- Email: [email protected]
Direct scattering and trapping–desorption of large water clusters from graphite
20 October 2000
Chemical Physics Letters 329 (2000) 200±206
www.elsevier.nl/locate/cplett
Direct scattering and trapping±desorption of large water clusters from graphite Anna Tomsic, Nikola Markovic, Jan B.C. Pettersson * Department of Chemistry, Physical Chemistry, Goteborg University, SE-41296 Goteborg, Sweden Received 23 June 2000; received in ®nal form 24 August 2000
Abstract Large water clusters are scattered from graphite in an angular and kinetic energy resolved collision experiment. Two types of collision processes dominate the cluster±surface interactions: direct inelastic scattering of clusters and desorption of trapped clusters mediated by rapid surface-induced heating. The previously observed formation of charged clusters is concluded to mainly be associated with the trapping±desorption channel. Classical molecular dynamics simulations of the collisions agree well with the trapping±desorption channel, while the direct scattering process is less well described. Ó 2000 Elsevier Science B.V. All rights reserved.
1. Introduction Interactions of van der Waals clusters and hydrogen-bonded clusters with solid surfaces have received considerable attention. The cluster±surface collisions result in a range of phenomena including scattering of neutral fragments [1±10], emission of charged fragments [11±16], chemical reactions within the cluster [17] or between the cluster and the surface [18], and the ejection of secondary particles [19,20]. The behavior of the dierent processes is often sensitive to cluster size and collision energy. Recent studies with slow neutral argon [2±6], water [7,14], ethanol [8] and ammonia [9,10] clusters have indicated that the initial kinetic energy of a cluster is eciently transferred into internal degrees of freedom during
*
Corresponding author. Fax: +46-31-772-3107. E-mail address: [email protected] (J.B.C. Pettersson).
a cluster±surface collision. This results in the clusters becoming trapped on the surface and they thereafter decompose by evaporation. If the surface temperature is suciently high and the surface relatively ¯at, the clusters may glide long distances on the surface during the decomposition process [21], and large cluster fragments can be emitted from the surface as a result of extensive evaporation and fragmentation [21,22]. The present Letter contributes to this picture by showing that collisions between water clusters and graphite also may result in direct scattering of clusters from the surface. Andersson et al. [7] recently studied the emission of small neutral fragments during collisions of n 6 4000 with graphite surfaces in order H2 On to characterize the cluster decomposition dynamics. At surface temperatures of 800±1400 K, angular distributions were well described by a model where fragments thermally evaporate from parent clusters gliding along the surface plane [3]. At lower temperatures, clusters were slowed down by
0009-2614/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 0 ) 0 1 0 0 2 - 2
A. Tomsic et al. / Chemical Physics Letters 329 (2000) 200±206
friction forces before complete evaporation took place. Andersson and Pettersson [13,14] observed the emission of positively and negatively charged cluster fragments during scattering of n 6 3700 from hot graphite surfaces. The H2 On incident velocity of 1380 msÿ1 combined with a large incident angle of 70° from the surface normal direction prevented the clusters from being immediately destroyed at surface impact. Cluster ions were found to be emitted in sharply peaked angular distributions close to the surface tangential direction, and the ionization probability depended on cluster size and surface temperature. The clusters fragmentized in surface contact, and 15±25% of a cluster survived as one unit for an initial cluster size of a few thousand molecules. The previous studies measured charged cluster fragments [13,14] and small neutral fragments [7] emitted from the graphite surface while we in the present study probe the neutral ¯ux of large water clusters leaving the surface. We present results from cluster beam experiments and molecular dynamics simulations, with the aim to better characterize the cluster±surface collision dynamics. The experimental results show that in addition to the trapping±desorption-like behavior observed earlier [7,13,14], water clusters may also undergo direct scattering from the surface. This has important implications for the interpretation of cluster charging processes observed in scattering of water clusters from graphite [13,14] and other surfaces [11,12]. The experimental data also set stricter limits for the modeling of cluster collision processes, and indicate that quantum eects are important for the dynamics of these large systems. 2. Experimental The cluster beam apparatus used in the present experiments has been described elsewhere [7,8]. It consists of a three-chamber dierentially pumped beam-line followed by a chamber for surface impact experiments and beam characterization. A continuous beam of neutral water clusters is generated by adiabatic supersonic expansion of water vapor into vacuum. Water from an external res-
