smr/1831-21 spring college on water in physics,...

SMR/1831-21

Spring College on Water in Physics, Chemistry and Biology

Hong-fei WANG

10 - 21 April 2007

State Key Lab. of Molecular Reaction Dynamics & Institute of ChemistryChinese Academy of Sciences,

Beijing, China

Nonlinear spectroscopy of water surfacesPart I - Second Harmonic Generation (SHG) from Liquid Interfaces

Nonlinear Spectroscopy of Water SurfacesPart I: Second Harmonic Generation (SHG)

from Liquid Interfaces

April 10-20, 2007@ICTP, Trieste, Italy

State Key Laboratory of Molecular Reaction Dynamics &

Institute of ChemistryChinese Academy of Sciences

http://[email protected]

Hong-fei Wang (王鸿飞)

LuGu Lake, Matrilineal Tribe, Sichuan & Yunnan Province, China October 30, 2004

The Matrilineal Tribe at LuGu Lake

LiJiang, China, Oct. 26, 200412th Laser Physics Conference

Prof. Ron Y.R. Shen and wife

I: Second Harmonic Generation (SHG) from Water Surfaces

– The liquid interface– Foundation of the surface specificity: SHG from the neat air/water

interface– SHG from the salt aqueous solution surfaces

II: Sum Frequency Generation VibrationalSpectroscopy (SFG-VS) from Liquid Surfaces

– SFG-VS as a polarizational, coherent and quantitative vibrationalspectroscopy

– Orientation measurement of the surface molecular groups– Anti-parallel double layer structure at dipolar liquid interfaces

III: Sum Frequency Generation VibrationalSpectroscopy (SFG-VS) of Water Surfaces

– Neat air/water interface– Neutral water surfaces– Salt aqueous solution surfaces– Charged aqueous solution surfaces

Outline : Part I

Structure of the Liquid Interface

Disordered? Dynamic? Molecularly thin?

Experimental measurement: Interface SpecificityWhat is in the Gibbs Adsorption layer of liquid interface?

Asymmetric forces at the interface

Bulk Surface

S.A. Rice, JCP, 82, 4391 (1985).

Structure & Role of the Liquid InterfaceThermodynamics: W. Gibbs, I. Langmuir

i ii

dG S dT V dp dA dnσ σ σ σ σγ μ= − + + +∑γ - surface tension OR surface energy

• γ is a measurable quantity, but• consensus is that γ is only a energy term

What is its microscopic or molecular origin?What is the role of the surface/interface?

Air/water: γ=73 mN/m, air/methanol: γ=23 mN/m

“Surface free energy due to the inward pull on the molecules at the surface is the fundamental property of surfaces; surface tension will be taken simply as its mathematical equivalent.”

What is Surface Tension?

Adam, N. K. The Physics and Chemistry of Surfaces, 3rd ed., 1941.

The axeltree modelAdamson, A. W.; Gast, A. P. Physical Chemistry of Surfaces, 6th ed., 1997.

How the inward pull becomes the lateral tension?

This model implies that strong orientational force at the interface, and the orientational force is the responsible for the surface tension.

However, Adam also stated in his book that it is impossible for the pure liquid surface to be well oriented.

Interface Properties

IndustrialApplications

CatalysisMaterials

EnvironmentBiological Interfaces

Structure,Conformation,

Dynamics

Fundamental Questions：Order or disorder at Interfaces?How to accurately measure?

OrientationAngle & Distribution

Molecular Structure, Conformation and Dynamics at Interfaces

X.W. Zhuang, Y.R. ShenPhys. Rev. B, 59, 12632, 1999

Water Interfaces: Motivations & BackgroundImportance of water interfaces:

biological, environmental, material…1. Protein folding 2. Membrane formation3. Micellar assembly 4. Wetting

• Hydrogen bonding structure• Hydrophobic & hydrophilic effect

Water, a comprehensive treatise：the physics and physical chemistry of waterF. Franks, Vol. 1-7, Plenum, New York 1972

air/water interface:spectroscopy, structure, dynamics

April, 2006

Second Harmonic Generaton (SHG) & Sum Frequency Generation (SFG)

SHG & SFG are Surface Specific Spectroscopic Probe

Sum Frequency Generation (SFG)Second Harmonic Generation (SHG)

S SH. -F. WangG G

X.W. Zhuang, Y.R. Shen, Phys. Rev. B 59, 12632, 1999

ωIRωVIS

ωSUMωIR

ωVISInterface

ωSUM=ωIR+ωVIS

The true laws cannot be linear nor can they be derived from such.

