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The influence of soft mode adsorbate vibrations on NEXAFS analyses: NO on Pd{110}
Volume 185,number 5,6
CHEMICAL PHYSICSLETTERS
25 October I99 1
The influence of soft mode adsorbate vibrations on NEXAFS analyses: NO on Pd{ 1lo} Jagriti Singh ‘, W.K. Walter, A. Atrei 2 and D.A. King Deparfmenl oJChemistry, University of Catnbrrdge, Lensfieid Road, Cambridge CB2 1EW UK
Received I 1 June 1991;in final form 19July 1991
Near-edgeX-ray absorption fine structure (NEXAFS) measurementshave been performedon NO adsorbed on Pd{1IO}using a polarized light source at a range of incidence angles. An analysis of intensities of o and n. resonance features is made which includes both the averageorientation of the moleculeand contributions arising from a low frequency, in-plane hindered-translation mode. The analysis indicates that at all coverages NO is upright, with a hindered-translation mode frequency of 68 cm-’ directed along the (001 ) azimuth. The normal frozen-moleculeanalysis would, incorrectly, yield a tilt angle of 22”; adsorbate molecularorientation cannot be reliably obtained from NEXAFSdata at 300 K without includingthe effect ofthese high-amplitude modes.
1. Introduction
modes which are outside the range of infrared studies using conventional light sources.
Near-edge
X-ray absorption fine structure has been used extensively to study the molecular orientation of adsorbed species on various metal surfaces [ 11. In the analysis of the data a frozen-molecule approach has been used to identify molecular orientation. The precision of the technique is found to be limited by problems with normalization and background subtraction [ 2-61. In presenting here the first direct study of the orientation of NO adsorbed on Pd{ 1lo} at both low and saturation coverage, both the data acquisition and the analysis have been simplified. For the first time the effect of molecular vibration has been quantitatively included in the NEXAFS data analysis for molecular orientation, and the potential of the technique is demonstrated for measuring high-amplitude vibrational modes of the adsorbate. This renders NEXAFS an extremely useful technique for measuring these low-frequency adsorbate vibrational (NEXAFS)
’ Nehru Memorial Trust Fellow at the Daresbury Synchrotron Source, UK, on leave from Physics Department, Rajasthan University, Jaipur, India. ’ Present address: Department of Chemistry, University of Florence, 50121 Florence, Italy.
426
2. Experimental The experiment was conducted at the Daresbury Synchrotron Radiation Source on the new undulator beam line ( 5U. 1; energy range 1OO- 1000 eV) . The high-intensity (~5 x IO” photons/s at N K-edge), tunable, monochromatic, elliptically polarized light from a beamline without entrance slits was allowed to fall perpendicular to the axis of rotation of the NOcovered Pd{ 110) surface at different incidence angles, and nitrogen K-edge NEXAFS was measured using a HA100 VSW multichannel electron energy analyser (used in single channel mode). The analyzer was tuned to the N Auger peak and NEXAFS spectra were recorded by measuring the strength of the Auger signal as a function of incident photon energy around the nitrogen K-edge. The Auger yield measurements were made at constant emission angle by keeping the analyzer at an angle of 45” to the incident beam as well as to the plane swept out by the surface normal. A grid was not installed between the light source and the crystal for flux measurements, as is normal [ 21. This removed the problem of any
0009-2614/9I/$ 03.50 0 1991 Elsevier Science Publishers B.V. All rights reserved.
Volume 185, number 5,6
CHEMICAL PHYSICS LETTERS
structure being introduced by the grid in the absorption spectra. This mode of operation is particularly suitable on the Daresbury light source, where the long radiation lifetime ( z 35 h) means that there is no need for decay normalization. At each incident angle of the beam NEXAFS spectra were recorded alternately for both the clean and the NO dosed Pd{ 1101 surface, without changing the position; both sets of data are therefore obtained at exactly the same position and with similar beam characteristics. Before each run the crystal was flashed clean and redosed with NO. The measurements were repeated to check the reproducibility. The photon energy was not calibrated on an absolute scale, although the linearity of the scale was checked over a wide energy range for every injection, a number of times. The manipulator position was set to a polar angle accuracy of 0.5’ [ 71. Three sets of data were recorded, two for saturation coverage of NO on Pd{ 1IO} at 300 K, one of these being with the horizontal component of the E-vector (El ) in the (001) direction, the second with ,??Iin the ( 1TO) direction; and the third set for low coverage (0.3 of saturation coverage) in the (001 ) direction.
3. Results Fig. 1 shows typical data obtained for both clean and NO-dosed surfaces. Peak A is the x resonance
Clean
1000 370
380
390
400
410
420
Photon energy (eV1
Fig. I. Directly recorded N-edge NEXAFS spectra from clean Pd{ 1IO} and the same crystal (with saturation coverage of NO at 300 K. for normal incidence with (El) along (001)). The photon energy scale has not been absolutely calibrated.
