- Email: [email protected]
Effects of morphology in electron-stimulated desorption: O− from O2 condensed on D2O films grown at 15–150 K on Pt
Surface Science 436 (1999) L671–L676 www.elsevier.nl/locate/susc
Surface Science Letters
Effects of morphology in electron-stimulated desorption: O− from O condensed on D O films grown at 15–150 K on Pt 2 2 R. Azria a, Y. Le Coat a, M. Lachgar a, M. Tronc b, L. Parenteau c, L. Sanche c, * a Laboratoire des Collisions Atomiques et Mole´culaires (LCAM), Baˆt 351 Universite´ Paris-Sud, F-91405 Orsay, France b Laboratoire de Chimie Physique, Universite´ Pierre et Marie Curie, 11 rue Pierre et Marie Curie, F-75231 Paris Cedex 05, France c Canadian Medical Research Council Group in Radiation Sciences, Faculty of Medicine, University of Sherbrooke, Sherbrooke, Que. J1H 5N4, Canada Received 31 March 1999; accepted for publication 29 April 1999
Abstract Electron-stimulated desorption of impact energy in the range 6–20 eV. (15–150 K ) of D O condensation on 2 The results are shown to be related rights reserved.
O− ions from O -covered multilayer ice films is investigated as a function of 2 The magnitude of the O− signal is found to strongly depend on temperature Pt, amount of condensed O (0.1–4 ML) and D O film thickness (4–100 ML). 2 2 to different morphologies of the ice layers. © 1999 Elsevier Science B.V. All
Keywords: Amorphous thin films; Desorption induced by electronic transitions (DIET ); Dissociative electron attachment; Electronstimulated desorption ( ESD); Low energy electrons; Polycrystalline thin films; Water
In previous electron-stimulated desorption ( ESD) experiments from O adsorbed onto 2 different spacer layer films deposited on Pt, at 20 K [1], it was shown that the magnitude of the O− signal was strongly influenced by the nature of the atoms or molecules forming the spacer. These investigations were based on a comparison of the magnitude of the O− dissociative electron attachment (DEA) yields, arising from 0.15 monolayer (ML) of O condensed on a 4 ML Kr spacer, 2 with that obtained from 0.15 ML of O deposited 2 onto different 4 ML spacer films consisting of * Corresponding author. Fax: +1-819-564-5442. E-mail address: [email protected] (L. Sanche)
series of atoms and molecules. Intrinsic parameters of the intermediate anion (symmetry, lifetime, orientation, new decay channels, charge transfer) processes, as well as extrinsic (electron interaction with solid prior to DEA, anion interaction with solid after dissociation) processes, were found to influence the observed signal and could account for the differences in O− yields from different substrates. In the case of O –H O two-component 2 2 targets, the extrinsic effects contributed to decrease the O− DEA signal but a significant change in the intrinsic parameters also had to be invoked. It was noted [1] that the magnitude of the lower O− DEA yields could be reproduced by an 80% decrease in the lifetime of the dissociative O− 1 2
0039-6028/99/$ – see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S0 0 39 - 6 0 28 ( 99 ) 0 06 4 2 -1
SU RF A LE CE TT S ER CIE S N CE
L672
R. Azria et al. / Surface Science 436 (1999) L671–L676
(2P , 2S+) resonances with respect to autodetachu ment, and that the reduction in lifetime could be due to the local dipole field of the adjacent H O 2 molecules. Quenching by H O molecules of the 2 N− 1 (2P ) transient negative ion had previously 2 g been proposed [2] to account for the suppression of the N stretching mode intensity in high-reso2 lution electron energy loss spectra obtained by coadsorption of one H O molecule for six N 2 2 molecules on Al (111). However, recent O− yield functions obtained from 0.1 ML O deposited on 2 4 ML of different polar molecules [CH OH ( m= 3 1.70 D); CD COOD ( m=1.74 D); CD CN ( m= 3 3 3.90 D)] [3] show that, in the case of O− ESD via O− 1 states, depletion of the O− signal is not 2 proportional to the dipole moment of molecules forming the spacer layer. Furthermore, measurements of D− DEA yields from pure D O films 2 formed at various temperatures on Pt, by Simpson et al. [4], clearly indicated that the magnitude of the desorbed signal was related to the morphology of the film and that structural changes had an influence on both the intrinsic and extrinsic parameters controlling the DEA process. In this Letter, we report similar studies for a two-component system, consisting of O con2 densed on multilayer D O films deposited