Symmetry dictates interactions.

Albert Einstein

Interfacial Specificity of SHG & SFG

ω1

ω2

ω2＋ω1

Interface(2) ( ) 0r ≠χ

anisotropicω1

ω2

(2) ( ) 0r ≠χω2＋ω1

isotropicω1

ω2

(2) ( ) 0r =χ(2) (2)( ) ( )r r= − −χ χ

(2) (2)( ) ( )r r= −χ χ

central symmetry

(2) ( ) 0r =χ

Interface specificityInterface sensitivity

0 : : ...E EE EEEμ μ α β γ= + + + +(1) (2) (3)

0 : : ...P P E EE EEEχ χ χ= + + + +

Y.R. Shen & T.F. Heinz et al., 1980’s

SFG-VS Spectra of Molecular InterfaceKey Information

Surface Density: Ns

Molecular Orientation

Molecular Spectroscopy

Kinetic & Dynamics,

etc…

Best for liquid surfaces and LB film studies

1. Liquid Interfaces Probed by Second-Harmonic and Sum-Frequency Spectroscopy, K. B. Eisenthal, Chem. Rev. 1996, 96, 1343-1360.

2. Liquid Interfaces: A Study by Sum-Frequency Vibrational Spectroscopy, P. B. Miranda and Y. R. Shen*, J. Phys. Chem. B 1999, 103, 3292-3307.

SFG

Int

ensi

ty (

a.u.

)

30002950290028502800Wavenumber (cm

-1)

ssp

sps

ppp

Air/acetone

M

GL

P

PMT

Ti:Sappire Femtosecond Laser

High Voltage

Interface

SR400A/D Card

Control Box

RS232

Mo

Se M

SM

SM

F F

M

B

S

T

PC

LL

HWP

OFT

PU

L

In-situ SHG & Surface Pressure Measurement

air/water interface@400nm ~200 counts/second

In-situ SHG & Surface Pressure Measurement

fs laser + single photon counting Fully computer controlled

BS

M7

M3

Sample holder

LD

VisIR

SF

M2

OPG

HARMONICUNIT

PL2143A

M1 M4

PDL1

L2

M5

M6

M9

M8ID2

ID3

ID4

W

GP

L4

F1

MN

PMT

COMPUTER

Magnetic base

θ-rotation

Spacer

X,Y,Z-stage

Tilt

M10

W

GP

L3

ID1

GP

Picosecond SFG Experimental Setup(EKSPLA, Lithuania)

Pulsewidth ～23ps Resolution < 6cm-1

IR &Vis tuning range 400nm-10μm

Picosecond SFG Spectrometer

EKSPLA, Lithuania

Improvement• Polarization control• Detection sensitivity

10x10-6

8

6

4

2

SFG

inte

nsity

(a.u

.)

37403720370036803660

Wavenumber(cm-1)

Ekspla Our

Broadband SFG Experimental Setup

“Vibrationally resolved sum-frequency generation with broad-bandwidth infrared pulses”, L. J. Richter, T. P. Petralli-Mallow, and J. C. Stephenson, Opt. Letts., 23, 1594, 1998.

Advantages: No scanning; Fast acquisation

Liquid Interface Studied with SHG

“The idea that raised my temperature was the possibility to actually “look at” the surface of a liquid, e.g., a solution or neat liquid/gas interface, something that had not been done before by a spectroscopic technique for reasons already noted.”

Liquid Interface Studies with SFG-VS

“In many cases, SFG is shown to be the only technique available that can provide detailed information about a liquid interface at the molecular level.”

