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Ups from adsorbed Hg on metal surfaces
L465
Surface Science 133 (1983) LMS-L468 North-Holland Publishing Company
SURFACE
SCIENCE
UPS FROM Shin-ichi
LETTERS
ADSORBED
ISHI and Yuichi
Hg ON METAL
SURFACES
OHNO
Research Institute for Catalysis, Hokkaido
University, Sapporo 060, Japan
Received 25 July 1983
The assignments for UPS from adsorbed mercury on metal surfaces are discussed by means of the molecular orbital dekption which reveals the characteristic features observed by Egelhoff et al. [Surface Sci. 54 (1976) 6701 in the region of 8-10 eV below the Fermi level.
In ultra-violet photoelectron spectroscopy (UPS) studies of mercury (Hg) on metal surfaces, two characteristic peaks derived from an adsorbed Hg are commonly observed at - 8 and - 10 eV below the Fermi level E,, and are indentified with the 5d,,, (*Ds,*) and 5d,,, ( *D3,*) states of Hg, respectively. The Hg 6s-like orbital observed at - 3.2 eV below E, by Egelhoff, Perry and Linnett [l] (in the following referred to as EPL) is not seen in the UP spectra taken by Brundle and Roberts [2] and Becker and Hagstrum [3]. From here on, we will focus our interest to a region 8-10 eV below E,, i.e., to the structure of the spectra in the region of the Hg 5d peaks which EPL have revealed from the angle-resolved UP (ARUP) spectra. They showed that the broadening of the 2D, z and *D3,z peaks could not be explained by instrumental factors and that i [l]. The purpose of this letter the D5,* was further split into two components is to explain the above characteristics from the UP spectra by EPL by means of our model which we propose to the study of UPS in the Xc/metal system [4]. Herbst has given an explanation for some of the observed features from ARUPS by EPL as well as for the Xe/W(lOO) system by means of the crystal field theory within spin-orbit (S-O) interaction [5], which, however, is rejected from both theoretical [6,7] and experimental [8] considerations at present. According to the above assignments of previous work [l-3,5], the electronic configuration (5d)” of an adsorbed Hg is degenerate as it is in the spherically symmetric field of gaseous Hg. The 5d levels upon ionization are split into the 5d,,, (j = 5/2) and the 5d,,, (j = 3/2) states - due to S-O interaction which are sixfold and fourfold degenerate, respectively. The 5d,,, are further split into mj = f 5/2 and mj = + 3/2 components due to perturbation. In the present treatment, the 5d levels of an adsorbed Hg before photoionization will be split into u, n and 6 sublevels by the cylindrically symmetric field in the 0039-6028/83/0000-0000/$03.00
0 1983 North-Holland
L466
S. Ishr, Y. Ohm / UPS from adsorbed Hg on merul
surfmes
adsorption system. In what follows, the - 8 and - 10 eV peaks below E, in the UP spectra are represented as p1 and pz, respectively. Let us recall the 5d electronic levels of a gaseous Hg before considering the adsorption system. The electronic configuration of the ground state of the Hg atom is a closed shell as . . .(5d)‘“(6s)2, which term is denoted as ‘S. For the Hg+ ion - by the ejection of one of the 5d electrons - the electronic configuration of the ground state is an open shell as . . .