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EELS of methane physisorbed on NaCl(100)
241
Surface Science 175 (1986) 241~24S North-Holland, Amsterdam
EELS OF METHANE
PHYSISURBER
ON NaClf100)
J.P. HARDY
P.A. COX inorganic Chemistry Laborafoty,
South Parks Road Oxford OXI 3QR, fJiY
G.E. EWING Departmenr of Chemistry, Indiana University, Bloomington> IN 47405, USA
and
Received
17 January
1986; accepted
for publication
28 April 1986
EELS spectra of CH,, CD, and CH,Q physisorbed on NaCl(100) at 40 K are presented. For the clean NaCl surface, increasing the incident beam energy to the 30 to SO eV range is sufficient to overcome char$ing effects. However, when adsorbate is present charging prevents extended signal averaging. All methane modes appear to contribute to the EELS spectra. Loss peaks corresponding to adsorbate vibrations are very broad (400 to 600 cm-‘, FWHH), the low resolution being probably due to a ~mbination of effects including surface disorder, scattering of electrons by g& phase molecules and charging effects.
1. Introduction EELS has been long established as a technique for the study of adsorbates on metals &2] and on semiconductors and metallic oxides [3-51. Problems associated with surface charging effects have prevented a similar detailed analysis of insulators. Thiry and coworkers have very recently reported HREELS of MeJo and other insulators. They use an auxiliary electron gun to counteract the effects of charging 161. However, heating effects associated with the presence af the second, unfocussed and relatively high energy electron beam preclude the study of physisorbed molecules on these surfaces [7]. In this account we report on the potential of conventional EELS as a method for ~39-6028/86/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V,
242
J. P. Hardy et al. / EELS
obtaining information surface, NaCl(100).
about
of methane physisorbed on NaCI(I 00)
the vibrations
of an adsorbate
on an insulating
2. Experimental Samples of NaCl (Hilger Analytical Ltd.) were cleaved in air to produce (100) surfaces. A crystal of size 2 X 10 X 20 mm was mounted under a copper flange onto a specially constructed helium cooled sample holder (Thor Cryogenics). The crystal and sample holder were placed in an ESCALAB 5 (VG Scientific, UK) UHV facility and baked out for 15 h at 15O“C. After bake out the pressure was about 2 X 10-i’ mbar. No traces of oxygen or carbon could be found in the XPS O(ls) region, or the C Auger emission near 250 eV either with the crystal at room temperature, or cooled to 40 K. The most likely impurities to occur under these conditions are CO and H,O. At 40 K the residence time for CO can be estimated [S] from the heat of adsorption which we have measured to be 14 + 1 kJ mall’ [9] and by taking the surface to molecule vibration frequency to be about 100 cm-‘. It turns out to be days while that for H,O will be even longer. However even if the pressure of these impurities rises from the lo-” mbar range to 1 x 10e9 mbar then these species would take an hour to form a monolayer. During the EELS experiments the apparatus was evacuated continually while the methane was bled into the system and after each experiment the apparatus was pumped down again to the 10-i” mbar range so these impurities should not contaminate the crystal surface during an experimental run. We carried out an experiment to see whether we would detect CO in our EELS experiments on methane if it had been present and formed a monolayer. From our previous studies we know that at 54 K and a pressure of 1 X lo-* mbar CO gives about 50% coverage on NaCl(100) and a full monolayer at a pressure of 1 x lo-’ mbar [9]. At 54 K we calculate the residence time for CO to be about t min and the time to form half a monolayer about 2 min. These results are compatible with the isotherm results taken under dynamic equilibrium conditions. At 54 K the vapour pressure of solid CO is much too high for this to be formed with the chamber pressure at 1 x 10F8, or 1 X lo-’ mbar [lo]. With CO at a pressure of 1 X lo-’ mbar adsorbed on the NaCl(100) surface at 50 K we observed a broad EELS loss peak centred at 2130 cm-’ of width 150 cm-‘. This frequency agrees with the IR measurements for CO adsorbed on NaCl films [11,12]. When the apparatus was pumped down below 1 x 1O-9 mbar this EELS loss peak disappeared. This is consistent with the calculated residence time for CO on NaCl(100) at 50 K of 2 min. This CO peak was not observed in the EELS spectrum of CH, on NaCl at 40 K so we consider that contamination by CO was not a serious problem.