J. Phys. Chem. B, 103, 3392-3307, 1999.

Surface SHG & SFG Studies (Molecular)1. Y.R. Shen Group (Physics, Berkeley) SHG(~1980) &

SFG(~1987), Tony F. Heinz2. Sipe Group (Physics Toronto) SHG(~1986) 3. Eisenthal Group (Chemistry, Columbia) SHG(~1984) &

SFG(~1992)4. Richmond Group (Chemistry, Oregon State) SHG(~1985)

& SFG(~1992)5. Bain Group (Chemistry, Oxford) & Davies Group

(Chemistry, Cambridge) SFG(~1994)6. Hirose Group (Chemistry, Tokyo) SFG(~1991)7. Laubereau Group (Physics, MPI, Germany) SFG(~1994)8. Shultz Group, Corn Group, Somorjai Group, etc……

1. Methodology with clear physical picture J. Chem. Phys., 119, 5226-36, 2003Chin. J. Chem. Phys., 17, 362-368, 2004

2. Quantitative treatment of local field factors J. Chem. Phys., to be submitted

3. Effective spectra assignment methodology J. Phys. Chem. B., 108, 7297-7306, 2004J. Phys. Chem. B., 109,14118-14129, 2005

4. Accurate determination of orientation Chin. Sci. Bull., 48(20), 2183-2187, 2003；49(9),899, 2004Chem. Phys. Lett., 406, 467-473, 2005 ; 408, 284-289, 2005

Problems & Solutions

* Quantitative spectral and orientational analysis in surface Sum Frequency Generation Vibrational Spectroscopy (SFG-VS), Hong-fei Wang* et al,Int. Rev. Phy. Chem., 24, 191-256, 2005.* Quantitative measurement and interpretation of optical second harmonicgeneration from molecular interfaces, Hong-fei Wang* et al, ,Phys. Chem. Chem. Phys., 8, 4041-4052, 2006.

Surface Specificity of SHG & SFG

Can SHG be effective surface probe of isotropic liquid?

Past consensus: No!

Rationale: Significant quadrupole/bulk contribution to SHG signal

Bloembergen et al, Phys. Rev., 174, 813 (1968) Eisenthal et al, J.P.C. 92, 5074 (1988)Shen et al, PRB, 35, 4420 (1987); PRB, 38, 7985 (1988) Shen et al, J.P.C. B. 104, 3349 (2000)

Surface contribution versus bulk contribution in surface nonlinear optical spectroscopy, Y.R. Shen, Appl. Phys. B, 68, 295–300 (1999)

SHG & SFG-VS as Interface Probe

1960-1980 Bleombergen et al Bulk contribution dominant1980-2000 Shen et al Bulk & surface comparable

Non-resonant： Quadrupole/Bulk > 60%Resonant & Reflective ： Surface Dipole > > 90%Resonant & transmission: Comparable & bulk significant

Precondition: Interface Dipolar Contribution

SHG from Air/Water InterfaceBenchmark case for non-resonant isotropic interface

Quadrupole/Bulk > 60%

Rationale:T dependence indicates

dipolar contribution The rest is quadrupole/bulk

T Dependence was an Artifact!

Phys. Rev. Lett. 70, 2313-2316 (1993)

This note was based on my experiment in the Eisenthalgroup. I found that the water vapor condensation on cell windows was the cause of SHG signal drop.

No Kleinman Symmetry at Water Surface

xzx zxxχ χ≠ Significant Quadrupole/Bulk Contributions

Kleinman symmetry：when far from resonance

xzx zxxχ χ=

What is Kleinman Symmetry?

Phys. Rev. 126, 1977-1979 (1962)

(2) (2)ijk jikχ χ=

(2)ijkχ

2ω ω

When both frequencies are far from electronic resonances

Rev. Mod. Phys. 35, 23-39 (1963)

Kleinman Symmetry or Conjecture?

(2) (2) (2)

2 2

2 2

21

4

ijk jik ijk

eg

eg

χ χ εχ

ω ωε

ω ω

− =

+≈ −

−

For Quartz:

694.3139.0

0.3eg

nmnm

ωωε

==

=

Quantum mechanical treatment:

Kleinman symmetry was not even validfor quartz crystal!!!

Franken & Shen called it only Kleinman Conjecture!!!

Symmetry is not Determined by Frequency

1. Calculation of nonlinear optical susceptibilities using diagrammatic perturbation theory,J. F. Ward, Rev. Mod. Phys., 37, 1-18, 1965.