on Pt. 2 By varying the amount of O and the thickness 2 and temperature of formation of the D O spacer 2 over wide limits, we show that the magnitude of O− yields from DEA is strongly influenced by the structural state and the degree of porosity of the D O spacer. The latter is found to have a strong 2 effect on extrinsic processes, particularly anion scattering prior to desorption. This conclusion, however, does not imply that intrinsic parameters are insensitive to the morphology of the spacer. To our knowledge, this is the first anion ESD study of a two-component system consisting of a small amount of molecules adsorbed on a thin spacer film substrate whose morphology (i.e. surface roughness, porosity and structural state) is modified in the experiment. It becomes obvious, from the results presented, that interpretation of anion ESD experiments from adsorbates must take into account the morphology of the substrate, which is sensitive to temperature and angle of adsorption [5]. The choice of the O –D O system 2 2
was dictated by our understanding of O− ESD from pure O films deposited onto metallic sub2 strates [1,6,7] and by the important role played by water in radiation chemistry and biology [8] and ice in atmospheric, planetary and interstellar chemistry [9–12]. The experiments were carried out at the University of Paris-Sud (Orsay) and at the University of Sherbrooke using similar apparatuses, which have been described elsewhere [7,13]. In brief, they consist of a UHV chamber (base pressure 2×10−10 Torr) equipped with a cryocooled polycrystalline Pt foil, a low-energy electron monochromator and a quadrupole mass spectrometer for ion detection. Target films are grown on the Pt substrate by condensing D O vapour and 2 O gas; their thicknesses are estimated from the 2 amount of gas needed to deposit 1 ML assuming no change in the sticking coefficient for the adlayers [7]. The angles of the axis of the dosing nozzles are ~85° (Orsay) and ~45° (Sherbrooke) with respect to the Pt surface normal. The D O spacer 2 layers are grown at temperatures ranging between 15 and 150 K (Orsay) or 20 to 120 K (Sherbrooke). Afterwards, they are cooled to 15 K (Orsay) or 20 K (Sherbrooke) before O deposition. O− ESD 2 data are always recorded at 15 or 20 K. Differences observed in O− relative intensities from measurements in Orsay and Sherbrooke performed under similar conditions are attributed to differences in ion transmission through the different quadrupoles and the different angle of incidence of the adsorbates. According to the results of Stevenson et al. [5], the porosity of the films grown with the D O 2 beam striking the Pt surface at an angle of 85° with respect to the normal would be about twice as large as that of films produced with the D O 2 beam striking the surface at 45°. Ice is known to exist in amorphous and crystalline phases, which are stable under UHV conditions within the temperature range 15–160 K. Ice films grown by vapour deposition at substrate temperatures below ~110 K are amorphous [4] and have a large surface-to-volume ratio arising from a high density of micropores [14]. These pores collapse if the film is gradually warmed to ~120 K, resulting in higher density amorphous ice [15–18]. Non-porous amorphous ice can also
CE N IE SC RS CE E A TT RF LE SU
R. Azria et al. / Surface Science 436 (1999) L671–L676
be grown by condensing water vapour at temperatures between ~110 and 140 K. Annealing an amorphous ice film above the amorphous-to-crystalline phase transition temperature (~155 K ) leads to crystallization of the film [19,20]. Crystalline films can also be grown directly by condensing water vapour at temperatures of ~150 K. The above temperatures may vary by as much as 10 K depending on the deposition rate [20,21]. Fig. 1 represents four O− ion yields spectra from various quantities of O (4, 1, 0.5 and 2 0.1 ML) deposited onto a 4 ML D O spacer layer 2 grown at 15 K. The O− yield function in Fig. 1a is very similar to that observed in pure O [6,7], 2 where the 6–10 eV structures have been associated with DEA via the gas-phase-allowed 2P , O− 1 u 2 resonance and gas-phase-forbidden 2S+(I ) resog
Fig. 1. O− ESD yields from a 4 ML D O spacer layer con2 densed at 15 K on Pt and covered with (a) 4 ML, (b) 1 ML, (c) 0.5 ML and (d) 0.1 ML of O . In curves (b) to (d) the 2 vertical intensity has been increased with respect to that of curve (a) by the factor appearing on the left.
