Introduction to Photoelectron spectroscopy- UPS & XPS

Photoelectron spectroscopy- UPS & XPS.

Photoelectron spectroscopy detects the kinetic
energy of the electron escaped from the surface

1. Chapter 3. Photoelectron
spectroscopy-UPS & XPS
<ref>
1.Introduction to photoelectron spectroscopy / P.K. Ghosh,
Wiley, 1983
2.http://sciborg.uwaterloo.ca/course_notes/chemistry/chem
129/pdfs/ c129notes_chapter_04.pdf
3.Chimia55 (2001) 759–762

2. The Photoelectric Effect
•Albert Einsteinconsidered electromagnetic energy to
be bundled into little packets called photons.
Energy of photon = E = hv
Where, h = Planck constant ( 6.62 x 10
-34
J s )
v = frequency (Hz) of the radiation
–Photons of light hit surface electrons and transfer their energy
hv= B.E.+K.E.
–The energized electrons overcome their attraction and escape
from the surface
•Photoelectron spectroscopydetects the kinetic
energy of the electron escaped from the surface.
hv
e
-
(K.E.)

3. KE = h?-BE

4. •X-ray Photoelectron Spectroscopy (XPS)
-using soft x-ray (200-2000 eV)radiation to
examine core-levels.
•Ultraviolet Photoelectron Spectroscopy (UPS)
-using vacuum UV (10-45 eV)radiation to
examine valence levels.
Photoelectron spectroscopy
-a single photon in/ electron outprocess

6. Light sources: a Helium lamp emitting at 21.2 eV(He I radiation)
or 40.8 eV(He II radiation)
Ultraviolet Photoelectron Spectroscopy
(UPS)
Koopmans’Theorem
For a closed-shell molecule, the ionization energy of an electron in a
particular orbital is approximately equal to the orbital energy.
I.E.= E
orbital
= B.E.
such as H
2
?H
2
+
+ e
-
.... ,
....
EKhvEIor
EIhvEK
-=
-=

7. VibrationalFine Structure in PES

8. Vibrationalenergy states
)v'(....
)
2
1
v()
2
1
v()v(
2
+
D+=-Þ
+-+=
vib
eee
EEIEKhv
xhvhvE
where £GE
vib
+
(v’)=E
+
(v= v’) -E
+
(v= 0) is the extra (i.e., energy above
v= 0) vibrationalenergy of the ion.
Lines corresponding to different
vibrationalenergy levels of H
2
+
,
v = 0, 1, 2, 3, . . .,

9. Figure 4.4: He(I) UPS spectrum of HClgas.
1.Loss of a bonding electron decreases the bond order, increasing the
bond length in the resulting cationcompared to the parent molecule.
2. Loss of a nonbonding electron has no effect on bond order or bond
length.
3. Loss of an antibondingelectron increases the bond order, decreasing the
bond length of the cationcompared to the parent molecule.

10. £GN = N
+
–J”where N
+
and J”
represent the rotational quantum numbers of the ion and the neutral, respectively.

11. 1.074 ¢XA(£m
1s)
2
(£m
1s*)
2
(£m
2s)
2
(£m
2s*)
1
(£k
2p)
4
(£m
2p)
2
N
2
+
(B)
1.1749 ¢XA(£m
1s)
2
(£m
1s*)
2
(£m
2s)
2
(£m
2s*)
2
(£k
2p)
3
(£m
2p)
2
N
2
+
(A)
1.11642 ¢XA(£m
1s)
2
(£m
1s*)
2
(£m
2s)
2
(£m
2s*)
2
(£k
2p)
4
(£m
2p)
1
N
2
+
(X)
1.09769 ¢XA(£m
1s)
2
(£m
1s*)
2
(£m
2s)
2
(£m
2s*)
2
(£k
2p)
4
(£m
2p)
2
N
2
Bond LengthElectronic
Configuration
Molecule

12. Vibrationalfrequencies from UPS spectra of CO
and N
2
1706CO
+
(B)
1549CO
+
(A)
2200 ?weakly antibondingCO
+
(X)
2157CO
u (cm
-1
)molecule
1936N
2
+
(A)
2331N
2

13. UPS of the valence bands of solid H
2O molecule
J. Phys. C: Solid State Phys., 15 (1982) 2549-2558.
•Peak shift-charging effect
•Broadening-molecular solid
bonding and relaxation effects.