(5d)9(6s)2, denoted as 2D, which gives rise to 2DS,2 and 2D1,2 states due to S-O interaction. The . . . ionization energies of the 2D5,2 and 2D3,2 states are 14.84 and 16.71 eV 191, respectively. The interval ]2D5,2-1Ds,2] between the ’ D5,2 and 2D3,2 states of S-O splitting is 1.87 eV. For the adsorption system (Hg/metal), the electronic configuration of the ground state is described as . . .(1~5d)~(m5d)~(s5d)~(o6s)‘, which term is denoted as ‘Z, where the ‘z-axis is perpendicular to the metal surface. The a5d, n5d and 65d orbitals are mainly composed of the atomic 5d orbitals, and the ~6s is composed of the 6s orbital of Hg and the metal orbitals with u-symmetry. The electronic terms with configurations . . . (u5d)‘( ~5d)~( 65d)4( u6s)‘, . . .(u5d)2(r5d)3(S5d)4(u6s)2 and . . .(u5d)2(r5d)4(s5d)3(u6s)2 for the ionic states of the adsorbate are denoted as 2.X, ‘II and 2A, respectively. The ‘II and 2A are split into 2113,2, 217 1,2 (inverted doublet, 211,,2 above 2113,2) and 2A5,2, 2A3/2 (“3/2 due to the S-O interaction, respectively. The above 2A 5,2 ) intervals 12113,2- 211,,2 1and 1‘A 5,2- 2A3,2 ] are 2a and ~CX,respectively, where (Y is the S-O interaction parameter which was derived from the experimental atomic levels so that the atomic 2D5,2 and 2D,,2 states have energies of - 2a and ~CX,respectively, i.e., (Y= 0.374 eV. In order to determine the energy levels for ionic states (final states in UPS), it is necessary to consider the electronic states which include S-O interaction. We have adopted a simple model which includes the S-O interaction, a procedure which has been used on diatomic molecules of rare gases [lo], Zn and Hg [ll], etc. The energy levels for ionic states are easily obtained by solving the secular equation IH,, - ES,,) = 0,
Hi, = EJjj
+ H,$,
(1)
where aii is Kronecker’s delta and the E, (i = 1, 2,. . . ,5) correspond to the spin-independent electronic energy eigenvalue for each of the states, e.g., E, for the 2z state, E, and E, (E, = E3) for 211, E, and E, ( E4 = E,) for ‘A. The Hi; is the S-O interaction matrix whose diagonal elements are 0, CY,-(Y, 2a and -20~ for the 2&,2, 2I7,,2, 2113,2, 2A3,2 and 2As,2 states, respectively. The non-zero off-diagonal elements are (2nl,21Ejso12zII,2)~
and their conjugate
(2n3,2iJjso12A3,2)~
terms. The secular equation
(1) is explicitly
written
by
I.467
S. Ishi, Y. Ohno / UPS from oakorbed Hg on metal surjuces
(A)
(El
(Cl
(A)
(Cl
(8)
(al
(b)
(c)
Fig. 1. Energy level splittings due to S-O interaction in the cylindrically symmetric field (in units of ev): (I) 12Z-2Al = 0.3 and 1217-*Al = 0.15; (II) 128-2A1 = 0.3 and 1217-2Al = 0; (A) without S-O interaction; (B) with S-O diagonal terms only; (C) with S-O off-diagonal terms included. (III) Energy level splittings by Herbst: (a) *D state; (b) with S-O interaction; (c) splittings due to perturbation (crystal field).
E, - E
-6ci
0
0
0
-&x
E,+a--E
0
0
0
0
0
E,-&-E
-213
0
0
0
-2a
E,+2a-E
0
0
0
0
0
ES-2a-E
=
0.
(2)
The resulting energy level schemes are shown in fig. 1, where we have assumed that the intervals ]*2- *A 1and ]*II- *A1 for the cases (I) and (II) in fig. 1 are 0.3, 0.15 eV and 0.3, 0 eV, respectively. For comparison, the energy level splittings by Herbst [5] are shown in case (III) in fig. 1. The pi and p2 peaks have three (*&,,, 2l7;,2, *As,*) and two (*II;,,, *A;,*) components, respectively, where the 2B;,2, *II;,*, *II;,* and 2A;,2 are expressed as follows: *z
l/2
=
4%2)
*III’1/2
=
-c(*q2)
*q/2
=
+7,2)
‘A;,,
=
-8(*J7,2)
The coefficients
+b(*G,*)?
+47,2)~
+f(ZA3,2)9
+q2A3,2).
a , . . . , h are determined
as eigenvector
problems.
