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High resolution photoelectron spectra of gasphase molecules using synchrotron radiation
Journal of Electron Spectroscopy and Related Phenomena, 41 (1988) 187-196 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
187
HIGH RESOLUTION PHOTOELECTRON SPECTRA OF GASPHASE MOLECULES USING SYNCHROTRON RADIATION*
G. MICHAEL BANCROFT, JOHN D. BOZEK, JEFF N. CUTLER and KIM H. TAN Department of Chemistry and Centre for Chemical Physics, The University of Western Ontario, London, Ontario N6A 5B7 (Canada) and Canadian &VZChFOtFOn Radiation Facility, Wisconsin, Madison, WI 53589 (U.S.A.)
SynChFOtFOn
Radiation Centre, University of
(Received 24 August 1987)
ABSTRACT The Canadian beamline has recently been transferred from the low energy, low intensity storage ring Tantalus to the higher energy, high brightness Aladdin ring. The high intensity from the beamline, and the good resolution of the Mark IV Grasshopper grazing incidence monochromator enables us to obtain high resolution core level and valence band spectra up to 200 eV photon energies. A Xe4d spectrum of Xe gas yields a total Xe4d,,, linewidth of 0.23 eV, and a Si2p spectrum of SiCl, yields a Si2p,,, linewidth of 0.33 eV. High resolution valence band spectra of CF,Cl up to 170 eV show that relative intensities fluctuate markedly at kinetic energies of > 80 eV. Both theoretical MS-Xa cross sections and branching ratios, and experimental branching ratios show fairly strong resonances between 80 eV and 110 eV kinetic energies.
INTRODUCTION
The development of monochromatized X-ray photoelectron (or ESCA) instruments by Gelius and Siegbahn et al. [ 1,2] for gas phase measurements showed, for the first time, the great importance of high resolution ( < 0.3 eV) for studying core levels and valence band spectra. For example, such resolution is critical for resolving vibrational structure on core levels and for resolving complex valence band spectra. In the last five years, a number of groups have used monochromatized synchrotron radiation between 10 eV and N 100 eV photon energies at good resolution ( < 0.5 eV) to study the energy dependence of valence level o andP values of polyatomic molecules [ 3-8 and refs. therein]. However, there have been no spectra of valence bands reported above N 115 eV photon energy even at moderate resolutions, and no reports of high reso*Submitted as part of the celebration of the 70th birthday of Professor Kai Siegbahn.
0368-2048/88/$03.50
0 1988 Elsevier Science Publishers B.V.
188 Au
Diode
Current
From
285 PHOTON
85 ENERGY
CSRF
l530
42 (eV)
Fig. 1. A comparison of the photon flux from the Aladdin ring to that from Tantalus as measured by a gold diode at the exit slit of the monochromator. Note the nonlinearity of the photon energy scale. Note that 10 nA current corresponds to about 6 X 10” photons s-l.
lution spectra of core levels having binding energies 250 eV. There is still usually insufficient intensity to operate monochromators and electron analyzers at their highest resolutions and, until very recently, the ultimate resolutions of monochromators above 300 eV photon energies has been > 0.3 eV. With the advent of new high intensity, high brightness storage rings such as Aladdin, BESSY and the 750 MeV NSLS ring, it has become possible to obtain high resolution ( -0.2 eV total instrumental width) gas phase spectra up to a300 eV photon energies. Herein, we describe our first high resolution gas phase photoelectron results after the Canadian Synchrotron Radiation Facility [g-11] was transferred from the 240 MeV Tantalus storage ring to Aladdin operating at 800 MeV. Total instrumental widths of 0.15-0.4 eV enable both high resolution core and valence band spectra up to N 200 eV photon energies to be obtained. The valence band spectra of CF,Cl between 70 eV and 170 eV shows interesting fluctuations which are well reproduced by our MS-Xcu calculations. EXPERIMENTAL
The windowless Canadian beamline, and the Leybold-Heraeus photoelectron spectrometer, have been described in detail in previous papers [g-11]. On transferring the beamline from Tantalus to Aladdin in 1986, the beamline optics were replaced. In particular a new elliptical Ml mirror yielded a much better focus on the entrance slit of the 2 metre grazing incidence “Grasshopper” monochromator [ 121. This monochromator yields very high intensities between 20 eV and 1000 eV (Fig. 1) and very high resolutions between 20 eV
189 TABLE 1 CSRF resolution (in eV) for different photon energies for 1Opm and 100pm monochromator slits with a 1200 ~~ve/mrn grating” Photon energy
40 eV 100 eV 200 eV 300 eV 500 eV 700 eV 1000 eV
Photon width (eV) 10 pm $its (0.04 A resolution)
100 fim slits (0.4 A resolution)
0.005 0.032 0.13 0.30 0.81 1.5 3.2
0.05 0.32 1.3 3.0 8.1 15 32
“These are theoretical values, but a number of spectra taken between 50 eV and 800 eV (e.g. Fig. 3) show that the experimental values are very close to the theoretical ones.
