Vacuum/volume 41/numbers 1-3/pages 234 to 236/1990 0042-207X/9053.00 + .00 Printed in Great Britain © 1990 Pergamon Press plc

Surface resonances in a CH4 monolayer physisorbed on a silver (111) surface M S a k u r a i , National Institute for Fusion Science, Furo-cho, Chikusa, Nagoya 464-01, Japan

T O k a n o , Institute of Industrial Science, University of Tokyo, RoppongL Minato, Tokyo 106, Japan

and

Y T u z i , UL VAC Corporation, Chigasaki 353, Japan

Surface resonances in the scattering of low energy electrons with a physisorbed monolayer on a Ag(111) surface were studied with a high resolution electron energy loss spectrometer. The energy dependence of the intensity of elastic scattering ( I -V curve) was measured on clean Ag(1 11), CH4-Ag (11 1), Xe-Ag( l 1 1) and CH4-Xe-Ag(11 1) at 40 K. The I -V curve from a CH4-Ag surface showed two dips at 3.7 and 4.0 eV which correspond to the threshold energy of surface wave excitation. The I -V curves for the inelastic scattering were measured via an energy loss process by the excitation of vibrational modes of CH4. As for the dipole active v,~ mode, resonant features similar to those for the elastic peak, were observed while resonance effects against the dipole inactive v2 mode were poor.

1. Introduction

Surface states and resonances observed in the incident energy dependence of diffraction intensity have been investigated on several crystal surfaces using high-resolution electron energy loss spectroscopy (EELS)1 5 low-energy electron diffraction (LEED) 6'7 and reflection high-energy electron diffraction (RHEED) 8. In the EELS measurements, the intensity variation of the (00) beam at surface resonances is more easily understood than with other methods because the number of diffraction spots which are allowed with the Bragg condition is limited at the energy region for the EELS measurement (5 eV), and fine structures in the I-V curves can be distinguished due to their energy resolution. From the observed minima in the incident energy dependence of (00) intensity, the dispersion relation of surface states/resonances has been obtained on clean and hydro- gen covered W(100) 2, Pd(100) 5 and Pd(111) 6 surfaces.

For the inelastic process, it has been reported that the dipole selection rule in the vibrational excitation is no longer valid at the surface resonance conditions, and the enhancement of vibra- tional excitation through impact scattering occurs 6.

In the physisorbed phase of rare gases, N2, CH4, H 2 and 0 2, two-dimensionally condensed phases with ordered structures appear as the first step of the layer growth. In case of a CH 4 monolayer adsorbed on Ag(111), hcp structure has been ob- served with LEED 9. Since, at surface resonance conditions, the electrons are trapped between the outermost layer and vacuum, the surface resonance effect inherent to the physisorbed mono- layer is expected. However, evidence of a surface resonance corresponding to the structure of an adsorbed layer has rarely been reported.

In the present paper, we report surface resonance effects observed in the I-V curves of elastic and vibrational excitation

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processes on CH 4 monolayers and CH4-Xe bilayers physisorbed on a Ag(111) surface.

2. Experimental

The experimental apparatus has been previously described ~°. The monochromator and analyser of the electron spectrometer were composed of a pair of hemispherical deflectors and electros- tatic lenses. The Ag(l I 1) surface was cleaned by several cycles of Xe ÷ bombardment and annealing. A monolayer of CH 4 was condensed on the substrate at 40 K under the admitted CH 4 pressure of I x 10-~-1 x 10 -5 Pa. Experiments were performed at this condition where the CH4 monolayer is in thermodynamic equilibrium with the gas phase. In the experiment on Xe mono- layers, the substrate temperature was raised to 60-70 K at the Xe pressure 1 × 10 -6 Pa to maintain the equilibrium condition between the gas phase and the adsorbed phase. After a mono- layer of Xe was completed, as monitored with the variation of the elastic intensity in EELS spectrum, the admittance of Xe was stopped and the substrate was cooled to 40 K. CH 4 was also adsorbed on Xe-Ag( l l l ) surface with the same condition as CH4-Ag(111) system. I-V curves for elastic and inelastic scatter- ing processes were measured for both CH4-Ag( l l l ) and CHa-Xe-Ag( l l l ) systems in the incident electron energy range of 2-12 eV with 10 meV steps. The energy resolution of the spectrometer was about 20 meV.