25 October 1991
arising from an atomic 1s to molecular K*bound-state transition of the photo-excited electron, and peak B is the CJresonance arising due to a 1s to o* transition. Since the photon energy is not calibrated the small shifts observed in the energy at which the A* resonance is observed are not real effects, but energy differences between the (5 and K resonances can be trusted. Fig. 2 shows the three sets of data after background removal. To achieve this, the spectrum of the dosed surface has been divided by the spectrum of the clean surface. This method eliminates any monochromator structure and transmission characteristics and also the effect of the detector response function. Every spectrum shown in fig. 2 is the sum of all the spectra recorded for a given geometry under the stated conditions, after background removal. Each of the three sets show that the o resonance gradually evolves as the angle of the photon beam is varied from nonnat to grazing incidence, indicating that NO is more upright than lying down on the Pd{ 110) surface. As the incidence angle moves towards the surface normal the o resonance shifts in energy towards the n resonance. The energy shift is ~2 eV. As can be seen in fig. 3a, the halfwidth of the small peak at the D resonance position at an incidence angle of 90” is smaller than that observed at 20”. Previous workers in the field [2,4-6,8-l I] have observed similar peaks at 90” in the o resonance position where an upright assignment of molecules is preferred, and similarly peaks in the region of the K resonance where a parallel assignment of adsorbate is preferred. These small peaks have been assigned to other artifacts, such as anomalous background. SEXAFS structure, additional resonance (although weaker) arising from adsorbate-substrate bonds, other ISor n: transitions, and a suggestion that the assumption regarding the strongly directional nature of the o resonance is incorrect. Thus, Somers et al. [4] suggest that in the case of a truly parallel-bonded species there may be a non-zero probability of excitation of the o resonance by the component of the E-vector perpendicular to the surface. All of these are tentative explanations for non-zero intensity in the o and II peak positions where the authors are led to believe that the adsorbed molecule is either exactly perpendicular or exactly parallel to the surface plane, and have no actual experimental or theoretical backing. This 427
Volume 185, number 5,6
CHEMICAL PHYSICS LETTERS
25 October 1991
additional resonances from adsorbate-substrate bonds would occur almost exactly underneath o or n: resonances of relevance for different adsorbates on different substrates, without exception. We believe that these peaks are not artifacts but must be assigned to the o resonance itself. In support of this, we have observed a similar shift ( z 2 eV) of the o resonance towards lower energy for very low coverages (0.055 of saturation coverage) of NO on Pd{ I lo}, as shown in fig. 3b. It may be argued that at very low coverage, since the CJresonance is very small at this coverage, the peak should be dominated by anomalous background features, which would increase the G/R intensity ratio. In fact the o/n intensity ratio at very low coverage is less than that observed at 0.3 of saturation coverage. Both the finite o intensity at 90” incidence and the energy shift are dealt with below.
4. Data analysis with vibrational motion
r
I
30
400
420
410
Photon
energy
(eV)
Fig. 2. NEXAFS spectra for different angles of incidence of the photon beam, after background normalization for (a) saturation coverage of NO on Pd{ 1IO), El along ( 00 I ) direction, (b ) saturation coverage with E” along ( I TO),(c) lowcoverage (0.3 of saturation) with El along (001). The angles marked on the spectra indicate the angle ofincidence of the photon beam to the ( 1 IO} surface. Each spectrum shown is the sum of a set of scans. The Y-axis is in arbitrary units so as to represent all the spectra separately.
is a particularly awkward situation for a technique which is explicitly intended to provide a measure of molecular orientation. It is rather improbable that an anomalous background, SEXAFS structure or 428
The influence of vibrational motions on NEXAFS analyses have only been dealt with qualitatively to date [ 121. The electron emission process takes place on a time scale which is short ( o 1O-l5 s) compared with that for vibrational motion (E lo-” s), and the NEXAFS spectrum therefore represents an integrated emission spectrum over the time-averaged motions of the adsorbate. For an upright molecule, NEXAFS resonance intensities are relatively insensitive to tilt angles less than =: 15”, and for small-amplitude, high-frequency modes the frozen-molecule approach is reasonable. However, the amplitude of some low-frequency modes can be large, and ignoring these modes can give very misleading orientational analyses. A possible explanation for the persistent o resonance at 90” is that it is due to bending modes parallel to the surface plane. The shift in peak position.may be due to structural alterations; thus, at lower coverages, due to stronger backbonding, the equilibrium interatomic distance (d) of the NO bond may increase, which would give rise to a shift in the o resonance peak to lower energy [ 131, as observed. Since increased backbonding strengthens the Pd-N
bond, this would lead to a loweringof the amplitude of the bending mode at very low coverages. This would therefore also account for a lower CT/Kinten-
Volume 185, number 5,6
25 October 1991
CHEMICAL PHYSICS LETTERS
LO{,
,
,
380
400
420
Photon energy (eV)
zoo{ , 400
,
,
,
410
420
430
Photon
energy
(eV)
Fig. 3. (a) N-edge NEXAFS spectra after background subtraction, for 90” and 20” incidence angle of the photon beam for saturation coverage of NO on Pd{ IIO}with E” along ( li0). The onset of the a resonance in both the spectra is at the same photon energy but the peak position is shifted by ~2 eV towards lower energy for the normal incidence spectrum. (b) Directly recorded N-edges NEXAFS spectra for low (0.3 ofsaturation) and very low (0.055 of saturation) coverage al grazing incidence (20” ) of the photon beam with E" along (001).