L468
S. Ishi, Y. Ohno /
UPSfromoakorbed
Hg on metul surjuces
From (C), for both cases (I) and (II) in fig. 1, it is evident that the interval 12X;,2-2A5,21 is larger than the interval 1217~,2-2A~,21, i.e., the pi peak is broadened more than the p2 peak, assuming that each of the states has the same full width at half maximum. Furthermore, the p2 peak observed is probably a single peak since the energy resolution of the 127” analyser, which EPL have employed, is 0.20 eV at 15 eV. If the interval l’A-‘IIl is smaller than the interval 12Z-‘ITl, the p, peak seems to be split into two components because of the reduced interval )211;,2- 2A5,2l, as EPL have observed. A limited case is case (II) in fig. I, where the ‘I7 and ‘A are degenerate. Finally, when the ‘2, ‘II and 2A are degenerate, S-O splittings reduce to (b) for case (III) in fig. 1. References [l] [2] [3] [4] [5] [6] (71 [8] [9] [lo] [ll]
W.F. Egelhoff, Jr., D.L. Perry and J.W. Linnett, Surface Sci. 54 (1976) 670. C.R. Brundle and M.W. Roberts, Proc. Roy. Sot. (London) A331 (1972) 383. G.E. Becker and H.D. Hagstrum, J. Vacuum Sci. Technol. 10 (1973) 31. S. Ishi and Y. Ohno, J. Electron Spectrosc. Related Phenomena, to be published. J.F. Herbst, Phys. Rev. B15 (1977) 3720. J.A.D. Matthew and M.G. Devey, J. Phys. C9 (1976) L413. P.R. Antoniewicz, Phys. Rev. Letters 38 (1977) 374. J.L. Erskine, Phys. Rev. B24 (1981) 2236. C.E. Moore, Atomic Energy Levels, Natl. Bur. Std. (US) Circ. 467 (1958). J.S. Cohen and B. Schneider, J. Chem. Phys. 61 (1974) 3230. P.J. Hay, T.H. Dunning, Jr. and R.C. Raffenetti, J. Chem. Phys. 65 (1976) 2679.
Surface Science 133 (1983) LMS-L468 North-Holland Publishing Company
SURFACE
SCIENCE
UPS FROM Shin-ichi
LETTERS
ADSORBED
ISHI and Yuichi
Hg ON METAL
SURFACES
OHNO
Research Institute for Catalysis, Hokkaido
University, Sapporo 060, Japan
Received 25 July 1983
The assignments for UPS from adsorbed mercury on metal surfaces are discussed by means of the molecular orbital dekption which reveals the characteristic features observed by Egelhoff et al. [Surface Sci. 54 (1976) 6701 in the region of 8-10 eV below the Fermi level.
In ultra-violet photoelectron spectroscopy (UPS) studies of mercury (Hg) on metal surfaces, two characteristic peaks derived from an adsorbed Hg are commonly observed at - 8 and - 10 eV below the Fermi level E,, and are indentified with the 5d,,, (*Ds,*) and 5d,,, ( *D3,*) states of Hg, respectively. The Hg 6s-like orbital observed at - 3.2 eV below E, by Egelhoff, Perry and Linnett [l] (in the following referred to as EPL) is not seen in the UP spectra taken by Brundle and Roberts [2] and Becker and Hagstrum [3]. From here on, we will focus our interest to a region 8-10 eV below E,, i.e., to the structure of the spectra in the region of the Hg 5d peaks which EPL have revealed from the angle-resolved UP (ARUP) spectra. They showed that the broadening of the 2D, z and *D3,z peaks could not be explained by instrumental factors and that i [l]. The purpose of this letter the D5,* was further split into two components is to explain the above characteristics from the UP spectra by EPL by means of our model which we propose to the study of UPS in the Xc/metal system [4]. Herbst has given an explanation for some of the observed features from ARUPS by EPL as well as for the Xe/W(lOO) system by means of the crystal field theory within spin-orbit (S-O) interaction [5], which, however, is rejected from both theoretical [6,7] and experimental [8] considerations at present. According to the above assignments of previous work [l-3,5], the electronic configuration (5d)” of an adsorbed Hg is degenerate as it is in the spherically symmetric field of gaseous Hg. The 5d levels upon ionization are split into the 5d,,, (j = 5/2) and the 5d,,, (j = 3/2) states - due to S-O interaction which are sixfold and fourfold degenerate, respectively. The 5d,,, are further split into mj = f 5/2 and mj = + 3/2 components due to perturbation. In the present treatment, the 5d levels of an adsorbed Hg before photoionization will be split into u, n and 6 sublevels by the cylindrically symmetric field in the 0039-6028/83/0000-0000/$03.00