L673
nances; the 13 eV peak has been ascribed to dissociation of a transient 2S+ anion state with undertermined g/u assignment. Moreover, it has been shown that kinetic energy ( KE ) distributions of the desorbing O− ions are very broad, extending to 3 eV and 5 eV in the energy range of the 8 eV and 13 eV peaks in the O− yield functions respectively [6,7]. The signal above 16 eV has been attributed to dipolar dissociation (DD, i.e. e−+O O++O−+e−) [6,7]. Comparing the 2 curves in Fig. 1, we find that: 1. the O− signal is reduced with decreasing O 2 coverage, with the relative intensities of the 8 and 13 eV peaks in Fig. 1b–d reversed from that in Fig. 1a and hence from that of the pure O films; 2 2. in going from Fig. 1b to Fig. 1d, the O− DEA signal decreases much faster than the decrease in amount of condensed O ; 2 3. for the 0.1 ML O –4 ML D O two-component 2 2 system, the DEA signal is very strongly suppressed; 4. the DD signal is roughly proportional to the quantity of condensed O . 2 Fig. 2 shows O− yields from 1 ML of O depos2 ited onto D O spacer layers of 4 ML, 40 ML and 2 100 ML thicknesses condensed at 15 K on the Pt foil. We see that increasing D O spacer thickness 2 results in a strong decrease and even the suppression of the O− signal at all energies. This observation can be associated with the porous amorphous structure of the D O spacer condensed at 15 K. In 2 fact, if the O molecules penetrate into the spacer, 2 then, in order to desorb, O− ions must suffer collisions which reduce their KE (for a review of anion scattering within thin molecular solid films see Ref. [22]). For the remaining KE lower than the polarisation energy induced at the film–vacuum interface, O− ions do not desorb. We can therefore reasonably assume that, due to these collisions, the deeper the O molecules penetrate into the 2 pores, the lower is the KE of O− ions and, consequently, the O− signal is lowered, as seen in Fig. 2. The O− yields functions shown in Fig. 1 can be understood as follows. When 0.1 ML of O is 2 deposited onto the porous amorphous 4 ML D O 2 spacer film, O molecules diffuse into the pores to 2 the Pt substrate. In addition to O− ion collisions
SU RF A LE CE TT S ER CIE S N CE
L674
R. Azria et al. / Surface Science 436 (1999) L671–L676
O –4 ML D O film the pores are completely filled, 2 2 so that a substantial amount of O molecules lies 2 on the surface of the film giving rise to an O− yield function similar to that obtained from pure O [7]. 2 This interpretation is supported by the yield functions in Fig. 3, which shows the O− yields from a 4 ML O film and also from 1 ML and 2 0.1 ML of O condensed on 4 ML D O films 2 2 grown at temperatures of 150 K, 15 K, 20 K, and 128 K. When D O is condensed at 128 K or 150 K 2 the O− 1 resonances are clearly visible; at these 2 temperatures the spacer layer is in a non-porous amorphous and crystalline geometry respectively,
Fig. 2. O− ESD yields from 1 ML of O condensed on D O 2 2 spacer layers of increasing thicknesses (4 ML, 40 ML and 100 ML) condensed at 15 K on a Pt substrate. The vertical gain is normalized to the bottom curve.
and a lesser contribution from electron energy losses prior to dissociation, two other effects contribute to suppression of the DEA O− desorbing signal, namely quenching of O− 1 resonances by 2 the metal [23] and an increase of polarization energy due to the stronger image force potential in the Pt substrate [23]. As the amount of deposited O increases (Fig. 1b and c) so does the O− 2 signal, but since these ions suffer collisions as they diffuse through the D O spacer, O− ions with high 2 KE (i.e. formed around 13 eV electron incident energy) [7] have a higher probability to emerge into vacuum. This explains the reversed relative intensities of the 8 eV and 13 eV peaks. Since the DD signal is not quenched at the metal surface and, in this case, O− does not lose KE from polarization, as shown by Sambe et al. [23], its intensity is more directly related to the number of condensed O molecules. Finally, in the 4 ML 2
Fig. 3. O− ESD yields from 4 ML O (a), from 1 ML O con2 2 densed on a 4 ML D O spacer layer condensed at 150 K (b) 2 and 15 K (c) on Pt and from 0.1 ML O condensed on a 4 ML 2 D O spacer condensed at 128 K (d ) and 20 K (e) on Pt. The 2 magnitude of the 6–10 eV integrated O− ESD signal from 0.3 ML O on 8 ML D O is plotted in the inset as a function 2 2 of the temperature of condensation of the D O spacer. 2
CE N IE SC RS CE E A TT RF LE SU
R. Azria et al. / Surface Science 436 (1999) L671–L676
so that O molecules are unable to diffuse through 2 the D O layer and remain on the D O surface. In 2 2 this case, O− emission arises from the surface, causing the O− yield function of the 1 ML O –4 ML H O target to appear similar to that 2 2 obtained with pure multilayer O . The temperature 2 dependence of the O− DEA signal integrated between 6–10 eV is shown in the inset of Fig. 3 for 0.3 ML O condensed on 8 ML D O. This 2 2 indicates that O diffusion through the spacer layer 2 is increasingly inhibited as the temperature of condensation of the D O spacer rises from 80 K 2 to 120 K. The results in Fig. 3 are interpreted to arise essentially from a reduction of the number and/or size of the pore in the D O film, with 2 increasing temperature, which restricts O diffu2 sion. They are consistent with the DEA D− yields, from pure D O films heated from 20 to 140 K 2 reported by Simpson et al. [21], who found an increase in the D− signal from 80 K up to 130 K due to collapsing of pores in amorphous ice layers deposited on Pt. We further note that, for yields measured under similar conditions (e.g. Fig. 1d versus Fig. 3e), the signal arising from the more porous sample due to the angle of the D O beam 2 is lower by a factor of ~2. Hence, there appears to be a good correlation between the magnitude of the D− signal and the porosity determined from the D O adsorption angle [5]. 2 Finally, we have invoked quenching of the O− 1 resonant states and a stronger image charge 2 effect at the Pt substrate in the case of a submonolayer 0.1 ML O –4ML porous D O film, to 2 2 account for O− suppression below 15 eV incident electron energy (Fig. 1d). These effects are evidenced in Fig. 4, which represents O− yield functions from 0.5 ML O –8 ML D O–Pt in Fig. 4a 2 2 and from 0.5 ML O –8 ML D O–4 ML crystalline 2 2 benzene in Fig. 4b. In both experiments the D O 2 spacers are condensed at 20 K. We see that in Fig. 4a the O diffusion to the Pt substrate leads 2 to quenching of the DEA and the subsequent reduction in O− desorption, whereas in Fig. 4b the influence of the Pt substrate is screened by the crystalline benzene spacer so that the O− 1 reso2 nances are clearly visible in the O− yields. Again, the observed relative intensity of the two peaks around 8 eV and 13 eV is related to collisions and differences in KE of desorbing O− ions.