sity ratio observed at these low coverages. The o resonance peak energy is lower at normal incidence than at other angles. This observation has also been made for many other systems. This can also be attributed to a change in the N-O d-spacing during bending, with d increasing as a function of increasing tilt angle. With adsorbates on metals, due to electron-hole pairing interatomic equilibrium spacings may become configuration-dependent. Larger tilt angles may be expected to result in an increase in dr* backbonding, and hence an increase in the spacing d. The observed rs resonance at close to normal incidence is only sensitive to molecules with a large instantaneous tilt angle, and reflects the larger d-spacing, whereas away from normal incidence the o resonance observed is dominated by emission from species with low instantaneous tilt angles. Both hindered-translation and rotational modes would contribute to the bending of the molecule. Excited levels of the hindered-translation mode will be more heavily populated at room temperature than those of the hindered-rotational mode because of the larger energy spacing for the latter, as concluded, for example, by Henderson et al. [ 141 in explaining the
thermal broadening of the CO ESDIAD pattern for CO desorbed from Pt{ 112}. In the quantitative analysis of the present data we have modelled the incidence-angle dependence of the ratio I,/ (I,$ I,), where I, and I, are the integrated intensities of G and n resonances obtained from the adsorbate-covered spectra ratioed to the clean surface spectra, and then linear-background subtracted. This ratio is independent of flux variations between runs: and also of detector geometry or absorption cross section. It is only dependent on (i) the polarization and incidence angle of the photon beam, and (ii) the polar and azimuthal tilt directions of the adsorbate molecular axis. The data are presented as intensity ratio versus incidence angle plots in fig. 4. The reason for the spread in the data points can be traced mainly to the change in polarization from one run to another due to beam movement. The Daresbury undulator beamline 5U. 1 has no entrance slit. This is to increase the photon flux for faster and better data collection but has the disadvantage that the polarization alters due to electron beam movement in the storage ring with time, and from one fill to another. These changes may be more than 10% [ 151. The bending magnet also has 429
----___ .______y. “\
Volume 185, number $6
x--..
.a._
0 60oso-
G _
0 v x .E 2 G1
(a)
0.30. 0.200.80
molecule we have assumed a normal distribution of 0.70-
z
OKI-
8 5
0.50-
5 E
0 40-
E
0.30-
.s$
0 zo-
m
3 ._ -3
E b Z
..
II \_
0.40-
z
variation is not due to inaccuracy in the measurements but because of the actual change in polarization due to source movement and the absence of an entrance slit. For beam lines with fixed polarization such a scatter would not occur, and the above form of curve fitting would provide greater precision. In the present work inaccuracies in the analyses with curve fitting can be attributed largely to the resultant spread in the data points. To include the effect of vibration of the adsorbate
1 . .:__
0.70-
2 0.70-
060oso-
_;;..._ -. \‘\
the population density of the only vibrational mode with a large amplitude. Z, and I, resonance intensities for instantaneous electron emission at a tilt angle 1y,have been previously derived in the frozen-molecule approach [ 11; then the normalized (3 resonance intensity obtained upon including the molecular vibration is given by
4
‘s....
(d
x
Azimuth 2
b
lb
satumon coverage
io io Incidence
40
io
$0
io
8b
sb
angle of beam (deg)
Fig. 4. Optimum theoretical fit as described in the text to the three sets of experimental data. (a) Data set for saturation coverage, El along ( I IO). Thebest fit theoreticalcurveis for horizontal polarization p=74% and an rms tilt angle of 25” along (00 I ) The two dashed curves are for rms tilt angles of 23 ’ and 27” respectively. (b) Data set for low coverage (0.3 ofsaturation coverage) along (001). The best fit is for p= 80% and an rms tilt angle of 25’ along (00 I ) The dashed curves are for rms tilt angles of 23” and 27’ respectively. (c) Data set for saturation cobrerage, El along ( I TO). The best lit is forp=68% and rms tilt angles of 25” along CODI) and 0” along (I i0). The dashed curves are for tilt angles of 23” and 27’ along (00 I ) The polarization of the undulator beamline lies in the range 74 & 6% in the horizontal direction.
the effect of decreasing the polarization while enhancing the flux as compared to ordinary synchrotron beam lines. From our NEXAFS data the polarization of the new undulator beam line was found to be 74+6% in the horizontal direction. The large 430
25 October 199 I
CHEMICAL PHYSICS LETTERS
where G(cu,) gives the population density distribution ofthe vibrational mode with respect to the equilibrium axial tilt angle (Yof the NO molecule; (Y,denotes the instantaneous tilt angle of the molecule to the surface normal, at the moment of photoelectron emission. 0 is the incidence angle of the photon beam to the surface plane, Q is the azimuthal angle of the resultant dynamic tilt of the adsorbed molecule and p is the horizontal polarization of the incident beam. The best-fit to our data obtained with the parallel component of the electric vector of the incident radiation in each of the azimuths is shown in fig. 4. This is obtained with the NO molecule vibrating around an upright axial position with its mean normal to the surface, and a single dominant vibrational mode at room temperature, a hindered translation along (001). For this fit the function G( a,) is taken as: G(rUi)= ---exp($/2&,,) %..‘fi
,
where ay,, is the rms value of the tilt angle. .4s shown elsewhere [7], for the incident radiation along the (001) azimuth, @=O, and
Volume 185, number 5,6
CHEMICAL PHYSICS LETTERS
25 October 1991
I
O-+ I
I
I
I
2 cos*Oco& + 2 sin2%sin*a = cos*8cos*atsin’Osin*at
I tR’
For the incident radiation along ( 1TO), @=x/2, and (a)
2 cos20 cos’a + 2R sin’s = ltRtcos2Bcos~atRsin2a’ where R = ( 1 -p) /p. A more rigorous approach would require replacement of the normal distribution by that given by the eigenfunction of the adsorbate molecule as an oscillator. This would be needed in the analysis of lowtemperature measurements, where the eigenfunctions of the most dominant modes of vibration are known. The fit obtained to the data in both azimuths using the above expressions gives an rms value of 25” for the tilt angle of NO to the surface normal in the (001) direction, across the atomic rows in the { 110) surface. This corresponds to a frequency for hindered translation of 68 cm-‘, assuming a spacing between the Pd-Pd axis and the 0 atom to be 2.6 to 2.7 A for bridge-bonded NO, obtained from the N-O and metal-N bond lengths [ 161, as calculated from the expression
02
(c)
Fig. 5. (a) The Pd( 1 IO} unit mesh. (b) The hindered-translation vibrational mode along ( 1 IO). (c) The low-frequency mode along (001).