201
ervoir is expanded into the ®rst chamber through a heated nozzle (capillary of 0.3 mm diameter and 18 mm length). In the present experiments the nozzle temperature was 530 K and the source pressure was 18 bar, resulting in an average cluster size n 3800 in the beam. The beam is square wave modulated by a mechanical chopper in the third chamber, before entering the scattering chamber that had a background pressure of 7 10ÿ9 mbar during the experiments. The cluster size distribution in the incident beam was measured with an axial molecular beam ionizer (30 eV electron impact) using the retarding potential method [23]. The surface used in the experiments was of `oriented' pyrolytic graphite (Grade ZYB type, Union Carbide) with the dimensions 20 3 mm2 . It was mounted on an XYZ-manipulator and could be resistively heated to 1500 K by a DC current through the sample. Neutral cluster fragments surviving the collision with the surface were ionized using the ionization unit (electron impact energy 30 eV) of a quadrupole mass spectrometer. The mass spectrometer could be rotated around the surface and had an angular resolution of less than 2°. Cluster ions with a given kinetic enegy were de¯ected by 90° by applying a voltage to a pair of de¯ection plates. The ions then passed through the quadrupole system, which was turned o in these experiments, and were ®nally detected by a dynode±scintillator± photomultiplier system. The rotatable detector could be placed in the beam path, and calibrated by comparison with the retarding potential detector used to measure the size distribution in the incident beam. The time-of-¯ight of the clusters from the surface to the detection system was measured, which allowed the mean ®nal velocity to be determined. The signal from the kinetic energy measurements was ampli®ed in a phase-locked ampli®er, while a multi-channel analyzer was used for the time-of-¯ight measurements. 3. Cluster±surface collision experiments Experimental studies of water cluster collisions with graphite have been performed focusing on the emission of large neutral clusters from the surface.
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The incident angle was kept at 70° with respect to the surface normal direction, which was previously found to give a maximum probability for the survival of large cluster ion fragments [13]. The mean velocity in the incident beam was 1375 20 msÿ1 . Fig. 1 shows typical angular distributions for water clusters scattering from a graphite surface at 1400 K. The two distributions correspond to clusters leaving the surface with a ®nal kinetic energy of 50 and 600 eV. The average cluster size in the incident beam was n 3800, which corresponds to an initial kinetic energy of 680 eV. The distributions are sharply peaked and shifted towards the surface tangential direction, and the distribution width increases slightly with decreasing kinetic energy. The results are directly comparable to the angular distributions previously observed for charged water clusters emitted from graphite surfaces [14]. Fig. 2 shows the kinetic energy distribution in the incident beam, together with ®nal kinetic energy distributions for neutral and charged clusters leaving the graphite surface. The incident beam distribution shown in Fig. 2a is rather broad, which directly re¯ects the cluster size distribution in the incident beam since the initial cluster velocity is essentially independent of cluster size [13]. Fig. 2b shows kinetic energy distributions for neutral clusters leaving the graphite surface at three dierent surface temperatures. The distribu-
Fig. 1. Angular distributions for large water cluster fragments produced during scattering of neutral water clusters from a graphite surface. The distributions are for cluster fragments with ®nal kinetic energies of 50 and 600 eV. The incident angle was 70°, the surface temperature was 1400 K, and the average cluster size in the incident beam n 3800. The inset shows a polar plot of the 600 eV distribution.
Fig. 2. Kinetic energy distributions for the incident cluster beam and cluster fragments scattering from the graphite surface: (a) the incident beam; (b) neutral cluster fragments; (c) positively and negatively charged cluster fragments. The distributions for scattered cluster fragments were measured at hf 84°, at or close to the scattering direction of maximum intensity. The incident angle was 70°, and the average cluster size in the incident beam n 3800. Surface temperatures are indicated in the ®gure.
tions are measured for a ®nal scattering angle of 84°, corresponding to the scattering direction with maximum intensity. The large peak around 0 eV is due to the ionization of water molecules (monomers) in the ¯ux from the surface. For a surface temperature of 300 K a broad peak with a maximum around 600 eV is observed. As the surface temperature is increased to 1200 and 1400 K, a second peak is observed at kinetic energies below 200 eV. This low energy peak increases in intensity with increasing surface temperature, while the high energy peak is not strongly aected by an increase in surface temperature.
