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Inelastic diffraction in coadsorbed periodic structures
Surface Science 418 (1998) 407–419
Inelastic diffraction in coadsorbed periodic structures B.G. Frederick a,*, T. Hildebrand b, C.C. Perry c, Q. Chen c, A.W. Munz c,1, Th. Bertrams c, V. Zielasek b, N.V. Richardson d, M. Henzler b a Laboratory for Surface Science and Technology and Dept. of Chemistry, University of Maine, Orono, ME 04469-5764, USA b Institut fu¨r Festko¨rperphysik der Universita¨t Hannover, Applestrasse 2, 30167 Hannover, Germany c IRC in Surface Science, University of Liverpool, Liverpool, L69 3BX, UK d School of Chemistry, University of St. Andrew’s, St. Andrew’s, Fife, KY16 9ST, UK Received 4 May 1998; accepted for publication 13 August 1998
Abstract We show for the first time that, with two coadsorbed periodic structures it is possible to observe in a diffraction condition unique to one structure a relative enhancement of the vibrational losses characteristic of the species contained in the respective periodic structure. A recently developed SPA-LEED instrument equipped with an electron monochromator and analyser was used to distinguish true elastic diffraction from inelastic diffraction in the vibrational losses of coadsorbed (2×3)N/Cu(110) and the a-phase 4 3 ) structure on Cu(110). Vibrational enhancements by factors of 4–20 were found in energy loss spectra and momentumbenzoate (−1 5 resolved spot profiles. Inelastic spot profiles are qualitatively consistent with a kinematic description of the energy–momentum conservation induced broadening from both loss-before-diffraction (L-D) and diffraction-before-loss (D-L) events. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Carboxylic acid; Copper; Electron energy loss spectroscopy; Electron–solid interactions; Low energy electron diffraction; Low index single crystal surfaces; Nitrogen; Vibrations of adsorbed molecules
1. Introduction Since the discovery of electron diffraction [1] it has been known that inelastic scattering by a dipole loss mechanism is enhanced near any diffraction condition, where Dq, the parallel momentum transfer into the surface excitation, is near zero and l=1/Dq becomes large. Although experimental work in electronic losses [1–3] was interpreted in terms of a kinematic approximation of loss* Corresponding author. Fax: (+1) 207 581 2255; e-mail: [email protected] 1 Current address: Fritz-Haber Institut der Max-PlanckGesellschaft, Faradayweg 4–6, 14192 Berlin, Germany.
before-diffraction (L-D) and diffraction-beforeloss (D-L) events, not until 1971 was a quantum field theory of inelastic diffraction developed [4,5] which showed that such two-step inelastic diffraction dominates over the one-step process. Although a dynamical theory, developed about the same time [6 ], is necessary to predict absolute intensities, angular profiles are given to a good approximation within a kinematic description for electronic excitations. However, most LEED-IV experimental and theoretical work continue to ignore the effects of inelastic scattering processes in structure determination. In the vibrational loss regime, much higher energy resolution (ca 10 meV, vis a` vis 0.5 eV ) is
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required and, while dramatic improvements have been made in the instrumentation for high resolution electron energy loss spectroscopy (HREELS) allowing sub-1 meV measured energy resolution [7], the ability to simultaneously scan scattering angles with high k-resolution, apart from within the scattering plane, has been rather difficult. Only a few HREELS instruments have been built which are capable of scanning both in-plane and out-of-plane [8–10]; however, these do not have sufficient k-resolution to measure spot profiles accurately. In a recent development, the high momentum resolution obtained with an octopole deflection unit of a SPA-LEED instrument has been combined with a single pass cylindrical electron monochromator and analyser in an instrument designated as the ELS-LEED [11]. The energy resolution obtained allows elastic from inelastic scattering to be distinguished in the vibrational loss regime, as demonstrated in recent applications to thin ionic films [12,13]. We will show that, by selecting a diffraction condition unique to one of two (or more) coadsorbed periodic structures, the dipole intensity of vibrational losses associated with that structure is relatively stronger than the losses of other species coexisting on the surface due to spatial correlation between diffraction and inelastic scattering events. In principle, this allows the measurement of a vibrational spectrum specific to that periodic structure. There are a number of situations where this distinction would be valuable. For example, interpretation of STM measurements of coadsorbed systems is limited by the difficulties of associating additional spectroscopic information with the features in a specific ordered region. The comparison is straightforward only if the surface contains one, and only one, periodic structure. Similarly, the coadsorption of molecules in disordered arrangements or other periodicities cannot be distinguished in reflection absorption infra-red spectroscopy or in specular HREELS and the superposition of the vibrational spectra from various configurations may be misleading. To the extent that the inelastic diffraction can be described (approximately) within a kinematic, two-step process, the diffraction and inelastic loss events are independent and, therefore, the observa-
tion of a vibrational enhancement effect depends on the spatial correlation between the domain from which the electron scatters inelastically and that from which it diffracts. Because the strength and area with which an electron interacts via the dipole mechanism depends on the distance from the surface and the interaction is thought to be ˚ [14], we significant at distances of up to 100 A expect the enhancement effect to be critically dependent on the domain size, being largest in the limit d&l(E, h), where d is the domain size and l(E, h) a characteristic length over which the dipole interaction between the electron of energy E and incident angle h decays. To establish the feasibility of exploiting this vibrational enhancement via inelastic diffraction, we chose the coadsorption system (2×3)N and a-phase benzoate/Cu(110). Nitrogen islands can ˚, be deposited first with sizes of order 100–1000 A leaving essentially clean Cu(110) regions [15]. The a-phase benzoate also adsorbs to form very large islands, limited apparently by the quality of the Cu(110) substrate [16 ]. We present STM work to demonstrate the morphology and extent of phase separation of the nitrogen and benzoate structure and inelastic diffraction results which reveal the desired vibrational enhancement effect.
2. Experimental ELS-LEED (Hannover) and STM measurements (Liverpool ) were performed in two UHV systems using the same Cu(110) crystal and nitrogen and benzoate dosing procedures and equipment. The Cu(110) crystal was mechanically and electrochemically polished and characterised via STM (Omicron Vakuumphysik, Liverpool ) and SPA-LEED (Hannover). The energy dependence of the true elastically diffracted spot widths, ca 1.5–2% of the surface Brillouin zone (SBZ), indicated that the surface quality limited the transfer ˚ . Nitrogen was deposited by width to about 100 A activation using a plasma discharge ion sputter gun ( VSW AS10), providing a nominal beam energy of 500 eV, with the sample held at 600 K and biased at +540 V, where the net ion current was near zero, for a duration of 5–60 s. Benzoic
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acid was sublimed under vacuum before introduction via a dosing system, described previously [17], with the surface temperature near 350 K. The ELSLEED instrument is described elsewhere [11].