beam from run to run. The dashed curves in fig. 4 indicate an error margin of 25 ?2”, which would in turn yield a margin on the estimated vibrational frequency of 68&8 cm-‘. Analyses of vibrational spectra for NO on Pd{ 110) are consistent with the dominance of a bridge-bonded species, attached to adjacent Pd atoms along the (00 1) rows [ 191. As indicated in fig. 5, this species will have two in-plane hindered-translation modes in orthogonal directions parallel to the surface plane. However, the mode along the ( li0) azimuth involves substantial Pd-N bond stretching, and would have a low amplitude and high frequency. The mode directed along ( 00 1) has a large amplitude and low frequency; it should be in the region 30-100 cm-’ [ 181. The frequency and tilt azimuth derived in the present work are therefore perfectly consistent with this assignment.
5. Conclusion where Q, is the vibrational normal coordinate, ui is the frequency of the vibrational mode, and T is the temperature [ 17,181. On the other hand, analysis of the same data with the frozen-molecule approach yields an NO molecule inclined at an angle of 22’ to the surface normal in the (00 1) direction [ 7 1. Since a diatomic molecule such as NO adsorbed in an essentially upright configuration must exhibit a highamplitude in-plane vibrational mode parallel to the surface plane, the former analysis is clearly more accurate. Estimates of errors on the rms tilt angle obtained are difficult to quantify, since much of the scatter in the data points around the theoretical lines arises from variations in the polarization of the photon
For upright NO on Pd{ 110) the hindered-translation vibrational mode along (001 ), with a frequency determined as 68 cm-‘, makes a significant contribution to the o resonance intensity in NEXAFS spectra at incidence angles close to the surface normal, and cannot be ignored in determinations of molecular orientation. Since soft vibrational modes in the surface plane will always produce largeamplitude motion for molecular adsorbates at temperatures of around 300 K, this conclusion has widespread implications for the analysis of NEXAFS data from molecular adsorbates. An optimal study, currently in progress in our work, would involve cooling the crystal to very low temperatures for an accurate 431
Volume 185, number 5,6
CHEMICAL PHYSICS LETTERS
determination of molecular orientation, and warming in successive stages to 300 K to determine the soft-mode frequency. NEXAFS thus becomes a powerful technique for the determination of the frequency, and, for a crystal plane with twofold symmetry, the bending-plane azimuth for low-frequency modes parallel to the surface.
25 October I99
I
[ 41 J. Somers, M.E. Kordesch, T.H. Lindner, H. Conrad, A.M. Bradshaw and G.P. Williams, SurfaceSci. 188 (1987) L693.
[ 51M. Bader, B. Hillert, A. Puschmann, J. Haase and A.M. Bradshaw, Europhys. Letters 5 ( 1988) 443. [ 61 R.J. Madlx, J.L. Solomon and J. StGhr, Surface Sci. 197 (1988) L253.
[ 71 J. Sit@, A. Atrei, W. Walter and D.A. King, to be published. [ 811. Stiihr, K. Baberschke, R. Jaeger, R. Treischlu and S. Brennan, Phys. Rev. Letters 47 (1981) 381.
[ 911. Stiihr and R. Jaeger, Phys. Rev. B 26 ( 1982) 41 I I. Acknowledgement
[IO] G. Tomillon, S. Raaen, T.A. Skotheim, M. Sagurton, R. Garrett and G.P. Williams, Surface Sci. 184 (1987) L345.
[I I] A. Puschmann, J. Haase, M.D. Crapper, C.E. Riley and D.P. The SERC is acknowledged for postdoctoral support to AA and WKW, for an equipment grant and for beam time at Daresbury. We are grateful to Sam Haq for providing a clean Pd{ 1 IO} crystal, and the Daresbury staff for skilled, efficient support, with special thanks to C. Mythen.
Woodruff, Phys. Rev. Letters 54 (1985) 2250.
[ 121 P.A. Stevens, R.J. Madix and J. Stiihr, Surface Sci. 230 (1990) I.
[ 131 D. Arvanitis, V. Dobler, L. Wenzel, K. Baberschke and J. Stiihr, Surface Sci. 178 ( 1986) 686. [ 141 M.A. Henderson, A. Szabo and J.T. Yates Jr., J. Chem. Phys. 91 (1989) 7255.
[ 151 HA. Padmore, private communication. [ 161 C.-T. Kao, G.S. Blackman, M.A. van Hove, GA. Somorjai References
and C.-M. Chan, Surface Scl. 224 ( 1989) 77.
[ 171 S.J. Cyrin, Molecular [I ] J. Stiihr and D.A. Outka, Phys. Rev. B 36 ( 1987) 7891. [I!] D.A. Outka and J. Stiihr, J. Chem. Phys. 88 ( 1988) 3539. [3] M. Bader, A. Puschmann and J. Haase, Phys. Rev. B 33 ( 1986) 7336.
432
wbratlons
and
mean
square
amplitudes (Elsevier, Amsterdam, 1968).