A. Tomsic et al. / Chemical Physics Letters 329 (2000) 200±206
Based on the experimental results we conclude that large water clusters colliding with a graphite surface may be emitted from the surface by two dierent mechanisms. First, the clusters may undergo direct inelastic scattering from the surface, resulting in the high-energy peak in the kinetic energy distribution. The initial cluster kinetic energy is partially transferred into internal degrees of freedom while the eect of surface temperature is limited, which leads to only minor fragmentation of the clusters. Secondly, clusters may become trapped on the surface and subsequently desorb due to extensive fragmentation caused by surface heating. This channel is represented by the lowenergy peak in the energy distribution and is in operation above 700 K. It has a strong surface temperature dependence and leads to a considerable decrease in cluster size. The results in Fig. 2c show that the previously reported emission of charged water clusters [13,14] is mainly associated with the trapping±desorption channel. This indicates that the charging process is favored by the long surface residence time and the high cluster temperatures involved. We have measured the velocity for clusters leaving the surface with dierent kinetic energies. Typical average ®nal velocities are 935 50 msÿ1 . This indicates that all clusters leaving the surface have lost a considerable amount of kinetic energy during the surface interaction. The high energy peak seen in Fig. 2b thus corresponds to clusters initially coming from the high energy part of the incident kinetic energy distribution in Fig. 2a. The data for the trapping±desorption channel are consistent with the results from earlier experimental studies of argon scattering from graphite [2±6] and with molecular dynamics simulations for the Arn =Pt (1 1 1) system, including both relatively small clusters n 6 26 [24,25] and very large clusters (n 6 4400) [21,22]. Our earlier H2 On /graphite studies of emitted charged fragments [13,14] and small neutral fragments [7] are mainly related to the trapping±desorption channel, and did not indicate the existence of a direct cluster scattering channel. The observation of direct scattering of water clusters is at ®rst surprising considering the relatively low binding energy of the clusters and the expected ecient transfer of energy from
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cluster translation to internal degrees of freedom. The results are, however, qualitatively comparable to recent investigations of collisions between NaCl particles with diameters of 15±80 nm and hot platinum surfaces [26], where both direct scattering and trapping followed by decomposition was observed. 4. Molecular dynamics simulations We have carried out molecular dynamics simulations of collisions between a H2 O4094 cluster with an initial temperature of 180 K and a graphite surface at 1400 K. The incident velocity was 470 msÿ1 directed along the surface normal, corresponding to the normal component of the experimental mean cluster velocity (hi 70°; mi 1375 msÿ1 ). The interaction between the water molecules was described using the SPC model [27]. The molecule-surface potential included Lennard±Jones terms, acting between pairs of atoms, and electrostatic contributions from the interaction between the molecular point charges and point quadrupoles on the carbon atoms. This potential has performed well in simulations of water monomer scattering (see potential model 4 in Ref. [28]). The graphite (0 0 0 1) surface consisted of 41 472 carbon atoms arranged in three layers. Within the sheets, the interaction was described using Brenner's potential [29], whereas the interlayer forces were modeled using Morse functions [30]. The water±water and water±carbon poten [28]. tials were smoothly switched o at 10 A Fig. 3 shows four snapshots from a simulation and in Fig. 4 the time dependence of the cluster± surface distance, the cluster size and the average temperature are shown. With `cluster' we refer to the largest fragment, and two molecules are considered to be connected if they are within the potential cut-o distance. The initial 180 K cluster is solid-like with a marked crystal structure (Fig. 3a). After 5 ps deformation of the cluster due to surface impact is essentially completed (Fig. 3b) and the cluster is now heated by the surface and a temperature gradient is created inside the cluster. The average temperature continues to increase and the cluster becomes severely deformed due to melting
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A. Tomsic et al. / Chemical Physics Letters 329 (2000) 200±206
Fig. 3. Snapshots from a trajectory calculation of H2 O4094 scattering from graphite. The incident velocity was 470 msÿ1 , the initial cluster temperature 180 K, and the surface temperature 1400 K. Only part of the topmost graphite layer is shown. The integration time is indicated in each panel.
of the part in contact with the surface, as illustrated by the conditions after 45 ps in Fig. 3c. Fig. 4b,c show that evaporation begins after about 10 ps when the average temperature is approaching 300 K. After about 50 ps, evaporation from the heated cluster becomes strong enough that the cluster is slowly forced away from the surface. At this stage, the heating and evaporation processes balance each other, keeping the cluster at a temperature close to 540 K. The temperature starts to
decrease when the cluster leaves the surface. The decrease in temperature is rather slow, as the lower temperature also slows down the rate of evaporation. After 130 ps (Fig. 3d) the largest cluster fragment, now consisting of 1300 molecules, is well outside the range of the surface potential. A few trajectories have been run under the same conditions, which showed that the cluster size and temperature graphs seen in Fig. 4 are quite insensitive to the precise geometry of the cluster at
A. Tomsic et al. / Chemical Physics Letters 329 (2000) 200±206
Fig. 4. Time dependence of some properties of the largest cluster fragment for the collision shown in Fig. 3 (solid curves), and for a 0 K cluster where the cluster±surface interaction has been reduced by a factor three (dashed curves): (a) the distance from the cluster center of mass to the surface; (b) the size of the largest cluster fragment; (c) the internal cluster temperature.