J.P. Hardy et al. / EELS
of methanephysisorbed on NaCl(IOO)
243
In order to obtain these EELS spectra the face of the NaCl crystal was orientated to optimize detection of electrons scattered in the specular direction. The helium cooled crystal mount does not allow rotation under vacuum so we could not follow electron loss intensity as a function of crystal angle. A He-Ne laser was used to match the crystal angle to that of a conventionally mounted MgO crystal which had been optimized for specular scattering by following electron loss peak intensity. After bake out optimization of the EELS signal from the NaCl crystal was achieved using the x, y and z adjustment available on the helium cooled probe mount. Spectra were recorded with 30 to 50 eV beam energies which are sufficient to eliminate charging effects on clean NaCl surfaces. However the presence of the adsorbate on the surface caused serious charging effects. Data could be collected only over about 5 to IO min before total loss of EELS signals occurred despite using relatively high beam energies of 30 to 50 eV. In consequence the loss spectra are very noisy, generally being accumulated only over 15 to 20 scans. Loss of EELS signal level and spectrometer resolution may have occurred also due to the presence of the methane at pressures between 1 X lO--’ and I X low7 mbar.
3. Results Fig. 1 shows the EELS spectrum of NaCl(1~) at 40 K with a background pressure of 1.5 X 10-‘” mbar. Phonon bands are clearly resolved, the loss peaks corresponding to a surface optical phonon frequency of 237 cm- ’ (29.4 meV). The room temperature spectrum for the same background pressure is very similar, except that a peak corresponding to a gain of 29.4 meV appears on the left hand side of the elastic peak. Strong bands due to surface optical phonons and corresponding to excitation of Fuchs-Kliewer modes are observed in EELS spectra of all dielectric materials. The energy of the NaCl phonon is close to that predicted by dielectric theory [13]. For adsorbed CH, neither the EELS nor the Auger C(ls) emission signals were sufficiently intense to be used to determine adsorption isotherms accurately. However the heats of adsorption of CH, on NaCI(100) have been measured as a function of coverage [14] and these data taken in conjunction with our results for the adsorption of CO and NaCl(IOO) [9] enable us to estimate the coverage by CH, for these EELS experiments. The heat of adsorption of CH, on NaCl(lOO) varies from 11.7 kJ mol-’ for low coverage to 9.6 kJ mol-’ for high coverage and about 10.5 kJ mol-’ for half coverage. Using these heats of adsorption we calculate the residence time at 40 K to be 10 min for the most firmly bound molecules at low coverage, quarter of a minute at half coverage and 1 s at high coverage. At 1 x 10 - * mbar we calculate that it takes 3 min to form a monolayer if 1 X lot5 sites are
244
f. P. Hardy et al. / EELS
0
of methane physisorbed on ~a~I(i~i
1000
2000
3000
LOSS ENERGY (cm-i) Fig. 1. EELS spectrum of NaCl(100) at 40 K, background present, a loss peak appears at the position
pressure 1.5 x lo-” mbar. When CO is marked in the spectrum.
available for the CH, and a shorter time if each CH, molecule occupies more than one site. At a pressure of 1 x lo-’ mbar this time will be reduced to about 0.3 mm. These calculations are only approximate, but they show that with the crystal at 40 K there will be an equilibrium established in a few minutes with about half coverage for a pressure of 1 X 10e8 mbar and about full monolayer coverage for a pressure of 1 x lo-’ mbar. Solid CH, will not be formed as its vapour pressure is 1 x 10e5 mbar at 40 K [lo]. We have measured adsorption isotherms for CO and found that an equilibrium is set up with about 50% coverage for a crystal temperature of 54 K and a pressure of CO of 1 X 1O-8 mbar. We measured the heat of adsorption at half coverage to be 14 * 1 kJ mol-‘. Now the rate of adsorption for CH, is nearly the same as that for CO at the same pressure assuming that both have a sticking probability of unity and the residence times are largely governed by /RT). For CH, with AH for half coverage of 10.5 the factor exp( A Hdesorption kJ mol-’ at 40 K, the factor AH/T is the same as for CO at 54 K. This comparison supports the conclusion which we reached above, namely that at 40 K equilibrium will be set up in a matter of minutes with about half
J.P. Hardy et al. / EELS of methane physisorbed on NaCl{loO)
t
I
I
t
1
0
1000
2000
3000
4000
LOSS ENERGY
245
(cm-')
Fig. 2. EELS spectrum of CH, physisorbed on NaCl(100). Gas phase vibrational positions are shown [Ml. CH, pressure 1 X 10W8mbar, temperature 40 K.