3. Results 3.1. (2×3)N/Cu(110) We first present results regarding the vibrational enhancement under diffraction conditions and morphology of sub-saturation nitrogen surfaces. Nitrogen was deposited first in the experiments in Hannover, and characterised by Auger and ELSLEED. Comparison of Auger spectra obtained after a 30 s dose and at saturation coverage indicated that a 30 s dose led to approximately 30% of the surface being covered by the (2×3)N reconstruction. STM measurements of similarly prepared surfaces in Liverpool (Fig. 1) show that large, elongated islands of nitrogen form, as reported previously [18], with the remainder of the surface consisting of essentially clean unreconstructed Cu(110) terraces. Similar to the p(2×1)O/Cu(110) system [19], we find that the island width varies with coverage. The islands, determined from images such as that shown in Fig. 1A following a 5 s exposure, had an average ˚ in [110] but a much narrower length of 345±190 A ˚ along [001]. For a width distribution of 57±12 A higher exposure of 20 s, under the preparation conditions in Liverpool, images such as that shown in Fig. 1B and line scans suggested that roughly half the surface was covered with the (2×3)N reconstruction and widths along [001] were ca ˚ . Lengths along [110] were generally 350±130 A ˚ ; the diagonal of a larger than 1400 A ˚ 2 image. Therefore, for the surface 1000×1000 A prepared in the ELS-LEED experiment (30% of saturation) island widths were probably in the ˚ range. 200–300 A Loss spectra, taken for an N-dosed surface to about 30% of saturation in the (0, −1) spot and 3 (0, 0) condition at energies between 6 and 12 eV, confirmed that the dipole cross-section was largest at low energies for the (0, 0) direction, as shown
(a)
(b) ˚ 2 STM image after 5 s exposure to Fig. 1. (A) 1000×1000 A nitrogen showing typical sizes of the nitrogen islands with clean copper terraces between (2.14 V; 0.48 nA.; high pass filtered). ˚ 2 image (−1.28 V; 1.0 nA; high pass filtered ) (B) 900×900 A showing increased island widths along [001] after 20 s exposure. Lengths along [110] were generally greater than the 1000× ˚ 2 image size limitations. 1000 A
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Fig. 2. (A) Energy loss spectra for a surface approximately 30% reconstructed with (2×3)N/Cu(110) measured in the (0, 0) direction (specular) at 6.4, 7.9 and 11.2 eV and (B) in the (0, 1)) spot, showing intensity due to the n(N–Cu) stretching modes at 45.5 and 3 54.5 meV, which are unresolved at the measured resolution in the tail off the plastic peak. (C ) Normalised loss intensity in the region of the N losses (50 meV ) as a function of parallel momentum in the [001] azimuth through the (0, ±1)) spots and the (0, 0) spot 3 illustrating the dipole enhancement in the Bragg conditions.
in Fig. 2A. Higher resolution spectra demonstrate that the major contribution to inelastic scattering is from nitrogen-related dipole losses, at 45.5 and 54.5 meV, as indicated in Fig. 2A and B, and assigned by Hannon et al. [20]. In the (0, −1) 3 direction, the n(N–Cu) stretching modes [20], were more easily distinguished at somewhat higher beam
energies, as shown in Fig. 2B, due to lower background in the tail of the elastic peak. Fig. 2C shows a profile at 50 meV loss energy through the [001] azimuth containing the (0, ±1) and (0, 0) 3 spots, normalised to the elastic intensity on specular. The dipole loss intensity scales approximately with the reflectivity in the Bragg condition and,
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although at 50 meV there is significant contribution from the tail of the elastically scattered electrons, the vibrational enhancement can be clearly observed in a spot profile. The use of relatively low energy monochromation was necessary to obtain sufficient intensity in the diffraction spots so that the vibrational losses could be measured. 3.2. Coadsorption of (2×3)N with a-phase benzoate/Cu(110) 3.2.1. STM measurements For the vibrational enhancement effect to be observable within coadsorbed structures of different periodicity, it was important to establish the extent to which the two structures remained phase separated and the size of the domains which were formed. In a sequential coadsorption experiment, nitrogen was deposited first and then benzoic acid was dosed. STM measurements revealed that, in most areas, large ordered islands of the a-phase benzoate structure formed around the nitrogen islands, as shown in the image of Fig. 3A. This structure has been identified previously with a periodicity described in matrix notation [16 ] as 4 3) resulting in a unit cell 23 times larger than (−1 5 the copper unit cell. Within the indicated unit cell, shown in Fig. 3C, are four circular features identified with flat-lying benzoate species, while the brighter features which adsorb at the corners of the chosen unit cell are assigned to upright species. Typically, near the nitrogen islands the benzoate remained difficult to image, indicating that the density may have been somewhat lower and a net repulsive interaction may exist between the nitrogen and a-phase structures. However, there was some evidence that additional upright species could adsorb or were mobile on top of the nitrogen islands, as indicated by the bright features shown in the image of Fig. 3B. Note the appearance of partial bright features in Fig. 3B where upright species have diffused into or out of the nitrogen island during the scanning process. In summary, the STM images indicate that the N(2×3) and flat-lying species in the a-phase benzoate system remain phase separated and contain large single crystalline domains of different periodicity, suit-
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able for the measurement of an inelastic diffraction effect, although the upright benzoate species may be distributed between both periodic structures. A 2-D SPA-LEED scan, taken at 11.2 eV, for a coadsorption surface prepared in the ELS-LEED system is shown in Fig. 4, together with grids illustrating the two degenerate domains of the a-phase benzoate structure related by reflection through the substrate mirror planes and the nitrogen overlayer spots. The spots observed in SPALEED can all be accounted for by a superposition of the diffraction patterns of the two structures; evidence of a double diffraction process involving the nitrogen (−1, 0) reciprocal lattice vector and 2 a −b (or −b∞ ) lattice vector or a nitrogen 1 1 (−1, 1) reciprocal lattice vector with a −b lattice 1 2 3 vector (or a nitrogen (−1, −1) with −b∞ ) is indi1 2 3 cated by the arrows labelled DD.
3.2.2. Energy loss scans Energy loss scans were acquired at several different beam energies in the (0, 0) condition, the N(−1, 0) and (0, 1) spots, and in several benzoate 2 3 diffraction spots, labelled in Fig. 4 for reference. We should note that the crystal was rotated in the [110] plane relative to the SPA-LEED octopole in order to access the relatively large momentum transfer vectors near (−1, 0) at these low primary 2 beam energies. The data shown in Fig. 5 are normalised to the zero loss intensity in each Bragg condition and offset vertically for clarity. The integrated intensities and nitrogen-to-benzoate loss ratios in the specular, nitrogen and benzoate diffraction conditions are summarised in Table 1 for two energies and five diffraction conditions, measured in the energy loss mode. Higher resolution data measured on a surface with only the a-phase structure (using a double pass cylindrical spectrometer system; VSW HIB 1000) is also shown at the bottom of Fig. 5. The intense peak at 90 meV is due to the benzoate c(CH ) out-ofplane wagging mode of flat-lying species. The feature at 33 meV is associated with motion of the molecule perpendicular to the surface and that at 56 meV is a low frequency out-of-plane buckling mode, which we will designate b(BA). The bands at 50 meV, 100 meV and 175 meV are respectively
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Fig. 4. A 2-D SPA-LEED scan measured at the zero-loss condition (energy resolution of 20 meV FWHM ). The positions of the 4 3 ) unit cell, the b =− 3 a − 4 a and substrate (0, 0) spot, the b∞ =− 3 a − 4 a and b∞ =−10 a + 2 a lattice vectors of the (−1 5 1 1 23 1 23 2 23 1 23 2 23 1 23 2 3 2 b =−10 a − 2 a lattice vectors of the (−4 ) unit cell, and the (2×3)N over layer spots (0, −1/3), (−1/2, 0) are identified. −1 −5 2 23 1 23 2 Double diffraction spots (see text) are indicated by DD.
the strongly dipole-active n(O–Cu) stretch, d(OCO) deformation and n (OCO) stretch associS ated with a minority of upright species [21]. In the lower resolution ELS-LEED data, however, we will focus predominantly on the c(CH ) (flatlying benzoate), n (OCO) stretch (upright benzoS
ate) and the n(N–Cu) stretches (cf. Fig. 2A and B), indicated by the dashed lines in Fig. 5. We first compare the ratio of the integrated intensities of the 50 and 90 meV peaks measured under different diffraction conditions. The near degeneracy of the bands at 45 and 54 meV, due to
˚ 2 STM image showing a-phase benzoate domains adsorbed after deposition of the nitrogen (2×3) islands Fig. 3. (A) A 500×500 A ˚ 2 image showing bright features on top the nitrogen islands which are identified as mobile (−1.78 V; 0.46 nA). (B) A 150×150 A ˚ 2 region showing the ( 4 3) unit cell containing four flat-lying benzoate species. Bright upright benzoate species. (C ) A 50×75 A −1 5 features, which only populate sites at the corners of the indicated unit cell, are identified as upright species [16 ].