impact, i.e., the maximum temperature is close to 540 K and the cluster size at the end of the trajectory is around 1300. The ®rst 90 ps of the cluster±surface distance plot in Fig. 4a is also very similar for dierent trajectories but the ®nal velocity of the cluster fragment varies between 40 and 120 msÿ1 . The simulations indicate that the trapping±desorption-like collision mechanism is strongly dependent on surface temperature. In simulations using a 700 K surface, the rate of heating is too slow to induce the strong evaporation that propels the cluster away from the surface. Under these conditions, the cluster completely disintegrates on the surface through slow evapo-
205
ration. We have not observed the direct scattering mechanism in simulations using the parameter values above. In collisions with lower incident velocity we can see a tendency that the cluster starts to rebound from the surface but too much of the initial translational energy is lost in the form of disordered molecular motion for the process to succeed. By arti®cially reducing the cluster± surface interaction and using colder clusters, energy dissipation is decreased. Under such conditions we do indeed observe direct scattering, and results from one trajectory calculation are included in Fig. 4. The simulations appear to give a good description of the trapping±desorption channel identi®ed in the experimental studies. The discrepancy between the experimental results and the calculations concerning the direct scattering channel may, to some extent, be explained by the presence of larger clusters due to the rather broad experimental size distribution. We expect energy dissipation to be less important for large clusters, which therefore should have better elastic properties. Most likely the discrepancy is related to limitations of the classical model used. The heat capacity of ice has been measured down to very low temperatures [31±33]. The temperature dependence can roughly be described with the equation CV AT ; where A 0:144 JKÿ2 molÿ1 . Between 0 and 250 K the heat capacity increases from 0 to 36 JKÿ1 molÿ1 , while the classical model has an almost constant heat capacity close to 6R 50 JKÿ1 molÿ1 in this temperature range R 8:3145 JKÿ1 molÿ1 is the molar gas constant). The internal energy exceeding the zero point energy is thus smaller for `true' water clusters than for the classical model system, and one may argue that a classical description will always overestimate the heat capacity and the ¯exibility of the cluster. It is also likely that the energy transfer rate is lower in a quantum mechanical system compared to the classical model system [34]. Acknowledgements This project was supported by the Swedish Natural Science Research Council.
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References [1] R.J. Holland, G.Q. Xu, J. Levko, A. Robertson Jr., S.L. Bernasek, J. Chem. Phys. 88 (1998) 8526. [2] M. Ch^atelet, A. De Martino, J.B.C. Pettersson, F. Pradere, H. Vach, Chem. Phys. Lett. 196 (1992) 563. [3] H. Vach, A. De Martino, M. Benslimane, M. Ch^ atelet, F. Pradere, J. Chem. Phys. 100 (1994) 8526. [4] M. Benslimane, M. Ch^ atelet, A. De Martino, F. Pradere, H. Vach, Chem. Phys. Lett. 237 (1995) 323. [5] M. Ch^atelet, M. Benslimane, A. De Martino, F. Pradere, H. Vach, Surf. Sci. 352 (1996) 50. [6] A. De Martino, M. Benslimane, M. Ch^ atelet, F. Pradere, H. Vach, J. Chem. Phys. 105 (1996) 7828. [7] P.U. Andersson, A. Tomsic, M.B. Andersson, J.B.C. Pettersson, Chem. Phys. Lett. 279 (1997) 100. [8] M.B. N ag ard, J.B.C Pettersson, Chem. Phys. Lett. 293 (1998) 535. [9] C. Menzel, A. Kn oner, J. Kutzner, H. Zacharias, Z. Phys. D 38 (1996) 179. [10] C. Menzel, R. Baumfalk, H. Zacharias, Chem. Phys. 239 (1998) 287. [11] A.A. Vostrikov, D.Y. Dubov, Z. Phys. D 20 (1991) 61. [12] A.A. Vostrikov, A.M. Zadorozhny, D.Y. Dubov, G. Witt, I.V. Kazakova, O.A. Bragin, V.G. Kazakov, V.N. Kikhtenko, A.A. Tyutin, Z. Phys. D 40 (1997) 542. [13] P.U. Andersson, J.B.C. Pettersson, Z. Phys. D 41 (1997) 57. [14] P.U. Andersson, J.B.C. Pettersson, J. Phys. Chem. B 102 (1998) 7428. [15] W. Christen, K.-L. Kompa, H. Schr oder, H. St ulpnagel, Ber. Bunsenges. Phys. Chem. 96 (1992) 1197. [16] C.R. Gebhardt, H. Schr oder, K.-L. Kompa, Nature 400 (1999) 544. [17] S. Koizumi, H. Yasumatsu, A. Terasaki, T. Kondow, J. Chem. Phys. 111 (1999) 6565.