monolayer coverage for a pressure of CH, of 1 x lo-’ mbar and full coverage for a pressure of 1 x 10W7mbar. It is possible that there might be multilayer coverage at the higher pressure but a layer of solid CH, will not form at 40 K and a pressure of 1 x lo-’ mbar. The EELS spectrum obtained with CH, at a pressure of 1 X lo-* mbar adsorbed on NaCl(1~) at 40 K is shown in fig. 2. There are two broad peaks which are about 500 cm- * wide whereas the elastic peak is about 160 cm-’ wide. This suggests that the first peak centred at approximately 1540 cm-’ contains two unresolved bands from the IR active band centred at 1306 cm-t and the Raman active band centred at 1534 cm-’ [15], while the second loss peak at 2970 cm-’ contains the IR active band centred at 3019 cm-’ and the Raman active band centred at 2917 cm-‘. The positions of the IR and Raman bands are indicated on the figure. On changing to CD, the lower frequency EELS peak would be expected to shift to about 1000 cm-’ corresponding to the IR active band at 996 cm- * and the Raman band at 1092 cm-‘. The higher frequency peak would be expected to shift to about 2200 cm-’ and contain the 2259 cm-’ IR active band and
246
J. P. Hardy et al. / EELS of mezhane physisor~d
L
on Na&&I#)
I
I
I
I
0
1000
2000
3000
LOSS ENERGY km-‘1 Fig. 3. EELS spectrum of CD, physisorbed on NaCl(l~). Gas phase vibrational positions are shown [15]. CD, pressure 1 X 10.. ’ mbar, temperature 40 K.
the 2109 cm-’ Raman active band. In fact this is what occurs as is shown in It is particularly fig. 3 where a pressure of 1 X 10m7 mbar is employed. noticeable that the stronger band for the EELS spectrum of CD, occurs at a position of minimum signal for the spectrum of CH,. The IR and Raman spectrum of CH,D, is more complicated than those of CH,, or CD, as it contains nine fundamentals instead of four. The intensity of the EELS spectra decrease at higher loss frequencies and so the stronger EELS bands are expected to occur in the region of 1000 cm-’ rather than 3000 cm-‘. Both the CD4 and the CH,D, spectra were obtained with pressures of 1 x lo-? mbar, charging problems were more severe than in the case of the mbar, and the resolution is poorer. spectrum for CH, run at 1 x lo-* However the CH,D, spectrum presented in fig. 4 shows a band at about 3000 cm-’ which is absent in that of CD,. This is to be expected as bands at about 3000 cm-’ are present in the spectrum of CH2D, but not CD, [15]. The main band in the EELS spectrum of CH,D, is broader than that in CD,, extending to higher frequencies. This suggests that the EELS spectrum of CH,D,
J. P. Hardy ef al. / EELS oj methane physisorbed on NaCI(I 00)
I
I
0
I
I
1000 2000 LOSS ENERGY (cm-‘)
247
t
3000
Fig. 4. EELS spectrum of CD2H, physisorhed on NaCl(100). Gas phase vibrationai positions are shown [15]. CD,H, pressure 1 x IO-’ mbar, temperature 40 K.
contains not only the strong IR bands centred at 1033 and 1090 cm-‘, also the weaker bands centred at 1234,1333 and 1436 cm-‘.