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Fig. 5. Energy loss spectra for a surface with coadsorbed benzoate and nitrogen measured at 11.2 eV incident energy in the specular (0, 0) condition, in benzoate (BA) (− 6 , 8 ) and 23 23 (−10 , 2 ) spots, and the nitrogen (−1 , 0) and (0, 1) spots. A 23 23 2 3 higher resolution spectrum (4 meV FWHM ) of a surface with only a-phase benzoate/Cu(110) is also shown for comparison. The positions of the n(N–Cu) stretch (50 meV ), the out-ofplane wag c(CH ) of flat-lying benzoate species (90 meV ) and the n (OCO) due to upright benzoate species (175 meV ) are S indicated by vertical dashed lines.
the n(N–Cu) stretches, and the weak benzoate bands near 56 meV is unfortunate and results in some residual intensity near 50 meV under all scattering conditions in the ELS-LEED data for the coadsorption system. Because the 50 meV {n(N–Cu)+b(BA)} to 90 meV {c(CH )} loss intensity ratio in the benzoate spots at 11.2 eV is similar to that in the (0, 0) spot, the 50 meV loss is probably dominated by benzoate features on specular (cf. Fig. 2A, where the n(N–Cu) stretch mode is weak on specular at 11.2 eV ). However, the dipole enhancement of the n(N–Cu) stretch, relative to the benzoate c(CH ) mode of flat-lying species is very pronounced when in the (2×3)N diffraction conditions. In the (0, 1) diffraction con3 dition, the 50 meV to 90 meV loss intensity ratio increased by a factor of 20 relative to the intensity in the benzoate diffraction conditions, although background subtraction is somewhat arbitrary and, therefore, introduces some uncertainty into the ratios quoted in Table 1. A factor of ten enhancement was found in the (−1, 0) spot. A 2 smaller set of data from spectra acquired at 7.8 eV beam energy is also included in Table 1, and shows a similar trend with an increase by a factor of about four. Having established that a clear vibrational enhancement effect exists in the nitrogen versus benzoate diffraction conditions when comparing the n(N–Cu) stretch+b(BA) intensity relative to the benzoate c(CH ), we should note that the other
Table 1 Integrated intensities of energy loss peaks (meV cts/s) for: n(N–Cu)+b(BA), I(50 meV ); flat-lying benzoate c(CH ), I(90 meV ); upright benzoate n (OCO), I(175 meV ) S I(50 meV )
I(90 meV )
I(175 meV )
I(50)/I(90)
11.2 eV N (0, 1) 3 N (−1, 0) 2 BA (−10, 2 ) 23 23 BA (− 6 , 8 ) 23 23 Spec (0, 0)
111 46.5 3.3 21.7 116.6
147 226 191 608 2967
307 142 71 234 1864
0.76 0.21 0.02 0.04 0.04
7.8 eV N (0, 1) 3 N (−1, 0) 2 BA (− 6 , 8 ) 23 23 Spec (0, 0)
66.3 3.8 14 254
230 143 205 738
294 131 170 1279
0.29 0.27 0.07 0.34
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Table 2 Ratios of the k-integrated inelastic spot intensities in the indicated loss and diffraction conditions relative to the elastic intensities in that spot, I(0 loss), for the n(N–Cu)+b(BA), I(50 meV ) and flat-lying benzoate c(CH ), I(90 meV ) losses at a primary beam energy of 11.2 eV Diffraction condition
I(50 meV )/I(0)
I(90 meV )/I(0)
I(50 meV )/I(90 meV )
N (0, 1) 3 N (0, −1) 3
0.0007 0.0011
0.0005 0.0008
1.50 1.40
0.0006 0.0005 0.0011 0.0009
0.0014 0.0015 0.0016 0.0015
0.46 0.34 0.71 0.62
0.0012±0.0001
0.0012±0.0001
0.99±0.02
BA BA BA BA
(2, −5) 23 23 (− 2 , 5 ) 23 23 (− 4 , 10) 23 23 (− 6 , 8 ) 23 23
Spec (0, 0)
major loss features of the benzoate species, the symmetric carboxylate stretch at 175 meV (1420 cm−1) and the n(CH ) stretch at 370 meV (3000 cm−1; not shown), do not display such an effect. The carboxylate stretch mode is strongly dipole active and the lack of an enhancement in the benzoate diffraction condition indicates that the upright species are not significantly phase separated. This macroscopic measurement correlates well with the microscopic evidence described above from the STM images. By contrast, the n(CH ) stretch is excited primarily by impact scattering and the variation in absolute count rates (normalised to the incident beam current impinging on the sample) as a function of angle in HREELS measurements typically shows little dipole contribution. 3.2.3. k-dependent scans A complementary set of data, measured as the integrated area of 1-D cuts through momentum space also shows the vibrational enhancement effect, as summarised in Table 2. The ratio of the integrated spot intensity in the loss condition to the elastic spot intensity is given, since for dipole scattering the cross-section sG (E, h) should vary d only due to primary beam energy and incident angle, which is determined by the reciprocal lattice vector G. The N(0, ±1) spots should be identical 3 and give an indication of reproducibility; the I(90 meV )/I(0) ratios in the N(0, ±1) are similar. 3 In the benzoate diffraction conditions, the reproducibility of the I(90 meV )/I(0) ratios are much
better. The variation in the I(50 meV )/I(0) ratios are within the reproducibility of the N(0, ±1) 3 ratios, but in principle could vary due to: (i) relative dipole contributions from the n(N–Cu) and b(BA) modes, which vary with incidence and emission angles, h(G, w); (ii) variation in any impact contribution s . The absolute dipole impact cross-section, typically 1% of the I(0) spot intensity, which is at most 1% of incident beam current for specular scattering {(0, 0)}, may vary from 10−4 down to 10−6 for weak diffraction conditions. By comparison, s varies strongly with beam impact energy, but, apart from nodes under special conditions, is expected to be relatively independent of scattering angles h, w and is expected to be 10−6 or less. Non-dipolar contributions can be eliminated by examination of the 50 meV/90 meV loss intensity ratio. The reproducibility in the N(0, ±1) spots, which should be identical by sym3 metry, is much better. There is a clear increase of the I(50)/I(90) ratio in the nitrogen diffraction conditions relative to specular and a significant decrease in the ratio under benzoate diffraction condition, as expected for correlated diffractionloss processes. Some variation in the I(50)/I(90) ratios within the benzoate diffraction conditions may also be physically meaningful. 3.2.4. Inelastic spot profile analysis We now compare the spot profiles in the elastic and inelastic conditions with the theoretical broadening of the dipolar loss process due to energy momentum conservation conditions. For a homo-
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geneous, isotropic surface, the momentum transfer k into a phonon is determined by the beam energy d E, incidence angles (h, w) and loss energy v according to the condition [22–24] (kv )3 ) (1) [(v−k · v )2+(kv )2]2 d ) where V(E, h, w) is the electron velocity prior to the inelastic event. This 2-D function is volcanoshaped with a zero at k=0. At normal incidence (h=0), the rim has constant probability in any azimuthal direction w, but at off-normal incidence forward scattering of the phonon is more likely. The measured spot profiles in k-space through the spot in a direction parallel and perpendicular to the scattering plane are shown in Fig. 6 for the elastically scattered N(−1, 0) diffraction spot, I(0), 2 and in the nitrogen, I , and benzoate-related, 50 meV I , losses, offset for clarity. Exact solutions of 90 meV the momentum conservation condition K(k , v) d for transfer into momentum along [110], k , and x along [001], k , are shown for the actual angles of y incidence (solid; L-D) and exit (dashed; D-L) as the lowest curves in each panel. To achieve large momentum transfer at relatively low beam energies, the ELS-LEED octopole tilts the bisector between the 12° angle formed by the incident and reflected beams away from the surface normal, as shown schematically in Fig. 6A. By the Ewald sphere construction, the elastically diffracted 11.2 eV electron beam would impinge at an angle of −29.6° and, after diffraction by a (−1, 0) lattice 2 vector, scatter from the surface at an angle of −17.6° with respect to the surface normal. In this case, if loss occurs before the diffraction event (L-D), then the dipole cross-section is higher than after (D-L) since the incidence angle is closer to K(k, v)=