[ 181 N.V. Richardson and A.M. Bradshaw, Surface Sci. 88 (1979) 255. [ 191 S. Haq, R. Raval and D.A. King. to be published.
CHEMICAL PHYSICSLETTERS
25 October I99 1
The influence of soft mode adsorbate vibrations on NEXAFS analyses: NO on Pd{ 1lo} Jagriti Singh ‘, W.K. Walter, A. Atrei 2 and D.A. King Deparfmenl oJChemistry, University of Catnbrrdge, Lensfieid Road, Cambridge CB2 1EW UK
Received I 1 June 1991;in final form 19July 1991
Near-edgeX-ray absorption fine structure (NEXAFS) measurementshave been performedon NO adsorbed on Pd{1IO}using a polarized light source at a range of incidence angles. An analysis of intensities of o and n. resonance features is made which includes both the averageorientation of the moleculeand contributions arising from a low frequency, in-plane hindered-translation mode. The analysis indicates that at all coverages NO is upright, with a hindered-translation mode frequency of 68 cm-’ directed along the (001 ) azimuth. The normal frozen-moleculeanalysis would, incorrectly, yield a tilt angle of 22”; adsorbate molecularorientation cannot be reliably obtained from NEXAFSdata at 300 K without includingthe effect ofthese high-amplitude modes.
1. Introduction
modes which are outside the range of infrared studies using conventional light sources.
Near-edge
X-ray absorption fine structure has been used extensively to study the molecular orientation of adsorbed species on various metal surfaces [ 11. In the analysis of the data a frozen-molecule approach has been used to identify molecular orientation. The precision of the technique is found to be limited by problems with normalization and background subtraction [ 2-61. In presenting here the first direct study of the orientation of NO adsorbed on Pd{ 1lo} at both low and saturation coverage, both the data acquisition and the analysis have been simplified. For the first time the effect of molecular vibration has been quantitatively included in the NEXAFS data analysis for molecular orientation, and the potential of the technique is demonstrated for measuring high-amplitude vibrational modes of the adsorbate. This renders NEXAFS an extremely useful technique for measuring these low-frequency adsorbate vibrational (NEXAFS)
’ Nehru Memorial Trust Fellow at the Daresbury Synchrotron Source, UK, on leave from Physics Department, Rajasthan University, Jaipur, India. ’ Present address: Department of Chemistry, University of Florence, 50121 Florence, Italy.
426
2. Experimental The experiment was conducted at the Daresbury Synchrotron Radiation Source on the new undulator beam line ( 5U. 1; energy range 1OO- 1000 eV) . The high-intensity (~5 x IO” photons/s at N K-edge), tunable, monochromatic, elliptically polarized light from a beamline without entrance slits was allowed to fall perpendicular to the axis of rotation of the NOcovered Pd{ 110) surface at different incidence angles, and nitrogen K-edge NEXAFS was measured using a HA100 VSW multichannel electron energy analyser (used in single channel mode). The analyzer was tuned to the N Auger peak and NEXAFS spectra were recorded by measuring the strength of the Auger signal as a function of incident photon energy around the nitrogen K-edge. The Auger yield measurements were made at constant emission angle by keeping the analyzer at an angle of 45” to the incident beam as well as to the plane swept out by the surface normal. A grid was not installed between the light source and the crystal for flux measurements, as is normal [ 21. This removed the problem of any
0009-2614/9I/$ 03.50 0 1991 Elsevier Science Publishers B.V. All rights reserved.
Volume 185, number 5,6
CHEMICAL PHYSICS LETTERS
structure being introduced by the grid in the absorption spectra. This mode of operation is particularly suitable on the Daresbury light source, where the long radiation lifetime ( z 35 h) means that there is no need for decay normalization. At each incident angle of the beam NEXAFS spectra were recorded alternately for both the clean and the NO dosed Pd{ 1101 surface, without changing the position; both sets of data are therefore obtained at exactly the same position and with similar beam characteristics. Before each run the crystal was flashed clean and redosed with NO. The measurements were repeated to check the reproducibility. The photon energy was not calibrated on an absolute scale, although the linearity of the scale was checked over a wide energy range for every injection, a number of times. The manipulator position was set to a polar angle accuracy of 0.5’ [ 71. Three sets of data were recorded, two for saturation coverage of NO on Pd{ 1IO} at 300 K, one of these being with the horizontal component of the E-vector (El ) in the (001) direction, the second with ,??Iin the ( 1TO) direction; and the third set for low coverage (0.3 of saturation coverage) in the (001 ) direction.
3. Results Fig. 1 shows typical data obtained for both clean and NO-dosed surfaces. Peak A is the x resonance
Clean
1000 370
380
390
400
410
420
Photon energy (eV1
Fig. I. Directly recorded N-edge NEXAFS spectra from clean Pd{ 1IO} and the same crystal (with saturation coverage of NO at 300 K. for normal incidence with (El) along (001)). The photon energy scale has not been absolutely calibrated.