[18] C. Mair, T. Fiegele, F. Biasoli, Z. Herman, T.D. M ark, J. Chem Phys. 111 (1999) 2770. [19] K. T oglhofer, F. Aumayr, H. Kurz, H.P. Winter, P. Scheier, T.D. M ark, J. Chem. Phys. 99 (1993) 8254. [20] E. Hendell, U. Even, J. Chem. Phys. 103 (1995) 9045. [21] M. Svanberg, N. Markovic, J.B.C. Pettersson, Chem. Phys. 220 (1997) 137. [22] M. Svanberg, J.B.C. Pettersson, Chem. Phys. Lett. 263 (1996) 661. [23] O.F. Hagena, W. Obert, J. Chem. Phys. 56 (1972) 1793. [24] G.Q. Xu, S.L. Bernasek, J.C. Tully, J. Chem. Phys. 88 (1988) 3376. [25] G.Q. Xu, R.J. Holland, S.L. Bernasek, J.C. Tully, J. Chem. Phys. 90 (1989) 3831. [26] J.G. Korsgren, J.B.C. Pettersson, J. Phys. Chem. B 103 (1999) 10425. [27] H.J.C Berendsen, J.P.M. Postma, W.F. van Gunsteren, J. Hermans, in: B. Pullman (Ed.), Intermolecular Forces, Reidel, Dordrecht, 1981, p. 331. [28] N. Markovic, P.U. Andersson, M.B. N ag ard, J.B.C. Pettersson, Chem. Phys. 247 (1999) 413; 252 (2000) 409 (erratum). [29] D.W. Brenner, Phys. Rev. B 42 (1990) 9458; 46 (1992) 1948 (erratum). [30] M.B. N ag ard, N. Markovic, J.B.C. Pettersson, J. Chem. Phys. 109 (1998) 10350. [31] W.F. Giauque, J.W. Stout, J. Am. Chem. Soc. 58 (1936) 1144. [32] P. Flubacher, A.J. Leadbetter, J.A. Morrison, J. Chem. Phys. 33 (1960) 1751. [33] M. Sugisaki, H. Suga, S. Seki, Bull. Chem. Soc. Japan 41 (1968) 2591. [34] E. Fredj, R.B. Gerber, M.A. Ratner, J. Chem. Phys. 109 (1998) 4833.
Chemical Physics Letters 329 (2000) 200±206
www.elsevier.nl/locate/cplett
Direct scattering and trapping±desorption of large water clusters from graphite Anna Tomsic, Nikola Markovic, Jan B.C. Pettersson * Department of Chemistry, Physical Chemistry, Goteborg University, SE-41296 Goteborg, Sweden Received 23 June 2000; received in ®nal form 24 August 2000
Abstract Large water clusters are scattered from graphite in an angular and kinetic energy resolved collision experiment. Two types of collision processes dominate the cluster±surface interactions: direct inelastic scattering of clusters and desorption of trapped clusters mediated by rapid surface-induced heating. The previously observed formation of charged clusters is concluded to mainly be associated with the trapping±desorption channel. Classical molecular dynamics simulations of the collisions agree well with the trapping±desorption channel, while the direct scattering process is less well described. Ó 2000 Elsevier Science B.V. All rights reserved.
1. Introduction Interactions of van der Waals clusters and hydrogen-bonded clusters with solid surfaces have received considerable attention. The cluster±surface collisions result in a range of phenomena including scattering of neutral fragments [1±10], emission of charged fragments [11±16], chemical reactions within the cluster [17] or between the cluster and the surface [18], and the ejection of secondary particles [19,20]. The behavior of the dierent processes is often sensitive to cluster size and collision energy. Recent studies with slow neutral argon [2±6], water [7,14], ethanol [8] and ammonia [9,10] clusters have indicated that the initial kinetic energy of a cluster is eciently transferred into internal degrees of freedom during
*
Corresponding author. Fax: +46-31-772-3107. E-mail address: [email protected] (J.B.C. Pettersson).