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grazing. The true elastic diffraction spot profiles, which are determined by the defect structure of the surface, are shown as the I curves. The 0 loss theoretical momentum conservation condition curves for the L-D and D-L events, K(k , v) were d convoluted with the measured elastic profiles I and then averaged to give an estimate, 0 loss K*I , of the asymmetry and broadening 0 loss expected in the spot profiles for the loss conditions. The maximum in the momentum transfer increases with loss energy and the ratio of the intensity of the forward to backward scattering lobes increases with angle of incidence (with respect to the surface normal ), predicting an asymmetric spot profile. For momentum transfer in a direction perpendicular to the scattering plane, k , the distribution is y symmetric in |k |: only the magnitude varies due y to the incidence (exit) angle. The I , K*I , 0 loss 0 loss and I , or I , spot profiles were then fit 50 meV 90 meV with a Lorentzian to allow quantitative comparison of experiment with theory. The widths of the inelastic peaks are always significantly broader than the elastic spot profiles, but the kinematical description of the D-L and L-D processes predicts spot shapes significantly different from experiment. If properly treated in the quantum field theory [4], of course, the amplitudes of the two processes must be added. To a good approximation, the shape of the dipole lobe is thought to be given by a superposition of the two separate two-step processes, since these dominate in both the quantum field theory and multiple scattering dynamical calculations [6 ]. However, Eq. (1) for the energy momentum conservation condition assumes a homogeneous, isotropic surface. A Green function treatment within the dielectric theory of EELS for an electron scattering from a two-medium sub-
Fig. 6. Comparison of 1-D cuts through the spot profiles measured in the elastic and inelastic scattering conditions for the N(−1 , 0) spot in the [110] direction at: (A) 50 meV (n(N–Cu)) and (B) at 90 meV ((c(CH )) loss; in the [001] direction at (C ) 50 meV 2 and (D) 90 meV loss for an incident beam energy of 11.2 eV. The energy momentum conservation condition, K(k , v), predicting d the shape of the dipolar lobes is shown for the incident beam (L-D; solid curve) and the diffracted curve (D-L; dashed curve). The elastic spot profile I(0 loss) has been convoluted with the energy momentum conservation terms for the L-D and D-L processes and averaged to generate the expected inelastic spot profile, K*I , which can then be compared with the measured inelastic spot 0 loss profiles, I or I . The 1-D cuts through the spot profiles are in the scattering plane for (A) and (B) and perpendicular 50 meV loss 90 meV loss to the scattering plane for (C ) and (D).
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strate with a boundary perpendicular to the surface has been given by Lambin et al. [25] and leads to an energy momentum conservation condition which varies with distance from the transverse boundary. The more complex island morphology of the present surface could be described by extension of the work by Lambin et al., for example to an array of parallel islands. This is beyond the scope of the present work, but may account for the discrepancy between the simple theoretical estimates and measured spot profiles. Finally, the magnitude of the vibrational enhancement effect is expected to depend on island size. There is little theoretical work available to estimate the magnitude of the effect; however, treatment of the transverse boundary model as a function of the distance from the boundary [25] ˚ offers some insight. At a distance of 200–300 A from the boundary of an GaAs/AlAs heterostructure, reflected electrons can still excite surface phonons on the opposite medium [25]. Since the scattering cross-section decreases with the thickness of the dielectric layer [23], the signal for a chemisorbed layer should be weaker and would reduce the interaction distances significantly from the estimates given by Lambin et al. [25]. Further theoretical work and experiments, in which the island size is determined from SPA-LEED measurements and the vibrational enhancement effect is monitored, are required to establish the limitations of this technique for obtaining a vibrational spectrum specific to a particular periodic structure in a coadsorption system. Nevertheless, we have established that the effect can be utilised for coads˚. orbed domains whose size is of order 200 A
4. Conclusions We have prepared surfaces of a-phase benzoate/Cu(110) coadsorbed with the (2×3)N/ Cu(110) structure and characterised them with STM. The a-phase islands remain phase separated from the nitrogen islands, although additional upright species appear mobile across the nitrogen islands. Using a recently developed ELS-LEED spectrometer, we were able to measure loss spectra
and spot profiles in the diffraction conditions appropriate to each periodicity structure and show that there is a vibrational enhancement due to spatial correlation between the dipolar inelastic scattering events and the diffraction process. The mechanism is expected to be dependent on domain size, but has been observed with nitrogen islands ˚ . The broadenof minimum dimension 200–300 A ing of the spot profiles in the vibrational loss condition, compared with the elastically diffracted beams, is qualitatively consistent with a kinematic description of the energy–momentum conservation constraint in the L-D and D-L events.
Acknowledgements This work was supported by the EPSRC in the UK and the Deutsche Forschungsgemeinschaft in Germany. Q.C. thanks the British Council for a Studentship.
References [1] C. Davisson, L.H. Germer, Phys. Rev. 30 (1927) 705. [2] J.C. Turnbull, H.E. Farnsworth, Phys. Rev. 54 (1958) 509. [3] P.P. Reichertz, H.E. Farnsworth, Phys. Rev. 75 (1949) 1902. [4] C.B. Duke, G.E. Laramore, Phys. Rev. B 3 (1971) 3183. [5] G.E. Laramore, C.B. Duke, Phys. Rev. B 3 (1971) 3198. [6 ] C.B. Duke, U. Landman, Phys. Rev. B 6 (1972) 2956. [7] H. Ibach, J. Electron Spectrosc. Relat. Phenom. 64/65 (1993) 819. [8] G.L. Nyberg, S.R. Bare, P. Hofmann, D.A. King, M. Surman, Appl. Surf. Sci. 22/23 (1985) 392. [9] J.E. Yater, A.D. Kukarni, F.W. DeWette, J.L. Erskine, J. Electron Spectrosc. Relat. Phenom. 54 (1990) 395. [10] J.L. Erskine, E.J. Jeong, J. Yater, Y. Chen, S.Y. Tong, J. Vac. Sci. Technol. A: 8 (1990) 2649. [11] H. Claus, A. Bu¨ssenschu¨tt, M. Henzler, Rev. Sci. Instrum. 63 (1992) 2195. [12] A. Bu¨ssenschu¨tt, H. Claus, M. Henzler, Phys. Rev. B 49 (1994) 7829. [13] V. Zielasek, A. Bu¨ssenschu¨tt, M. Henzler, App. Surf. Sci. 90 (1995) 117. [14] H. Ibach, D.L. Mills, Electron Energy Loss Spectroscopy and Surface Vibrations, Academic Press, New York, 1982.