2. A Unified Treatment of Selection Rules and Symmetry Relations for Sum-Frequency and Second Harmonic SpectroscopiesA. J. Moad and G. J. Simpson, J. Phys. Chem. B, 108, 3548-62, 2004.

Recent Discussion on Kleinman Symmetry

1. Using Kramer-Kronig dispersion relationship2. Universally used Uniaxial treatment was the reason3. Did not know of Franken’s early work

Missing Key Spot on the Original SHG PaperFranken et al., Phys. Rev. Lett. 7, 118-119 (1961).

Editor of PRL thought it was a small unwanted black spot, and simply removed it without consulting the authors.PRL never made any corrections.

Macroscopic & Microscopic Kleinman Symmetry

Conclusion: KleinmanSymmetry is rotationalinvariant!!!

(2)

1 ' ' '

(2)

1 ' ' '

NI I I

ijk il jm kn lmnI lmn i j k

NI I Ijm il kn mln jik

I lmn i j k

R R R

R R R

χ β

β χ

= =

= =

=

= =

∑ ∑

∑ ∑

For arbitrary rotational operation

lnlmn mβ β=if and only if

Directly make measurement on macroscopic symmetry properties a measurement on microscopic symmetry properties.

W. Zhang et al., J. Chem. Phys., 123, 224713, 2006.

Symmetry & Non-zero Polarizability Tensors

are asymmetric,Chiral terms are non-symmetric,Rest are symmetric terms

, , ,xzx xxz y zy yyzβ β β β(2) (2)

' ' ' ' ' '' ' '

ijk s ii jj kk i j ki j k

N R R R β= ∑χ

Water Symmetry & Polarizability TensorsWater H2OSymmetry C2v

Ground state1st Excited state

1 1X A1 1

2 1B A←

An Asymmetric Transition

acaβ the largest

No Molecular Kleinman Symmetry!!!

H H

a

bc

xzx zxxχ χ≠ Significant Quadrupole/Bulk Contributions

3

cos,

coscaa cbb

aca bcb

R Dθβ β

β β θ+

= =+

( 2) ( 2)( 2)

2( 2)( 1)( 2)

zxx

xzx

zzz

xzx

R D RR D R

R DR D R

χχχχ

− + +=− ∗ + +

+ −=− ∗ + +

acaβ bcbβ dominant

Then:

1400

1200

1000

800

600

400

200SHG signal intensity (arbitrary unit)

350300250200150100500Angle of incidence laser polarization (uint degree)

p detection s detection

Direct Data Treatment


Most accurate SHG measurement on the air/water Interface

• Two measurements• Two unknown parameters

Results

1. No evidence supports quadrupole/bulk contribution 2. Full treatment with dipole contribution satisfactory3. Consistent with SFG-VS results


W idth of Distribution (σ)0 10 20 30 40 50 60 70

App

aren

t Mol

ecul

ar T

ilt A

ngle

(deg

.)0

10

20

30

40

50

60

70

80

90

SHG magic angle

θo=0o

θo=10o

θo=20o

θo=30o

θo=40o

θo=50o

θo=60o

θo=70o

θo=80o

θo=89o

“ An SHG Magic Angle: Dependence of Second Harmonic Generation Orientation Measurements on the Width of the Orientation Distribution”G. J. Simpson and K. L. Rowlen, J. Am. Chem. Soc. , 121, 2635-2636 (1999).

SHG “Magic Angle” 39.2

Very broad Water

orientational

distribution?

80x10-3

60

40

20

0

-20

-40

-60

Effective Hyperpolarizability (Arb. Uni.)

908070605040302010

Orientation Angle(Deg)

PP 0 SP 0 45S 0 PP 10 SP 10 45S 10 PP 30 SP 30 45S 30

80x10-3

60

40

20

0

-20

-40

-60

Effective Hyperp

olarizability (Arb Uni)

908070605040302010

Orientation Angle(Deg)

PP SP 45S

J. Chem. Phys., to be submitted.

Orientational Distribution- phase and distribution analysis

speffχ 45s

effχ ppeffχ are with the same phase

With phase information, distribution may not be as broad.