but
4. Discussion and conclusion Despite the low resolution of these EELS spectra several conclusions can be reached. Clearly it is possible to obtain EELS spectra for molecules adsorbed on dielectric materials despite charging problems. With a resolution of only to work with small molecules containing well about 150 cm-’ it is important separated IR and Raman bands in order to be able to distinguish which frequencies contribute to the EELS spectra. This point is brought out by a comparison between our work and that of Jennings et al. [16] who obtained EELS spectra from a conducting polymer. In this case broad bands were obtained corresponding to the excitation of a great many vibrational modes. With such a complicated spectrum it is very hard to know just which frequencies are responsible for the observed EELS spectrum. The evidence which we have given suggests that the selection rules for the
248
J.P. Hardy et al. / EELS
of methane physisorbed
on NaCl(lOO)
EELS spectra of molecules adsorbed on dielectric surfaces are different from those for a molecule weakly adsorbed on a metallic surface. In the latter case the strongest EELS bands are for those IR active modes in the gas phase which generate a dipole moment perpendicular to the surface [l]. The selection rule for metallic surfaces that dipole moment changes in the plane of the surface do not contribute to the dipole scattering will not apply to an insulating surface. We do not know the true line width of the EELS bands but the IR bands for CH, adsorbed on glass [ 171 and on NaCl powder [lS] give band widths of 10 to 30 cm-’ and so if the resolution of the EELS spectra can be improved it should be possible to see whether both the IR band at 1306 cm-’ and the Raman band at 1534 cm-’ are excited in the EELS spectrum when CH, is physisorbed on NaCl. In a future experiment it may be possible to improve the resolution of the EELS experiment by first adsorbing CH, at 40 K and then reducing the temperature to 34 K while also evacuating the chamber. This would not be an easy experiment to perform as even at 34 K the more weakly adsorbed molecules would be expected to desorb in 3 min. However it could be attempted. It would also be interesting to compare EELS spectra for CH, physisorbed onto a metal and onto a dielectric surface. References [I] H. Ibach and D.L. Mills, Etectron Energy Loss Spectroscopy and Surface Vibrations (Academic Press, New York, 1982). S. Lehwald and H. Ibach, Surface Sci. 89 (1979) 425. H. Ibach, Phys. Rev. Letters 24 (1970) 1416. P.A. Cox, R.G. Egdell, P.D. Naylor, .I. Electron Spectrosc. Related Phenomena 29 (1983) 247. D.G. Aitken, P.A. Cox, R.G. EgdeII, M.D. Hill and I. Sach, Vacuum 33 (1983) 753. P.A. Thiry, M. Liehr, J.J. Pireaux and R. Caudano, Phys. Rev. B29 (1984) 4824. P.A. Thiry, Vibrations at Surfaces IV, private communication. G.A. Somorjai, Chemistry in Two Dimensions: Surfaces (Corneli University Press, Ithaca, NY, 1981). [9] J.P. Hardy. G.E. Ewing, R. Stables and C.J.S.M. Simpson, Surface Sci. 159 (1985) L474. [lo] R.E. Honig and H.O. Hook, RCA Rev. 21 (1960) 360. [ll] R. Gevirzman, Y. Kozirovski and M. Folman, Trans. Faraday Sot. 65 (1969) 2206. 1121 J. Heidberg and I. Hussia, in: Proc. Intern. Topical Conference on Vibrations at Surfaces (Plenum Press, New York, 1982) p. 323. (131 P.A. Cox, W.R. FlaveII and A.A. Williams, Vibrations at Surfaces IV, Conf. Proc., to be published. [14] S. Ross and H. Clark, J. Am. Chem. Sot. 76 (1954) 4291. 1151 Tables of Molecular Vibrational Frequencies, Consolidated Vol. 1 (National Bureau of Standards. Washington, DC, 1972). (16) W.D. Jennings, G.S. Chottiner, C. Natarajan, A.V. Melo, R.W. Hoffman, W.E. O’Grady, I. Lundstrom and W.R. Salaneck, Appt. Surface Sci. 21 (1985) 80. [17] N. Sheppard and D.J.C. Yates, Proc. Roy. Sot. (London) A238 (1956) 69. [18] G. Ewing, unpublished data. [2] [3] [4] [5] [6] [7] [S]
Surface Science 175 (1986) 241~24S North-Holland, Amsterdam
EELS OF METHANE
PHYSISURBER
ON NaClf100)
J.P. HARDY
P.A. COX inorganic Chemistry Laborafoty,
South Parks Road Oxford OXI 3QR, fJiY
G.E. EWING Departmenr of Chemistry, Indiana University, Bloomington> IN 47405, USA
and
Received
17 January
1986; accepted
for publication
28 April 1986
EELS spectra of CH,, CD, and CH,Q physisorbed on NaCl(100) at 40 K are presented. For the clean NaCl surface, increasing the incident beam energy to the 30 to SO eV range is sufficient to overcome char$ing effects. However, when adsorbate is present charging prevents extended signal averaging. All methane modes appear to contribute to the EELS spectra. Loss peaks corresponding to adsorbate vibrations are very broad (400 to 600 cm-‘, FWHH), the low resolution being probably due to a ~mbination of effects including surface disorder, scattering of electrons by g& phase molecules and charging effects.
