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The intrinsic linewidth of the 4f levels in gold as determined by photoemission
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a a a a a a Volume 54A, number 1
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11 August 1975 a a a
THE INTRINSIC LINEWIDTH OF THE 4f LEVELS IN GOLD AS DETERMINED BY PHOTOEMISSION I. LINDAU, P. PIANETTA, K. YU and W.E. SPICER Stanford Electronics Laboratories, Stanford University, Stanford, Ca. 94305, USA Received 23 May 1975 High resolution photoemission experiments have been performed using synchrotron radiation at two different photo energies, 8000 eV and 160 eV, on the 4f doublet of Au giving an inherent linewidth of less than 0.3 eV in good agreement with McGuire’s estimate of 0.23 eV.
In this letter we with to report on determination of the inherent linewidth of the 4f levels of gold by means of the photoemission technique. The 4f levels of gold have been studied extensively [11with conventional X-ray sources, Al Ka-radiation at hi.’ = 1486.7 eV and Mg K 0-radiation at hi.’ = 1253.6 eV. These techniques have an instrumental resolution of about 1 eV for studies of solids (0.5—0.6 eV for monochromatized Xrays [1]) and are too gross to give us reliable linewidths and shapes for inherently very narrow photolines. In this work we report on studies at two completely dif ferent photon energy ranges, 8 keV and obtained with synchrotron radiation and100—200 where theeV, instrumental resolution is considerably improved, Synchrotron radiation from the newly established SSRP (Stanford Synchrotron Radiation Project) was used as the light source. A description of this facility is given elsewhere [21. For the measurements reported here the intense, continuous synchrotron radiation was monochromatized with two different monochromators: 8 keV photons were obtained with a non-dispersive crystal (Si(220)) monochromator [3,4]. Tunable radiation in the photon energy region 100—300 eV was provided by a grazing incidence grating monochromator [5,6]. The energy analysis of the photoemitted electrons was performed with a double-gas cylindrical mirror analyzer [4]. The polycrystalline gold samples were prepared by evaporation in situ under ultrahigh vacuum conditions, the pressure being better than 2 X l0~10torr. In fig. 1 we show an energy distribution curve for *
.
S
Research supported by the National Science Foundation (DMR 74-22230 and DMR 73-07692 A02) in cooperation with the Stanford Linear Accelerator Center and the Energy Research and Development Administration,
Au 45-DOUBLET
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-____________________________________ 6~INDI~G°ENE~Y 5eV) 80 70 Fig. 1. 90 The energy distribution of photocmitted electrons from gold at a photon energy of 160 eV is shown with the binding
energy referred to the Fermi level. The 5d (valence band), 5p and 4f levels are observed. Inserted is a detail of the 4fdoublet obtained at hv = 8000 eV showing a measured FWHM of .
e
a gold spectrum taken at a photon energy of 160 eV. Spectra for photon energies between 80 eV and 180 eV have been presented elsewhere [7]. 1-lere we want to concentrate on the linewidth of the 4f levels observed at binding energies of 84.0 eV (4f 712) and 87.7 eV (4f512). The spectrum shown in fig. I for binding energies 0—100 eV is taken with 0.8 eV instrumental resolution. When the resolution is improved to 0.25 eV the full width at half maximum (FWHM) value for each of two peaks in the doublet is 0.42 eV. Three factors contribute to the observed linewidth: the inherent width of the 4fphotoline; the resolution of the energy analyzer; and the resolution of the monochromator. The instrumental contributions can .