25 October 1991
arising from an atomic 1s to molecular K*bound-state transition of the photo-excited electron, and peak B is the CJresonance arising due to a 1s to o* transition. Since the photon energy is not calibrated the small shifts observed in the energy at which the A* resonance is observed are not real effects, but energy differences between the (5 and K resonances can be trusted. Fig. 2 shows the three sets of data after background removal. To achieve this, the spectrum of the dosed surface has been divided by the spectrum of the clean surface. This method eliminates any monochromator structure and transmission characteristics and also the effect of the detector response function. Every spectrum shown in fig. 2 is the sum of all the spectra recorded for a given geometry under the stated conditions, after background removal. Each of the three sets show that the o resonance gradually evolves as the angle of the photon beam is varied from nonnat to grazing incidence, indicating that NO is more upright than lying down on the Pd{ 110) surface. As the incidence angle moves towards the surface normal the o resonance shifts in energy towards the n resonance. The energy shift is ~2 eV. As can be seen in fig. 3a, the halfwidth of the small peak at the D resonance position at an incidence angle of 90” is smaller than that observed at 20”. Previous workers in the field [2,4-6,8-l I] have observed similar peaks at 90” in the o resonance position where an upright assignment of molecules is preferred, and similarly peaks in the region of the K resonance where a parallel assignment of adsorbate is preferred. These small peaks have been assigned to other artifacts, such as anomalous background. SEXAFS structure, additional resonance (although weaker) arising from adsorbate-substrate bonds, other ISor n: transitions, and a suggestion that the assumption regarding the strongly directional nature of the o resonance is incorrect. Thus, Somers et al. [4] suggest that in the case of a truly parallel-bonded species there may be a non-zero probability of excitation of the o resonance by the component of the E-vector perpendicular to the surface. All of these are tentative explanations for non-zero intensity in the o and II peak positions where the authors are led to believe that the adsorbed molecule is either exactly perpendicular or exactly parallel to the surface plane, and have no actual experimental or theoretical backing. This 427
Volume 185, number 5,6
CHEMICAL PHYSICS LETTERS
25 October 1991
additional resonances from adsorbate-substrate bonds would occur almost exactly underneath o or n: resonances of relevance for different adsorbates on different substrates, without exception. We believe that these peaks are not artifacts but must be assigned to the o resonance itself. In support of this, we have observed a similar shift ( z 2 eV) of the o resonance towards lower energy for very low coverages (0.055 of saturation coverage) of NO on Pd{ I lo}, as shown in fig. 3b. It may be argued that at very low coverage, since the CJresonance is very small at this coverage, the peak should be dominated by anomalous background features, which would increase the G/R intensity ratio. In fact the o/n intensity ratio at very low coverage is less than that observed at 0.3 of saturation coverage. Both the finite o intensity at 90” incidence and the energy shift are dealt with below.
4. Data analysis with vibrational motion
r
I
30
400
420
410
Photon
energy
(eV)
Fig. 2. NEXAFS spectra for different angles of incidence of the photon beam, after background normalization for (a) saturation coverage of NO on Pd{ 1IO), El along ( 00 I ) direction, (b ) saturation coverage with E” along ( I TO),(c) lowcoverage (0.3 of saturation) with El along (001). The angles marked on the spectra indicate the angle ofincidence of the photon beam to the ( 1 IO} surface. Each spectrum shown is the sum of a set of scans. The Y-axis is in arbitrary units so as to represent all the spectra separately.
is a particularly awkward situation for a technique which is explicitly intended to provide a measure of molecular orientation. It is rather improbable that an anomalous background, SEXAFS structure or 428
The influence of vibrational motions on NEXAFS analyses have only been dealt with qualitatively to date [ 121. The electron emission process takes place on a time scale which is short ( o 1O-l5 s) compared with that for vibrational motion (E lo-” s), and the NEXAFS spectrum therefore represents an integrated emission spectrum over the time-averaged motions of the adsorbate. For an upright molecule, NEXAFS resonance intensities are relatively insensitive to tilt angles less than =: 15”, and for small-amplitude, high-frequency modes the frozen-molecule approach is reasonable. However, the amplitude of some low-frequency modes can be large, and ignoring these modes can give very misleading orientational analyses. A possible explanation for the persistent o resonance at 90” is that it is due to bending modes parallel to the surface plane. The shift in peak position.may be due to structural alterations; thus, at lower coverages, due to stronger backbonding, the equilibrium interatomic distance (d) of the NO bond may increase, which would give rise to a shift in the o resonance peak to lower energy [ 131, as observed. Since increased backbonding strengthens the Pd-N
bond, this would lead to a loweringof the amplitude of the bending mode at very low coverages. This would therefore also account for a lower CT/Kinten-
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LO{,
,
,
380
400
420
Photon energy (eV)
zoo{ , 400
,
,
,
410
420
430
Photon
energy
(eV)
Fig. 3. (a) N-edge NEXAFS spectra after background subtraction, for 90” and 20” incidence angle of the photon beam for saturation coverage of NO on Pd{ IIO}with E” along ( li0). The onset of the a resonance in both the spectra is at the same photon energy but the peak position is shifted by ~2 eV towards lower energy for the normal incidence spectrum. (b) Directly recorded N-edges NEXAFS spectra for low (0.3 ofsaturation) and very low (0.055 of saturation) coverage al grazing incidence (20” ) of the photon beam with E" along (001).