1. Introduction EELS has been long established as a technique for the study of adsorbates on metals &2] and on semiconductors and metallic oxides [3-51. Problems associated with surface charging effects have prevented a similar detailed analysis of insulators. Thiry and coworkers have very recently reported HREELS of MeJo and other insulators. They use an auxiliary electron gun to counteract the effects of charging 161. However, heating effects associated with the presence af the second, unfocussed and relatively high energy electron beam preclude the study of physisorbed molecules on these surfaces [7]. In this account we report on the potential of conventional EELS as a method for ~39-6028/86/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V,
242
J. P. Hardy et al. / EELS
obtaining information surface, NaCl(100).
about
of methane physisorbed on NaCI(I 00)
the vibrations
of an adsorbate
on an insulating
2. Experimental Samples of NaCl (Hilger Analytical Ltd.) were cleaved in air to produce (100) surfaces. A crystal of size 2 X 10 X 20 mm was mounted under a copper flange onto a specially constructed helium cooled sample holder (Thor Cryogenics). The crystal and sample holder were placed in an ESCALAB 5 (VG Scientific, UK) UHV facility and baked out for 15 h at 15O“C. After bake out the pressure was about 2 X 10-i’ mbar. No traces of oxygen or carbon could be found in the XPS O(ls) region, or the C Auger emission near 250 eV either with the crystal at room temperature, or cooled to 40 K. The most likely impurities to occur under these conditions are CO and H,O. At 40 K the residence time for CO can be estimated [S] from the heat of adsorption which we have measured to be 14 + 1 kJ mall’ [9] and by taking the surface to molecule vibration frequency to be about 100 cm-‘. It turns out to be days while that for H,O will be even longer. However even if the pressure of these impurities rises from the lo-” mbar range to 1 x 10e9 mbar then these species would take an hour to form a monolayer. During the EELS experiments the apparatus was evacuated continually while the methane was bled into the system and after each experiment the apparatus was pumped down again to the 10-i” mbar range so these impurities should not contaminate the crystal surface during an experimental run. We carried out an experiment to see whether we would detect CO in our EELS experiments on methane if it had been present and formed a monolayer. From our previous studies we know that at 54 K and a pressure of 1 X lo-* mbar CO gives about 50% coverage on NaCl(100) and a full monolayer at a pressure of 1 x lo-’ mbar [9]. At 54 K we calculate the residence time for CO to be about t min and the time to form half a monolayer about 2 min. These results are compatible with the isotherm results taken under dynamic equilibrium conditions. At 54 K the vapour pressure of solid CO is much too high for this to be formed with the chamber pressure at 1 x 10F8, or 1 X lo-’ mbar [lo]. With CO at a pressure of 1 X lo-’ mbar adsorbed on the NaCl(100) surface at 50 K we observed a broad EELS loss peak centred at 2130 cm-’ of width 150 cm-‘. This frequency agrees with the IR measurements for CO adsorbed on NaCl films [11,12]. When the apparatus was pumped down below 1 x 1O-9 mbar this EELS loss peak disappeared. This is consistent with the calculated residence time for CO on NaCl(100) at 50 K of 2 min. This CO peak was not observed in the EELS spectrum of CH, on NaCl at 40 K so we consider that contamination by CO was not a serious problem.