3. Results and discussion

Figure 1 shows the I-V curve of elastic scattering from clean and CH 4 covered Ag(111). The scattering plane is nearly parallel to (10) direction of the surface reciprocal lattice. The incident angle and the detection angle are 45 ° (specular reflection condition).

M Sakurai et al: Surface resonances in CH4-Ag(111 )

. e l

. e l

113

I-.-4

CH4/Ag (JJJ)

clean Ag (ttt1

' 6 " ' - 0 ' • 0 4 .0 .0 8 . 0 I .0 t2 .0

Incident electron energy (eV)

Figure 1. Incident electron energy dependence of elastic intensity under specular reflection condition for clean Ag(111) and CH~, covered Ag(I 11) surfaces.

The incident electron energy was scanned keeping the energy resolution of spectrometer constant• The spectrum from the clean surface shows some minima due to surface state/resonances near the (01) beam emergence threshold (9eV). As the variation in the transmission of spectrometer with the electron energy is proved to be monotonic, the structure observed reflects a resonance feature in the electron scattering. These minima correspond to image potential states of various binding energies. An additional minimum at 4.3 eV is ascribed to an intrinsic surface state of Ag( l l l ) 5. These two types of surface states have been investi- gated with both EELS and inverse photoemission techniques I 1.

For the CH 4 monolayer, new minima at 3.7 and 4.0 eV were observed as well as a broad minimum at 8 eV due to resonance from substrate. The surface state at 4.3 eV and the resonance near the (01) beam threshold disappeared• From the LEED observa- tion of CH4-Ag( l l l ) 9, CH4 molecules form a close-packed lattice with a nearest neighbour distance of 4.2 /~,, and are orientationally epitaxial with the substrate lattice. Then the (01) beam threshold for CH4 lattice is calculated to 3.9 eV which probably corresponds to either of the two minima. The doublet structure suggests the presence of two different phases in CH 4 monolayer on Ag( l l l ) which could not be resolved in the previous LEED observations• It has been reported that two rotationally different phases were observed for the CH4-graphite system, and two phases coexist in a temperature range of 51-60

K 12. If the coexistence of rotationally commensurate x/~ phase

and x/~-R30 ° phase is assumed, resonance energies for each phase are calculated at 3.9 and 4.3 eV through kinematical diffraction conditions, taking into account the mismatch ( ~ 8 °) of the scattering plane against ( I0 ) direction of Ag(111). Then the energy difference between the two dips is well explained by the two phase model, however for the absolute value, further

assumption of lattice expansion ( ~ 4 ~ ) is needed. The intensity ratio and the difference in the position of two minima varied with ambient pressure of CH4 in the range of 10 -8 Pa. At the CH4 pressure of 3 x 10 - s Pa, the dips were observed at 3.6 and 4.0 eV, and the ratio of peak height was 3, while at the pressure higher than 1 x 10 - 7 Pa, the peak height ratio was ~ 1.3 as

shown in Figure 1. This means that the x/~ phase mainly exists at lower pressure, a tendency which is along the lines of CH4-graphite system 12. However, the absence of the diffraction

spots for x//3-R30 ° phase in the LEED pattern conflicts with above discussion. It may be reconciled by the difference in either coherence length of electrons or diffraction conditions.

For Xe monolayer, the I-V curve for elastic scattering showed two minima in the range of 2-12 eV. One at 8 eV arises from the resonance at the substrate, while the other at 2.9 eV comes from the (01) beam threshold of the Xe lattice. Xe atoms form a close- packed structure on a Ag(111) surface with a lattice constant of 4.45 ,~;1 from which (01) beam threshold is calculated as 3.6 eV. The difference between the predicted threshold energy for mono- layer Xe lattice and the observed value may be explained by the variation of work function during Xe adsorption 14.

The I-V curves for CH 4 overlayer on Xe monolayer was measured in the energy range of 2.5-4.5 eV. Figure 2 shows the I-V curves for different CH 4 exposures• In accordance with the increase of CH 4 coverage, the minimum at 2.9 eV diminishes, while two minima at 3.3 and 3.6 eV grow. At 0.5 L exposure where the monolayer was completed, no resonance effect from underlying Xe monolayer was observed• This illustrates that only the ordered structure of the uppermost layer contributes to the surface resonance phenomena. As for the energy shift in the two minima between CH4-Ag( l l l ) and CH4-Xe-Ag(I l l ) systems, work function changes may also explain the results.