sity ratio observed at these low coverages. The o resonance peak energy is lower at normal incidence than at other angles. This observation has also been made for many other systems. This can also be attributed to a change in the N-O d-spacing during bending, with d increasing as a function of increasing tilt angle. With adsorbates on metals, due to electron-hole pairing interatomic equilibrium spacings may become configuration-dependent. Larger tilt angles may be expected to result in an increase in dr* backbonding, and hence an increase in the spacing d. The observed rs resonance at close to normal incidence is only sensitive to molecules with a large instantaneous tilt angle, and reflects the larger d-spacing, whereas away from normal incidence the o resonance observed is dominated by emission from species with low instantaneous tilt angles. Both hindered-translation and rotational modes would contribute to the bending of the molecule. Excited levels of the hindered-translation mode will be more heavily populated at room temperature than those of the hindered-rotational mode because of the larger energy spacing for the latter, as concluded, for example, by Henderson et al. [ 141 in explaining the
thermal broadening of the CO ESDIAD pattern for CO desorbed from Pt{ 112}. In the quantitative analysis of the present data we have modelled the incidence-angle dependence of the ratio I,/ (I,$ I,), where I, and I, are the integrated intensities of G and n resonances obtained from the adsorbate-covered spectra ratioed to the clean surface spectra, and then linear-background subtracted. This ratio is independent of flux variations between runs: and also of detector geometry or absorption cross section. It is only dependent on (i) the polarization and incidence angle of the photon beam, and (ii) the polar and azimuthal tilt directions of the adsorbate molecular axis. The data are presented as intensity ratio versus incidence angle plots in fig. 4. The reason for the spread in the data points can be traced mainly to the change in polarization from one run to another due to beam movement. The Daresbury undulator beamline 5U. 1 has no entrance slit. This is to increase the photon flux for faster and better data collection but has the disadvantage that the polarization alters due to electron beam movement in the storage ring with time, and from one fill to another. These changes may be more than 10% [ 151. The bending magnet also has 429
----___ .______y. “\
Volume 185, number $6
x--..
.a._
0 60oso-
G _
0 v x .E 2 G1
(a)
0.30. 0.200.80
molecule we have assumed a normal distribution of 0.70-
z
OKI-
8 5
0.50-
5 E
0 40-
E
0.30-
.s$
0 zo-
m
3 ._ -3
E b Z
..
II \_
0.40-
z
variation is not due to inaccuracy in the measurements but because of the actual change in polarization due to source movement and the absence of an entrance slit. For beam lines with fixed polarization such a scatter would not occur, and the above form of curve fitting would provide greater precision. In the present work inaccuracies in the analyses with curve fitting can be attributed largely to the resultant spread in the data points. To include the effect of vibration of the adsorbate
1 . .:__
0.70-
2 0.70-
060oso-
_;;..._ -. \‘\
the population density of the only vibrational mode with a large amplitude. Z, and I, resonance intensities for instantaneous electron emission at a tilt angle 1y,have been previously derived in the frozen-molecule approach [ 11; then the normalized (3 resonance intensity obtained upon including the molecular vibration is given by
4
‘s....
(d
x
Azimuth 2
b
lb
satumon coverage
io io Incidence
40
io
$0
io
8b
sb
angle of beam (deg)
Fig. 4. Optimum theoretical fit as described in the text to the three sets of experimental data. (a) Data set for saturation coverage, El along ( I IO). Thebest fit theoreticalcurveis for horizontal polarization p=74% and an rms tilt angle of 25” along (00 I ) The two dashed curves are for rms tilt angles of 23 ’ and 27” respectively. (b) Data set for low coverage (0.3 ofsaturation coverage) along (001). The best fit is for p= 80% and an rms tilt angle of 25’ along (00 I ) The dashed curves are for rms tilt angles of 23” and 27’ respectively. (c) Data set for saturation cobrerage, El along ( I TO). The best lit is forp=68% and rms tilt angles of 25” along CODI) and 0” along (I i0). The dashed curves are for tilt angles of 23” and 27’ along (00 I ) The polarization of the undulator beamline lies in the range 74 & 6% in the horizontal direction.
the effect of decreasing the polarization while enhancing the flux as compared to ordinary synchrotron beam lines. From our NEXAFS data the polarization of the new undulator beam line was found to be 74+6% in the horizontal direction. The large 430
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CHEMICAL PHYSICS LETTERS
where G(cu,) gives the population density distribution ofthe vibrational mode with respect to the equilibrium axial tilt angle (Yof the NO molecule; (Y,denotes the instantaneous tilt angle of the molecule to the surface normal, at the moment of photoelectron emission. 0 is the incidence angle of the photon beam to the surface plane, Q is the azimuthal angle of the resultant dynamic tilt of the adsorbed molecule and p is the horizontal polarization of the incident beam. The best-fit to our data obtained with the parallel component of the electric vector of the incident radiation in each of the azimuths is shown in fig. 4. This is obtained with the NO molecule vibrating around an upright axial position with its mean normal to the surface, and a single dominant vibrational mode at room temperature, a hindered translation along (001). For this fit the function G( a,) is taken as: G(rUi)= ---exp($/2&,,) %..‘fi
,
where ay,, is the rms value of the tilt angle. .4s shown elsewhere [7], for the incident radiation along the (001) azimuth, @=O, and
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CHEMICAL PHYSICS LETTERS
25 October 1991
I
O-+ I
I
I
2 cos*Oco& + 2 sin2%sin*a = cos*8cos*atsin’Osin*at
I tR’
For the incident radiation along ( 1TO), @=x/2, and (a)
2 cos20 cos’a + 2R sin’s = ltRtcos2Bcos~atRsin2a’ where R = ( 1 -p) /p. A more rigorous approach would require replacement of the normal distribution by that given by the eigenfunction of the adsorbate molecule as an oscillator. This would be needed in the analysis of lowtemperature measurements, where the eigenfunctions of the most dominant modes of vibration are known. The fit obtained to the data in both azimuths using the above expressions gives an rms value of 25” for the tilt angle of NO to the surface normal in the (001) direction, across the atomic rows in the { 110) surface. This corresponds to a frequency for hindered translation of 68 cm-‘, assuming a spacing between the Pd-Pd axis and the 0 atom to be 2.6 to 2.7 A for bridge-bonded NO, obtained from the N-O and metal-N bond lengths [ 161, as calculated from the expression
02
(c)
Fig. 5. (a) The Pd( 1 IO} unit mesh. (b) The hindered-translation vibrational mode along ( 1 IO). (c) The low-frequency mode along (001).