a cluster±surface collision. This results in the clusters becoming trapped on the surface and they thereafter decompose by evaporation. If the surface temperature is suciently high and the surface relatively ¯at, the clusters may glide long distances on the surface during the decomposition process [21], and large cluster fragments can be emitted from the surface as a result of extensive evaporation and fragmentation [21,22]. The present Letter contributes to this picture by showing that collisions between water clusters and graphite also may result in direct scattering of clusters from the surface. Andersson et al. [7] recently studied the emission of small neutral fragments during collisions of n 6 4000 with graphite surfaces in order H2 On to characterize the cluster decomposition dynamics. At surface temperatures of 800±1400 K, angular distributions were well described by a model where fragments thermally evaporate from parent clusters gliding along the surface plane [3]. At lower temperatures, clusters were slowed down by
0009-2614/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 0 ) 0 1 0 0 2 - 2
A. Tomsic et al. / Chemical Physics Letters 329 (2000) 200±206
friction forces before complete evaporation took place. Andersson and Pettersson [13,14] observed the emission of positively and negatively charged cluster fragments during scattering of n 6 3700 from hot graphite surfaces. The H2 On incident velocity of 1380 msÿ1 combined with a large incident angle of 70° from the surface normal direction prevented the clusters from being immediately destroyed at surface impact. Cluster ions were found to be emitted in sharply peaked angular distributions close to the surface tangential direction, and the ionization probability depended on cluster size and surface temperature. The clusters fragmentized in surface contact, and 15±25% of a cluster survived as one unit for an initial cluster size of a few thousand molecules. The previous studies measured charged cluster fragments [13,14] and small neutral fragments [7] emitted from the graphite surface while we in the present study probe the neutral ¯ux of large water clusters leaving the surface. We present results from cluster beam experiments and molecular dynamics simulations, with the aim to better characterize the cluster±surface collision dynamics. The experimental results show that in addition to the trapping±desorption-like behavior observed earlier [7,13,14], water clusters may also undergo direct scattering from the surface. This has important implications for the interpretation of cluster charging processes observed in scattering of water clusters from graphite [13,14] and other surfaces [11,12]. The experimental data also set stricter limits for the modeling of cluster collision processes, and indicate that quantum eects are important for the dynamics of these large systems. 2. Experimental The cluster beam apparatus used in the present experiments has been described elsewhere [7,8]. It consists of a three-chamber dierentially pumped beam-line followed by a chamber for surface impact experiments and beam characterization. A continuous beam of neutral water clusters is generated by adiabatic supersonic expansion of water vapor into vacuum. Water from an external res-
201
ervoir is expanded into the ®rst chamber through a heated nozzle (capillary of 0.3 mm diameter and 18 mm length). In the present experiments the nozzle temperature was 530 K and the source pressure was 18 bar, resulting in an average cluster size n 3800 in the beam. The beam is square wave modulated by a mechanical chopper in the third chamber, before entering the scattering chamber that had a background pressure of 7 10ÿ9 mbar during the experiments. The cluster size distribution in the incident beam was measured with an axial molecular beam ionizer (30 eV electron impact) using the retarding potential method [23]. The surface used in the experiments was of `oriented' pyrolytic graphite (Grade ZYB type, Union Carbide) with the dimensions 20 3 mm2 . It was mounted on an XYZ-manipulator and could be resistively heated to 1500 K by a DC current through the sample. Neutral cluster fragments surviving the collision with the surface were ionized using the ionization unit (electron impact energy 30 eV) of a quadrupole mass spectrometer. The mass spectrometer could be rotated around the surface and had an angular resolution of less than 2°. Cluster ions with a given kinetic enegy were de¯ected by 90° by applying a voltage to a pair of de¯ection plates. The ions then passed through the quadrupole system, which was turned o in these experiments, and were ®nally detected by a dynode±scintillator± photomultiplier system. The rotatable detector could be placed in the beam path, and calibrated by comparison with the retarding potential detector used to measure the size distribution in the incident beam. The time-of-¯ight of the clusters from the surface to the detection system was measured, which allowed the mean ®nal velocity to be determined. The signal from the kinetic energy measurements was ampli®ed in a phase-locked ampli®er, while a multi-channel analyzer was used for the time-of-¯ight measurements. 3. Cluster±surface collision experiments Experimental studies of water cluster collisions with graphite have been performed focusing on the emission of large neutral clusters from the surface.