L675
Fig. 4. O− ESD yield from 0.5 ML O , condensed on a 8 ML 2 H O film condensed at 20 K on Pt (a) and from 0.5 ML O 2 2 deposited on 8 ML H O condensed at 20 K on 4 ML of crystal2 line benzene.
In conclusion, we have shown that O− ESD from O condensed on the surface of an ice film 2 depends strongly on its morphology. Similar observations have recently been made for other twocomponent systems, consisting of N and Ar 2 adsorbed on thin n-hexane spacer films, condensed at 20–70 K on Pt (111) (see Ref. [24] and results to be published). For these systems, it was the magnitude of the metastable N and Ar desorption 2 yields that strongly depended on the morphology of the spacer.
Acknowledgements We thank Drs. D. Teillet-Billy, M. Sizun, M.A. Huels and A.D. Bass for helpful discussions. This
SU RF A LE CE TT S ER CIE S N CE
L676
R. Azria et al. / Surface Science 436 (1999) L671–L676
research is financed by the CNRS and the Canadian MRC.
References [1] M.A. Huels, L. Parenteau, L. Sanche, J. Chem. Phys. 100 (1994) 3940. [2] K. Jacobi, M. Bertolo, P. Geng, W. Hansen, C. Astaldin, Chem. Phys. Lett. 173 (1990) 97. [3] M. Lachgar, Y. Le Coat, R. Azria, M. Tronc, E. Illenberger, in preparation. [4] W.C. Simpson, M.T. Sieger, T.M. Orlando, L. Parenteau, K. Nagesha, L. Sanche, J. Chem. Phys. 107 (1997) 8668. [5] K.P. Stevenson, G.A. Kimmel, Z. Dohna´lek, R. Scott Smith, B.D. Kay, Science 283 (1999) 1505. [6 ] R. Azria, Y. Le Coat, J.P. Ziesel, J.P. Guillotin, B. Mharzi, M. Tronc, Chem. Phys. Lett. 220 (1994) 417. [7] M.A. Huels, L. Parenteau, M. Michaud, L. Sanche, Phys. Rev. A 51 (1995) 337. [8] C. von Sonntag, The Chemical Basis for Radiation Biology, Taylor and Francis, London, 1987. [9] M.T. Sieger, W.C. Simpson, T.M. Orlando, Nature 394 (1998) 554.
[10] [11] [12] [13]
E. Herbst, Annu. Rev. Phys. Chem. 46 (1995) 27. D.C. Clary, Science 271 (1996) 1509. R.E. Johnson, Rev. Mod. Phys. 68 (1996) 305. M. Tronc, R. Azria, Y. Le Coat, E. Illenberger, J. Phys. Chem. 100 (1996) 14 745. [14] R.S. Smith, G. Huang, E.K.L. Weng, B.D. Kay, Surf. Sci. 367 (1996) L13. [15] B.S. Berland, D.E. Brown, M.A. Tolbert, S.M. George, Geophys. Res. Lett. 22 (1995) 3493. [16 ] M.A. Zonlo, T.B. Onasch, M.S. Warshawsky, M.A. Tolbert, G. Mallick, P. Arentz, M.A. Robinson, J. Phys. Chem. 101 (1997) 10 887. [17] J.P. Devlin, V. Buch, J. Phys. Chem. 101 (1995) 16 534. [18] B. Rowland, J.P. Devlin, J. Chem. Phys. 94 (1991) 812. [19] G.P. Johari, A. Hallbrucken, E. Mayer, J. Chem. Phys. 92 (1990) 6742. [20] R.J. Speedy, P.G. Debenedetti, R. Smith, C. Huang, B.D. Kay, J. Chem. Phys. 105 (1996) 240. [21] W.C. Simpson, L. Parenteau, R.S. Smith, L. Sanche, T.M. Orlando, Surf. Sci. 390 (1997) 86. [22] M. Akbulut, N.J. Sack, T.E. Madey, Surf. Sci. Rep. 28 (1997) 177. [23] H. Sambe, D.E. Ramaker, L. Parenteau, L. Sanche, Phys. Rev. Lett. 59 (1987) 236. [24] E. Vichnevetski, P. Cloutier, L. Sanche, J. Chem. Phys. 110 (1999) 1112.