14. UPS of the valence bands of solid CO
2molecule
J. Phys. C: Solid State Phys., 15 (1982) 2549-2558.

17. Commonly employed x-ray sources :
Al K
aradiation
: hn= 1486.6 eVMg K
aradiation
: hn= 1253.6 eV

18. E
k
= hn–E
b
–j
j= work function
E
k
(KL
1
L
2
)= [E
b
(K) –E
b
(L
1
)]
–E
b
(L
2
) –j
X-ray

19. http://www.chem.qmw.ac.uk/surfaces/scc/scat5_3.htm
XPS spectrum obtained from a Pd metal sample
4d,5s
4p
4s

20. Spin-Orbit Splitting
in the region of the 3demission
(1s)
2
(2s)
2
(2p)
6
(3s)
2
(3p)
6
(3d)
10
....
¡÷(1s)
2
(2s)
2
(2p)
6
(3s)
2
(3p)
6
(3d)
9
....
L=2 andS=1/2 ?J= 5/2 and 3/2
Ground state

21. 1.the formal oxidation state of the atom
2.the local chemical and physical environment
Chemical Shifts

25. Pt metal
Pt(0)
Pt(II)
Pt(IV)

28. XPS and UPS Characterization of the
TiO
2
/ZnPcGly Heterointerface:
Alignment of Energy Levels
J. Phys. Chem. B 2002, 106, 5814-58
Molecular structure of dye ZnPcGly

29. Figure 1. X-ray photoelectron spectra of (a) TiO2 film and (b) TiO2/ ZnPcGly
interface deposited on transparent conducting oxide (TCO) glass.

30. Figure 2. X-ray photoelectron spectra in the Ti2p region (a) and O1s region (b)
for unsputteredand sputtered surface of TiO2 film. Spectra from bottom to top
correspond to cases where TiO2 was unsputtered, after sputtering for 0.5, 1.0,
1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 6.0, 8.5, and 9.0 min, respectively.
2p
3/2
(459.4 eV)
Ti(IV)2p
1/2
(465.1 eV)
Ti(II)
Ti(III)531.5 eV-hydroxyl groups or defective oxides
533.1eV-adsorbed water

31. Figure 4. XPS core level spectra of the TiO2 substrate and the dye
(ZnPcGly) overlayer. (a,b) Ti 2p and O 1s level of TiO2 film (spectra a)
and TiO2/ZnPcGly interface (spectra b); (c-e) C 1s, N 1s, and Zn 2p
emission of the dye ZnPcGly.

32. Figure 3. (a) Ultraviolet photoelectron spectra (He I) for the surface of unsputtered
and sputtered TiO2 films. (b) Ultraviolet photoelectron spectra (He II) for the
surface of unsputteredand sputtered TiO2 films.
VCB: valence band maximum
(3.28 eV)
E
g
= 3.28 eV
SO: secondary electron onset
(17.1 eV)
E
F
= 21.2 –17.1 = 4.1 eV
Defect Ti
3+
3d at ca. 0.8 eV

33. Figure 5. (a) Ultraviolet photoelectron spectra (He I) for surface of bare
TiO2 and TiO2/ZnPcGly. Inset: UPS in the valence band region forTiO2
and TiO2/ZnPcGly. (b) Ultraviolet photoelectron spectra (He II) for surface
of bare TiO2 and TiO2/ZnPcGly. Inset: UPS in the valence band region for
TiO2 and TiO2/ZnPcGly.
HOMOmax= 1.62 eV
Eg(ZnPcGly) = 1.82 eVfrom
a optical measurements.
?LUMOmax= -0.20 eV

34. Figure 6. Energy diagram for nanoporousTiO2 surface and
TiO2/ ZnPcGlyinterface determined from XPS and UPS
measurements.