J.P. Hardy et al. / EELS
of methanephysisorbed on NaCl(IOO)
243
In order to obtain these EELS spectra the face of the NaCl crystal was orientated to optimize detection of electrons scattered in the specular direction. The helium cooled crystal mount does not allow rotation under vacuum so we could not follow electron loss intensity as a function of crystal angle. A He-Ne laser was used to match the crystal angle to that of a conventionally mounted MgO crystal which had been optimized for specular scattering by following electron loss peak intensity. After bake out optimization of the EELS signal from the NaCl crystal was achieved using the x, y and z adjustment available on the helium cooled probe mount. Spectra were recorded with 30 to 50 eV beam energies which are sufficient to eliminate charging effects on clean NaCl surfaces. However the presence of the adsorbate on the surface caused serious charging effects. Data could be collected only over about 5 to IO min before total loss of EELS signals occurred despite using relatively high beam energies of 30 to 50 eV. In consequence the loss spectra are very noisy, generally being accumulated only over 15 to 20 scans. Loss of EELS signal level and spectrometer resolution may have occurred also due to the presence of the methane at pressures between 1 X lO--’ and I X low7 mbar.
3. Results Fig. 1 shows the EELS spectrum of NaCl(1~) at 40 K with a background pressure of 1.5 X 10-‘” mbar. Phonon bands are clearly resolved, the loss peaks corresponding to a surface optical phonon frequency of 237 cm- ’ (29.4 meV). The room temperature spectrum for the same background pressure is very similar, except that a peak corresponding to a gain of 29.4 meV appears on the left hand side of the elastic peak. Strong bands due to surface optical phonons and corresponding to excitation of Fuchs-Kliewer modes are observed in EELS spectra of all dielectric materials. The energy of the NaCl phonon is close to that predicted by dielectric theory [13]. For adsorbed CH, neither the EELS nor the Auger C(ls) emission signals were sufficiently intense to be used to determine adsorption isotherms accurately. However the heats of adsorption of CH, on NaCI(100) have been measured as a function of coverage [14] and these data taken in conjunction with our results for the adsorption of CO and NaCl(IOO) [9] enable us to estimate the coverage by CH, for these EELS experiments. The heat of adsorption of CH, on NaCl(lOO) varies from 11.7 kJ mol-’ for low coverage to 9.6 kJ mol-’ for high coverage and about 10.5 kJ mol-’ for half coverage. Using these heats of adsorption we calculate the residence time at 40 K to be 10 min for the most firmly bound molecules at low coverage, quarter of a minute at half coverage and 1 s at high coverage. At 1 x 10 - * mbar we calculate that it takes 3 min to form a monolayer if 1 X lot5 sites are
244
f. P. Hardy et al. / EELS
0
of methane physisorbed on ~a~I(i~i
1000
2000
3000
LOSS ENERGY (cm-i) Fig. 1. EELS spectrum of NaCl(100) at 40 K, background present, a loss peak appears at the position
pressure 1.5 x lo-” mbar. When CO is marked in the spectrum.
available for the CH, and a shorter time if each CH, molecule occupies more than one site. At a pressure of 1 x lo-’ mbar this time will be reduced to about 0.3 mm. These calculations are only approximate, but they show that with the crystal at 40 K there will be an equilibrium established in a few minutes with about half coverage for a pressure of 1 X 10e8 mbar and about full monolayer coverage for a pressure of 1 x lo-’ mbar. Solid CH, will not be formed as its vapour pressure is 1 x 10e5 mbar at 40 K [lo]. We have measured adsorption isotherms for CO and found that an equilibrium is set up with about 50% coverage for a crystal temperature of 54 K and a pressure of CO of 1 X 1O-8 mbar. We measured the heat of adsorption at half coverage to be 14 * 1 kJ mol-‘. Now the rate of adsorption for CH, is nearly the same as that for CO at the same pressure assuming that both have a sticking probability of unity and the residence times are largely governed by /RT). For CH, with AH for half coverage of 10.5 the factor exp( A Hdesorption kJ mol-’ at 40 K, the factor AH/T is the same as for CO at 54 K. This comparison supports the conclusion which we reached above, namely that at 40 K equilibrium will be set up in a matter of minutes with about half
J.P. Hardy et al. / EELS of methane physisorbed on NaCl{loO)
t
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I
t
1
0
1000
2000
3000
4000
LOSS ENERGY
245
(cm-')
Fig. 2. EELS spectrum of CH, physisorbed on NaCl(100). Gas phase vibrational positions are shown [Ml. CH, pressure 1 X 10W8mbar, temperature 40 K.