and 300 eV (Table 1) . Figure 1 illustrates the intensity with 100 pm entrance and exit slits (0.4 A resolution using a 1200 g/mm JY grating) at the exit slit of the monochromator, as recorded by a gold diode. The 10 nA Au diode current corresponds to - 6 x 1O’l photons see -’ at a beam current of 27 mA on Aladdin but substantially higher intensities are now obtained due to the larger beam currents circulating in Aladdin (up to 200 mA) . These larger beam currents are required when the ultimate resolution of 0.04 A is used (10 pm slits) because the intensity at 0.04 A resolution is close to two orders of magnitude lower than at 0.4 A resolution. It is important to note that the intensity from the beamline on the Aladdin ring is much higher than that from Tantalus at all energies. At 100 eV, we have - 50 times as much intensity as on Tantalus; and by 200 eV, the Aladdin ring yields orders of magnitude more intensity than Tantalus. The intensity from the beamline on the Aladdin ring is much larger at all energies at 0.4 A than is available, for example, from a He1 lamp (typically 1-3 x 10” photons s-l). The decreased intensity at the C and 0 K edges is due to contamination and grows as the optics and grating become contaminated. The intensity drops off above 600 eV, and the scattered light contribution becomes large at - 900 eV. It is important to comment on the photon resolution resulting from the Grasshopper monochromator (Table 1) . Below 100 eV, the resolution is excellent, and above 200 eV, the resolution and intensity of CSRF is competitive with any existing beamline. Above w 500 eV, such a grazing incidence monochromator is not competitive because of the poor resolution. The Kr3d gas phase absorption spectrum at - 90 eV (Fig. 2) taken with 20 pm slits resolves peaks which are separated by - 130 meV, and shows that the monochromator
I.
90.3
91.3
91.3
93.3
94.3
95.3
Photon Energy (eV) Fig. 2. Gas phase photoabsorption spectrum of krypton obtained using - 55 meV photon resolution. The assigned spectral features correspond to electronic excitations from the 3d spin-orbit pair of orbitals into p-type Rydberg orbitals [ 121.
is yielding very close to the theoretical resolution of 0.08 A (55 meV at 90 eV) . The absorbance features illustrated correspond to electronic excitations from the core Kr3cl spin orbit pair of orbitals to thep type Rydberg Kr orbitals [ 131. Such absorption spectra do not require the high intensities that are necessary for high resolution photoelectron spectra. The diverging beam from the monochromator exit slit is refocused to a spot size of - 5 mm horizontal by - 1 mm vertical in the sample chamber through a very efficient two stage differential pumping chamber, and a high vacuum gate value with a rectangular glass light guide (1mm x 10 mm X 190 mm long) . This light guide enables us to run photoelectron spectra without windows at sample chamber pressures of - 10e5 torr [ lo]. The sample chamber and combined lens-analyzer system were purchased from Leybold-Heraeus. The lens analyzer is mounted at the “magic angle” so that electron intensities are independent of /I and the polarization of the incident radiation. The gas was leaked to the interaction region through either a single hole of N 50 pm or a multicapillary array [lo], RESULTS
Core levels On the Aladdin ring, the intensity is great enough that we are able to decrease the monochromator slit widths to near the 10 pm minimum and still obtain reasonable photoelectron intensities. The Xe4d spectrum taken at Aladdin (Fig. 3a) at photon and electron resolutions yielded a total width of
191
71.0 70.0 69.0 68.0 67.0 66.
Binding Energy (eV) Fig. 3. High resolution Xe4d photoelectron spectra of xenon gas measured at CSRF at: Aladdin ring (top) and Tantalus (bottom) both using 94 eV photons. The top spectrum was taken with 25 pm slits and 12.5 eV pass energy (0.1 eV photon and electron resolutions), and took - 15 minutes to record. The bottom spectrum was taken with 30 pm slits and 25 eV pass energy (0.2 eV electron resolution) and took - 3 hours to record.
Binding
Energy (eV)
Fig. 4. The Si2p photoelectron spectrum of gaseous SiCl, obtained at the Aladdin ring using a photon energy of 125 eV and photon and electron resolutions of -0.15 eV each.
