are 50, 100, and 200 ev. Fig. 3. Attenuation of the scattered Cs intensity as a function of water coverage at four di erent Cs beam energies. I s (Cs ) indicates the scattered intensity from the surface of frozen water, and I 0 (Cs ) from clean Si(1 1 1). The amount of water exposure is read with an ionization gauge, and the actual exposure at the sample is much greater (see the text).

5 T.-H. Shin et al. / Nucl. Instr. and Meth. in Phys. Res. B 157 (1999) 191± Many studies have been done for the structure of frozen water layer deposited on a cold-temperature surface [10±13]. The frozen water layers are considered [12,13] grow in an amorphous phase below the glass transition temperature of ice (120± 140 K). The amorphous phase ice lm has a more open structure than crystalline ice phases and may contain a signi cant amount of micropores. We believe that such an amorphous lm was formed in the present work as the deposition temperature was 110±120 K. The amorphous ice overlayer most likely maintained its originally prepared structure during the measurement time, because the experiments were carried out immediately after the lm deposition (<1 min) in order to avoid the glass transition. The most striking observation in the reactive scattering from the ice overlayer is the extremely large yield for clustering reaction, exempli ed by a value Y rs (H 2 O) ˆ 0.3±1.0. In attempt to rationalize this observation, we rst consider the two-step mechanism of reactive scattering (reactions (1) and (2)) proposed in previous work on chemisorbed systems. The yield for reactive scattering in this case is expressed by Eq. (4). Y rs ˆ Y d Y assoc 4 Fig. 4. The attenuation factor of the Cs intensity scattered from a frozen water overlayer shown for various beam energies. The number for the attenuation factor corresponds to the negative slope of the plots in Fig. 3. respectively, c is the attenuation factor given by the negative slope of the straight lines, and d is the layer thickness. The attenuation factor increases with increasing Cs incidence energy, as summarized in Fig Discussion For chemisorbed molecules Y d is typically 0.1±1 and Y assoc is 10 3 [5±8]. We may extend the concept of the two-step mechanism to the formation of clusters with n > 1, by assuming that clusters grow through successive Cs ± H 2 O association reactions (reactions (5) to (6)). Cs g H 2 O g! O g 5. 1 g H 2O g! g 6 Then one can write for the yield of large cluster formation, Y rs Š ˆ Y d H 2 O Yn Y assoc Cs H 2 O n 1 H 2 OŠ 7 iˆ1 Eq. (7) is used to t the cluster distribution presented in Fig. 2(b), from which we obtain Y d (H 2 O) to be and Y assoc [Cs (H 2 O) n 1 H 2 O] to be in the range of 0.16±0.43. Evidently, these numbers are very far from the Y d and Y assoc values obtained for chemisorbed systems. The convexshaped curves in Fig. 2(b) suggest that the value for Y assoc [Cs (H 2 O) n 1 H 2 O] decreases with increasing n, in the range of 0.16±0.43. However, small extent of deviation from linearity is within the experimental uncertainty of cluster intensity distribution, possibly resulting from an uncalibrated QMS, and thus may have no signi cance. In a SIMS study of a frozen water layer using kev noble gas ion bombardment [14], H H 2 clusters have been observed up to n ˆ 51. For these protonated clusters, the logarithmic plot of their intensities like the ones in Fig. 2(b) gives a reasonable straight line over the entire mass range. Judging from the value of Y assoc, we interpret that the clusters are created in a condensed phase, not in the gaseous desorbing ux as is the case for the chemisorbed H 2 O