monolayer coverage for a pressure of CH, of 1 x lo-’ mbar and full coverage for a pressure of 1 x 10W7mbar. It is possible that there might be multilayer coverage at the higher pressure but a layer of solid CH, will not form at 40 K and a pressure of 1 x lo-’ mbar. The EELS spectrum obtained with CH, at a pressure of 1 X lo-* mbar adsorbed on NaCl(1~) at 40 K is shown in fig. 2. There are two broad peaks which are about 500 cm- * wide whereas the elastic peak is about 160 cm-’ wide. This suggests that the first peak centred at approximately 1540 cm-’ contains two unresolved bands from the IR active band centred at 1306 cm-t and the Raman active band centred at 1534 cm-’ [15], while the second loss peak at 2970 cm-’ contains the IR active band centred at 3019 cm-’ and the Raman active band centred at 2917 cm-‘. The positions of the IR and Raman bands are indicated on the figure. On changing to CD, the lower frequency EELS peak would be expected to shift to about 1000 cm-’ corresponding to the IR active band at 996 cm- * and the Raman band at 1092 cm-‘. The higher frequency peak would be expected to shift to about 2200 cm-’ and contain the 2259 cm-’ IR active band and
246
J. P. Hardy et al. / EELS of mezhane physisor~d
L
on Na&&I#)
I
I
I
I
0
1000
2000
3000
LOSS ENERGY km-‘1 Fig. 3. EELS spectrum of CD, physisorbed on NaCl(l~). Gas phase vibrational positions are shown [15]. CD, pressure 1 X 10.. ’ mbar, temperature 40 K.
the 2109 cm-’ Raman active band. In fact this is what occurs as is shown in It is particularly fig. 3 where a pressure of 1 X 10m7 mbar is employed. noticeable that the stronger band for the EELS spectrum of CD, occurs at a position of minimum signal for the spectrum of CH,. The IR and Raman spectrum of CH,D, is more complicated than those of CH,, or CD, as it contains nine fundamentals instead of four. The intensity of the EELS spectra decrease at higher loss frequencies and so the stronger EELS bands are expected to occur in the region of 1000 cm-’ rather than 3000 cm-‘. Both the CD4 and the CH,D, spectra were obtained with pressures of 1 x lo-? mbar, charging problems were more severe than in the case of the mbar, and the resolution is poorer. spectrum for CH, run at 1 x lo-* However the CH,D, spectrum presented in fig. 4 shows a band at about 3000 cm-’ which is absent in that of CD,. This is to be expected as bands at about 3000 cm-’ are present in the spectrum of CH2D, but not CD, [15]. The main band in the EELS spectrum of CH,D, is broader than that in CD,, extending to higher frequencies. This suggests that the EELS spectrum of CH,D,
J. P. Hardy ef al. / EELS oj methane physisorbed on NaCI(I 00)
I
I
0
I
I
1000 2000 LOSS ENERGY (cm-‘)
247
t
3000
Fig. 4. EELS spectrum of CD2H, physisorhed on NaCl(100). Gas phase vibrationai positions are shown [15]. CD,H, pressure 1 x IO-’ mbar, temperature 40 K.
contains not only the strong IR bands centred at 1033 and 1090 cm-‘, also the weaker bands centred at 1234,1333 and 1436 cm-‘.