CH4/Xe/Ag (JJJ)

J \ Exposure (" \ of c. 4 CLJ

0.1 - e l ~ 0 . o

2.s 310 410 415 Incident electron energy (eV)

Figure 2. Incident electron energy dependence of elastic intensity for an overlayer of CH4 on Xe monolayer with various CH4 doses.

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M S a k u r a i e t a l : Surface resonances in CH4-Ag(111 )

V ibra t iona l loss spectra of CH4 on Ag(111) were observed at the incident electron energies under surface resonance and off- resonance conditions. For all the incident energies except for the resonance condition, dipole allowed 74 mode was excited most intensively, while weak excitation of )'2 mode (dipole inactive) and 73 mode (dipole active) were also observable. At 3.7 eV which is the surface resonance condition, 7,~ mode was weaker compared to 72 mode. Combinational mode of 72 and 73 mode was observable, and was pronounced at 3.7 eV.

Though the resonant excitation of dipole inactive mode or combinational mode via temporary negative-ion resonance state is known for some molecules in gas phase and condensed phase,O. ,5. ,6, the negative-ion resonance of gaseous CHa in the present energy range has not been reported. Furthermore, the resonance width in the incident energy dependence of the com- binational excitation was much narrower than that of negative- ion resonance (shape resonance) reported to date. Then, the enhancement is considered to come from surface resonance.

Incident energy dependence of vibrational loss intensities at 162 meV (74 mode) and 190 meV (72 mode) is shown in Figure 3 together with the intensity of elastic scattering. Variation of the intensity of 74 mode is similar to the variation of elastic intensity, while the intensity of Y2 mode varied to lesser extent. This difference in the energy dependence agrees with the results on H-W(100) system4; the cross-section of a symmetric stretch mode which is dipole active is suppressed, while impact modes (v~y and vw~8), which are forbidden from selection rules of impact scattering, are observed. As discussed by Sunjic ~ ~, the mechanism for vibrational excitation under surface resonance conditions consists of three elementary process: diffraction into the surface state, following inelastic scattering at the surface, and diffraction from the surface into vacuum. Though the mechanism works for either of two inelastic processes: dipole and impact scattering, impact losses are more pronounced compared to dipole losses since the short range interaction of impact scattering is much more benefited by the localization of electrons at the surface. The enhancement in the loss intensity of the combinational mode observed at the resonance condition is explained by this mechan- ism. For the suppression of dipole losses, the intensity of the electrons which have experienced dipole scattering will decrease at the resonance condition in the same manner as the elastic intensity, since the dipole scattering is concentrated to forward direction.

In conclusion, the I-V curve obtained with EELS reflects only the structure of the uppermost layer near the surface resonance condition, and it may provide more detailed information on the structure of the surface monolayer than LEED. The I-V curves from CH4-Ag(I 11) and CH4-Xe-Ag(111) surfaces showed two dips which correspond to the surface resonance effect inherent to the CH 4 structure. It is also shown that studies of the energy loss spectra at the surface resonance conditions are useful for the investigation of vibrational excitation processes of physisorbed monolayers. For the I-V curve of the vibrational excitation of CH4, the intensity variation of dipole active 74 mode followed

QJ

t - - i

CH4/Ag (J. J J.)

XJO0

- , , : / . : : . .::. v 2 mode -:,,: ?;::~ :, , '~ i~

x loo . . . .

• ".~ .,. ; ; : .. v 4 mode .:~" ~

~ - / :. • ~.~". "~;..~,: . . . . . . . :::~. , . ,

3 315 410 415 510 Incident electron energy (eV)

Figure 3. Incident electron energy dependence of elastic intensity and vibrational loss intensities for 74 mode (162 meV) and 72 mode (190 meV).

that of elastic scattering, while the energy dependence of dipole inactive 72 mode was poor. They could be explained by the difference in the effect of surface resonance against the dipole and impact scattering processes.

References

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