0.23eV. This spectrum was taken at electron and photon resolutions of 0.1 eV. This appears to be the narrowest core level width yet observed, and the spectrum took only N 15 minutes to record. A very recent spectrum taken with even better total instrumental resolution ~0.1 eV gave a total Xe4d width of 0.20
250
j&xl
--
1120 .-
ff$I
no rv
25.1
- VALENCE BAND
pa
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vmev
26.1
u.3
n.6
l4.3
ll.6
h
7.2.4
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Emw
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Fig. 5. Photoelectron spectra of the valence bands of CF&l at 170 eV, 130 eV, and 100 eV taken with -0.4 eV resolution for electrons and between 0.12 eV (100 eV) and 0.4 eV (170 eV) for photons.
eV. Although there may be a small amount of charge broadening ( < 0.05 eV) , these spectra indicate that the inherent width of the Xe4d line is larger than the 0.13 eV measured by King et al. [ 141. The previously published spectrum [lo] taken at Tantalus (Fig. 3b) was taken at poorer resolution; and took much longer to accumulate a poorer quality spectrum. The difference between the two spectra is consistent with the fifty times more flux at Aladdin relative to Tantalus (Fig. 1) . Similarly, the Si2p photoelectron spectrum of Sic& (Fig. 4) exhibits a linewidth of 0.33 eV, and enables a clear resolution of the Si2p spin-orbit splitting. This spectrum is broadened somewhat by both vibrational and charging effects. Similar resolution has recently enabled Brion et al. [ 151 to resolve the multitude of many body lines associated with the Ar3s ionization. This resolution should now enable resolution of vibrational [ 1,2] and ligand field effects [ 161 on several core levels with binding energies < 150 eV [ 171. Valence bands
Valence band cross sections decrease strongly as the exciting photon energy increases [ 181. With CSRF on Tantalus, we had difficulty obtaining high resolution spectra above 70 eV photon energy for any molecule. Our recent results [ 191 indicated that valence band cross sections, as well as core level cross
193
sections, can show resonant-type behaviour at high kinetic energies ( > 50 eV) . With the increased intensities on Aladdin, we have obtained high resolution valence band spectra of CF, and CF&l up to 200 eV photon energy. Figure 5 shows the photoelectron spectra of CF&l at 100 eV, 130 eV and 170 eV. These spectra show similar resolution to the He1 spectra taken earlier [ 201, and the low energy synchrotron radiation spectra [ 21,221. Experimental branching ratios have been obtained from the fitted areas. MS-Xa! calculations on CF&l have been performed in an analogous fashion to that for CFJ [ 41 and these will be published elsewhere [ 221. The theoretical cross sections are shown in Figs. 6-11, as well as the theoretical and experimental and branching ratios. There are three interesting observations from these data. Firstly, there are resonant-like structures between 80 eV and 110 eV kinetic energy in the theoretical cross sections for many orbitals. Secondly, there are large changes in both experimental and theoretical branching ratios mainly due to these resonant-like structures. Thirdly, the agreement between experiment and theory is surprisingly good, since at high kinetic energies MS-Xa calculations underestimate the exchange potential [ 231. The branching ratios will not be nearly as sensitive as the cross sections to any errors in the theoretical cross section trend. However, even the cross sections and their trends with photon energy are in reasonable agreement with those expected on the basis of atomic cross sections [ 181 and the Gelius model [ 24,221. The resonant structures are probably due to electron scattering such as is normally seen in EXAFS of core levels
1251. CONCLUSIONS
High resolution core and valence level spectra can now be obtained out to 200 eV using monochromatized synchroton radiation at the Canadian Synchrotron Radiation Facility. Total linewidths of 0.2-0.3 eV are readily obtainable. Such resolution is imperative for studying splittings on core levels, and obtaining branching ratios and partial cross sections for valence levels at photon energies above N 70 eV. For CF&l valence bands, both theoretical MS-Xa and experimental branching ratios show high energy resonant-like structures, the origin of which needs to be further examined. With new high intensity wiggler and undulator beamlines [ 261, it will be possible to obtain much higher fluxes than are available now, and therefore even higher photon resolutions will be available. Along with new high resolution monochromators at > 200 eV photon energies, it should soon be possible to take spectra up to 500 or 600 eV photon energy at total photon plus electron resolutions of N 0.1 eV.
194 Photoelectron 60
70
80
FJ
Energy
a0
lx)
Photoelectron
(eV)
120
130
!a
60
60 6.75
’
70
Energy
80
90
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lx)
’
’
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’
(eV) ls0
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-
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oi 70
M
90
ml
lx)
Photon
120
ls0
Energy
!a
60
lso
no
Photon
(eV)
Energy
(eV)
Fig. 6. Plots of theoretical MS-Xa! cross sections and theoretical and experimental branching ratio for the 5e orbital from 70 eV to 170 eV. Note the experimental error bars on the branching ratios.