YesYes (Shen)

Yes

Not supported Not supported

SummaryConsistent with SFG-VS measurementSHG only measures non-straddle type water

Shen et al., PRB, 66, 2051, 2002; JPCB, 104, 3349, 2000.

SFG-VS as Interface ProbeSignificant bulk contribution?

SHG & SFG-VS as Interface Probe

1960-1980 Bleombergen et al. Bulk contribution dominant

1980-present Shen et al Bulk & surface comparableNon-resonant： Quadrupole/Bulk > 60% ???Resonant & Reflective ： Surface Dipole > > 90%Resonant & transmission: Comparable ???

Precondition: Interface Dipolar Contribution

Surface contribution versus bulk contribution in surface nonlinear optical spectroscopy, Y.R. Shen, Appl. Phys. B, 68, 295–300 (1999)

The AnswerCan SHG be effective surface probe of isotropic liquid?

Answer: Yes!

Why: No Significant quadrupole/bulk contribution to SHG signal !!

J. Chem. Phys., 123, 224713, 2006.

Unified Symmetry Analysis of SHG & SFG Tensors

H.F. Wang* et. al., Phys. Chem. Chem. Phys., 8, 4041-4052, 2006.

( ) symmetric β1, β2, < > and [ ] asymmetric β3, { } chiral

1 2 3 1 2 3

1 2 1 2 3

2 3 1 2 3

1 2 1 2 3

( 2 ) ( 2 )( ) ( 2 )

2( 2 ) 2( 2 )( ) ( 2 )

zxx

xzx

zzz

xzx

DDDD

χ β β β β β βχ β β β β β

β β β β βχχ β β β β β

+ − − − −=− − − −

+ + − −=− − − −

3cos / cosD θ θ= ⟨ ⟩ ⟨ ⟩

Unified Treatment of SHG & SFG Tensors

1 2 3 4 6 1 2 3 4 6( , , , , , , , , , , , )v v v v v vC C C C C C C C C C C C∞ ∞

For a rotationally symmetric interface or film, molecules with all kinds of symmetry satisfy:

Orientational parameter:

Only two ratios between the β1, β2, β3 are needed


Unified Treatment of SHG (SFG) Tensors

Dominant βiSymmetry of the transition dipole

Resonance Frequency

β2, β3B or B1 or B2 or E

β1, β3A or A1ω

β3B or B1 or B2

β1, β2A or A12ω

β1, β2, β3 are further simplified by molecular symmetry

the Uniaxial assumption: Only β1 is non-zero

Generally assumed & used, but is never correct!!


Langmuir or L-B film SHG studies were almost based on

Whole practice in thin film SHG study needs re-evaluation.

the Kleinman Conjecture: β2=β3 when far from resonance

Jungwirth & Tobias, Chem. Rev., 2006, 106, 1259-1281

Recent Interests on the Electrolyte Aqueous Surface

“On the nature of ions at the liquid water surface”, Peterson & Saykally, Annu. Rev. Phys. Chem., 57, 333–64, 2006.

Directly Probe Surface Ions with Resonant SHG?Charge-transfer-to-solvent (CTTS) band

Iodide:Ferrocynide:

0 6.2 0.2 /adsG kcal molΔ = − ±0 6.8 0.3 /adsG kcal molΔ = − ± ?!

What about the even larger organic anions, such as phenolate anion?

22 3( ) ( ) cos cosR r cθ θ θ θ= = < > − < >

-δ distribution

1. Orientational distribution analysis

2. Orientational sensitive & insensitive measurements

3. Phase & interference analysis

2 2* * ( )* * *SFG s vis IRI A d R N I Iθ=

J. Chem. Phys., 119, 5226-36, 2003

Non-Resonant SHG Probe Water Molecule at the Air/Water Interface of Electrolyte Solution

10x10-6

8

6

4

2

0

SF

Inte

ns

ity

(a

.u.)

30002950290028502800Wavenumber ( cm

-1 )

Pure Water NaBr 3.0M NaBr 3.0M Sample Baked

SSP

Organic Contamnations

Liu & Allen et al., J. Phys. Chem. B, 108, 2252-60, 2004.

