Chapter 3. Photoelectron spectroscopy- UPS & XPS

<ref>1. Introduction to photoelectron spectroscopy / P.K. Ghosh,

Wiley, 19832. http://sciborg.uwaterloo.ca/course_notes/chemistry/chem

129/pdfs/ c129notes_chapter_04.pdf3. Chimia 55 (2001) 759–762

The Photoelectric Effect• Albert Einstein considered 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 spectroscopy detects the kinetic energy of the electron escaped from the surface.

hve- (K.E.)

KE = h? - BE

• 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 out process

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. = Eorbital = B.E.

such as H2 ? H2+ + e-

.... ,....

EKhvEIorEIhvEK−=

−=

Vibrational Fine Structure in PES

Vibrational energy states

)v'(....

)21

v()21

v()v( 2

+∆+=−⇒

+−+=

vib

eee

EEIEKhv

xhvhvE

where ΔEvib+(v’) = E+(v = v’) - E+(v = 0) is the extra (i.e., energy above

v = 0) vibrational energy of the ion.Lines corresponding to different vibrational energy levels of H2

+ , v = 0, 1, 2, 3, . . .,

Figure 4.4: He(I) UPS spectrum of HCl gas.

1. Loss of a bonding electron decreases the bond order, increasing the bond length in the resulting cation compared to the parent molecule.

2. Loss of a nonbonding electron has no effect on bond order or bond length.

3. Loss of an antibonding electron increases the bond order, decreasing the bond length of the cation compared to the parent molecule.

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

1.074 °A(σ1s)2(σ1s*)2 (σ2s)2

(σ2s*)1 (π2p)4(σ2p)2N2

+ (B)

1.1749 °A(σ1s)2(σ1s*)2 (σ2s)2

(σ2s*)2 (π2p)3(σ2p)2N2

+ (A)

1.11642 °A(σ1s)2(σ1s*)2 (σ2s)2

(σ2s*)2 (π2p)4(σ2p)1N2

+ (X)

1.09769 °A(σ1s)2(σ1s*)2 (σ2s)2

(σ2s*)2 (π2p)4(σ2p)2N2

Bond LengthElectronic Configuration

Molecule

Vibrational frequencies from UPS spectra of CO and N2

1706CO+ (B)

1549CO+ (A)

2200 ? weakly antibondingCO+ (X)2157COυ (cm-1)molecule

1936N2+ (A)

2331N2

UPS of the valence bands of solid H2O moleculeJ. Phys. C: Solid State Phys., 15 (1982) 2549-2558.

•Peak shift- charging effect•Broadening- molecular solid bonding and relaxation effects.

UPS of the valence bands of solid CO2 moleculeJ. Phys. C: Solid State Phys., 15 (1982) 2549-2558.

Commonly employed x-ray sources :

Al Kα radiation : hν = 1486.6 eVMg Kα radiation : hν = 1253.6 eV

Ek = hν – Eb – ϕϕ = work function

Ek(KL1L2) = [Eb(K) – Eb(L1)] – Eb(L2) – ϕ

X-ray

http://www.chem.qmw.ac.uk/surfaces/scc/scat5_3.htm

XPS spectrum obtained from a Pd metal sample

4d,5s4p

4s

Spin-Orbit Splitting

in the region of the 3d emission(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 and S = 1/2 ? J = 5/2 and 3/2

Ground state

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

Chemical Shifts

Pt metal

Pt(0)

Pt(II)

Pt(IV)

XPS and UPS Characterization of the TiO2/ZnPcGly Heterointerface: Alignment of Energy Levels

J. Phys. Chem. B 2002, 106, 5814-58

Molecular structure of dye ZnPcGly

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

Figure 2. X-ray photoelectron spectra in the Ti2p region (a) and O1s region (b) for unsputtered and 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.

2p3/2 (459.4 eV)Ti(IV) 2p1/2 (465.1 eV)

Ti(II)

Ti(III) 531.5 eV- hydroxyl groups or defective oxides 533.1eV- adsorbed water

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.

Figure 3. (a) Ultraviolet photoelectron spectra (He I) for the surface of unsputteredand sputtered TiO2 films. (b) Ultraviolet photoelectron spectra (He II) for the surface of unsputtered and sputtered TiO2 films.

VCB: valence band maximum(3.28 eV)Eg = 3.28 eV

SO: secondary electron onset(17.1 eV)EF = 21.2 – 17.1 = 4.1 eV

Defect Ti3+ 3d at ca. 0.8 eV

Figure 5. (a) Ultraviolet photoelectron spectra (He I) for surface of bareTiO2 and TiO2/ZnPcGly. Inset: UPS in the valence band region for TiO2 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 eV from a optical measurements.? LUMOmax = -0.20 eV

Figure 6. Energy diagram for nanoporous TiO2 surface and TiO2/ ZnPcGly interface determined from XPS and UPS measurements.