but
4. Discussion and conclusion Despite the low resolution of these EELS spectra several conclusions can be reached. Clearly it is possible to obtain EELS spectra for molecules adsorbed on dielectric materials despite charging problems. With a resolution of only to work with small molecules containing well about 150 cm-’ it is important separated IR and Raman bands in order to be able to distinguish which frequencies contribute to the EELS spectra. This point is brought out by a comparison between our work and that of Jennings et al. [16] who obtained EELS spectra from a conducting polymer. In this case broad bands were obtained corresponding to the excitation of a great many vibrational modes. With such a complicated spectrum it is very hard to know just which frequencies are responsible for the observed EELS spectrum. The evidence which we have given suggests that the selection rules for the
248
J.P. Hardy et al. / EELS
of methane physisorbed
on NaCl(lOO)
EELS spectra of molecules adsorbed on dielectric surfaces are different from those for a molecule weakly adsorbed on a metallic surface. In the latter case the strongest EELS bands are for those IR active modes in the gas phase which generate a dipole moment perpendicular to the surface [l]. The selection rule for metallic surfaces that dipole moment changes in the plane of the surface do not contribute to the dipole scattering will not apply to an insulating surface. We do not know the true line width of the EELS bands but the IR bands for CH, adsorbed on glass [ 171 and on NaCl powder [lS] give band widths of 10 to 30 cm-’ and so if the resolution of the EELS spectra can be improved it should be possible to see whether both the IR band at 1306 cm-’ and the Raman band at 1534 cm-’ are excited in the EELS spectrum when CH, is physisorbed on NaCl. In a future experiment it may be possible to improve the resolution of the EELS experiment by first adsorbing CH, at 40 K and then reducing the temperature to 34 K while also evacuating the chamber. This would not be an easy experiment to perform as even at 34 K the more weakly adsorbed molecules would be expected to desorb in 3 min. However it could be attempted. It would also be interesting to compare EELS spectra for CH, physisorbed onto a metal and onto a dielectric surface. References [I] H. Ibach and D.L. Mills, Etectron Energy Loss Spectroscopy and Surface Vibrations (Academic Press, New York, 1982). S. Lehwald and H. Ibach, Surface Sci. 89 (1979) 425. H. Ibach, Phys. Rev. Letters 24 (1970) 1416. P.A. Cox, R.G. Egdell, P.D. Naylor, .I. Electron Spectrosc. Related Phenomena 29 (1983) 247. D.G. Aitken, P.A. Cox, R.G. EgdeII, M.D. Hill and I. Sach, Vacuum 33 (1983) 753. P.A. Thiry, M. Liehr, J.J. Pireaux and R. Caudano, Phys. Rev. B29 (1984) 4824. P.A. Thiry, Vibrations at Surfaces IV, private communication. G.A. Somorjai, Chemistry in Two Dimensions: Surfaces (Corneli University Press, Ithaca, NY, 1981). [9] J.P. Hardy. G.E. Ewing, R. Stables and C.J.S.M. Simpson, Surface Sci. 159 (1985) L474. [lo] R.E. Honig and H.O. Hook, RCA Rev. 21 (1960) 360. [ll] R. Gevirzman, Y. Kozirovski and M. Folman, Trans. Faraday Sot. 65 (1969) 2206. 1121 J. Heidberg and I. Hussia, in: Proc. Intern. Topical Conference on Vibrations at Surfaces (Plenum Press, New York, 1982) p. 323. (131 P.A. Cox, W.R. FlaveII and A.A. Williams, Vibrations at Surfaces IV, Conf. Proc., to be published. [14] S. Ross and H. Clark, J. Am. Chem. Sot. 76 (1954) 4291. 1151 Tables of Molecular Vibrational Frequencies, Consolidated Vol. 1 (National Bureau of Standards. Washington, DC, 1972). (16) W.D. Jennings, G.S. Chottiner, C. Natarajan, A.V. Melo, R.W. Hoffman, W.E. O’Grady, I. Lundstrom and W.R. Salaneck, Appt. Surface Sci. 21 (1985) 80. [17] N. Sheppard and D.J.C. Yates, Proc. Roy. Sot. (London) A238 (1956) 69. [18] G. Ewing, unpublished data. [2] [3] [4] [5] [6] [7] [S]