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The incident angle was kept at 70° with respect to the surface normal direction, which was previously found to give a maximum probability for the survival of large cluster ion fragments [13]. The mean velocity in the incident beam was 1375 20 msÿ1 . Fig. 1 shows typical angular distributions for water clusters scattering from a graphite surface at 1400 K. The two distributions correspond to clusters leaving the surface with a ®nal kinetic energy of 50 and 600 eV. The average cluster size in the incident beam was n 3800, which corresponds to an initial kinetic energy of 680 eV. The distributions are sharply peaked and shifted towards the surface tangential direction, and the distribution width increases slightly with decreasing kinetic energy. The results are directly comparable to the angular distributions previously observed for charged water clusters emitted from graphite surfaces [14]. Fig. 2 shows the kinetic energy distribution in the incident beam, together with ®nal kinetic energy distributions for neutral and charged clusters leaving the graphite surface. The incident beam distribution shown in Fig. 2a is rather broad, which directly re¯ects the cluster size distribution in the incident beam since the initial cluster velocity is essentially independent of cluster size [13]. Fig. 2b shows kinetic energy distributions for neutral clusters leaving the graphite surface at three dierent surface temperatures. The distribu-
Fig. 1. Angular distributions for large water cluster fragments produced during scattering of neutral water clusters from a graphite surface. The distributions are for cluster fragments with ®nal kinetic energies of 50 and 600 eV. The incident angle was 70°, the surface temperature was 1400 K, and the average cluster size in the incident beam n 3800. The inset shows a polar plot of the 600 eV distribution.
Fig. 2. Kinetic energy distributions for the incident cluster beam and cluster fragments scattering from the graphite surface: (a) the incident beam; (b) neutral cluster fragments; (c) positively and negatively charged cluster fragments. The distributions for scattered cluster fragments were measured at hf 84°, at or close to the scattering direction of maximum intensity. The incident angle was 70°, and the average cluster size in the incident beam n 3800. Surface temperatures are indicated in the ®gure.
tions are measured for a ®nal scattering angle of 84°, corresponding to the scattering direction with maximum intensity. The large peak around 0 eV is due to the ionization of water molecules (monomers) in the ¯ux from the surface. For a surface temperature of 300 K a broad peak with a maximum around 600 eV is observed. As the surface temperature is increased to 1200 and 1400 K, a second peak is observed at kinetic energies below 200 eV. This low energy peak increases in intensity with increasing surface temperature, while the high energy peak is not strongly aected by an increase in surface temperature.
A. Tomsic et al. / Chemical Physics Letters 329 (2000) 200±206
Based on the experimental results we conclude that large water clusters colliding with a graphite surface may be emitted from the surface by two dierent mechanisms. First, the clusters may undergo direct inelastic scattering from the surface, resulting in the high-energy peak in the kinetic energy distribution. The initial cluster kinetic energy is partially transferred into internal degrees of freedom while the eect of surface temperature is limited, which leads to only minor fragmentation of the clusters. Secondly, clusters may become trapped on the surface and subsequently desorb due to extensive fragmentation caused by surface heating. This channel is represented by the lowenergy peak in the energy distribution and is in operation above 700 K. It has a strong surface temperature dependence and leads to a considerable decrease in cluster size. The results in Fig. 2c show that the previously reported emission of charged water clusters [13,14] is mainly associated with the trapping±desorption channel. This indicates that the charging process is favored by the long surface residence time and the high cluster temperatures involved. We have measured the velocity for clusters leaving the surface with dierent kinetic energies. Typical average ®nal velocities are 935 50 msÿ1 . This indicates that all clusters leaving the surface have lost a considerable amount of kinetic energy during the surface interaction. The high energy peak seen in Fig. 2b thus corresponds to clusters initially coming from the high energy part of the incident kinetic energy distribution in Fig. 2a. The data for the trapping±desorption channel are consistent with the results from earlier experimental studies of argon scattering from graphite [2±6] and with molecular dynamics simulations for the Arn =Pt (1 1 1) system, including both relatively small clusters n 6 26 [24,25] and very large clusters (n 6 4400) [21,22]. Our earlier H2 On /graphite studies of emitted charged fragments [13,14] and small neutral fragments [7] are mainly related to the trapping±desorption channel, and did not indicate the existence of a direct cluster scattering channel. The observation of direct scattering of water clusters is at ®rst surprising considering the relatively low binding energy of the clusters and the expected ecient transfer of energy from
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cluster translation to internal degrees of freedom. The results are, however, qualitatively comparable to recent investigations of collisions between NaCl particles with diameters of 15±80 nm and hot platinum surfaces [26], where both direct scattering and trapping followed by decomposition was observed. 4. Molecular dynamics simulations We have carried out molecular dynamics simulations of collisions between a H2 O4094 cluster with an initial temperature of 180 K and a graphite surface at 1400 K. The incident velocity was 470 msÿ1 directed along the surface normal, corresponding to the normal component of the experimental mean cluster velocity (hi 70°; mi 1375 msÿ1 ). The interaction between the water molecules was described using the SPC model [27]. The molecule-surface potential included Lennard±Jones terms, acting between pairs of atoms, and electrostatic contributions from the interaction between the molecular point charges and point quadrupoles on the carbon atoms. This potential has performed well in simulations of water monomer scattering (see potential model 4 in Ref. [28]). The graphite (0 0 0 1) surface consisted of 41 472 carbon atoms arranged in three layers. Within the sheets, the interaction was described using Brenner's potential [29], whereas the interlayer forces were modeled using Morse functions [30]. The water±water and water±carbon poten [28]. tials were smoothly switched o at 10 A Fig. 3 shows four snapshots from a simulation and in Fig. 4 the time dependence of the cluster± surface distance, the cluster size and the average temperature are shown. With `cluster' we refer to the largest fragment, and two molecules are considered to be connected if they are within the potential cut-o distance. The initial 180 K cluster is solid-like with a marked crystal structure (Fig. 3a). After 5 ps deformation of the cluster due to surface impact is essentially completed (Fig. 3b) and the cluster is now heated by the surface and a temperature gradient is created inside the cluster. The average temperature continues to increase and the cluster becomes severely deformed due to melting
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Fig. 3. Snapshots from a trajectory calculation of H2 O4094 scattering from graphite. The incident velocity was 470 msÿ1 , the initial cluster temperature 180 K, and the surface temperature 1400 K. Only part of the topmost graphite layer is shown. The integration time is indicated in each panel.