beam from run to run. The dashed curves in fig. 4 indicate an error margin of 25 ?2”, which would in turn yield a margin on the estimated vibrational frequency of 68&8 cm-‘. Analyses of vibrational spectra for NO on Pd{ 110) are consistent with the dominance of a bridge-bonded species, attached to adjacent Pd atoms along the (00 1) rows [ 191. As indicated in fig. 5, this species will have two in-plane hindered-translation modes in orthogonal directions parallel to the surface plane. However, the mode along the ( li0) azimuth involves substantial Pd-N bond stretching, and would have a low amplitude and high frequency. The mode directed along ( 00 1) has a large amplitude and low frequency; it should be in the region 30-100 cm-’ [ 181. The frequency and tilt azimuth derived in the present work are therefore perfectly consistent with this assignment.
5. Conclusion where Q, is the vibrational normal coordinate, ui is the frequency of the vibrational mode, and T is the temperature [ 17,181. On the other hand, analysis of the same data with the frozen-molecule approach yields an NO molecule inclined at an angle of 22’ to the surface normal in the (00 1) direction [ 7 1. Since a diatomic molecule such as NO adsorbed in an essentially upright configuration must exhibit a highamplitude in-plane vibrational mode parallel to the surface plane, the former analysis is clearly more accurate. Estimates of errors on the rms tilt angle obtained are difficult to quantify, since much of the scatter in the data points around the theoretical lines arises from variations in the polarization of the photon
For upright NO on Pd{ 110) the hindered-translation vibrational mode along (001 ), with a frequency determined as 68 cm-‘, makes a significant contribution to the o resonance intensity in NEXAFS spectra at incidence angles close to the surface normal, and cannot be ignored in determinations of molecular orientation. Since soft vibrational modes in the surface plane will always produce largeamplitude motion for molecular adsorbates at temperatures of around 300 K, this conclusion has widespread implications for the analysis of NEXAFS data from molecular adsorbates. An optimal study, currently in progress in our work, would involve cooling the crystal to very low temperatures for an accurate 431
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determination of molecular orientation, and warming in successive stages to 300 K to determine the soft-mode frequency. NEXAFS thus becomes a powerful technique for the determination of the frequency, and, for a crystal plane with twofold symmetry, the bending-plane azimuth for low-frequency modes parallel to the surface.
25 October I99
I
[ 41 J. Somers, M.E. Kordesch, T.H. Lindner, H. Conrad, A.M. Bradshaw and G.P. Williams, SurfaceSci. 188 (1987) L693.
[ 51M. Bader, B. Hillert, A. Puschmann, J. Haase and A.M. Bradshaw, Europhys. Letters 5 ( 1988) 443. [ 61 R.J. Madlx, J.L. Solomon and J. StGhr, Surface Sci. 197 (1988) L253.
[ 71 J. Sit@, A. Atrei, W. Walter and D.A. King, to be published. [ 811. Stiihr, K. Baberschke, R. Jaeger, R. Treischlu and S. Brennan, Phys. Rev. Letters 47 (1981) 381.
[ 911. Stiihr and R. Jaeger, Phys. Rev. B 26 ( 1982) 41 I I. Acknowledgement
[IO] G. Tomillon, S. Raaen, T.A. Skotheim, M. Sagurton, R. Garrett and G.P. Williams, Surface Sci. 184 (1987) L345.
[I I] A. Puschmann, J. Haase, M.D. Crapper, C.E. Riley and D.P. The SERC is acknowledged for postdoctoral support to AA and WKW, for an equipment grant and for beam time at Daresbury. We are grateful to Sam Haq for providing a clean Pd{ 1 IO} crystal, and the Daresbury staff for skilled, efficient support, with special thanks to C. Mythen.
Woodruff, Phys. Rev. Letters 54 (1985) 2250.
[ 121 P.A. Stevens, R.J. Madix and J. Stiihr, Surface Sci. 230 (1990) I.
[ 131 D. Arvanitis, V. Dobler, L. Wenzel, K. Baberschke and J. Stiihr, Surface Sci. 178 ( 1986) 686. [ 141 M.A. Henderson, A. Szabo and J.T. Yates Jr., J. Chem. Phys. 91 (1989) 7255.
[ 151 HA. Padmore, private communication. [ 161 C.-T. Kao, G.S. Blackman, M.A. van Hove, GA. Somorjai References
and C.-M. Chan, Surface Scl. 224 ( 1989) 77.
[ 171 S.J. Cyrin, Molecular [I ] J. Stiihr and D.A. Outka, Phys. Rev. B 36 ( 1987) 7891. [I!] D.A. Outka and J. Stiihr, J. Chem. Phys. 88 ( 1988) 3539. [3] M. Bader, A. Puschmann and J. Haase, Phys. Rev. B 33 ( 1986) 7336.
432
wbratlons
and
mean
square
amplitudes (Elsevier, Amsterdam, 1968).
[ 181 N.V. Richardson and A.M. Bradshaw, Surface Sci. 88 (1979) 255. [ 191 S. Haq, R. Raval and D.A. King. to be published.