B.G. Frederick et al. / Surface Science 418 (1998) 407–419 [15] F.M. Leibsle, Surf. Sci. 311 (1994) 45. [16 ] B.G. Frederick, F.M. Leibsle, S. Haq, N.V. Richardson, Surf. Rev. Lett. 3 (1996) 1523. [17] B.G. Frederick, Q. Chen, F.M. Leibsle, M.B. Lee, K.J. Kitching, N.V. Richardson, Surf. Sci. 394 (1997) 1. [18] F.M. Leibsle, R. Davis, A.W. Robinson, Phys. Rev. B 49 (1994) 8290. [19] K. Kern, H. Niehus, A. Schatz, P. Zeppenfeld, J. George, G. Comsa, Phys. Rev. Lett. 67 (1991) 855. [20] J.B. Hannon, H.A. Du¨rr, E.W. Plummer, Phys. Rev. B 54 (1996) 16444.
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[21] P.D.A. Pudney, B.G. Frederick, N.V. Richardson, Surf. Sci. 307–309 (1994) 46. [22] A.A. Lucas, J.P. Vigneron, P. Lambin, P.A. Thiry, M. Liehr, J.J. Pireaux, R. Caudano, Int. J. Quant. Chem. S 19 (1986) 687. [23] B.G. Frederick, G. Apai, T.N. Rhodin, Phys. Rev. B 44 (1991) 1880. [24] M.B. Lee, B.G. Frederick, G. Apai, Surf. Rev. Lett. 1 (1994) 635. [25] P. Lambin, T. Laloyaux, A.A. Lucas, J.P. Vigneron, Phys. Rev. B 35 (1987) 5621.
Inelastic diffraction in coadsorbed periodic structures B.G. Frederick a,*, T. Hildebrand b, C.C. Perry c, Q. Chen c, A.W. Munz c,1, Th. Bertrams c, V. Zielasek b, N.V. Richardson d, M. Henzler b a Laboratory for Surface Science and Technology and Dept. of Chemistry, University of Maine, Orono, ME 04469-5764, USA b Institut fu¨r Festko¨rperphysik der Universita¨t Hannover, Applestrasse 2, 30167 Hannover, Germany c IRC in Surface Science, University of Liverpool, Liverpool, L69 3BX, UK d School of Chemistry, University of St. Andrew’s, St. Andrew’s, Fife, KY16 9ST, UK Received 4 May 1998; accepted for publication 13 August 1998
Abstract We show for the first time that, with two coadsorbed periodic structures it is possible to observe in a diffraction condition unique to one structure a relative enhancement of the vibrational losses characteristic of the species contained in the respective periodic structure. A recently developed SPA-LEED instrument equipped with an electron monochromator and analyser was used to distinguish true elastic diffraction from inelastic diffraction in the vibrational losses of coadsorbed (2×3)N/Cu(110) and the a-phase 4 3 ) structure on Cu(110). Vibrational enhancements by factors of 4–20 were found in energy loss spectra and momentumbenzoate (−1 5 resolved spot profiles. Inelastic spot profiles are qualitatively consistent with a kinematic description of the energy–momentum conservation induced broadening from both loss-before-diffraction (L-D) and diffraction-before-loss (D-L) events. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Carboxylic acid; Copper; Electron energy loss spectroscopy; Electron–solid interactions; Low energy electron diffraction; Low index single crystal surfaces; Nitrogen; Vibrations of adsorbed molecules
1. Introduction Since the discovery of electron diffraction [1] it has been known that inelastic scattering by a dipole loss mechanism is enhanced near any diffraction condition, where Dq, the parallel momentum transfer into the surface excitation, is near zero and l=1/Dq becomes large. Although experimental work in electronic losses [1–3] was interpreted in terms of a kinematic approximation of loss* Corresponding author. Fax: (+1) 207 581 2255; e-mail: [email protected] 1 Current address: Fritz-Haber Institut der Max-PlanckGesellschaft, Faradayweg 4–6, 14192 Berlin, Germany.
before-diffraction (L-D) and diffraction-beforeloss (D-L) events, not until 1971 was a quantum field theory of inelastic diffraction developed [4,5] which showed that such two-step inelastic diffraction dominates over the one-step process. Although a dynamical theory, developed about the same time [6 ], is necessary to predict absolute intensities, angular profiles are given to a good approximation within a kinematic description for electronic excitations. However, most LEED-IV experimental and theoretical work continue to ignore the effects of inelastic scattering processes in structure determination. In the vibrational loss regime, much higher energy resolution (ca 10 meV, vis a` vis 0.5 eV ) is
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required and, while dramatic improvements have been made in the instrumentation for high resolution electron energy loss spectroscopy (HREELS) allowing sub-1 meV measured energy resolution [7], the ability to simultaneously scan scattering angles with high k-resolution, apart from within the scattering plane, has been rather difficult. Only a few HREELS instruments have been built which are capable of scanning both in-plane and out-of-plane [8–10]; however, these do not have sufficient k-resolution to measure spot profiles accurately. In a recent development, the high momentum resolution obtained with an octopole deflection unit of a SPA-LEED instrument has been combined with a single pass cylindrical electron monochromator and analyser in an instrument designated as the ELS-LEED [11]. The energy resolution obtained allows elastic from inelastic scattering to be distinguished in the vibrational loss regime, as demonstrated in recent applications to thin ionic films [12,13]. We will show that, by selecting a diffraction condition unique to one of two (or more) coadsorbed periodic structures, the dipole intensity of vibrational losses associated with that structure is relatively stronger than the losses of other species coexisting on the surface due to spatial correlation between diffraction and inelastic scattering events. In principle, this allows the measurement of a vibrational spectrum specific to that periodic structure. There are a number of situations where this distinction would be valuable. For example, interpretation of STM measurements of coadsorbed systems is limited by the difficulties of associating additional spectroscopic information with the features in a specific ordered region. The comparison is straightforward only if the surface contains one, and only one, periodic structure. Similarly, the coadsorption of molecules in disordered arrangements or other periodicities cannot be distinguished in reflection absorption infra-red spectroscopy or in specular HREELS and the superposition of the vibrational spectra from various configurations may be misleading. To the extent that the inelastic diffraction can be described (approximately) within a kinematic, two-step process, the diffraction and inelastic loss events are independent and, therefore, the observa-
tion of a vibrational enhancement effect depends on the spatial correlation between the domain from which the electron scatters inelastically and that from which it diffracts. Because the strength and area with which an electron interacts via the dipole mechanism depends on the distance from the surface and the interaction is thought to be ˚ [14], we significant at distances of up to 100 A expect the enhancement effect to be critically dependent on the domain size, being largest in the limit d&l(E, h), where d is the domain size and l(E, h) a characteristic length over which the dipole interaction between the electron of energy E and incident angle h decays. To establish the feasibility of exploiting this vibrational enhancement via inelastic diffraction, we chose the coadsorption system (2×3)N and a-phase benzoate/Cu(110). Nitrogen islands can ˚, be deposited first with sizes of order 100–1000 A leaving essentially clean Cu(110) regions [15]. The a-phase benzoate also adsorbs to form very large islands, limited apparently by the quality of the Cu(110) substrate [16 ]. We present STM work to demonstrate the morphology and extent of phase separation of the nitrogen and benzoate structure and inelastic diffraction results which reveal the desired vibrational enhancement effect.