/Y t
Fig. 7. Plots of theoretical MS-Xa cross sections and theoretical and experimental branching ratios for the next three orbitals 5al + laz + 4e. All three theoretical cross sections show resonances between 100 and 120 eV. Photoelectron en
70
90
80
M
Energy a0
Ill
lvl llo
Photon
Energy
(eV) 120
M
!A0
(eV)
l.30
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160 l-n
Photon
Energy
(eV)
Fig. 8. Plots of theoretical MS-XCI cross sections and theoretical and experimental branching ratios for the 3e orbital.
Fig. 9. Plots of theoretical MS-Xcz cross sections and theoretical and experimental branching ratios for the 4a, orbital.
195 PhoMectron 50
M
70
0
Energf
90
lw
IO
(eV) PO
!30
MO
50
p
60
Pholoetectron
Energy
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(eV) 120
w
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qw’m
M
39
m in m w ua Photon
Enerq
(eV)
60
60
no
m8090mmuowua~ww Photon
Energy
(eV)
Fig. 10. Plots of theoretical MS-Xa cross sections and theoretical and experimental branching ratios for the 2e orbital. Fig. 11. Plots of theoretical MS-X& cross sections and theoretical and experimental branching ratios for the 3a, orbital. ACKNOWLEDGEMENTS
The authors would like to acknowledge the assistance of the staff at the Synchrotron Radiation Center (Stoughton) , and the helpful comments of J.S. Tse. We are grateful to the National Research Council (NRC) of Canada, the Natural Sciences and Engineering Research Council (NSERC ) of Canada, and the University of Western Ontario for financial support. REFERENCES U. Gelius, E. Basilier, S. Svensson, T. Bergmark and K. Siegbahn, J. Electron Spectrosc. Relat. Phenom., 2 (1973) 279. U. Gelius, L. Asplund, E. Basilier, S. Hedman, K. Helenelund and K. Siegbahn, Nucl. Instrum. Methods, 31 (1984) 85. T.A. Carlson, M.O. Krause, W.A. Svensson, P. Gerard, F.A. Grimm, T.A. Whitley and B.P. Pullen, Z. Phys. D, 2 (1986) 309 and references cited therein. B.W. Yates, K.H. Tan, G.M. Bancroft, L.L. Coatsworth, J.S. Tse and G.J. Schrobilgen, J. Chem. Phys., 84 (1986) 3603; 85 (1986) 3840 and references cited therein. G.G. Be de Souza, P. Morin and I. Nenner, Phys. Rev. A, 34 (1986) 4770 and references cited therein. I. Novak, J.M. Benson and A.W. Potts, Chem. Phys., 104 (1986) 153 and references cited therein. G. Cooper, J.C. Green, M.P. Payne, B.R. Dobson and I.H. Hellier, J. Am. Chem. Sot., 109 (1987) 3836 and references cited therein.
196
8 MS. Banna, A. Kossman and V. Schmidt, Chem. Phys., 114 (1987) 157 and references cited 9
10 11 12 13 14 15 16 17 18 19 20 21
22 23 24 25 26
therein. K.H. Tan, G.M. Bancroft, L.L. Coatsworth and B.W. Yates, Can. J. Phys., 60 (1982) 131. B.W. Yates, K.H. Tan, L.L. Coatsworth and G.M. Bancroft, Phys. Rev. A, 31 (1985) 1529. K.H. Tan, P.C. Cheng, G.M. Bancroft and J.Wm. McGowan, Can. J. Spectrosc., 29 (1984) 1381. F.C. Brown, R.Z. Bachrach and N. Lien, Nucl. Instrum. Methods, 152 (1978) 73. R. Haensel, G. Reitel, P. Schreiber and C. Kunz, Phys. Rev., 188 (1969) 1375. G.C. King, M. Tronc, F.H. Read and R.O. Bradford, J. Phys. B, 10 (1977) 2479. C.E. Brion, A.O. Bawagan and K.H. Tan, Chem. Phys. Lett., 134 (1987) 76. G.M. Bancroft and J.S. Tse, Comments Inorg. Chem., V (1986) 89. R.P. Gupta, J.S. Tse and G.M. Bancroft, Philos. Trans. R. Sot. London, 293 (1980) 535. I. Lindau and J.J. Yeh, At. Nucl. Data Tables, 32 (1985) 1. B.M. Addison, K.H. Tan, B.W. Yates, J.N. Cutler, G.M. Bancroft and J.S. Tse, J. Chem. Phys., submitted. T. Curtas, H. Gusten and L. Klasina L., J. Chem. Phys., 67 (1977) 2687. A.W. Potts, I. Novak, F. Quinn, G.V. Marr, B. Dobson, I.H. Hillier and J.B. Mist, J. Phys. B., 18 (1985) 3177; I. Novak, J.M. Benson and A.W. Potts, J. Electron Spectrosc. Relat. Phenom., 41 (1986) 175. J.D. Bozek, J.N. Cutler, K.H. Tan, B.W. Yates, G.M. Bancroft and J.S. Tse, to be published. S.A. Chou, F.W. Kutzler, D.E. Ellis, G.K. Shenay, T.I. Morrison and P.A. Montano, Phys. Rev. B, 331 (1985) 1069. U. Gelius, J. Electron Spectrosc. Relat. Phenom., 5 (1974) 985. P.A. Lee, P.H. Citrin, P. Eisenburger and B.M. Kencaid, Rev. Mod. Phys., 53 (1981) 769. P. Eisenburger, Science, 231 (1986) 687.