Surface Science Letters
Effects of morphology in electron-stimulated desorption: O− from O condensed on D O films grown at 15–150 K on Pt 2 2 R. Azria a, Y. Le Coat a, M. Lachgar a, M. Tronc b, L. Parenteau c, L. Sanche c, * a Laboratoire des Collisions Atomiques et Mole´culaires (LCAM), Baˆt 351 Universite´ Paris-Sud, F-91405 Orsay, France b Laboratoire de Chimie Physique, Universite´ Pierre et Marie Curie, 11 rue Pierre et Marie Curie, F-75231 Paris Cedex 05, France c Canadian Medical Research Council Group in Radiation Sciences, Faculty of Medicine, University of Sherbrooke, Sherbrooke, Que. J1H 5N4, Canada Received 31 March 1999; accepted for publication 29 April 1999
Abstract Electron-stimulated desorption of impact energy in the range 6–20 eV. (15–150 K ) of D O condensation on 2 The results are shown to be related rights reserved.
O− ions from O -covered multilayer ice films is investigated as a function of 2 The magnitude of the O− signal is found to strongly depend on temperature Pt, amount of condensed O (0.1–4 ML) and D O film thickness (4–100 ML). 2 2 to different morphologies of the ice layers. © 1999 Elsevier Science B.V. All
Keywords: Amorphous thin films; Desorption induced by electronic transitions (DIET ); Dissociative electron attachment; Electronstimulated desorption ( ESD); Low energy electrons; Polycrystalline thin films; Water
In previous electron-stimulated desorption ( ESD) experiments from O adsorbed onto 2 different spacer layer films deposited on Pt, at 20 K [1], it was shown that the magnitude of the O− signal was strongly influenced by the nature of the atoms or molecules forming the spacer. These investigations were based on a comparison of the magnitude of the O− dissociative electron attachment (DEA) yields, arising from 0.15 monolayer (ML) of O condensed on a 4 ML Kr spacer, 2 with that obtained from 0.15 ML of O deposited 2 onto different 4 ML spacer films consisting of * Corresponding author. Fax: +1-819-564-5442. E-mail address: [email protected] (L. Sanche)
series of atoms and molecules. Intrinsic parameters of the intermediate anion (symmetry, lifetime, orientation, new decay channels, charge transfer) processes, as well as extrinsic (electron interaction with solid prior to DEA, anion interaction with solid after dissociation) processes, were found to influence the observed signal and could account for the differences in O− yields from different substrates. In the case of O –H O two-component 2 2 targets, the extrinsic effects contributed to decrease the O− DEA signal but a significant change in the intrinsic parameters also had to be invoked. It was noted [1] that the magnitude of the lower O− DEA yields could be reproduced by an 80% decrease in the lifetime of the dissociative O− 1 2
0039-6028/99/$ – see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S0 0 39 - 6 0 28 ( 99 ) 0 06 4 2 -1
SU RF A LE CE TT S ER CIE S N CE
L672
R. Azria et al. / Surface Science 436 (1999) L671–L676
(2P , 2S+) resonances with respect to autodetachu ment, and that the reduction in lifetime could be due to the local dipole field of the adjacent H O 2 molecules. Quenching by H O molecules of the 2 N− 1 (2P ) transient negative ion had previously 2 g been proposed [2] to account for the suppression of the N stretching mode intensity in high-reso2 lution electron energy loss spectra obtained by coadsorption of one H O molecule for six N 2 2 molecules on Al (111). However, recent O− yield functions obtained from 0.1 ML O deposited on 2 4 ML of different polar molecules [CH OH ( m= 3 1.70 D); CD COOD ( m=1.74 D); CD CN ( m= 3 3 3.90 D)] [3] show that, in the case of O− ESD via O− 1 states, depletion of the O− signal is not 2 proportional to the dipole moment of molecules forming the spacer layer. Furthermore, measurements of D− DEA yields from pure D O films 2 formed at various temperatures on Pt, by Simpson et al. [4], clearly indicated that the magnitude of the desorbed signal was related to the morphology of the film and that structural changes had an influence on both the intrinsic and extrinsic parameters controlling the DEA process. In this Letter, we report similar studies for a two-component system, consisting of O con2 densed on multilayer D O films deposited on Pt. 2 By varying the amount of O and the thickness 2 and temperature of formation of the D O spacer 