2. Experimental ELS-LEED (Hannover) and STM measurements (Liverpool ) were performed in two UHV systems using the same Cu(110) crystal and nitrogen and benzoate dosing procedures and equipment. The Cu(110) crystal was mechanically and electrochemically polished and characterised via STM (Omicron Vakuumphysik, Liverpool ) and SPA-LEED (Hannover). The energy dependence of the true elastically diffracted spot widths, ca 1.5–2% of the surface Brillouin zone (SBZ), indicated that the surface quality limited the transfer ˚ . Nitrogen was deposited by width to about 100 A activation using a plasma discharge ion sputter gun ( VSW AS10), providing a nominal beam energy of 500 eV, with the sample held at 600 K and biased at +540 V, where the net ion current was near zero, for a duration of 5–60 s. Benzoic
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acid was sublimed under vacuum before introduction via a dosing system, described previously [17], with the surface temperature near 350 K. The ELSLEED instrument is described elsewhere [11].
3. Results 3.1. (2×3)N/Cu(110) We first present results regarding the vibrational enhancement under diffraction conditions and morphology of sub-saturation nitrogen surfaces. Nitrogen was deposited first in the experiments in Hannover, and characterised by Auger and ELSLEED. Comparison of Auger spectra obtained after a 30 s dose and at saturation coverage indicated that a 30 s dose led to approximately 30% of the surface being covered by the (2×3)N reconstruction. STM measurements of similarly prepared surfaces in Liverpool (Fig. 1) show that large, elongated islands of nitrogen form, as reported previously [18], with the remainder of the surface consisting of essentially clean unreconstructed Cu(110) terraces. Similar to the p(2×1)O/Cu(110) system [19], we find that the island width varies with coverage. The islands, determined from images such as that shown in Fig. 1A following a 5 s exposure, had an average ˚ in [110] but a much narrower length of 345±190 A ˚ along [001]. For a width distribution of 57±12 A higher exposure of 20 s, under the preparation conditions in Liverpool, images such as that shown in Fig. 1B and line scans suggested that roughly half the surface was covered with the (2×3)N reconstruction and widths along [001] were ca ˚ . Lengths along [110] were generally 350±130 A ˚ ; the diagonal of a larger than 1400 A ˚ 2 image. Therefore, for the surface 1000×1000 A prepared in the ELS-LEED experiment (30% of saturation) island widths were probably in the ˚ range. 200–300 A Loss spectra, taken for an N-dosed surface to about 30% of saturation in the (0, −1) spot and 3 (0, 0) condition at energies between 6 and 12 eV, confirmed that the dipole cross-section was largest at low energies for the (0, 0) direction, as shown
(a)
(b) ˚ 2 STM image after 5 s exposure to Fig. 1. (A) 1000×1000 A nitrogen showing typical sizes of the nitrogen islands with clean copper terraces between (2.14 V; 0.48 nA.; high pass filtered). ˚ 2 image (−1.28 V; 1.0 nA; high pass filtered ) (B) 900×900 A showing increased island widths along [001] after 20 s exposure. Lengths along [110] were generally greater than the 1000× ˚ 2 image size limitations. 1000 A
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Fig. 2. (A) Energy loss spectra for a surface approximately 30% reconstructed with (2×3)N/Cu(110) measured in the (0, 0) direction (specular) at 6.4, 7.9 and 11.2 eV and (B) in the (0, 1)) spot, showing intensity due to the n(N–Cu) stretching modes at 45.5 and 3 54.5 meV, which are unresolved at the measured resolution in the tail off the plastic peak. (C ) Normalised loss intensity in the region of the N losses (50 meV ) as a function of parallel momentum in the [001] azimuth through the (0, ±1)) spots and the (0, 0) spot 3 illustrating the dipole enhancement in the Bragg conditions.
in Fig. 2A. Higher resolution spectra demonstrate that the major contribution to inelastic scattering is from nitrogen-related dipole losses, at 45.5 and 54.5 meV, as indicated in Fig. 2A and B, and assigned by Hannon et al. [20]. In the (0, −1) 3 direction, the n(N–Cu) stretching modes [20], were more easily distinguished at somewhat higher beam
energies, as shown in Fig. 2B, due to lower background in the tail of the elastic peak. Fig. 2C shows a profile at 50 meV loss energy through the [001] azimuth containing the (0, ±1) and (0, 0) 3 spots, normalised to the elastic intensity on specular. The dipole loss intensity scales approximately with the reflectivity in the Bragg condition and,
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although at 50 meV there is significant contribution from the tail of the elastically scattered electrons, the vibrational enhancement can be clearly observed in a spot profile. The use of relatively low energy monochromation was necessary to obtain sufficient intensity in the diffraction spots so that the vibrational losses could be measured. 3.2. Coadsorption of (2×3)N with a-phase benzoate/Cu(110) 3.2.1. STM measurements For the vibrational enhancement effect to be observable within coadsorbed structures of different periodicity, it was important to establish the extent to which the two structures remained phase separated and the size of the domains which were formed. In a sequential coadsorption experiment, nitrogen was deposited first and then benzoic acid was dosed. STM measurements revealed that, in most areas, large ordered islands of the a-phase benzoate structure formed around the nitrogen islands, as shown in the image of Fig. 3A. This structure has been identified previously with a periodicity described in matrix notation [16 ] as 4 3) resulting in a unit cell 23 times larger than (−1 5 the copper unit cell. Within the indicated unit cell, shown in Fig. 3C, are four circular features identified with flat-lying benzoate species, while the brighter features which adsorb at the corners of the chosen unit cell are assigned to upright species. Typically, near the nitrogen islands the benzoate remained difficult to image, indicating that the density may have been somewhat lower and a net repulsive interaction may exist between the nitrogen and a-phase structures. However, there was some evidence that additional upright species could adsorb or were mobile on top of the nitrogen islands, as indicated by the bright features shown in the image of Fig. 3B. Note the appearance of partial bright features in Fig. 3B where upright species have diffused into or out of the nitrogen island during the scanning process. In summary, the STM images indicate that the N(2×3) and flat-lying species in the a-phase benzoate system remain phase separated and contain large single crystalline domains of different periodicity, suit-
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able for the measurement of an inelastic diffraction effect, although the upright benzoate species may be distributed between both periodic structures. A 2-D SPA-LEED scan, taken at 11.2 eV, for a coadsorption surface prepared in the ELS-LEED system is shown in Fig. 4, together with grids illustrating the two degenerate domains of the a-phase benzoate structure related by reflection through the substrate mirror planes and the nitrogen overlayer spots. The spots observed in SPALEED can all be accounted for by a superposition of the diffraction patterns of the two structures; evidence of a double diffraction process involving the nitrogen (−1, 0) reciprocal lattice vector and 2 a −b (or −b∞ ) lattice vector or a nitrogen 1 1 (−1, 1) reciprocal lattice vector with a −b lattice 1 2 3 vector (or a nitrogen (−1, −1) with −b∞ ) is indi1 2 3 cated by the arrows labelled DD.