be estimated to be 0.25 eV, With arbitrary assumption of Gaussian line shapes the inherent linewidth would then be 0.2—0.3 eV. It can be assumed, however, 47
Volume 54A, number 1
PHYSICS LETTERS
that the line shapes are neigher pure Gaussian nor Lorentzian. We can, therefore, conclude that the intrinsic linewidth is certainly less than 0.3 eV but probably 0.10—0.2 eV. Our observations of the earlier reported [3,8,9]Au spectrum at a much higher photon energy, 8000 eV, support this conclusion. The insert in fig. I shows the 4f doublet obtained by the double crystal monochrornator and with a total instrumental resolution estimated to lie between 0.25 eV and 0.35 eV. The FWHM appears the same, 0.42 eV, as for ho = 160 eV, again indicating an inherent linewidth of 0.10—0.20 eV which is narrower than most photolines of other materials and other orbital symmetries [10]. No asymmetry is observed in the line shape of the doublet. A detailed knowledge of the exact form of the instrumental response function is required for a more accurate determination of the 4f inherent linewidth. In this connection we want to point out that Hufner and Wertheim [11] in a recent work have tried to eliminate part of the instrumental resolution with a deconvolution technique. The deconvolution leads to a FWHM of 0.4 eV for Au 4f 712 and Hufner and Wertheirn [11] draws the conclusion that the inherent linewidth is about 0.25 eV, in good agreement with our earlier published results [3,9]. The symmetric line shape is also in accord with our observation. McGuire [121 has recently published a theoretical calculation within the LS coupling scheme of, among other things, the 4flinewidth due to radiative and nonradiative Auger transitions. McGuire [12] obtains a linewidth of 0.232 eV for the Au 4f levels in very good agreement with our experimental determination. The narrow 4f level width is ascribed to the absence of Coster—Kronig transitions, it thus appears probable that McGuire’s model of the Auger transitions as the dominant decay mechanism for N-shell holes is correct and confirmed by our experiments. In conclusion we have established by two high-
Il August 1975
resolution photoemission experiments at two widely different photon energies (8000 eV and 160 eV. respectively) that the inherent linewidth of the gold 4f levels is certainly less than 0.3 eV, but probably 0.10.2 eV, in good agreement with McGuire’s theoretical estimate of 0.23 eV. The authors are grateful to the staff of the Stanford Synchrotron Radiation Project for their cooperation and assistance in this study, and to Drs. F.C. Brown, R.Z. Bach rach and M. Skibowski for fruitful help with operating the grazing incidence monochromator.
References [1] See K. Siegbahn, J. Electron Spectr. 5 (1974) 3 for a review and references to earlier work.
121 S. Doniach, I. Lindau, WE. Spicer and H. Winich, .1. Vac. Sd. Tech., to be published.
[31 1.ture Lindau, P. Pianetta, 250 (1974) 214. S. Doniach and WE. Spicer, Na[4] I. Lindau and P. Pianetta, unpublished. [51 F.C. Brown, R.Z. Bachrach, S.B.M. Ilagstrom, N. Lien (H. Pruett, in Vacuum Ultraviolet Physics, eds. FE. Koch, R. Haensel and C. Kunz, (Pergamon l’ress, 1974) p. 785~787. [6] R.Z. Bachrach, F.C. Brown and S.B.M. Hagstrom. i. Vac. Sd. Tech,. 12 (1975) 309.
[71 1. Lindau, P. Pianetta, K. Yu and WE. Spicer, Bull. Am. Phys. Soc., 20 (1975) 475 and to be published. [8] 1. Lindau, P. Pianetta, S. Doniach and WE. Spicer, Thirty-fourth Annual Conf. on Physical Electronics, Bell Laboratories, Murray Hill, N.J., February 1974. unpubushed. [9] 1. 1,indau, P. Pianeita, S. Doniach and WE. Spicer in Vacuum Ultraviolet Physics, eds. F..E. Koch, R. Haensel and C. Kunz, (Pergamon Press, 1974) p. 805. [10] W. Bambymek et al., Rev. Mod. Phys. 44 (1973) 716. ]t 1] S. Hufner and G.K. Wertheim. Phys. Rev. Bit (1975) 678. [121 Li. McGuire, Phys. Rev. A9 (1974) 1840.