6 196 T.-H. Shin et al. / Nucl. Instr. and Meth. in Phys. Res. B 157 (1999) 191±197 system [5,6]. In order to have a value Y assoc [Cs (H 2 O) n 1 H 2 O] ˆ 0.16±0.43, it is required that the water molecular density in the reaction media reaches approximately 1 molecule A 3, assuming a cross-section for Cs ±H 2 O ion±molecule association to be 10 A 2 and the time scale for reactive scattering process s [5,6]. Such a molecular density clearly points to water in a condensed phase. To compare, a molecular density of 0.1±0.01 molecule A 3 was estimated for the gaseous desorbing ux produced from an H 2 O± chemisorbed Si surface within s after Cs impact [5,6]. Another aspect of the cluster formation reaction is that when the reaction occurs in a condensed phase, Y d (H 2 O) in Eq. (7) should be interpreted di erently; it does not represent the yield of reaction (1) or the collisional desorption yield. Indeed Y d (H 2 O) ˆ is too small to indicate the desorption yield of frozen water by the Cs collision. Water molecules are weakly bound to the ice surface through hydrogen bonding, and their collisional desorption should be very e cient. When clusters up to n ˆ 5 are observed, the number of collisionally desorbed H 2 O molecules should be much greater than 5 because those left unassociated with Cs have a huge population. Note that the desorption process (reaction (1)) is not needed for the cluster formation in a condensed phase. In analogy to the de nition in the gas-phase mechanism that Y d means the number of molecules desorbed and made available for Cs ±molecule interaction, we interpret that Y d (H 2 O) of Eq. (7) is related to the number of H 2 O molecules accessible for the Cs ±H 2 O clustering interaction in the condensed phase. A Cs ion has direct interaction only with a limited number of nearest-neighbor water molecules in a condensed phase, and as such, the Y d (H 2 O) value can be small. The argument that the clustering reactions occur in a condensed phase is also supported by the substantial attenuation in scattered Cs beam intensity. When a Cs projectile collides with an amorphous ice layer, it can penetrate into a certain depth of an ice layer due to the large mass of Cs and the open structure of amorphous ice. The Cs ion undergoes many collisions with water molecules along its trajectory through the surface region, changing its moving direction and losing momentum. The scattered Cs intensity going into the detector is decreased by such processes as outof-plane scattering and trapping in the ice layer. The longer the path of Cs inside a solid, the more the attenuation of the scattered Cs intensity. Such a scenario agrees with the experimental ndings that scattered Cs intensity decreases as the thickness of the ice layer increases (Fig. 3) and that the attenuation factor increases with Cs incident energy, i.e., ion penetration depth (Fig. 4). Another possible cause for the decreased Cs intensity is neutralization of Cs during travel through the ice layer. Neutralization of low energy O and F ions transporting through frozen water overlayer has been investigated by Madey and coworkers [15,16]. They have found strong suppression of O intensity by an H 2 O layer, 1 ML of H 2 O suppressing the O signal to <0.1% [16]. Due to low ionization energy of Cs (3.89 ev) and lack of a resonant charge transfer channel with H 2 O, Cs will not be able to be as e ciently neutralized. However, a certain fraction of Cs ions will inevitably be neutralized in the ice layer. The neutralization probability of Cs will increase with collision energy for a non-resonant process [17], and this expectation is consistent with the trend shown in Fig. 4. In the above discussion we have assumed that a sequence of association reactions between Cs and H 2 O molecules (reactions (5) to (6)) are solely responsible for the cluster ion growth, similar to the clustering reactions in a supersonic nozzle expansion of gases. A Cs ion moving through the surface region of frozen water generates collision cascades along its trajectory, which eventually result in activated regions and environment that can provide larger clusters through molecular association. The proposed model of sequential molecular association in a condensed phase qualitatively accounts for the observed cluster intensity distribution. However, it does not prove that this model is entirely correct. We consider that many mechanistic aspects are still fairly imaginative for this new type of clustering reaction. The result can be interpreted from a di erent direction as well. For instance, multiply hydrated Cs ions are initially

7 T.-H. Shin et al. / Nucl. Instr. and Meth. in Phys. Res. B 157 (1999) 191± ejected from the ice surface upon Cs impact (reaction (8)). Cs g H 2 O s! g Š! O m g n m H 2 O g ; m < n 8 g Š represents a cluster ion with high internal excitation. This species is cooled by fragmenting in the gas phase, and rise to smaller clusters. Acknowledgements We thank Dr. Lahaye for many useful comments. This work was nancially supported by the Creative Research Initiatives Project and by KOSEF ( ). References [1] H.A. Storms, K.F. Brown, J.D. Stein, Anal. Chem. 49 (1977) [2] Y. Gao, J. Appl. Phys. 64 (1988) [3] K. Wittmaack, Nucl. Instr. and Meth. B 85 (1994) 374. [4] H. Gnaser, Int. J. Mass Spectrom. Ion Proc. 174 (1998) 119. [5] M.C. Yang, H.W. Lee, H. Kang, J. Chem. Phys. 103 (1995) [6] M.C. Yang, C.H. Hwang, H. Kang, J. Chem. Phys. 107 (1997) [7] H. Kang, K.D. Kim, K.Y. Kim, J. Am. Chem. Soc. 119 (1997) [8] H. Kang, M.C. Yang, K.D. Kim, K.Y. Kim, Int. J. Mass Spectrom. Ion Proc. 174 (1998) 143. [9] K.-Y. Kim, T.-H. Shin, S.-J. Han, H. Kang, Phys. Rev. Lett. 82 (1999) [10] J.A. McMillan, S.C. Los, Nature 206 (1965) 806. [11] N. Materer, U. Starke, A. Barbieri, M.A. Van Hove, G.A. Somorjai, G.-J. Kroes, C. Minot, J. Phys. Chem. 99 (1995) [12] P. Jenniskens, S.F. Banham, D.F. Blake, M.R.S. McCoustra, J. Chem. Phys. 107 (1997) [13] F.E. Livingston, G.C. Whipple, S.M. George, J. Chem. Phys. 108 (1998) [14] G.M. Lancaster, F. Honda, Y. Fukuda, J.W. Rabalais, J. Am. Chem. Soc. 101 (1979) [15] M. Akbulut, N.J. Sack, T.E. Madey, Surf. Sci. Rep. 28 (1997) 177. [16] M. Akbulut, N.J. Sack, T.E. Madey, J. Chem. Phys. 103 (1995) [17] D. Rapp, W.E. Francis, J. Chem. Phys. 37 (1962) 2631.