2 over wide limits, we show that the magnitude of O− yields from DEA is strongly influenced by the structural state and the degree of porosity of the D O spacer. The latter is found to have a strong 2 effect on extrinsic processes, particularly anion scattering prior to desorption. This conclusion, however, does not imply that intrinsic parameters are insensitive to the morphology of the spacer. To our knowledge, this is the first anion ESD study of a two-component system consisting of a small amount of molecules adsorbed on a thin spacer film substrate whose morphology (i.e. surface roughness, porosity and structural state) is modified in the experiment. It becomes obvious, from the results presented, that interpretation of anion ESD experiments from adsorbates must take into account the morphology of the substrate, which is sensitive to temperature and angle of adsorption [5]. The choice of the O –D O system 2 2
was dictated by our understanding of O− ESD from pure O films deposited onto metallic sub2 strates [1,6,7] and by the important role played by water in radiation chemistry and biology [8] and ice in atmospheric, planetary and interstellar chemistry [9–12]. The experiments were carried out at the University of Paris-Sud (Orsay) and at the University of Sherbrooke using similar apparatuses, which have been described elsewhere [7,13]. In brief, they consist of a UHV chamber (base pressure 2×10−10 Torr) equipped with a cryocooled polycrystalline Pt foil, a low-energy electron monochromator and a quadrupole mass spectrometer for ion detection. Target films are grown on the Pt substrate by condensing D O vapour and 2 O gas; their thicknesses are estimated from the 2 amount of gas needed to deposit 1 ML assuming no change in the sticking coefficient for the adlayers [7]. The angles of the axis of the dosing nozzles are ~85° (Orsay) and ~45° (Sherbrooke) with respect to the Pt surface normal. The D O spacer 2 layers are grown at temperatures ranging between 15 and 150 K (Orsay) or 20 to 120 K (Sherbrooke). Afterwards, they are cooled to 15 K (Orsay) or 20 K (Sherbrooke) before O deposition. O− ESD 2 data are always recorded at 15 or 20 K. Differences observed in O− relative intensities from measurements in Orsay and Sherbrooke performed under similar conditions are attributed to differences in ion transmission through the different quadrupoles and the different angle of incidence of the adsorbates. According to the results of Stevenson et al. [5], the porosity of the films grown with the D O 2 beam striking the Pt surface at an angle of 85° with respect to the normal would be about twice as large as that of films produced with the D O 2 beam striking the surface at 45°. Ice is known to exist in amorphous and crystalline phases, which are stable under UHV conditions within the temperature range 15–160 K. Ice films grown by vapour deposition at substrate temperatures below ~110 K are amorphous [4] and have a large surface-to-volume ratio arising from a high density of micropores [14]. These pores collapse if the film is gradually warmed to ~120 K, resulting in higher density amorphous ice [15–18]. Non-porous amorphous ice can also
CE N IE SC RS CE E A TT RF LE SU
R. Azria et al. / Surface Science 436 (1999) L671–L676
be grown by condensing water vapour at temperatures between ~110 and 140 K. Annealing an amorphous ice film above the amorphous-to-crystalline phase transition temperature (~155 K ) leads to crystallization of the film [19,20]. Crystalline films can also be grown directly by condensing water vapour at temperatures of ~150 K. The above temperatures may vary by as much as 10 K depending on the deposition rate [20,21]. Fig. 1 represents four O− ion yields spectra from various quantities of O (4, 1, 0.5 and 2 0.1 ML) deposited onto a 4 ML D O spacer layer 2 grown at 15 K. The O− yield function in Fig. 1a is very similar to that observed in pure O [6,7], 2 where the 6–10 eV structures have been associated with DEA via the gas-phase-allowed 2P , O− 1 u 2 resonance and gas-phase-forbidden 2S+(I ) resog
Fig. 1. O− ESD yields from a 4 ML D O spacer layer con2 densed at 15 K on Pt and covered with (a) 4 ML, (b) 1 ML, (c) 0.5 ML and (d) 0.1 ML of O . In curves (b) to (d) the 2 vertical intensity has been increased with respect to that of curve (a) by the factor appearing on the left.