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a a a a a a Volume 54A, number 1
PHYSICS LETTERS
11 August 1975 a a a
THE INTRINSIC LINEWIDTH OF THE 4f LEVELS IN GOLD AS DETERMINED BY PHOTOEMISSION I. LINDAU, P. PIANETTA, K. YU and W.E. SPICER Stanford Electronics Laboratories, Stanford University, Stanford, Ca. 94305, USA Received 23 May 1975 High resolution photoemission experiments have been performed using synchrotron radiation at two different photo energies, 8000 eV and 160 eV, on the 4f doublet of Au giving an inherent linewidth of less than 0.3 eV in good agreement with McGuire’s estimate of 0.23 eV.
In this letter we with to report on determination of the inherent linewidth of the 4f levels of gold by means of the photoemission technique. The 4f levels of gold have been studied extensively [11with conventional X-ray sources, Al Ka-radiation at hi.’ = 1486.7 eV and Mg K 0-radiation at hi.’ = 1253.6 eV. These techniques have an instrumental resolution of about 1 eV for studies of solids (0.5—0.6 eV for monochromatized Xrays [1]) and are too gross to give us reliable linewidths and shapes for inherently very narrow photolines. In this work we report on studies at two completely dif ferent photon energy ranges, 8 keV and obtained with synchrotron radiation and100—200 where theeV, instrumental resolution is considerably improved, Synchrotron radiation from the newly established SSRP (Stanford Synchrotron Radiation Project) was used as the light source. A description of this facility is given elsewhere [21. For the measurements reported here the intense, continuous synchrotron radiation was monochromatized with two different monochromators: 8 keV photons were obtained with a non-dispersive crystal (Si(220)) monochromator [3,4]. Tunable radiation in the photon energy region 100—300 eV was provided by a grazing incidence grating monochromator [5,6]. The energy analysis of the photoemitted electrons was performed with a double-gas cylindrical mirror analyzer [4]. The polycrystalline gold samples were prepared by evaporation in situ under ultrahigh vacuum conditions, the pressure being better than 2 X l0~10torr. In fig. 1 we show an energy distribution curve for *
.
S
Research supported by the National Science Foundation (DMR 74-22230 and DMR 73-07692 A02) in cooperation with the Stanford Linear Accelerator Center and the Energy Research and Development Administration,
Au 45-DOUBLET
41 455 217/2 / 1
~ It Il ~ It It
~Yr~T~UTION huu~I6OeV
25 1~u~8OOO 20
~ / tJ ~
J
042ev
g ~
~
[_~~3/~~
VA~DCE
-____________________________________ 6~INDI~G°ENE~Y 5eV) 80 70 Fig. 1. 90 The energy distribution of photocmitted electrons from gold at a photon energy of 160 eV is shown with the binding
energy referred to the Fermi level. The 5d (valence band), 5p and 4f levels are observed. Inserted is a detail of the 4fdoublet obtained at hv = 8000 eV showing a measured FWHM of .
e
a gold spectrum taken at a photon energy of 160 eV. Spectra for photon energies between 80 eV and 180 eV have been presented elsewhere [7]. 1-lere we want to concentrate on the linewidth of the 4f levels observed at binding energies of 84.0 eV (4f 712) and 87.7 eV (4f512). The spectrum shown in fig. I for binding energies 0—100 eV is taken with 0.8 eV instrumental resolution. When the resolution is improved to 0.25 eV the full width at half maximum (FWHM) value for each of two peaks in the doublet is 0.42 eV. Three factors contribute to the observed linewidth: the inherent width of the 4fphotoline; the resolution of the energy analyzer; and the resolution of the monochromator. The instrumental contributions can .