of the part in contact with the surface, as illustrated by the conditions after 45 ps in Fig. 3c. Fig. 4b,c show that evaporation begins after about 10 ps when the average temperature is approaching 300 K. After about 50 ps, evaporation from the heated cluster becomes strong enough that the cluster is slowly forced away from the surface. At this stage, the heating and evaporation processes balance each other, keeping the cluster at a temperature close to 540 K. The temperature starts to
decrease when the cluster leaves the surface. The decrease in temperature is rather slow, as the lower temperature also slows down the rate of evaporation. After 130 ps (Fig. 3d) the largest cluster fragment, now consisting of 1300 molecules, is well outside the range of the surface potential. A few trajectories have been run under the same conditions, which showed that the cluster size and temperature graphs seen in Fig. 4 are quite insensitive to the precise geometry of the cluster at
A. Tomsic et al. / Chemical Physics Letters 329 (2000) 200±206
Fig. 4. Time dependence of some properties of the largest cluster fragment for the collision shown in Fig. 3 (solid curves), and for a 0 K cluster where the cluster±surface interaction has been reduced by a factor three (dashed curves): (a) the distance from the cluster center of mass to the surface; (b) the size of the largest cluster fragment; (c) the internal cluster temperature.
impact, i.e., the maximum temperature is close to 540 K and the cluster size at the end of the trajectory is around 1300. The ®rst 90 ps of the cluster±surface distance plot in Fig. 4a is also very similar for dierent trajectories but the ®nal velocity of the cluster fragment varies between 40 and 120 msÿ1 . The simulations indicate that the trapping±desorption-like collision mechanism is strongly dependent on surface temperature. In simulations using a 700 K surface, the rate of heating is too slow to induce the strong evaporation that propels the cluster away from the surface. Under these conditions, the cluster completely disintegrates on the surface through slow evapo-
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ration. We have not observed the direct scattering mechanism in simulations using the parameter values above. In collisions with lower incident velocity we can see a tendency that the cluster starts to rebound from the surface but too much of the initial translational energy is lost in the form of disordered molecular motion for the process to succeed. By arti®cially reducing the cluster± surface interaction and using colder clusters, energy dissipation is decreased. Under such conditions we do indeed observe direct scattering, and results from one trajectory calculation are included in Fig. 4. The simulations appear to give a good description of the trapping±desorption channel identi®ed in the experimental studies. The discrepancy between the experimental results and the calculations concerning the direct scattering channel may, to some extent, be explained by the presence of larger clusters due to the rather broad experimental size distribution. We expect energy dissipation to be less important for large clusters, which therefore should have better elastic properties. Most likely the discrepancy is related to limitations of the classical model used. The heat capacity of ice has been measured down to very low temperatures [31±33]. The temperature dependence can roughly be described with the equation CV AT ; where A 0:144 JKÿ2 molÿ1 . Between 0 and 250 K the heat capacity increases from 0 to 36 JKÿ1 molÿ1 , while the classical model has an almost constant heat capacity close to 6R 50 JKÿ1 molÿ1 in this temperature range R 8:3145 JKÿ1 molÿ1 is the molar gas constant). The internal energy exceeding the zero point energy is thus smaller for `true' water clusters than for the classical model system, and one may argue that a classical description will always overestimate the heat capacity and the ¯exibility of the cluster. It is also likely that the energy transfer rate is lower in a quantum mechanical system compared to the classical model system [34]. Acknowledgements This project was supported by the Swedish Natural Science Research Council.
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