0 1983 North-Holland
L466
S. Ishr, Y. Ohm / UPS from adsorbed Hg on merul
surfmes
adsorption system. In what follows, the - 8 and - 10 eV peaks below E, in the UP spectra are represented as p1 and pz, respectively. Let us recall the 5d electronic levels of a gaseous Hg before considering the adsorption system. The electronic configuration of the ground state of the Hg atom is a closed shell as . . .(5d)‘“(6s)2, which term is denoted as ‘S. For the Hg+ ion - by the ejection of one of the 5d electrons - the electronic configuration of the ground state is an open shell as . . .(5d)9(6s)2, denoted as 2D, which gives rise to 2DS,2 and 2D1,2 states due to S-O interaction. The . . . ionization energies of the 2D5,2 and 2D3,2 states are 14.84 and 16.71 eV 191, respectively. The interval ]2D5,2-1Ds,2] between the ’ D5,2 and 2D3,2 states of S-O splitting is 1.87 eV. For the adsorption system (Hg/metal), the electronic configuration of the ground state is described as . . .(1~5d)~(m5d)~(s5d)~(o6s)‘, which term is denoted as ‘Z, where the ‘z-axis is perpendicular to the metal surface. The a5d, n5d and 65d orbitals are mainly composed of the atomic 5d orbitals, and the ~6s is composed of the 6s orbital of Hg and the metal orbitals with u-symmetry. The electronic terms with configurations . . . (u5d)‘( ~5d)~( 65d)4( u6s)‘, . . .(u5d)2(r5d)3(S5d)4(u6s)2 and . . .(u5d)2(r5d)4(s5d)3(u6s)2 for the ionic states of the adsorbate are denoted as 2.X, ‘II and 2A, respectively. The ‘II and 2A are split into 2113,2, 217 1,2 (inverted doublet, 211,,2 above 2113,2) and 2A5,2, 2A3/2 (“3/2 due to the S-O interaction, respectively. The above 2A 5,2 ) intervals 12113,2- 211,,2 1and 1‘A 5,2- 2A3,2 ] are 2a and ~CX,respectively, where (Y is the S-O interaction parameter which was derived from the experimental atomic levels so that the atomic 2D5,2 and 2D,,2 states have energies of - 2a and ~CX,respectively, i.e., (Y= 0.374 eV. In order to determine the energy levels for ionic states (final states in UPS), it is necessary to consider the electronic states which include S-O interaction. We have adopted a simple model which includes the S-O interaction, a procedure which has been used on diatomic molecules of rare gases [lo], Zn and Hg [ll], etc. The energy levels for ionic states are easily obtained by solving the secular equation IH,, - ES,,) = 0,
Hi, = EJjj
+ H,$,
(1)
where aii is Kronecker’s delta and the E, (i = 1, 2,. . . ,5) correspond to the spin-independent electronic energy eigenvalue for each of the states, e.g., E, for the 2z state, E, and E, (E, = E3) for 211, E, and E, ( E4 = E,) for ‘A. The Hi; is the S-O interaction matrix whose diagonal elements are 0, CY,-(Y, 2a and -20~ for the 2&,2, 2I7,,2, 2113,2, 2A3,2 and 2As,2 states, respectively. The non-zero off-diagonal elements are (2nl,21Ejso12zII,2)~
and their conjugate
(2n3,2iJjso12A3,2)~
terms. The secular equation
(1) is explicitly
written
by
I.467
S. Ishi, Y. Ohno / UPS from oakorbed Hg on metal surjuces
(A)
(El
(Cl
(A)
(Cl
(8)
(al
(b)
(c)
Fig. 1. Energy level splittings due to S-O interaction in the cylindrically symmetric field (in units of ev): (I) 12Z-2Al = 0.3 and 1217-*Al = 0.15; (II) 128-2A1 = 0.3 and 1217-2Al = 0; (A) without S-O interaction; (B) with S-O diagonal terms only; (C) with S-O off-diagonal terms included. (III) Energy level splittings by Herbst: (a) *D state; (b) with S-O interaction; (c) splittings due to perturbation (crystal field).