187
HIGH RESOLUTION PHOTOELECTRON SPECTRA OF GASPHASE MOLECULES USING SYNCHROTRON RADIATION*
G. MICHAEL BANCROFT, JOHN D. BOZEK, JEFF N. CUTLER and KIM H. TAN Department of Chemistry and Centre for Chemical Physics, The University of Western Ontario, London, Ontario N6A 5B7 (Canada) and Canadian &VZChFOtFOn Radiation Facility, Wisconsin, Madison, WI 53589 (U.S.A.)
SynChFOtFOn
Radiation Centre, University of
(Received 24 August 1987)
ABSTRACT The Canadian beamline has recently been transferred from the low energy, low intensity storage ring Tantalus to the higher energy, high brightness Aladdin ring. The high intensity from the beamline, and the good resolution of the Mark IV Grasshopper grazing incidence monochromator enables us to obtain high resolution core level and valence band spectra up to 200 eV photon energies. A Xe4d spectrum of Xe gas yields a total Xe4d,,, linewidth of 0.23 eV, and a Si2p spectrum of SiCl, yields a Si2p,,, linewidth of 0.33 eV. High resolution valence band spectra of CF,Cl up to 170 eV show that relative intensities fluctuate markedly at kinetic energies of > 80 eV. Both theoretical MS-Xa cross sections and branching ratios, and experimental branching ratios show fairly strong resonances between 80 eV and 110 eV kinetic energies.
INTRODUCTION
The development of monochromatized X-ray photoelectron (or ESCA) instruments by Gelius and Siegbahn et al. [ 1,2] for gas phase measurements showed, for the first time, the great importance of high resolution ( < 0.3 eV) for studying core levels and valence band spectra. For example, such resolution is critical for resolving vibrational structure on core levels and for resolving complex valence band spectra. In the last five years, a number of groups have used monochromatized synchrotron radiation between 10 eV and N 100 eV photon energies at good resolution ( < 0.5 eV) to study the energy dependence of valence level o andP values of polyatomic molecules [ 3-8 and refs. therein]. However, there have been no spectra of valence bands reported above N 115 eV photon energy even at moderate resolutions, and no reports of high reso*Submitted as part of the celebration of the 70th birthday of Professor Kai Siegbahn.
0368-2048/88/$03.50
0 1988 Elsevier Science Publishers B.V.
188 Au
Diode
Current
From
285 PHOTON
85 ENERGY
CSRF
l530
42 (eV)
Fig. 1. A comparison of the photon flux from the Aladdin ring to that from Tantalus as measured by a gold diode at the exit slit of the monochromator. Note the nonlinearity of the photon energy scale. Note that 10 nA current corresponds to about 6 X 10” photons s-l.
