Geok Mei CHONGMaster Candidate of Advanced Spectroscopy in Chemistry University of Leipzig, ASC Network

4th December 2009

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Outline1. Principle of XPS & ARXPS

Instrumentation

2. Depth Profile by ARXPS3. XPS and ARXPS applied to fluid analysis

Experimental setup for fluid analysis

4. Application of in research Surfactant Water Biological molecules

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Principle of XPS & ARXPSPhotoelectric effect

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Required spectrometer components

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Electron energy analyzer & detector

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1. Radius of curvature is dependent on kinetic energy of electron.

2. Channel electron multipliers

What information is learned from XPS?

1. Elemental Identification2. Chemical State Identification3. Quantification4. Mapping5. Depth profile

ARXPS

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Depth profile

z = depthλ = mean free pathθ = emission angleλ’ = observation depth

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How is z related to Is ?

λ’ = λ cos θ

Io attenuated exponentially

according to Beer Lambert law

the observed depth information varies with photoelectron detection angle θ

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Angular resolved XPS

z = depthλ = mean free pathθ = emission angle (relative to surface

normal)

λ’ = observation depthλ’ = λ cos θ

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Angular resolved XPS

λ’ = λ cos θz = depth

λ = mean free pathθ = emission angle (relative to

surface normal)

λ’ = observation depth

the observed depth information varies with photon energy

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Angular resolved XPSQuantification

The Observed photoelectron intensity of element A:

A fitting processIcal -> Iobs

XPS & ARXPS applied to fluid

First performed by H. Siegbahn, K. Siegbahn and colleagues.

Complete separation between PE signals from liquid and vapour using a beam of liquid formamide.

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H. Siegbahn, K. Siegbahn, J. Electron Spectrosc. Rel. Phenomena 2 1973, 319

XPS & ARXPS applied to fluidA challenging investigation

Needs for producing “well-behaved” liquid beam in vacuum

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1. Liquids of sufficient low vapour pressure (< 1 Torr).• Cooled to -40 0C• Droplet formation for high vapour

pressure• Loss of PE when absorbed by the vapor

2. Surface smoothness3. Sample charging effect

XPS & ARXPS applied to fluidHow to produce “well behaved” liquid beam?

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Rotating metal disc

• Allowed studies of liquids with low vapour pressure.

Liquid lamella

• Produced flat liquid surface

XPS & ARXPS applied to fluidHow to produce “well behaved” liquid beam?

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Liquid microjet

• Vacuum jet consists of a smooth continuous region of liquid water, which decays into droplet at a distance of approximately 5mm.

• Allowed studies of liquids with higher vapour pressure, example: water.

• However, using HeI radiation, only the outer valence region could be probed.

The size of the jet was reduced to the μ size range.

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Angular resolved XPSQuantification

The Observed photoelectron intensity of element A:

(Eq 1)

Requires accurate knowledge of photoionization cross section and angular characteristics of emission direction

A fitting process

Ical -> Iobs

Application of ARXPS in researchSurfactant

Concentration depth profile of TBAI in FA from C 1s

Used chemical shift to evaluate the relative intensities due to TBAI and FA.

The contributions from TBAI, FAliq and FAgas are separated.

The ratio of the peak area of TBAI to that of FAliq are determined for many combinations of photonenergies and observation angles. 16

F.Eschen, M. Heyerhoff, H. Morgner, J. Vogt, J. Phys. Condens. Matter 7 (1995) 1961

Application of ARXPS in researchSurfactant

Concentration depth profile of TBAI in FA from C 1s

Single molecular layer is assumed to be 1.5 Å thick.

Large decrease in salt conc. after 3rd layer.

The thickness of the enhanced salt conc. was estimated to be about 12 Å.

Given diameter of TBA+ is 9.5 Å, the thickness of enhanced salt corresponds to 1 monolayer of salt.

TBA+ ions have preferred orientation near the surface17

F.Eschen, M. Heyerhoff, H. Morgner, J. Vogt, J. Phys. Condens. Matter 7 (1995) 1961

Application of ARXPS in researchBehaviour of hydroxide at the water interface

O1s XPS (microjet) spectra of NaOH 0.2 – 2M aqueous solutions

Spectral contributions from H2O(gas), H2O(aq), and OH-(aq) @ 600eV were assigned.

Zoom into the OH-(aq) 2pπ.

Fully quantitative of OH-

intensity was not visible here as the intensity of O1s peak was small.

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Bernd Winter et. al., Chemical Physics Letters 474 (2009) 241–247

Application of ARXPS in researchBehaviour of hydroxide at the water/vapour interface

Oxygen 1s XPS spectra of NaOH 0.2 – 2 M aqueous solutions

OH-(aq) 2pπ and OH-

(aq) O1s photoelectron signal as function of OH- conc.

Linear dependence of the interfacial OH-

density on bulk conc.

MD results support PE experiments findings.

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Bernd Winter et. al., Chemical Physics Letters 474 (2009) 241–247

Application of ARXPS in researchBehaviour of hydroxide at the water/vapour interface

Experimental and computational calculations suggest that:

OH- do not have any special surface binding site.

There is linear dependence of the interfacial OH- signal on its bulk concentration.

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Bernd Winter et. al., Chemical Physics Letters 474 (2009) 241–247

Some earlier studies suggest that OH- strongly accumulates within the interfacial region (cluster?). The debates are still

on going …

Applications in biological MoleculesN1s PE spectral of 0.5m lysine at diff. pH

Biological molecules in water environment is very challenging in monitoring local charge density.

Microscopic structure of aa is sensitive to pH

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D. Nolting, E.F. Aziz, N. Ottosson, M. Faubel, I.V. Hertel, B. Winter, J. Am.Chem. Soc. 129 (2007) 14068

Applications in biological MoleculesN1s PE spectral of 2m imidazole aqueous at diff. pH

Structural changes can be faster than time resolution of NMR (10-5 s).

At high pH, proton exchange between the 2 N site on time scale of 10-12 s.

The 2 chemically pseudo-equivalent N atoms resolved.

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D. Nolting, N. Ottosson, M. Faubel, I.V. Hertel, B. Winter, J. Am. Chem. Soc. 130 (2008) 8150.

ConclusionARXPS is highly surface sensitive. Possible to probe

depth profile as small as 1.5 nm.

ARXPS is very sensitive to study interfacial at various depths at microscopic scale.

Still challenging to deal with fluid samples, especially high vapor pressure solution.

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An interesting and challenging field …

Quiz !

1. Which peak is caused by inelastic scattering?

2. Why XPS is surface sensitive?

3. What is the main factor that affect the spatial resolution of XPS?

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References1. H. Siegbahn, K. Siegbahn, J. Electron Spectrosc. Rel.

Phenomena, 2 (1973), 319

2. H. Siegbahn, S. Svensson and M. Lundholm, J. Electron Spectrosc. Rel. Phenomena 24 (1981), p. 205

3. Eschen F, Heyerhoff M, Morgner H and Vogt J (1995) J. Phys.: Condens. Matter 7 1961

4. Faubel M and Steiner B Ber. Bunsenges. Phys. Chem. 96 (1992)1167

5. Bernd Winter et. al., Chemical Physics Letters 474 (2009) 241–247

6. B. Winter, M. Faubel, Chem. Rev. 106 (2006) 1176

7. D. Nolting, E.F. Aziz, N. Ottosson, M. Faubel, I.V. Hertel, B. Winter, J. Am. Chem. Soc. 129 (2007) 14068

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Just kidding

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