3.2.2. Energy loss scans Energy loss scans were acquired at several different beam energies in the (0, 0) condition, the N(−1, 0) and (0, 1) spots, and in several benzoate 2 3 diffraction spots, labelled in Fig. 4 for reference. We should note that the crystal was rotated in the [110] plane relative to the SPA-LEED octopole in order to access the relatively large momentum transfer vectors near (−1, 0) at these low primary 2 beam energies. The data shown in Fig. 5 are normalised to the zero loss intensity in each Bragg condition and offset vertically for clarity. The integrated intensities and nitrogen-to-benzoate loss ratios in the specular, nitrogen and benzoate diffraction conditions are summarised in Table 1 for two energies and five diffraction conditions, measured in the energy loss mode. Higher resolution data measured on a surface with only the a-phase structure (using a double pass cylindrical spectrometer system; VSW HIB 1000) is also shown at the bottom of Fig. 5. The intense peak at 90 meV is due to the benzoate c(CH ) out-ofplane wagging mode of flat-lying species. The feature at 33 meV is associated with motion of the molecule perpendicular to the surface and that at 56 meV is a low frequency out-of-plane buckling mode, which we will designate b(BA). The bands at 50 meV, 100 meV and 175 meV are respectively
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Fig. 4. A 2-D SPA-LEED scan measured at the zero-loss condition (energy resolution of 20 meV FWHM ). The positions of the 4 3 ) unit cell, the b =− 3 a − 4 a and substrate (0, 0) spot, the b∞ =− 3 a − 4 a and b∞ =−10 a + 2 a lattice vectors of the (−1 5 1 1 23 1 23 2 23 1 23 2 23 1 23 2 3 2 b =−10 a − 2 a lattice vectors of the (−4 ) unit cell, and the (2×3)N over layer spots (0, −1/3), (−1/2, 0) are identified. −1 −5 2 23 1 23 2 Double diffraction spots (see text) are indicated by DD.
the strongly dipole-active n(O–Cu) stretch, d(OCO) deformation and n (OCO) stretch associS ated with a minority of upright species [21]. In the lower resolution ELS-LEED data, however, we will focus predominantly on the c(CH ) (flatlying benzoate), n (OCO) stretch (upright benzoS
ate) and the n(N–Cu) stretches (cf. Fig. 2A and B), indicated by the dashed lines in Fig. 5. We first compare the ratio of the integrated intensities of the 50 and 90 meV peaks measured under different diffraction conditions. The near degeneracy of the bands at 45 and 54 meV, due to
˚ 2 STM image showing a-phase benzoate domains adsorbed after deposition of the nitrogen (2×3) islands Fig. 3. (A) A 500×500 A ˚ 2 image showing bright features on top the nitrogen islands which are identified as mobile (−1.78 V; 0.46 nA). (B) A 150×150 A ˚ 2 region showing the ( 4 3) unit cell containing four flat-lying benzoate species. Bright upright benzoate species. (C ) A 50×75 A −1 5 features, which only populate sites at the corners of the indicated unit cell, are identified as upright species [16 ].
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Fig. 5. Energy loss spectra for a surface with coadsorbed benzoate and nitrogen measured at 11.2 eV incident energy in the specular (0, 0) condition, in benzoate (BA) (− 6 , 8 ) and 23 23 (−10 , 2 ) spots, and the nitrogen (−1 , 0) and (0, 1) spots. A 23 23 2 3 higher resolution spectrum (4 meV FWHM ) of a surface with only a-phase benzoate/Cu(110) is also shown for comparison. The positions of the n(N–Cu) stretch (50 meV ), the out-ofplane wag c(CH ) of flat-lying benzoate species (90 meV ) and the n (OCO) due to upright benzoate species (175 meV ) are S indicated by vertical dashed lines.
the n(N–Cu) stretches, and the weak benzoate bands near 56 meV is unfortunate and results in some residual intensity near 50 meV under all scattering conditions in the ELS-LEED data for the coadsorption system. Because the 50 meV {n(N–Cu)+b(BA)} to 90 meV {c(CH )} loss intensity ratio in the benzoate spots at 11.2 eV is similar to that in the (0, 0) spot, the 50 meV loss is probably dominated by benzoate features on specular (cf. Fig. 2A, where the n(N–Cu) stretch mode is weak on specular at 11.2 eV ). However, the dipole enhancement of the n(N–Cu) stretch, relative to the benzoate c(CH ) mode of flat-lying species is very pronounced when in the (2×3)N diffraction conditions. In the (0, 1) diffraction con3 dition, the 50 meV to 90 meV loss intensity ratio increased by a factor of 20 relative to the intensity in the benzoate diffraction conditions, although background subtraction is somewhat arbitrary and, therefore, introduces some uncertainty into the ratios quoted in Table 1. A factor of ten enhancement was found in the (−1, 0) spot. A 2 smaller set of data from spectra acquired at 7.8 eV beam energy is also included in Table 1, and shows a similar trend with an increase by a factor of about four. Having established that a clear vibrational enhancement effect exists in the nitrogen versus benzoate diffraction conditions when comparing the n(N–Cu) stretch+b(BA) intensity relative to the benzoate c(CH ), we should note that the other
Table 1 Integrated intensities of energy loss peaks (meV cts/s) for: n(N–Cu)+b(BA), I(50 meV ); flat-lying benzoate c(CH ), I(90 meV ); upright benzoate n (OCO), I(175 meV ) S I(50 meV )
I(90 meV )
I(175 meV )
I(50)/I(90)
11.2 eV N (0, 1) 3 N (−1, 0) 2 BA (−10, 2 ) 23 23 BA (− 6 , 8 ) 23 23 Spec (0, 0)
111 46.5 3.3 21.7 116.6
147 226 191 608 2967
307 142 71 234 1864
0.76 0.21 0.02 0.04 0.04
7.8 eV N (0, 1) 3 N (−1, 0) 2 BA (− 6 , 8 ) 23 23 Spec (0, 0)
66.3 3.8 14 254
230 143 205 738
294 131 170 1279
0.29 0.27 0.07 0.34
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Table 2 Ratios of the k-integrated inelastic spot intensities in the indicated loss and diffraction conditions relative to the elastic intensities in that spot, I(0 loss), for the n(N–Cu)+b(BA), I(50 meV ) and flat-lying benzoate c(CH ), I(90 meV ) losses at a primary beam energy of 11.2 eV Diffraction condition
I(50 meV )/I(0)
I(90 meV )/I(0)
I(50 meV )/I(90 meV )
N (0, 1) 3 N (0, −1) 3
0.0007 0.0011
0.0005 0.0008
1.50 1.40
0.0006 0.0005 0.0011 0.0009
0.0014 0.0015 0.0016 0.0015
0.46 0.34 0.71 0.62
0.0012±0.0001
0.0012±0.0001
0.99±0.02
BA BA BA BA
(2, −5) 23 23 (− 2 , 5 ) 23 23 (− 4 , 10) 23 23 (− 6 , 8 ) 23 23
Spec (0, 0)
major loss features of the benzoate species, the symmetric carboxylate stretch at 175 meV (1420 cm−1) and the n(CH ) stretch at 370 meV (3000 cm−1; not shown), do not display such an effect. The carboxylate stretch mode is strongly dipole active and the lack of an enhancement in the benzoate diffraction condition indicates that the upright species are not significantly phase separated. This macroscopic measurement correlates well with the microscopic evidence described above from the STM images. By contrast, the n(CH ) stretch is excited primarily by impact scattering and the variation in absolute count rates (normalised to the incident beam current impinging on the sample) as a function of angle in HREELS measurements typically shows little dipole contribution. 3.2.3. k-dependent scans A complementary set of data, measured as the integrated area of 1-D cuts through momentum space also shows the vibrational enhancement effect, as summarised in Table 2. The ratio of the integrated spot intensity in the loss condition to the elastic spot intensity is given, since for dipole scattering the cross-section sG (E, h) should vary d only due to primary beam energy and incident angle, which is determined by the reciprocal lattice vector G. The N(0, ±1) spots should be identical 3 and give an indication of reproducibility; the I(90 meV )/I(0) ratios in the N(0, ±1) are similar. 3 In the benzoate diffraction conditions, the reproducibility of the I(90 meV )/I(0) ratios are much