lution spectra of core levels having binding energies 250 eV. There is still usually insufficient intensity to operate monochromators and electron analyzers at their highest resolutions and, until very recently, the ultimate resolutions of monochromators above 300 eV photon energies has been > 0.3 eV. With the advent of new high intensity, high brightness storage rings such as Aladdin, BESSY and the 750 MeV NSLS ring, it has become possible to obtain high resolution ( -0.2 eV total instrumental width) gas phase spectra up to a300 eV photon energies. Herein, we describe our first high resolution gas phase photoelectron results after the Canadian Synchrotron Radiation Facility [g-11] was transferred from the 240 MeV Tantalus storage ring to Aladdin operating at 800 MeV. Total instrumental widths of 0.15-0.4 eV enable both high resolution core and valence band spectra up to N 200 eV photon energies to be obtained. The valence band spectra of CF,Cl between 70 eV and 170 eV shows interesting fluctuations which are well reproduced by our MS-Xcu calculations. EXPERIMENTAL
The windowless Canadian beamline, and the Leybold-Heraeus photoelectron spectrometer, have been described in detail in previous papers [g-11]. On transferring the beamline from Tantalus to Aladdin in 1986, the beamline optics were replaced. In particular a new elliptical Ml mirror yielded a much better focus on the entrance slit of the 2 metre grazing incidence “Grasshopper” monochromator [ 121. This monochromator yields very high intensities between 20 eV and 1000 eV (Fig. 1) and very high resolutions between 20 eV
189 TABLE 1 CSRF resolution (in eV) for different photon energies for 1Opm and 100pm monochromator slits with a 1200 ~~ve/mrn grating” Photon energy
40 eV 100 eV 200 eV 300 eV 500 eV 700 eV 1000 eV
Photon width (eV) 10 pm $its (0.04 A resolution)
100 fim slits (0.4 A resolution)
0.005 0.032 0.13 0.30 0.81 1.5 3.2
0.05 0.32 1.3 3.0 8.1 15 32
“These are theoretical values, but a number of spectra taken between 50 eV and 800 eV (e.g. Fig. 3) show that the experimental values are very close to the theoretical ones.
and 300 eV (Table 1) . Figure 1 illustrates the intensity with 100 pm entrance and exit slits (0.4 A resolution using a 1200 g/mm JY grating) at the exit slit of the monochromator, as recorded by a gold diode. The 10 nA Au diode current corresponds to - 6 x 1O’l photons see -’ at a beam current of 27 mA on Aladdin but substantially higher intensities are now obtained due to the larger beam currents circulating in Aladdin (up to 200 mA) . These larger beam currents are required when the ultimate resolution of 0.04 A is used (10 pm slits) because the intensity at 0.04 A resolution is close to two orders of magnitude lower than at 0.4 A resolution. It is important to note that the intensity from the beamline on the Aladdin ring is much higher than that from Tantalus at all energies. At 100 eV, we have - 50 times as much intensity as on Tantalus; and by 200 eV, the Aladdin ring yields orders of magnitude more intensity than Tantalus. The intensity from the beamline on the Aladdin ring is much larger at all energies at 0.4 A than is available, for example, from a He1 lamp (typically 1-3 x 10” photons s-l). The decreased intensity at the C and 0 K edges is due to contamination and grows as the optics and grating become contaminated. The intensity drops off above 600 eV, and the scattered light contribution becomes large at - 900 eV. It is important to comment on the photon resolution resulting from the Grasshopper monochromator (Table 1) . Below 100 eV, the resolution is excellent, and above 200 eV, the resolution and intensity of CSRF is competitive with any existing beamline. Above w 500 eV, such a grazing incidence monochromator is not competitive because of the poor resolution. The Kr3d gas phase absorption spectrum at - 90 eV (Fig. 2) taken with 20 pm slits resolves peaks which are separated by - 130 meV, and shows that the monochromator
I.
90.3
91.3
91.3
93.3
94.3
95.3
Photon Energy (eV) Fig. 2. Gas phase photoabsorption spectrum of krypton obtained using - 55 meV photon resolution. The assigned spectral features correspond to electronic excitations from the 3d spin-orbit pair of orbitals into p-type Rydberg orbitals [ 121.
is yielding very close to the theoretical resolution of 0.08 A (55 meV at 90 eV) . The absorbance features illustrated correspond to electronic excitations from the core Kr3cl spin orbit pair of orbitals to thep type Rydberg Kr orbitals [ 131. Such absorption spectra do not require the high intensities that are necessary for high resolution photoelectron spectra. The diverging beam from the monochromator exit slit is refocused to a spot size of - 5 mm horizontal by - 1 mm vertical in the sample chamber through a very efficient two stage differential pumping chamber, and a high vacuum gate value with a rectangular glass light guide (1mm x 10 mm X 190 mm long) . This light guide enables us to run photoelectron spectra without windows at sample chamber pressures of - 10e5 torr [ lo]. The sample chamber and combined lens-analyzer system were purchased from Leybold-Heraeus. The lens analyzer is mounted at the “magic angle” so that electron intensities are independent of /I and the polarization of the incident radiation. The gas was leaked to the interaction region through either a single hole of N 50 pm or a multicapillary array [lo], RESULTS
Core levels On the Aladdin ring, the intensity is great enough that we are able to decrease the monochromator slit widths to near the 10 pm minimum and still obtain reasonable photoelectron intensities. The Xe4d spectrum taken at Aladdin (Fig. 3a) at photon and electron resolutions yielded a total width of
191
71.0 70.0 69.0 68.0 67.0 66.