L673
nances; the 13 eV peak has been ascribed to dissociation of a transient 2S+ anion state with undertermined g/u assignment. Moreover, it has been shown that kinetic energy ( KE ) distributions of the desorbing O− ions are very broad, extending to 3 eV and 5 eV in the energy range of the 8 eV and 13 eV peaks in the O− yield functions respectively [6,7]. The signal above 16 eV has been attributed to dipolar dissociation (DD, i.e. e−+O O++O−+e−) [6,7]. Comparing the 2 curves in Fig. 1, we find that: 1. the O− signal is reduced with decreasing O 2 coverage, with the relative intensities of the 8 and 13 eV peaks in Fig. 1b–d reversed from that in Fig. 1a and hence from that of the pure O films; 2 2. in going from Fig. 1b to Fig. 1d, the O− DEA signal decreases much faster than the decrease in amount of condensed O ; 2 3. for the 0.1 ML O –4 ML D O two-component 2 2 system, the DEA signal is very strongly suppressed; 4. the DD signal is roughly proportional to the quantity of condensed O . 2 Fig. 2 shows O− yields from 1 ML of O depos2 ited onto D O spacer layers of 4 ML, 40 ML and 2 100 ML thicknesses condensed at 15 K on the Pt foil. We see that increasing D O spacer thickness 2 results in a strong decrease and even the suppression of the O− signal at all energies. This observation can be associated with the porous amorphous structure of the D O spacer condensed at 15 K. In 2 fact, if the O molecules penetrate into the spacer, 2 then, in order to desorb, O− ions must suffer collisions which reduce their KE (for a review of anion scattering within thin molecular solid films see Ref. [22]). For the remaining KE lower than the polarisation energy induced at the film–vacuum interface, O− ions do not desorb. We can therefore reasonably assume that, due to these collisions, the deeper the O molecules penetrate into the 2 pores, the lower is the KE of O− ions and, consequently, the O− signal is lowered, as seen in Fig. 2. The O− yields functions shown in Fig. 1 can be understood as follows. When 0.1 ML of O is 2 deposited onto the porous amorphous 4 ML D O 2 spacer film, O molecules diffuse into the pores to 2 the Pt substrate. In addition to O− ion collisions
SU RF A LE CE TT S ER CIE S N CE
L674
R. Azria et al. / Surface Science 436 (1999) L671–L676
O –4 ML D O film the pores are completely filled, 2 2 so that a substantial amount of O molecules lies 2 on the surface of the film giving rise to an O− yield function similar to that obtained from pure O [7]. 2 This interpretation is supported by the yield functions in Fig. 3, which shows the O− yields from a 4 ML O film and also from 1 ML and 2 0.1 ML of O condensed on 4 ML D O films 2 2 grown at temperatures of 150 K, 15 K, 20 K, and 128 K. When D O is condensed at 128 K or 150 K 2 the O− 1 resonances are clearly visible; at these 2 temperatures the spacer layer is in a non-porous amorphous and crystalline geometry respectively,
Fig. 2. O− ESD yields from 1 ML of O condensed on D O 2 2 spacer layers of increasing thicknesses (4 ML, 40 ML and 100 ML) condensed at 15 K on a Pt substrate. The vertical gain is normalized to the bottom curve.
and a lesser contribution from electron energy losses prior to dissociation, two other effects contribute to suppression of the DEA O− desorbing signal, namely quenching of O− 1 resonances by 2 the metal [23] and an increase of polarization energy due to the stronger image force potential in the Pt substrate [23]. As the amount of deposited O increases (Fig. 1b and c) so does the O− 2 signal, but since these ions suffer collisions as they diffuse through the D O spacer, O− ions with high 2 KE (i.e. formed around 13 eV electron incident energy) [7] have a higher probability to emerge into vacuum. This explains the reversed relative intensities of the 8 eV and 13 eV peaks. Since the DD signal is not quenched at the metal surface and, in this case, O− does not lose KE from polarization, as shown by Sambe et al. [23], its intensity is more directly related to the number of condensed O molecules. Finally, in the 4 ML 2
Fig. 3. O− ESD yields from 4 ML O (a), from 1 ML O con2 2 densed on a 4 ML D O spacer layer condensed at 150 K (b) 2 and 15 K (c) on Pt and from 0.1 ML O condensed on a 4 ML 2 D O spacer condensed at 128 K (d ) and 20 K (e) on Pt. The 2 magnitude of the 6–10 eV integrated O− ESD signal from 0.3 ML O on 8 ML D O is plotted in the inset as a function 2 2 of the temperature of condensation of the D O spacer. 2
CE N IE SC RS CE E A TT RF LE SU
R. Azria et al. / Surface Science 436 (1999) L671–L676
so that O molecules are unable to diffuse through 2 the D O layer and remain on the D O surface. In 2 2 this case, O− emission arises from the surface, causing the O− yield function of the 1 ML O –4 ML H O target to appear similar to that 2 2 obtained with pure multilayer O . The temperature 2 dependence of the O− DEA signal integrated between 6–10 eV is shown in the inset of Fig. 3 for 0.3 ML O condensed on 8 ML D O. This 2 2 indicates that O diffusion through the spacer layer 2 is increasingly inhibited as the temperature of condensation of the D O spacer rises from 80 K 2 to 120 K. The results in Fig. 3 are interpreted to arise essentially from a reduction of the number and/or size of the pore in the D O film, with 2 increasing temperature, which restricts O diffu2 sion. They are consistent with the DEA D− yields, from pure D O films heated from 20 to 140 K 2 reported by Simpson et al. [21], who found an increase in the D− signal from 80 K up to 130 K due to collapsing of pores in amorphous ice layers deposited on Pt. We further note that, for yields measured under similar conditions (e.g. Fig. 1d versus Fig. 3e), the signal arising from the more porous sample due to the angle of the D O beam 2 is lower by a factor of ~2. Hence, there appears to be a good correlation between the magnitude of the D− signal and the porosity determined from the D O adsorption angle [5]. 2 Finally, we have invoked quenching of the O− 1 resonant states and a stronger image charge 2 effect at the Pt substrate in the case of a submonolayer 0.1 ML O –4ML porous D O film, to 2 2 account for O− suppression below 15 eV incident electron energy (Fig. 1d). These effects are evidenced in Fig. 4, which represents O− yield functions from 0.5 ML O –8 ML D O–Pt in Fig. 4a 2 2 and from 0.5 ML O –8 ML D O–4 ML crystalline 2 2 benzene in Fig. 4b. In both experiments the D O 2 spacers are condensed at 20 K. We see that in Fig. 4a the O diffusion to the Pt substrate leads 2 to quenching of the DEA and the subsequent reduction in O− desorption, whereas in Fig. 4b the influence of the Pt substrate is screened by the crystalline benzene spacer so that the O− 1 reso2 nances are clearly visible in the O− yields. Again, the observed relative intensity of the two peaks around 8 eV and 13 eV is related to collisions and differences in KE of desorbing O− ions.