Treatment

1. Baking2. Activated carbon

800

600

400

200

0

SH

In

ten

sit

y (

a.u

.)

6005004003002001000Time (s)

Pure Water

PP Polarization

800

600

400

200

0

SH

In

ten

sit

y (

a.u

.)

6005004003002001000Time (s)

NaBr 0.1M

PP Polarization

800

600

400

200

0

SH

In

ten

sit

y (

a.u

.)

6005004003002001000Time (s)

NaBr 0.1M After Filter 0.22um

PP Polarization

SHG fluctuation measurementZhao & Eisenthal et al., Phys. Rev. Lett., 67, 2025, 1991.

Surface Particle Detection & Cleaning

• Baking: get rid of organic molecules• Filtering: get rid of surface particle

50

45

40

35

30

Ori

enta

tion

ang

le (

Deg

.)

543210Concentration (mol/l)

Na F NaCl NaBr Na I

Orientation angle vs Concentration3

2

1

0

Val

ue

of D


Na F NaCl NaBr Na I

Orientational parameter D vs Concentration

Water at Electrolyte Solution Interface

• Orientational parameter D: No Change• Conclusion: No significant change of average orientation

3cos / cosD θ θ= ⟨ ⟩ ⟨ ⟩

1000

800

600

400

200

Effe

ctiv

e W

ater

Den

sity

(a.

u.)


Na F NaCl NaBr Na I

Number of water vs Concentration

Water at Electrolyte Solution Interface

Surface water detected with SHG: increase with larger anions

• Consistent with SFG-VS measurement by Allen & Richmond• Inconsistent with the surface tension treend

Jungwirth & Tobias, Chem. Rev., 2006, 106, 1259-1281

Liu and Allen et al., JPCB, 2004, 108, 2252.Richmond et al., JPCB, 2004, 108, 5061.

Water at Electrolyte Solution Interface(SFG-VS at ssp polarization combination)

“Vibrational Spectroscopic Studies of Aqueous Interfaces: Salts, Acids, Bases, and Nanodrops”, Allen & Shultz et al., Chem. Rev. 2006, 106, 1155-1175

• Free O-H almost non-disturbed• Hydrogen bonded O-H changes

Facts: • Water orientation parameter:

no change2. Surface water detected by SHG:

increase more with larger anion3. Anti-correlation to surface tension results

Facts & Questions

Questions:Why thickness of the water layer increase?

Why anti-correlation with surface tension?

Are ions at the top layer?

Summary (Part I)1. SHG and SFG-VS is more surface specific than you think,

good for neat liquid surface studies.

2. SHG measures the average orientation of the water molecules (below the first layer).

3. SHG measurement for surface and thin films has to be reexamined from previous simple assumptions, e.g. uniaxialapproximation,…. Unified symmetry analysis is now available.

4. Surface water layer thickness increase with larger anions Consistent with SFG-VS measurement

I: Second Harmonic Generation (SHG) from Water Surfaces

– The liquid interface– Foundation of the surface specificity: SHG from the neat air/water

interface– SHG from the salt aqueous solution surfaces

II: Sum Frequency Generation VibrationalSpectroscopy (SFG-VS) from Liquid Surfaces

– SFG-VS as a polarizational, coherent and quantitative vibrationalspectroscopy

– Orientation measurement of the surface molecular groups– Anti-parallel double layer structure at dipolar liquid interfaces

III: Sum Frequency Generation VibrationalSpectroscopy (SFG-VS) of Water Surfaces

– Neat air/water interface– Neutral water surfaces– Salt aqueous solution surfaces– Charged aqueous solution surfaces

Outline : Part I

Graduate Students:SHGRao, Yi Zheng, De-shengZhou, Hong-tao Zhang, Wen-kaiBian, Hong-tao Xu, Yan-yan

SFG-VSLu, Rong Gan, WeiWu, Bao-hua Chen, HuaZhang, Zhen Feng, Ran-ranWu, Hui

Undergraduate Students:Lan, Zheng-gang Tao, Yi-songGuo, Xun-min Wu, DanZhang, Kai Ma, Jian-qiang

Staff member:Guo, Yuan

$$ NSFC, MOST, CAS $$

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