Binding Energy (eV) Fig. 3. High resolution Xe4d photoelectron spectra of xenon gas measured at CSRF at: Aladdin ring (top) and Tantalus (bottom) both using 94 eV photons. The top spectrum was taken with 25 pm slits and 12.5 eV pass energy (0.1 eV photon and electron resolutions), and took - 15 minutes to record. The bottom spectrum was taken with 30 pm slits and 25 eV pass energy (0.2 eV electron resolution) and took - 3 hours to record.
Binding
Energy (eV)
Fig. 4. The Si2p photoelectron spectrum of gaseous SiCl, obtained at the Aladdin ring using a photon energy of 125 eV and photon and electron resolutions of -0.15 eV each.
0.23eV. This spectrum was taken at electron and photon resolutions of 0.1 eV. This appears to be the narrowest core level width yet observed, and the spectrum took only N 15 minutes to record. A very recent spectrum taken with even better total instrumental resolution ~0.1 eV gave a total Xe4d width of 0.20
250
j&xl
--
1120 .-
ff$I
no rv
25.1
- VALENCE BAND
pa
l6.7
22.4
vmev
26.1
u.3
n.6
l4.3
ll.6
h
7.2.4
(9.7 m&l
Emw
l7.0 WI
Fig. 5. Photoelectron spectra of the valence bands of CF&l at 170 eV, 130 eV, and 100 eV taken with -0.4 eV resolution for electrons and between 0.12 eV (100 eV) and 0.4 eV (170 eV) for photons.
eV. Although there may be a small amount of charge broadening ( < 0.05 eV) , these spectra indicate that the inherent width of the Xe4d line is larger than the 0.13 eV measured by King et al. [ 141. The previously published spectrum [lo] taken at Tantalus (Fig. 3b) was taken at poorer resolution; and took much longer to accumulate a poorer quality spectrum. The difference between the two spectra is consistent with the fifty times more flux at Aladdin relative to Tantalus (Fig. 1) . Similarly, the Si2p photoelectron spectrum of Sic& (Fig. 4) exhibits a linewidth of 0.33 eV, and enables a clear resolution of the Si2p spin-orbit splitting. This spectrum is broadened somewhat by both vibrational and charging effects. Similar resolution has recently enabled Brion et al. [ 151 to resolve the multitude of many body lines associated with the Ar3s ionization. This resolution should now enable resolution of vibrational [ 1,2] and ligand field effects [ 161 on several core levels with binding energies < 150 eV [ 171. Valence bands
Valence band cross sections decrease strongly as the exciting photon energy increases [ 181. With CSRF on Tantalus, we had difficulty obtaining high resolution spectra above 70 eV photon energy for any molecule. Our recent results [ 191 indicated that valence band cross sections, as well as core level cross
193
sections, can show resonant-type behaviour at high kinetic energies ( > 50 eV) . With the increased intensities on Aladdin, we have obtained high resolution valence band spectra of CF, and CF&l up to 200 eV photon energy. Figure 5 shows the photoelectron spectra of CF&l at 100 eV, 130 eV and 170 eV. These spectra show similar resolution to the He1 spectra taken earlier [ 201, and the low energy synchrotron radiation spectra [ 21,221. Experimental branching ratios have been obtained from the fitted areas. MS-Xa! calculations on CF&l have been performed in an analogous fashion to that for CFJ [ 41 and these will be published elsewhere [ 221. The theoretical cross sections are shown in Figs. 6-11, as well as the theoretical and experimental and branching ratios. There are three interesting observations from these data. Firstly, there are resonant-like structures between 80 eV and 110 eV kinetic energy in the theoretical cross sections for many orbitals. Secondly, there are large changes in both experimental and theoretical branching ratios mainly due to these resonant-like structures. Thirdly, the agreement between experiment and theory is surprisingly good, since at high kinetic energies MS-Xa calculations underestimate the exchange potential [ 231. The branching ratios will not be nearly as sensitive as the cross sections to any errors in the theoretical cross section trend. However, even the cross sections and their trends with photon energy are in reasonable agreement with those expected on the basis of atomic cross sections [ 181 and the Gelius model [ 24,221. The resonant structures are probably due to electron scattering such as is normally seen in EXAFS of core levels
1251. CONCLUSIONS
High resolution core and valence level spectra can now be obtained out to 200 eV using monochromatized synchroton radiation at the Canadian Synchrotron Radiation Facility. Total linewidths of 0.2-0.3 eV are readily obtainable. Such resolution is imperative for studying splittings on core levels, and obtaining branching ratios and partial cross sections for valence levels at photon energies above N 70 eV. For CF&l valence bands, both theoretical MS-Xa and experimental branching ratios show high energy resonant-like structures, the origin of which needs to be further examined. With new high intensity wiggler and undulator beamlines [ 261, it will be possible to obtain much higher fluxes than are available now, and therefore even higher photon resolutions will be available. Along with new high resolution monochromators at > 200 eV photon energies, it should soon be possible to take spectra up to 500 or 600 eV photon energy at total photon plus electron resolutions of N 0.1 eV.