L675
Fig. 4. O− ESD yield from 0.5 ML O , condensed on a 8 ML 2 H O film condensed at 20 K on Pt (a) and from 0.5 ML O 2 2 deposited on 8 ML H O condensed at 20 K on 4 ML of crystal2 line benzene.
In conclusion, we have shown that O− ESD from O condensed on the surface of an ice film 2 depends strongly on its morphology. Similar observations have recently been made for other twocomponent systems, consisting of N and Ar 2 adsorbed on thin n-hexane spacer films, condensed at 20–70 K on Pt (111) (see Ref. [24] and results to be published). For these systems, it was the magnitude of the metastable N and Ar desorption 2 yields that strongly depended on the morphology of the spacer.
Acknowledgements We thank Drs. D. Teillet-Billy, M. Sizun, M.A. Huels and A.D. Bass for helpful discussions. This
SU RF A LE CE TT S ER CIE S N CE
L676
R. Azria et al. / Surface Science 436 (1999) L671–L676
research is financed by the CNRS and the Canadian MRC.
References [1] M.A. Huels, L. Parenteau, L. Sanche, J. Chem. Phys. 100 (1994) 3940. [2] K. Jacobi, M. Bertolo, P. Geng, W. Hansen, C. Astaldin, Chem. Phys. Lett. 173 (1990) 97. [3] M. Lachgar, Y. Le Coat, R. Azria, M. Tronc, E. Illenberger, in preparation. [4] W.C. Simpson, M.T. Sieger, T.M. Orlando, L. Parenteau, K. Nagesha, L. Sanche, J. Chem. Phys. 107 (1997) 8668. [5] K.P. Stevenson, G.A. Kimmel, Z. Dohna´lek, R. Scott Smith, B.D. Kay, Science 283 (1999) 1505. [6 ] R. Azria, Y. Le Coat, J.P. Ziesel, J.P. Guillotin, B. Mharzi, M. Tronc, Chem. Phys. Lett. 220 (1994) 417. [7] M.A. Huels, L. Parenteau, M. Michaud, L. Sanche, Phys. Rev. A 51 (1995) 337. [8] C. von Sonntag, The Chemical Basis for Radiation Biology, Taylor and Francis, London, 1987. [9] M.T. Sieger, W.C. Simpson, T.M. Orlando, Nature 394 (1998) 554.
[10] [11] [12] [13]
E. Herbst, Annu. Rev. Phys. Chem. 46 (1995) 27. D.C. Clary, Science 271 (1996) 1509. R.E. Johnson, Rev. Mod. Phys. 68 (1996) 305. M. Tronc, R. Azria, Y. Le Coat, E. Illenberger, J. Phys. Chem. 100 (1996) 14 745. [14] R.S. Smith, G. Huang, E.K.L. Weng, B.D. Kay, Surf. Sci. 367 (1996) L13. [15] B.S. Berland, D.E. Brown, M.A. Tolbert, S.M. George, Geophys. Res. Lett. 22 (1995) 3493. [16 ] M.A. Zonlo, T.B. Onasch, M.S. Warshawsky, M.A. Tolbert, G. Mallick, P. Arentz, M.A. Robinson, J. Phys. Chem. 101 (1997) 10 887. [17] J.P. Devlin, V. Buch, J. Phys. Chem. 101 (1995) 16 534. [18] B. Rowland, J.P. Devlin, J. Chem. Phys. 94 (1991) 812. [19] G.P. Johari, A. Hallbrucken, E. Mayer, J. Chem. Phys. 92 (1990) 6742. [20] R.J. Speedy, P.G. Debenedetti, R. Smith, C. Huang, B.D. Kay, J. Chem. Phys. 105 (1996) 240. [21] W.C. Simpson, L. Parenteau, R.S. Smith, L. Sanche, T.M. Orlando, Surf. Sci. 390 (1997) 86. [22] M. Akbulut, N.J. Sack, T.E. Madey, Surf. Sci. Rep. 28 (1997) 177. [23] H. Sambe, D.E. Ramaker, L. Parenteau, L. Sanche, Phys. Rev. Lett. 59 (1987) 236. [24] E. Vichnevetski, P. Cloutier, L. Sanche, J. Chem. Phys. 110 (1999) 1112.