E, - E
-6ci
0
0
0
-&x
E,+a--E
0
0
0
0
0
E,-&-E
-213
0
0
0
-2a
E,+2a-E
0
0
0
0
0
ES-2a-E
=
0.
(2)
The resulting energy level schemes are shown in fig. 1, where we have assumed that the intervals ]*2- *A 1and ]*II- *A1 for the cases (I) and (II) in fig. 1 are 0.3, 0.15 eV and 0.3, 0 eV, respectively. For comparison, the energy level splittings by Herbst [5] are shown in case (III) in fig. 1. The pi and p2 peaks have three (*&,,, 2l7;,2, *As,*) and two (*II;,,, *A;,*) components, respectively, where the 2B;,2, *II;,*, *II;,* and 2A;,2 are expressed as follows: *z
l/2
=
4%2)
*III’1/2
=
-c(*q2)
*q/2
=
+7,2)
‘A;,,
=
-8(*J7,2)
The coefficients
+b(*G,*)?
+47,2)~
+f(ZA3,2)9
+q2A3,2).
a , . . . , h are determined
as eigenvector
problems.
L468
S. Ishi, Y. Ohno /
UPSfromoakorbed
Hg on metul surjuces
From (C), for both cases (I) and (II) in fig. 1, it is evident that the interval 12X;,2-2A5,21 is larger than the interval 1217~,2-2A~,21, i.e., the pi peak is broadened more than the p2 peak, assuming that each of the states has the same full width at half maximum. Furthermore, the p2 peak observed is probably a single peak since the energy resolution of the 127” analyser, which EPL have employed, is 0.20 eV at 15 eV. If the interval l’A-‘IIl is smaller than the interval 12Z-‘ITl, the p, peak seems to be split into two components because of the reduced interval )211;,2- 2A5,2l, as EPL have observed. A limited case is case (II) in fig. I, where the ‘I7 and ‘A are degenerate. Finally, when the ‘2, ‘II and 2A are degenerate, S-O splittings reduce to (b) for case (III) in fig. 1. References [l] [2] [3] [4] [5] [6] (71 [8] [9] [lo] [ll]
W.F. Egelhoff, Jr., D.L. Perry and J.W. Linnett, Surface Sci. 54 (1976) 670. C.R. Brundle and M.W. Roberts, Proc. Roy. Sot. (London) A331 (1972) 383. G.E. Becker and H.D. Hagstrum, J. Vacuum Sci. Technol. 10 (1973) 31. S. Ishi and Y. Ohno, J. Electron Spectrosc. Related Phenomena, to be published. J.F. Herbst, Phys. Rev. B15 (1977) 3720. J.A.D. Matthew and M.G. Devey, J. Phys. C9 (1976) L413. P.R. Antoniewicz, Phys. Rev. Letters 38 (1977) 374. J.L. Erskine, Phys. Rev. B24 (1981) 2236. C.E. Moore, Atomic Energy Levels, Natl. Bur. Std. (US) Circ. 467 (1958). J.S. Cohen and B. Schneider, J. Chem. Phys. 61 (1974) 3230. P.J. Hay, T.H. Dunning, Jr. and R.C. Raffenetti, J. Chem. Phys. 65 (1976) 2679.