be estimated to be 0.25 eV, With arbitrary assumption of Gaussian line shapes the inherent linewidth would then be 0.2—0.3 eV. It can be assumed, however, 47
Volume 54A, number 1
PHYSICS LETTERS
that the line shapes are neigher pure Gaussian nor Lorentzian. We can, therefore, conclude that the intrinsic linewidth is certainly less than 0.3 eV but probably 0.10—0.2 eV. Our observations of the earlier reported [3,8,9]Au spectrum at a much higher photon energy, 8000 eV, support this conclusion. The insert in fig. I shows the 4f doublet obtained by the double crystal monochrornator and with a total instrumental resolution estimated to lie between 0.25 eV and 0.35 eV. The FWHM appears the same, 0.42 eV, as for ho = 160 eV, again indicating an inherent linewidth of 0.10—0.20 eV which is narrower than most photolines of other materials and other orbital symmetries [10]. No asymmetry is observed in the line shape of the doublet. A detailed knowledge of the exact form of the instrumental response function is required for a more accurate determination of the 4f inherent linewidth. In this connection we want to point out that Hufner and Wertheim [11] in a recent work have tried to eliminate part of the instrumental resolution with a deconvolution technique. The deconvolution leads to a FWHM of 0.4 eV for Au 4f 712 and Hufner and Wertheirn [11] draws the conclusion that the inherent linewidth is about 0.25 eV, in good agreement with our earlier published results [3,9]. The symmetric line shape is also in accord with our observation. McGuire [121 has recently published a theoretical calculation within the LS coupling scheme of, among other things, the 4flinewidth due to radiative and nonradiative Auger transitions. McGuire [12] obtains a linewidth of 0.232 eV for the Au 4f levels in very good agreement with our experimental determination. The narrow 4f level width is ascribed to the absence of Coster—Kronig transitions, it thus appears probable that McGuire’s model of the Auger transitions as the dominant decay mechanism for N-shell holes is correct and confirmed by our experiments. In conclusion we have established by two high-
Il August 1975
resolution photoemission experiments at two widely different photon energies (8000 eV and 160 eV. respectively) that the inherent linewidth of the gold 4f levels is certainly less than 0.3 eV, but probably 0.10.2 eV, in good agreement with McGuire’s theoretical estimate of 0.23 eV. The authors are grateful to the staff of the Stanford Synchrotron Radiation Project for their cooperation and assistance in this study, and to Drs. F.C. Brown, R.Z. Bach rach and M. Skibowski for fruitful help with operating the grazing incidence monochromator.
References [1] See K. Siegbahn, J. Electron Spectr. 5 (1974) 3 for a review and references to earlier work.
121 S. Doniach, I. Lindau, WE. Spicer and H. Winich, .1. Vac. Sd. Tech., to be published.
[31 1.ture Lindau, P. Pianetta, 250 (1974) 214. S. Doniach and WE. Spicer, Na[4] I. Lindau and P. Pianetta, unpublished. [51 F.C. Brown, R.Z. Bachrach, S.B.M. Ilagstrom, N. Lien (H. Pruett, in Vacuum Ultraviolet Physics, eds. FE. Koch, R. Haensel and C. Kunz, (Pergamon l’ress, 1974) p. 785~787. [6] R.Z. Bachrach, F.C. Brown and S.B.M. Hagstrom. i. Vac. Sd. Tech,. 12 (1975) 309.
[71 1. Lindau, P. Pianetta, K. Yu and WE. Spicer, Bull. Am. Phys. Soc., 20 (1975) 475 and to be published. [8] 1. Lindau, P. Pianetta, S. Doniach and WE. Spicer, Thirty-fourth Annual Conf. on Physical Electronics, Bell Laboratories, Murray Hill, N.J., February 1974. unpubushed. [9] 1. 1,indau, P. Pianeita, S. Doniach and WE. Spicer in Vacuum Ultraviolet Physics, eds. F..E. Koch, R. Haensel and C. Kunz, (Pergamon Press, 1974) p. 805. [10] W. Bambymek et al., Rev. Mod. Phys. 44 (1973) 716. ]t 1] S. Hufner and G.K. Wertheim. Phys. Rev. Bit (1975) 678. [121 Li. McGuire, Phys. Rev. A9 (1974) 1840.
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