194 Photoelectron 60
70
80
FJ
Energy
a0
lx)
Photoelectron
(eV)
120
130
!a
60
60 6.75
’
70
Energy
80
90
m
lx)
’
’
1
’
(eV) ls0
120
CF$I
-
MO
M)
5qtlqt4e
,;;I
oi 70
M
90
ml
lx)
Photon
120
ls0
Energy
!a
60
lso
no
Photon
(eV)
Energy
(eV)
Fig. 6. Plots of theoretical MS-Xa! cross sections and theoretical and experimental branching ratio for the 5e orbital from 70 eV to 170 eV. Note the experimental error bars on the branching ratios.
/Y t
Fig. 7. Plots of theoretical MS-Xa cross sections and theoretical and experimental branching ratios for the next three orbitals 5al + laz + 4e. All three theoretical cross sections show resonances between 100 and 120 eV. Photoelectron en
70
90
80
M
Energy a0
Ill
lvl llo
Photon
Energy
(eV) 120
M
!A0
(eV)
l.30
w
MO
WI
c
;’
l70
160 l-n
Photon
Energy
(eV)
Fig. 8. Plots of theoretical MS-XCI cross sections and theoretical and experimental branching ratios for the 3e orbital.
Fig. 9. Plots of theoretical MS-Xcz cross sections and theoretical and experimental branching ratios for the 4a, orbital.
195 PhoMectron 50
M
70
0
Energf
90
lw
IO
(eV) PO
!30
MO
50
p
60
Pholoetectron
Energy
70
XI0
M
90
ID
(eV) 120
w
la
1
@;2-iz
04.10..
O.O221
fq-w;;:;
qw’m
M
39
m in m w ua Photon
Enerq
(eV)
60
60
no
m8090mmuowua~ww Photon
Energy
(eV)
Fig. 10. Plots of theoretical MS-Xa cross sections and theoretical and experimental branching ratios for the 2e orbital. Fig. 11. Plots of theoretical MS-X& cross sections and theoretical and experimental branching ratios for the 3a, orbital. ACKNOWLEDGEMENTS
The authors would like to acknowledge the assistance of the staff at the Synchrotron Radiation Center (Stoughton) , and the helpful comments of J.S. Tse. We are grateful to the National Research Council (NRC) of Canada, the Natural Sciences and Engineering Research Council (NSERC ) of Canada, and the University of Western Ontario for financial support. REFERENCES U. Gelius, E. Basilier, S. Svensson, T. Bergmark and K. Siegbahn, J. Electron Spectrosc. Relat. Phenom., 2 (1973) 279. U. Gelius, L. Asplund, E. Basilier, S. Hedman, K. Helenelund and K. Siegbahn, Nucl. Instrum. Methods, 31 (1984) 85. T.A. Carlson, M.O. Krause, W.A. Svensson, P. Gerard, F.A. Grimm, T.A. Whitley and B.P. Pullen, Z. Phys. D, 2 (1986) 309 and references cited therein. B.W. Yates, K.H. Tan, G.M. Bancroft, L.L. Coatsworth, J.S. Tse and G.J. Schrobilgen, J. Chem. Phys., 84 (1986) 3603; 85 (1986) 3840 and references cited therein. G.G. Be de Souza, P. Morin and I. Nenner, Phys. Rev. A, 34 (1986) 4770 and references cited therein. I. Novak, J.M. Benson and A.W. Potts, Chem. Phys., 104 (1986) 153 and references cited therein. G. Cooper, J.C. Green, M.P. Payne, B.R. Dobson and I.H. Hellier, J. Am. Chem. Sot., 109 (1987) 3836 and references cited therein.
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8 MS. Banna, A. Kossman and V. Schmidt, Chem. Phys., 114 (1987) 157 and references cited 9
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