better. The variation in the I(50 meV )/I(0) ratios are within the reproducibility of the N(0, ±1) 3 ratios, but in principle could vary due to: (i) relative dipole contributions from the n(N–Cu) and b(BA) modes, which vary with incidence and emission angles, h(G, w); (ii) variation in any impact contribution s . The absolute dipole impact cross-section, typically 1% of the I(0) spot intensity, which is at most 1% of incident beam current for specular scattering {(0, 0)}, may vary from 10−4 down to 10−6 for weak diffraction conditions. By comparison, s varies strongly with beam impact energy, but, apart from nodes under special conditions, is expected to be relatively independent of scattering angles h, w and is expected to be 10−6 or less. Non-dipolar contributions can be eliminated by examination of the 50 meV/90 meV loss intensity ratio. The reproducibility in the N(0, ±1) spots, which should be identical by sym3 metry, is much better. There is a clear increase of the I(50)/I(90) ratio in the nitrogen diffraction conditions relative to specular and a significant decrease in the ratio under benzoate diffraction condition, as expected for correlated diffractionloss processes. Some variation in the I(50)/I(90) ratios within the benzoate diffraction conditions may also be physically meaningful. 3.2.4. Inelastic spot profile analysis We now compare the spot profiles in the elastic and inelastic conditions with the theoretical broadening of the dipolar loss process due to energy momentum conservation conditions. For a homo-
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geneous, isotropic surface, the momentum transfer k into a phonon is determined by the beam energy d E, incidence angles (h, w) and loss energy v according to the condition [22–24] (kv )3 ) (1) [(v−k · v )2+(kv )2]2 d ) where V(E, h, w) is the electron velocity prior to the inelastic event. This 2-D function is volcanoshaped with a zero at k=0. At normal incidence (h=0), the rim has constant probability in any azimuthal direction w, but at off-normal incidence forward scattering of the phonon is more likely. The measured spot profiles in k-space through the spot in a direction parallel and perpendicular to the scattering plane are shown in Fig. 6 for the elastically scattered N(−1, 0) diffraction spot, I(0), 2 and in the nitrogen, I , and benzoate-related, 50 meV I , losses, offset for clarity. Exact solutions of 90 meV the momentum conservation condition K(k , v) d for transfer into momentum along [110], k , and x along [001], k , are shown for the actual angles of y incidence (solid; L-D) and exit (dashed; D-L) as the lowest curves in each panel. To achieve large momentum transfer at relatively low beam energies, the ELS-LEED octopole tilts the bisector between the 12° angle formed by the incident and reflected beams away from the surface normal, as shown schematically in Fig. 6A. By the Ewald sphere construction, the elastically diffracted 11.2 eV electron beam would impinge at an angle of −29.6° and, after diffraction by a (−1, 0) lattice 2 vector, scatter from the surface at an angle of −17.6° with respect to the surface normal. In this case, if loss occurs before the diffraction event (L-D), then the dipole cross-section is higher than after (D-L) since the incidence angle is closer to K(k, v)=
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grazing. The true elastic diffraction spot profiles, which are determined by the defect structure of the surface, are shown as the I curves. The 0 loss theoretical momentum conservation condition curves for the L-D and D-L events, K(k , v) were d convoluted with the measured elastic profiles I and then averaged to give an estimate, 0 loss K*I , of the asymmetry and broadening 0 loss expected in the spot profiles for the loss conditions. The maximum in the momentum transfer increases with loss energy and the ratio of the intensity of the forward to backward scattering lobes increases with angle of incidence (with respect to the surface normal ), predicting an asymmetric spot profile. For momentum transfer in a direction perpendicular to the scattering plane, k , the distribution is y symmetric in |k |: only the magnitude varies due y to the incidence (exit) angle. The I , K*I , 0 loss 0 loss and I , or I , spot profiles were then fit 50 meV 90 meV with a Lorentzian to allow quantitative comparison of experiment with theory. The widths of the inelastic peaks are always significantly broader than the elastic spot profiles, but the kinematical description of the D-L and L-D processes predicts spot shapes significantly different from experiment. If properly treated in the quantum field theory [4], of course, the amplitudes of the two processes must be added. To a good approximation, the shape of the dipole lobe is thought to be given by a superposition of the two separate two-step processes, since these dominate in both the quantum field theory and multiple scattering dynamical calculations [6 ]. However, Eq. (1) for the energy momentum conservation condition assumes a homogeneous, isotropic surface. A Green function treatment within the dielectric theory of EELS for an electron scattering from a two-medium sub-
Fig. 6. Comparison of 1-D cuts through the spot profiles measured in the elastic and inelastic scattering conditions for the N(−1 , 0) spot in the [110] direction at: (A) 50 meV (n(N–Cu)) and (B) at 90 meV ((c(CH )) loss; in the [001] direction at (C ) 50 meV 2 and (D) 90 meV loss for an incident beam energy of 11.2 eV. The energy momentum conservation condition, K(k , v), predicting d the shape of the dipolar lobes is shown for the incident beam (L-D; solid curve) and the diffracted curve (D-L; dashed curve). The elastic spot profile I(0 loss) has been convoluted with the energy momentum conservation terms for the L-D and D-L processes and averaged to generate the expected inelastic spot profile, K*I , which can then be compared with the measured inelastic spot 0 loss profiles, I or I . The 1-D cuts through the spot profiles are in the scattering plane for (A) and (B) and perpendicular 50 meV loss 90 meV loss to the scattering plane for (C ) and (D).
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strate with a boundary perpendicular to the surface has been given by Lambin et al. [25] and leads to an energy momentum conservation condition which varies with distance from the transverse boundary. The more complex island morphology of the present surface could be described by extension of the work by Lambin et al., for example to an array of parallel islands. This is beyond the scope of the present work, but may account for the discrepancy between the simple theoretical estimates and measured spot profiles. Finally, the magnitude of the vibrational enhancement effect is expected to depend on island size. There is little theoretical work available to estimate the magnitude of the effect; however, treatment of the transverse boundary model as a function of the distance from the boundary [25] ˚ offers some insight. At a distance of 200–300 A from the boundary of an GaAs/AlAs heterostructure, reflected electrons can still excite surface phonons on the opposite medium [25]. Since the scattering cross-section decreases with the thickness of the dielectric layer [23], the signal for a chemisorbed layer should be weaker and would reduce the interaction distances significantly from the estimates given by Lambin et al. [25]. Further theoretical work and experiments, in which the island size is determined from SPA-LEED measurements and the vibrational enhancement effect is monitored, are required to establish the limitations of this technique for obtaining a vibrational spectrum specific to a particular periodic structure in a coadsorption system. Nevertheless, we have established that the effect can be utilised for coads˚. orbed domains whose size is of order 200 A
4. Conclusions We have prepared surfaces of a-phase benzoate/Cu(110) coadsorbed with the (2×3)N/ Cu(110) structure and characterised them with STM. The a-phase islands remain phase separated from the nitrogen islands, although additional upright species appear mobile across the nitrogen islands. Using a recently developed ELS-LEED spectrometer, we were able to measure loss spectra
and spot profiles in the diffraction conditions appropriate to each periodicity structure and show that there is a vibrational enhancement due to spatial correlation between the dipolar inelastic scattering events and the diffraction process. The mechanism is expected to be dependent on domain size, but has been observed with nitrogen islands ˚ . The broadenof minimum dimension 200–300 A ing of the spot profiles in the vibrational loss condition, compared with the elastically diffracted beams, is qualitatively consistent with a kinematic description of the energy–momentum conservation constraint in the L-D and D-L events.
Acknowledgements This work was supported by the EPSRC in the UK and the Deutsche Forschungsgemeinschaft in Germany. Q.C. thanks the British Council for a Studentship.
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